book-data communication

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(Devender Mahajan)

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Table of Contents

CHAPTER-01 DATA COMMUNICATION ____________________________________________________ 4

CLASSIFICATION OF DATA ___________________________________________________________________ 4 DATA REPRESENTATION ____________________________________________________________________ 4 DATA COMMUNICATION ____________________________________________________________________ 5 COMMUNICATION CHANNEL _________________________________________________________________ 5 MODES OF DATA TRANSMISSION _____________________________________________________________ 5 TRANSMISSION MODES _____________________________________________________________________ 6

CHAPTER-02 TRANSMISSION MEDIA ______________________________________________________ 7

FACTOR RELATING TRANSMISSION MEDIA ______________________________________________________ 7 TRANSMISSION IMPAIRMENTS ________________________________________________________________ 7 GUIDED MEDIA ___________________________________________________________________________ 7 FIBER OPTICS _____________________________________________________________________________ 9 OPTICAL MODES _________________________________________________________________________ 10 UNGUIDED TRANSMISSION MEDIA ___________________________________________________________ 11 WAVE PROPAGATION _____________________________________________________________________ 11

CHAPTER-03 NETWORK CONCEPTS AND TOPOLOGY ____________________________________ 15

WHAT IS NETWORK? ______________________________________________________________________ 15 NETWORK ARCHITECTURE _________________________________________________________________ 15 OBJECTIVES OF GOOD NETWORK ____________________________________________________________ 16 TOPOLOGY ______________________________________________________________________________ 16 APPLICATION OF NETWORKING ______________________________________________________________ 18

CHAPTER-04 MULTI-CHANNEL DATA COMMUNICATION _________________________________ 19

MODULATION ___________________________________________________________________________ 19 MODEMS ______________________________________________________________________________ 20 TYPE OF MODEMS ________________________________________________________________________ 21 MULTIPLEXING __________________________________________________________________________ 21 TYPE OF MULTIPLEXING ___________________________________________________________________ 22 PULSE CODE MODULATION (PCM) ___________________________________________________________ 22

CHAPTER-05 HARDWARE AND SOFTWARE COMPONENTS _______________________________ 24

NETWORK INTERFACE CARD ________________________________________________________________ 24 REPEATERS _____________________________________________________________________________ 25 BRIDGES ________________________________________________________________________________ 25 ROUTERS _______________________________________________________________________________ 26 SWITCHES ______________________________________________________________________________ 27

CHAPTER-06 SWITCHED NETWORK _____________________________________________________ 29

SWITCHED DATA SUB-NETWORK ____________________________________________________________ 29 TYPES OF SWITCHED DATA NETWORK ________________________________________________________ 29 ROUTING OF DATA PACKETS ________________________________________________________________ 30

CHAPTER-07 NETWORK PROTOCOLS ____________________________________________________ 31

WHAT IS A PROTOCOL? ____________________________________________________________________ 31 ELEMENTS OF A PROTOCOL _________________________________________________________________ 31 CHARACTERISTICS OF PROTOCOLS ___________________________________________________________ 31 FUNCTIONS OF A PROTOCOL ________________________________________________________________ 32

CHAPTER-08 ISO/OSI LAYERED NETWORK ARCHITECTURE _____________________________ 33

COMMUNICATION CATEGORIES ______________________________________________________________ 33 OSI TERMS ____________________________________________________________________________ 33 ISO/OSI REFERENCE MODEL _______________________________________________________________ 34 PHYSICAL LAYER _________________________________________________________________________ 34 DATA LINK LAYER ________________________________________________________________________ 35 NETWORK LAYER ________________________________________________________________________ 36 SUB-LAYERING OF NETWORK LAYER _________________________________________________________ 37 TRANSPORT LAYER _______________________________________________________________________ 37 SESSION LAYER __________________________________________________________________________ 37

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PRESENTATION LAYER ____________________________________________________________________ 38 APPLICATION LAYER ______________________________________________________________________ 38

CHAPTER-09 DATA LINK PROTOCOLS ___________________________________________________ 40

FRAME DESIGN CONSIDERATIONS ____________________________________________________________ 40 TYPES OF FRAME FORMATS _________________________________________________________________ 40 TYPE OF DATA LINK PROTOCOLS ____________________________________________________________ 41 BINARY SYNCHRONOUS DATA LINK PROTOCOL (BISYNC) _______________________________________ 41 HDLC (HIGH LEVEL DATA LINK CONTROL) ___________________________________________________ 41

CHAPTER-10 LOCAL AREA NETWORK ___________________________________________________ 43

LAN ATTRIBUTES ________________________________________________________________________ 43 ARCHITECTURE OF LAN ___________________________________________________________________ 43 ETHERNET PROTOCOL _____________________________________________________________________ 44 ETHERNET AND TOKEN RINGS _______________________________________________________________ 45

CHAPTER-11 X.25 INTERFACE ___________________________________________________________ 47

X.25 ARCHITECTURE ______________________________________________________________________ 47 X.25 SERVICES _________________________________________________________________________ 48 X.25 PACKET FORMATS ____________________________________________________________________ 48 LOGICAL CHANNELS ______________________________________________________________________ 48

CHAPTER-12 ISDN (INTEGRATED SERVICES DIGITAL NETWORK) ________________________ 49

ISDN CHANNELS _________________________________________________________________________ 49 ISDN ACCESS RATES _____________________________________________________________________ 49 ISDN SERVICES __________________________________________________________________________ 49 ISDN ARCHITECTURE _____________________________________________________________________ 49 ISDN ADVANTAGES ______________________________________________________________________ 50 ISDN DISADVANTAGES ____________________________________________________________________ 50

CHAPTER-13 TCP/IP SUITE _______________________________________________________________ 52

TCP/IP APPROACH _______________________________________________________________________ 52 ARCHITECTURE OF TCP/IP PROTOCOL SUITE ___________________________________________________ 52 IP PROTOCOL __________________________________________________________________________ 53 IP ADDRESSES ___________________________________________________________________________ 53 ARP ___________________________________________________________________________________ 55 ICMP __________________________________________________________________________________ 55 TCP PROTOCOL __________________________________________________________________________ 55 UDP PROTOCOL __________________________________________________________________________ 55 SNMP _________________________________________________________________________________ 55

CHAPTER-14 FDDI (FIBRE DISTRIBUTED DATA INTERFACE) ______________________________ 57

TYPE OF SERVICES ________________________________________________________________________ 57 TRAFFIC CONTROL ________________________________________________________________________ 57 PRIORITY MANAGEMENT ___________________________________________________________________ 57 RING MANAGEMENT ______________________________________________________________________ 57 FDDI PHYSICAL LAYER SPECIFICATION _______________________________________________________ 57

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Chapter-01 Data Communication Communication stands for exchange of information between two parties via some media. The development in the field of communication techniques is spread over the history of human civilization.

Early men used pictures and sign language to express their views, feelings to each other. With the spread of human race, new methods for communicating were introduced such as smoke signals and carrier pigeons. Existence of postal services was also found and was treated as much efficient way of communication.

In modern world, the communication system underwent a total change with the discovery of Telegraph System in which electricity was used for sending and receiving messages. After this invention of telephone system, facsimile facilities improved the quality and scope of communication system.

Introduction of computer system in this field further enhanced not only the capabilities of the communication system but also introduced the era of electronic communication. The blend of telecommunication features and computer technology provided more diversified communication system that can transmit any form of information

Source: The originator of the message.

Transmitter: Converts incoming digital signals from source into terms of transmission media.

Transmission System: Communication system that provides backbone for data communication.

Receiver: Converts incoming signals from Transmission System back to digital format.

Destination: Receiver of the message transmitted by Source.

Message: Content transmitted from Source to Destination.

Classification of Data

Analog Data: It is a kind of data that is treated as physical quantities in continuous form. For example voice, voltage.

Digital Data: It is a kind of data that is treated as discrete values and is represented as the string of zeros’ and ones’.

Data Representation

A code set is the set of code representing the symbols that could be used to make up meaningful data. There are several code set used in data communication, some are used for specific purposes while other are proprietary code set of computer manufacturers. The following two code set are used commonly:

ANSI’s 7-bit American Standard Code for Information Interchange (ASCII)

ASCII is developed and defined by American National Standards Institute (ANSI). It is 7-bit code and contains 128 codes. The code set consists of the following symbols:

� 96 graphic symbols, comprising of 94 printable characters, SPACE and Delete characters

� 32 control symbols. Some important symbols are:

� CR (Carriage Return)

� LF (Line Feed)

� ACK (Acknowledgement)

� NAK (Negative Acknowledgement)

� STX (Start of Text)

� ETX (End of Text)

ASCII is often used with an eighth bit call the parity bit. This bit is used for detecting transmission errors.

Source Transmitter Destination Receiver

Transmission

System

Message

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IBM’s 8-bit Extended Binary Coded Decimal Interchange Code (EBCDIC)

It is developed and defined by International Business Machines (IBM). It is 8-bit code set and contains 256 codes; but all the codes are not used and even not yet defined.

Data Communication

Data communication refers to the electronic transmission data over communication channels. Communication is simple concept that involves transmitter or sender, a medium of transmission and the receiver. In case of data communication, the sender could be an electronic sensor or a computer, which transmits the data electronically over a predefined medium. The receiver again could be a computer.

Data communication through computer involves:

� The physical medium

� The hardware and software supporting data communication functions

� Procedures for detecting and recovering from errors

� Rules and protocols to ensure the discipline exchange of information

In case of data communication, the sender could be an electronic sensor or a computer that transmits the data electronically. The receiver end again could be a computer.

Communication Channel

A communication channels transport the electrical signals from the transmitter to the receiver. In other words, communication channels are the ‘Data Highways’ carrying signals from sending end to the receiving station. It is categorized by two basic parameters – bandwidth and signal to noise ratio. These parameters determine the information carrying capacity of the channel.

Data Rates

Bit Rate: Bit rate can be defined as number of bits transmitted per second (bps). If t is the duration of a bit, then bit rate is 1/t.

Baud Rate: It measurement unit for number of time signal changes its value over a unit time. There is one-to-one correspondence between bits and electrical signals.

Modes of Data Transmission

Data transmission can be defined as the movement of bits on some physical medium connecting two digital devices. The modes of data transmission are the methods of timing control for reception of bits.

Asynchronous Transmission

It is also known as start-stop transmission. In this kind of transmission sender can transmit a character at any instant and the receiver will accept it. The terminals are the best examples of this kind of transmission. In asynchronous transmission data is transmitted character-by-character at unequal interval.

Stop Bits Data Bits Start Bit

1 1 0 1 0 1 0 0 1 0 1 Line is quiet

Direction of data ----------------------->

Data: 01001010

In order to enable the receiver to understand the character when it arrives, the sender encloses each character by stat and stop bits. The preceding the character is start bit and the following is stop bit (may be of 1, 1½ or 2 bits duration).

• Sending end can commence transmission of bytes at any instant of time.

• Only one byte is sent at a time but there is no time relation between consecutive bytes. That is there could be arbitrary delay between the transmissions of two bytes.

• In idle state, the polarity of electrical signal corresponds to 1.

• Receiving end needs to be synchronized repeatedly for each byte. This can be achieved by using two extra bits a start bit and a stop bit.

Start bit is prefixed to each byte and is always 0, i.e. before transmission of a byte, it ensures that the electrical signal changes for 1 to 0. Stop bit is used to ensure that the transition for 1 to 0 is always present at the beginning of a byte. It is necessary that the electrical signal should correspond to 1. But if the last bit of the byte is 0, transition never occurs. Stop bit is suffixed to each byte and it is always 1 and its duration is usually 1, 1.5 or 2 bits.

Advantages:

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� �

i) There no need of synchronized clocks

ii) Each character can be transmitted independently

iii) Less Expensive

Disadvantages:

i) Transmission line is idle during the time interval between transmitting characters.

Synchronous Transmission

In this kind of transmission messages are sent into a block (group of characters) which is further framed with the header and trailer information. The header usually contains synchronizing information which is used by the receiving end to set its clock in accordance with sending end clock. The header also contains the information identifying the sender and receiver. Following the header is the block of characters that contains the actual message to be transmitted. The block of the message is terminated by a trailer which contains end-of-message flag, check character for error detection. Hence, instead of transmitting single character at one time as in asynchronous method a complete block of characters framed and transmitted together.

This type of transmission is best suitable for remote communication, computer and printer and buffered storage media

• Transmission is carried out under the control of a time source.

• Bits are always synchronized to the reference clock irrespective of the byte they belong to.

• Bytes are transmitted as block in a continuous stream. Inter-block idle time is filled with idle character.

• A unique sequence of fixed number of bits called flag is prefixed to each block to identify bytes’ boundaries

Advantages:

i) Efficient use of transmission line

ii) High data transmission rates

Disadvantages:

i) Requires local buffered storage at the two ends.

ii) Requires accurately synchronized clock.

iii) More expensive

Transmission Modes

Transmission mode specifies data flow between two points. The direction of the data flow can be described as:

Simplex Transmission

In simplex mode data flows in only one direction on the data communication line (medium). Examples are radio and television broadcasts. They go from the TV station to your home television.

Half-Duplex

In half duplex mode, data flows in both directions but only one direction at a time on the data communication line. For example, a conversation on walkie-talkies is a half-duplex data flow. Each person takes turns talking. If both talk at once – nothing occurs! Bi-directional but only 1 direction at a time!

Full-Duplex

In full-duplex mode, data flows in both directions simultaneously. Modems are configured to flow data in both directions. Bi-directional both directions simultaneously!

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Chapter-02 Transmission Media The transmission media is the physical path between the transmitter and receiver in the data transmission system. There are 2 basic categories of transmission media: guided and unguided.

Guided transmission media uses a cabling system that guides the data signals along a specific path. The data signals are bound by the cabling system. Guided media is also known as bound media.

Unguided transmission media consists of a means for the data signals to travel but nothing to guide them along a specific path. The data signals are not bound to a cabling media and are therefore often called unbound media.

Factor relating transmission media

1. Bandwidth: Bandwidth can be defined as the difference between the highest and the lowest frequencies available for transmission in any given range. Greater the bandwidth of the signal, higher the data rates.

2. Transmission Impairments: Impairment implies the way signal may lose its strength, such as attenuation, limit the distance. Least the impairments, smoother the transmission.

3. Interference: Interference means the overlapping of frequency band that can distort or wipe out a signal. It is particularly related with unguided media. This may also occur in case of guide media when cables are closely placed.

4. No of Receivers: In guided media a link is shared with multiple attachments. In this case some attenuation and distortion of line is possible.

Transmission Impairments

� For analog signals, impairments introduce various random changes that degrade the signal quality.

� Digital signals, bit errors are found, i.e., interchange of 0 and 1 while transmission.

Types of Impairments

Attenuation and Attenuation Distortion

Attenuation stands for reduction in the strength of a signal. It poses three problems before concerned personnel.

a) To provide strength to electronics to detect and intercepts the signals

b) Level of signal must be high than that of noise

c) Attenuation is an increasing function of frequency

Delay Distortion

This impairment is introduced due to variations in velocity of propagation of a signal through a guided media as for varying frequency

Noise

Undesired signals that travel along with transmitted signal is referred as noise. Following are the common type of noises exist in transmission

a) Thermal Noise: This is caused by agitation of electrons in a conductor. It cannot be eliminated. It is also called as white noise

b) Intermodulation Noise: This is produced when there is some non-linearity in the transmitter, receiver or intervening transmission system.

c) Cross Talks

d) Impulse Noise: It is a minor noise annoyance for analog data such as short clicks and crackles experienced while voice transmission.

Guided Media

The signal are guided alone a solid medium. In this case, transmission capacity is measured in term of either data rates or bandwidth. The transmission capacity solely depends upon the distance between two nodes and nature of network, i.e., point-to-point or multi-point network. The three guided media commonly used are

� Twisted Pair

� Coaxial Cable

� Optical Fiber

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Twisted Pair

It is also abbreviated as TP. A pair of TP consists of two insulated copper wires used for both analog and digital. For analog signal amplifier is required about every 5-6 KM. For digital signals repeater are required about every 2-3 KM.

Physical Description

• Consists of two insulated copper wires arranged in regular spiral pattern

• The twisting tends to decrease the crosstalk interference between adjacent pair in a cable

• The twist length typically varies from two to six inches

• Wires in pair have thickness of from 0.016 to 0.036 inches

Applications

• Commonly used in telephone network.

• Commonly used within building for local area networks providing digital signaling

Transmission Characteristics

• Data rate varies from 10 Mbps to 100 Mbps.

• For long distance application 14 Mbps data rate could be achieved.

• For analog transmission, amplifiers are required about 5 to 6 KM.

• For digital transmission, repeaters are required every 2-3 KM.

• TP is limited in distance, bandwidth and data rate

Types

TP comes in two flavors: Shielded TP (STP) and Unshielded TP (UTP). The difference between both is that the STP has a foil around individual wires whereas in UTP it is not present. Moreover, STP is provide better protection from external interference and is comparatively more expensive. UTP is commonly used form of TP whether its telephone communication or digital LAN.

Category 3 and 5 UTP

In 1991, Electronic Industries Association published standard EIA-568 specifying the use of voice grade UTP and STP for in-building application and it was revised in 1995 with improved standard with respect to cable and connector design (EIA-568-A)

Features CAT-3 CAT-5

Frequency: 16 MHz 100 MHz

Data Rate: 16 Mbps 100 Mbps

Twists: 3-4 per foot 3-4 per inches

Difference between Category 3 and Category 5 UTP/STP

It is popular due to its low cost and moderate performance. It is commonly used in telecommunication. It eradicates the problem of problems of mixing signals and thus compensates the loss of information.

Advantages:

� Cheaper Installation

Disadvantage:

� Requires specialized hardware called hubs to connect more than two computers.

� It covers very short distance data communication

Coaxial or Co-ax Cable


Coaxial cable is a two element cable, but is constructed to differently to permit it to operate over a wider range of frequencies. A central copper core is wrapped by an insulator which is again wrapped by an outer conductor- a wire braid which is surrounded by insulation cover. The data is carried by

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the copper core and the copper mesh acts as a shielded against external electrical interface. The co-axial cable is resistant to noise and transmit data at higher rates. It can be used over longer distances and support more stations on a shared line than twisted pair.

It can be used for both analog and digital signals. Coaxial Cable has higher frequency characteristic then TP and, hence effectively used for higher frequencies and data rates. For analog signal amplifier is required about every few KM. For digital signals repeater are required about every KM.

Applications

• Television Distribution

• Long Distance Telephone Transmission

• Short-run computer system links

• Local Area Network


• It is used for both analog and digital signal.

• High frequency spectrum allows higher data rate.

• Using frequency division multiplexing over 10,000 voice channels could be transmitted on single cable simultaneously.

• Performance is affected be attenuation, thermal noise and intermodulation noise

Advantages:

� It is resistant to electrical and magnetic interface.

Disadvantages:

� It is thick and relatively stiff.

� It requires expensive, specialized connectors and trained professional for its installation.

Fiber Optics

Board band channel are required when large volume of data has to transmit at high speeds. But exiting broadband channels are very expensive. A new technology for data transmission is in developing to maturing stage. Through expensive it can be viewed as of future. Instead of using electromagnetic ways, it uses light as a source to transmit data. The cable is made of stand glass as thin as a human hair each surround by a protective coating. The light pulse is transmitted over the channel and a processing device at the receiver and reconverts. Advantages of this cable are that it is highly resistant to noise the transmission errors are reduced to negligible probability. It also allows data transmission over long distances without regeneration by high which means the capacity to size per unit of data transmission. The cost of manufacturing the cable is very high due to cost sophistication and specialization required while handling the production process.

A light pulse can be used to signal 1 bit, the absence of a pulse signal a 0 bit the absence of a pulse signals a 0 bit.


• A thin (2 to 125µm), flexible media capable of conducting an optical ray

• Glass, plastic or superfine fused silica is use to make fibre

• It has three concentric section: the core, the cladding and the jacket

• Core consists of one or more very thin strands or fibre

• Each fibre is surrounded by it own cladding (a glass or plastic coating)

• Jacket covers one or more bundles of clad fibre. It gives protection against moisture, abrasion, crushing and other environmental dangers.

• Optical Fibre system operates in the range of about 1014

to 1015

Hz covering portions of infrared and visible spectrum.

Applications

• Long haul trunk: 900 miles coverage with 20000 to 60000 voice channels

• Metropolitan trunk: 7.8 miles coverage with 100,000 voice channels

• Rural exchange truck

• Subscriber loop

• Local Area Network

Advantage over TP and Coaxial Cable

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• Greater Capacity: Data rate of 2 Gbps over 10 KM as compare to 100 Mbps/KM for coaxial and 10 Mbps to 100 Mbps per 10 KM for TP.

• Smaller size and lighter weight

• Lower attenuation: 0.2 dB

• Electromagnetic isolation

• Greater repeater spacing


• Light beam from the source enters the core. Rays at shallow angle are reflected and propagate along the fibre; other rays are absorbed by the surrounding cladding. This is known as multimode where variety of angles are reflected

• When the fibre core radius is reduced to the order of wavelength, such that only a single angle can pass that is the axial ray. This is known as single mode

An optical transmission system mainly consists of the components:

Light Source: It is used to emit light whenever an electric pulse is applied. For example, LED (Light Emitting Diode) and ILD (Injection Laser Diode)

LED

- Stands for Light Emitting Diode

- Cost effective

- Can operate on wide range of temperature.

- Longer operational life

ILD

- Stands for Injection Laser Diode

- Operates upon laser principle

- More efficient

- Greater data rate

Transmission Media: An ultra-thin wire of glass fibre or fused silica.

Light Detector: It is a function to generate electric push when light falls on it. For example, photo-diode

Optical Modes

As the light enters the core, rays at shallow angles are reflected and propagated along the fibre; other rays are absorbed by the surrounding material. This kind of propagation is called multimode. There are three primary types of transmission modes using optical fiber. They are:

� Step Index

� Graded Index

� Single Mode

Step Index

Step index has a large core, so the light rays tend to bounce around inside the core, reflecting off the cladding. This causes some rays to take a longer or shorter path through the core. Some take the direct path with hardly any reflections while others bounce back and forth taking a longer path. The

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result is that the light rays arrive at the receiver at different times. The signal becomes longer than the original signal. LED light sources are used. Typical Core: 62.5 microns.

Graded Index

Graded index has a gradual change in the core's refractive index. This causes the light rays to be gradually bent back into the core path. This is represented by a curved reflective path in the attached drawing. The result is a better receive signal than with step index. LED light sources are used. Typical Core: 62.5 microns.

Single Mode

Single mode has separate distinct refractive indexes for the cladding and core. The light ray passes through the core with relatively few reflections off the cladding. Single mode is used for a single source of light (one color) operation. It requires a laser and the core is very small: 9 microns.

Advantages

� Noise immunity: RFI and EMI immune (RFI - Radio Frequency Interference, EMI -Electromagnetic Interference)

� Security: cannot tap into cable.

� Large Capacity due to BW (bandwidth)

� No corrosion

� Longer distances than copper wire

� Smaller and lighter than copper wire

� Faster transmission rate

Disadvantages

� Physical vibration will show up as signal noise!

� Limited physical arc of cable. Bend it too much and it will break!

� Difficult to splice

� The cost of optical fiber is a trade-off between capacity and cost. At higher transmission capacity, it is cheaper than copper. At lower transmission capacity, it is more expensive.

Unguided Transmission Media

Unguided transmission media is data signals that flow through the air. They are not guided or bound to a channel to follow. Transmission (electromagnetic radiations released in the air) and reception (picking electromagnetic radiations from the environment) take place by the means of antenna.

Configurations

Unguided transmission works on the two type of configurations:

Directional: Transmitting and receiving antenna must be perfectly aligned.

Omni Directional: Transmitted signal spreads out in all directions and can be received by many antennas.

Frequencies

Microwave Frequencies fall in the range of 20GHz to 40 GHz. These are highly directional beams. It is suitable for point-to-point communication and satellite communication.

Broadcast Radio Frequencies fall in the range of 30 MHz to 1 GHz. These are suitable for omni-directional applications.

Infrared Frequencies fall in the range of 3 X 1011

Hz to 2 X 1014

Hz. It is suitable for point-to-point and multipoint application within confined area.

Wave Propagation

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RF Propagation

There are three types of RF (radio frequency) propagation:

� Ground Wave

� Ionospheric

� Line of Sight (LOS)

Ground wave propagation

It follows the curvature of the Earth. Ground waves have carrier frequencies up to 2 MHz. AM radio is an example of ground wave propagation.

Ionospheric propagation

It bounces off of the Earth's ionospheric layer in the upper atmosphere. It is sometimes called double hop propagation. It operates in the frequency range of 30 - 85 MHz. Because it depends on the Earth's ionosphere, it changes with the weather and time of day. The signal bounces off of the ionosphere and back to earth. Ham radios operate in this range.

Line of sight propagation

It transmits exactly in the line of sight. The receive station must be in the view of the transmit station. It is sometimes called space waves or tropospheric propagation. It is limited by the curvature of the Earth for ground based stations (100 km, from horizon to horizon). Reflected waves can cause problems. Examples of line of sight propagation are: FM radio, microwave and satellite.

Radio Frequencies

These are in the range of 300 kHz to 10 GHz. We are seeing an emerging technology called wireless LANs. Some use radio frequencies to connect the workstations together, some use infrared technology.

The frequency spectrum operates from 0 Hz (DC) to gamma rays (1019 Hz).

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Terrestrial Microwave

Microwave transmission is line of sight transmission. The transmit station must be in visible contact with the receive station. This sets a limit on the distance between stations depending on the local geography. Typically the line of sight due to the Earth's curvature is only 50 km to the horizon! Repeater stations must be placed so the data signal can hop, skip and jump across the country.


• Parabolic dish type microwave antenna.

• Typical size is of 10 feet in diameter.

• Fixed and focused a narrow beam to achieve line of sight transmission.

• Maximum distance between antennas should confirm to:

D = 7.14 √√√√K.h

Where K is adjustment factor usually 4/3 and h is the height of antenna in meters

• Microwaves bent with the curvature of earth

Applications

• Long haul telecommunication

• Television transmission

• Short point-to-point links between links between buildings


• Fewer amplifiers or repeaters

• Require line of sight transmission

• Attenuation can be expressed as

• Attenuation increases with rainfall

• Interference is another source of impairment. Therefore, assignment of frequencies band has to be strictly adhered.

• Band Assignment

- 4 GHz to 6 GHz: Long haul telecommunication

- 12 GHz: Cable television

- Higher frequencies such as 22 GHz are used for point-to-point communication

Microwaves operate at high operating frequencies of 3 to 10 GHz. This allows them to carry large quantities of data due to their large bandwidth.

Advantages:

� They require no right of way acquisition between towers.

� They can carry high quantities of information due to their high operating frequencies.

� Low cost land purchase: each tower occupies only a small area.

� High frequency/short wavelength signals require small antennae.

Disadvantages:

� Attenuation by solid objects: birds, rain, snow and fog.

� Reflected from flat surfaces like water and metal.

� Diffracted (split) around solid objects.

� Refracted by atmosphere, thus causing beam to be projected away from receiver.

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Satellite Communication

Satellites are transponders (units that receive on one frequency and retransmit on another) that are set in geostationary orbits directly over the equator. These geostationary orbits are 36,000 km from the Earth's surface. At this point, the gravitational pull of the Earth and the centrifugal force of Earth's rotation are balanced and cancel each other out. Centrifugal force is the rotational force placed on the satellite that wants to fling it out into space.

Satellite work as a switch to the earth stations

Features

� It is a microwave relay station

� It is used to convert two or more ground-base microwaves transmitters/receivers.

� Uplink is receiving signal from one station

� Downlink is amplifying and transmitting signal to other station

� Satellite operates on numerous frequency bands known as transponder channels or simply transponders

� Supports point-to-point and point-to-multipoint communication

� It must be stationary with respect to its position in order to be aligned with the line of sight of its earth stations.

� Two satellites working on same frequency band must be properly spaced. For 4/6 GHz band, angular displacement from the earth must of 4 degrees and 12/14 GHz band, angular displacement from the earth must be 3 degrees.

Application

� Television Distribution

� Long distance telephone transmission

� Private business network

Broadcast Radio


• Radio waves with frequency in range of 3 KHz to 300 GHz come under this category.

• Does not require disc-shape antenna and any precise alignment.

• VHF is widely used for high quality voice transmission, video transmission. It is also used in police wireless transmission.

Application

• Cover VHF and a part of the UHF: 30 MHz to 1 GHz

• Cover FM radio and UHF and VHF television


• Main source of impairment is multipath

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Chapter-03 Network Concepts and Topology

What is network?

Network is a group of computer and associated peripherals connected by communication channel capable of sharing files and other resources between several users. Networks are an interconnection of computers. These computers can be linked together using a wide variety of different cabling types, and for a wide variety of different purposes.

A network can range from peer-to-peer networks connecting a small number of users in office or department, to local area network connection many users over permanently installed cables and dial-up line or to a wide area network connecting users on several networks spread over a wide range of geographic area.

Take for example a typical office scenario where a number of users in a small business require access to common information. As long as all user computers are connected via a network, they can share their files, exchange mail, schedule meetings, send faxes and print documents all from any point of the network.

It would not be necessary for users to transfer files via electronic mail or floppy disk, rather, each user could access all the information they require, thus leading to less wasted time and hence greater productivity.

Imagine the benefits of a user being able to directly fax the Word document they are working on, rather than print it out, then feed it into the fax machine, dial the number etc.

Network Architecture

The design of the network including the hardware, software, access method and protocols in use, is called Network Architecture

Peer-to-Peer Networking

Network architecture in which drives, files and printer on every PC can be available to every other PC on the network, eliminating the need of costly, dedicated server.

Local Area Network

LAN is a group of computers and associated peripherals connected by a communication channel capable of sharing files and others resources between several users.

Moreover LANs typically comprise only one transmission media type: Coaxial cable or twisted pair but not both. Local area networks are characterized by comparatively high speed communication. The high speeds are possible because of usage of one type of cable and distance limitation which is generally 10 Km. or less.

More then any of the network model; the most importing thing about LANs is that they must successfully balance network hardware and software. The LANs hardware gives the system its processing

Metropolitan Area Network

MAN is a public high speed network, operating at 100 megabits per second, capable of voice and data transmission over a distances of up 50 miles. A MAN a smaller than a wide area network but larger than a local area network (LAN).

MAN often enables users in several local geographical locations to use the shared network resources as if they were all part of the same local network. MAN are all local network, however; they do not necessarily have to use routers (devices responsible for figuring out which data should says inside the local network and which data should stay inside the local network and which data should stay inside the local network and which data should be passed on to other network).

Wide Area Network (WAN)

WAN are those which may span over entire cities, countries, and continents. Such huge connection calls for different protocols connection-less. A wide area network is a LAN of many LANs. WANs consist of interconnected LANs that may be in different buildings, cities or even countries around the world. A WAN network may use two types connectivity: Connection-oriented and Connection-less.

Connection-oriented: A connection-oriented is one in which there is a physical connection between two terminals and fixed sequences of event-initializations, data transfer and disconnection are performed for each transmission. This is analogous to traditional telephone system, where a connection involves initialization (dialing), data transfer (speech), and hang-up (disconnect).

Connection-Less: A connection-less is one in which there is not physical connection between the two entities. The data contents with it all the addressing information required to reach the destination. This is analogous to traditional postal system.

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Since WAN were initially developed on packet-switched data network they mostly connection-less in contrast to LAN which are connection-oriented.

There are two type of WAN called Enterprise WAN and Global WAN respectively. An Enterprise WAN connects all the LANs situated at different physical locations but belonging to same organization. WAN consisting of LANs belonging to widespread organization like corporations universals, government or multination companies come under category of Enterprise WAN. All the LANs of Enterprise WAN may be in different parts of country or even world but must be belongs to a single company or institution. WANS have two standards namely CCITT X.25 and ISDN

CCIIT X.25

CCIIT X.25 forms the basic of all packet switching networks & uncompress the tower there layers of OSI model.

� Physical layers deals with the routines to initialize, maintain & disconnect a physical connection .

� The link layer is responsible for the efficient data transfer over the physical over the physical link.

� The packet level is responsible for the routing of data switching of circuits and communication between the user and the network system.

ISDN

Digital data communication is all set to revolutionize the way information was been transmitted across a network specially WANS due to their unprecedented of unparalleled reliability. Integrated Services Digital network (ISDN)is one such new coming technology which promise to after amazingly efficient transmission of voice, data , graphics video and other digital services, all in one integrated transmission system.

Objectives of Good Network

The major criteria that a data communication network must meet are:

� Performance: Performance is the defined as the rate of transference of error-free data. It is measured by the response time. Response time is the elapsed time between the end of an inquiry and the beginning of a response, e.g. requesting a file transfer and starting the file transfer. Factors that affect response time are:

� Number of Users: The more users are on a network, the slower the network will run

� Transmission Speed: The speed that the data will be transmitted at measured in bits per second (bps)

� Media Type: The type of physical connection used to connect nodes together

� Hardware Type: Slow computers such as XT, or fast ones such as Pentiums

� Consistency: Consistency is the predictability of response time and accuracy of data.

� Users prefer to have consistent response times; they develop a feel for normal operating conditions. For example, if the "normal" response time is 3 seconds for printing to a network printer but a response time of over 30 seconds occurs, we know that there is a problem in the system!

� Accuracy of data determines if the network is reliable! If a system loses data, then the users will not have confidence in the information and will often not use the system.

� Reliability: Reliability is the measure of how often a network is usable. MTBF (Mean Time Between Failures) is a measure of the average time a component is expected to operate between failures, and is normally provided by the manufacturer. A network failure can be caused by a problem with the hardware, the data carrying medium, or the Network Operating System.

� Recovery: Recovery is the network's ability to return to a prescribed level of operation after a network failure. This level is where the amount of lost data is nonexistent or at a minimum. Recovery is based on having back-up files.

� Security: Security is the protection of hardware, software and data from unauthorized access. Restricted physical access to computers, password protection, limiting user privileges and data encryption are common security methods. Anti-virus monitoring programs to defend against computer viruses are also a security measure.

Topology

Topology refers to the way in which the network of computers is connected. Each topology is suited to specific tasks and has its own advantages and disadvantages. Topology can be classified as physical topology and logical topology: Physical Topology describes where the cables are run and the terminals or nodes are located. Logical Topology refers to the path that the messages take to get from one user to another user.

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Bus Topology

In this kind of topology all workstations connect to the same cable segment commonly used cable is terminated at each end wiring is normally done point to point a faulty cable or workstation will take the entire LAN down.

The bus cable carries the transmitted message along the cable. As the message arrives at each workstation, the workstation computer checks the destination address contained in the message to see if it matches it's own. If the workstations address matches that contained in the message, the workstation processes the message. The message is transmitted along the cable and is visible to all computers connected to that cable.

Advantages Disadvantages

a) Easy to implement a) Limits on cable length and Workstation numbers

b) Low Cost b) Difficult to isolate network faults

c) A cable fault affects all workstations

d) As the number of workstations increase, the speed of the network slows down

Ring Topology

In ring topology workstations connected through repeaters called Ring Interface Unit (RIU) to form the ring. Main features of ring topology:

� Uni-directional transmission � Each RIU receives the signal and forward it after regeneration � Previous RIU station retains the copy of the data until it is received by next RIU station or the

recipient completely


a) Cable failures affect limited users a) Costly Wiring

b) Equal access for all users b) Difficult Connections

c) Each workstation has full access speed to the ring

c) Expensive Adaptor Cards

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d) As workstation numbers increase performance diminishes slightly

Star Topology

In this type of topology, there are dedicated links from the station to the central controller, usually a hub. Each interconnection supports two-way communication. The central controller acts as a switch to route the data from the source to destination.


a) Easy to add new workstations a) Single point of network failure

b) Centralized control b) No sharing of transmission media

c) Centralized network/hub monitoring

Application of Networking

1. Electronic Mail (e-mail or Email) replaces snail mail. E-mail is the forwarding of electronic files to an electronic post office for the recipient to pick up.

2. Scheduling Programs allow people across the network to schedule appointments directly by calling up their fellow worker's schedule and selecting a time!

3. Videotext is the capability of having a two-way transmission of picture and sound. Games like doom and Hearts, distance education lectures, etc. use video text.

4. Groupware is the latest network application. It allows user groups to share documents, schedules databases, etc. (ex. Lotus Notes)

5. Teleconferencing allows people in different regions to "attend" meetings using telephone lines.

6. Telecommuting allows employees to perform office work at home by "Remote Access" to the network.

7. Automated Banking Machines allow banking transactions to be performed everywhere: at grocery stores, drive-in machines etc.

8. Information Service Providers provide connections to the Internet and other information services. Examples are CompuServe, Genie, Prodigy, America online (AOL), etc.

9. Electronic Bulletin Boards (BBS - Bulletin Board Services) are dialup connections (using a modem and phone lines) that offer a range of services for a fee.

10. Value Added Networks are common carriers such as AGT, Bell Canada, etc. (they can be private or public companies) who provide additional leased line connections to their customers. These can be Frame Relay, ATM (Asynchronous Transfer Mode), X.25, etc. The leased line is the Value Added Network.

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Chapter-04 Multi-channel Data Communication The common problem that is faced in long haul communication is the convergence of signals that could be efficiently transmitted over the network. It is not feasible to have every machine connected physically to the other machine in the network. It would be unrealistic approach to implement data communication. Moreover, detecting point of failure and recovering from it will be difficult. To solve this problem, the solution lies in the using existing transmission technique and media. Firstly, modulation is done to convert signals according to the transmission media. This helps two different types of devices to accomplish data communication task. On the other hand, multiplexing is implemented to properly consume the transmission capacity of the media. Circuit can be defined as physical or virtual connection between two communicating devices. Channel can be defined as single route (slot) availed of the circuit to transmit/receive the data. Thus, on the single wired circuit or non-physical circuit, there can exist more than one channel.

Modulation

Modulation can be defined as technique by which a digital signal is converted into its analog form for transmission over analog facility.

The transmission media can either physical connection such as wires or virtual connection such as microwaves. The data transmitted by an electrical pulse or wave form which possess following properties:

1. Amplitude: It can be defined a maximum value of a varying quantity from the BASE value.

2. Frequency: The frequency of a signal is the number of times the same form of signal is repeated. That is no of repetitions in wave like periodic process per unit time

3. Phase: Points having similar locations on the time amplitude wave form are said to be in the same phase. In other words, relative position of the two waves having same frequency.

In modulation, a transmission media is usually divided into different independent data paths called bands. And each band can accommodate a range of frequency on a transmission media. This quality is known as bandwidth.

When bands are used, there is a requirement of a device that converts constant level direct current of the sender equipment into signals suitable for transmission media and performs reverse process for the receiving end. This conversion is called modulation and demodulation.

The carrier signal is called sinusoidal signal. Basically, there are three techniques by which the digital signals can be converted and transmitted on the transmission. These techniques are based upon the three characteristics of the sinusoidal wave viz, amplitude, frequency and phase

Amplitude Shift Key (ASK)

This is the simplest form of the digital modulation. In this method the carries amplitude is multiplied by the binary 1, or 0, i.e. Binary 1 is represented by one amplitude and binary 0 is represented by the another amplitude. This method allows medium speed transmission.

ASK is very sensitive to noise and finds limited application in data transmission. It is used at very low bit rate of less than 100 bps.

Frequency Shift Key (FSK)

In FSK one frequency of the carrier signal during bit time represents a binary one and another frequency represents a binary zero. That is in FSK, the frequency of the carrier is shifted between two

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direct values; one represents binary one and other binary zero. In the FSK carrier amplitude does not change. It is relatively simple to implement. It used extensively in low speed modems having bit rate below 1200 bps.

FSK is not very efficient in its use of available transmission channel bandwidth.

Phase Shift Key (PSK)

PSK is the most efficient of the three modulation methods and is used for high bit rates. A binary 1 is assumed until a phase shift occurs.

The phase shift indicates a binary 0. Binary states 0 and 1 are represented by the negative and positive polarities of the signal.

Differential PSK (DPSK)

The problem of generating the carrier with a fixed absolute phase can be overcome by encoding the digital information as the phase change rather than the absolute phase. This method is known as differential PSK.

If ¬¬¬¬T-1 is the pervious state and ¬¬¬¬T is the new phase state carrier, when the data bit modulate the

carrier, the phase change is defined as ∆¬¬¬¬ = ¬¬¬¬t - ¬¬¬¬t-1

For demodulating the DPSK signal, it is merely necessary to detect the carrier phase variation

MODEMS

The term modem has been derived from the words Modulator and Demodulator. Its basic function is

to prepare digital data for transmission over analog voice band services offered by the telecommunication network. The

Block Diagram of MODEM

Dig

ita

l

Inte

rfa

ce

Lin

e

Inte

rface

Transmitter (Modulator)

Receiver (Demodulator)

To

DTE

To Telephone

To Other

Modem

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transmission media between two modems is always a dedicated lease circuit or a switched telephone network

Acoustic Coupler: A special type of modem will allows an ordinary telephone to be used within in the computer for data transmission

Type of Modems

Directional Capability wise

Half Duplex Modem: These types of modems allow transmission only in one direction at a time. Thus, if the modem detects a carrier on the line, it indicates the DTE about incoming single and does not let the DTE to transmit until the incoming data bits are not received completely.

Full Duplex Modem: These types of modems allow simultaneous transmission in either directions. Hence it comprises two carriers on the line: one outgoing (transmitter) and other incoming (receiver).

Transmission Wise

Asynchronous Modem: These types of modems transmit or receive data bit-wise. That is, single byte at a time which is framed with start and stop bits.

Synchronous Modem: These types of modems handle a continuous stream of data bytes, but require a clock signal. The data bits are always synchronized to the clock signal. There is separate clock for transmitting and receiving data.

Data Rate Wise

Low Speed Modems: These modems operate on bit rate up to 1200 bps. These are asynchronous and employ FSK modulation of carrier.

Medium Speed Modems: These modems operate within the range of 2400 bps to 4800 bps. These are synchronous and employ differential PSK modulation of carrier.

High Speed Modems: These modems operate at 9600 bps or above. These are synchronous and employ QAM modulation of carrier.

Interconnection with computer

Internal Modems: These types of modems are installed within the computer system like card devices.

External Modems: These types of modems are like other peripherals to the system. They are connected to the system via a external port.

Multiplexing

It is the method of breaking single physical channel into several logical sub-channels, so that number of independent signal may be transmitted simultaneously on it. This technique is called multiplexing. A modem is an intermediary device that interconnects two DTE (Data Terminal Equipment) at larger distance. But, in case there are many applications in which several terminals are interconnected it is not feasible to install modems twice the number of the terminals for data communication.

Multiplexer

Another data transmission intermediary device which allows sharing the transmission medium called multiplexer is feasible and best suited. A multiplexer takes several data communication channels and converts them into one single data communication channel at the sending end. Thus, with multiplexing it is possible to use single transmission line to concurrently transmit data between several transmitters and receivers. Advantage: � Economical and efficient use of available communication channel � Multiplexer can be equipped with the diagnostic hardware and software for monitoring the

performance of individual data channel

Disadvantage � The major drawback of this

technique is, if any of the multiplexer or leased lines fails, all the terminals will be cut of the host.

Demultiplexer

Demultiplexing involves separating the samples from the

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different channels. Since multiplexing is done sequentially, the demultiplexer utilize the flag (special symbol identifier for each data channel) with sample to identify the corresponding channel

Type of Multiplexing

Frequency Division Multiplexing (FDM)

The lease line is usually provides speech channel with bandwidth of 300-3400 Hz. Most of multiplexers take advantage of this band. In FDM, the frequency band is sub-divided into several sub-channels separated by guard bands. Each channel is translated to different band and then all the channels are combined to form a FDM signal. In the FDM, the speech channels are stacked at the interval of 4 kHz to provide a guard band between the adjacent channels. Frequency transmission is done by employing a carrier which is modulated by the speech signal using suppressed amplitude modulation.

All sub-channels use FSK modulation of the carrier. Since aggregate of all sub-channels ranges within the speech channel bandwidth and is an analog signal, the multiplexer does not requires any modem to connect it to the line. Bandwidth of sub-channels depends upon the baud rates. Usually, FDM provides baud rates from 50 to 600 bauds

Advantages:

� Failure of on channel does not effects other channels

Disadvantages:

� High production cost due to analog components � Total capacity is limited to 2400 bps due to large bandwidth is wasted in the guard band. � Does not allow different bit rates of sub-channels. � Small variation requires complete hardware reconfiguration.

Time Division Multiplexing (TDM)

In TDM two or more signals are transmitted as single composite signal by using time sharing technique. TDM uses fixed allocation of time slots to the sub-channels, rather than assigning them different frequencies as in FDM. One complete cycle of time slot is called frame and the beginning of a frame is marked by a synchronized word which help the demultiplexer to identify the time slot and their boundaries.

If all the sub-channels have same bit rate, all the time slots are of uniform length. Otherwise, if multiplexer permits speed flexibility the higher speed sub-channel have longer time slot. There are two types of TDM.

a) Bit Interleaved TDM: In this type of TDM, each time-slot is one bit long. Hence, user data streams are interleaved taking one bit from each stream

b) Byte Interleaved TDM: In this type of TDM, each time-slot is one byte long. Hence, the multiplexed signal consists of series of interleaved character (byte) of the successive channels.

FEATURE:

� TDM allows the mixing of bit rates

� Better utilization of line capacity

� A better bit rate of 9600 bps is possible

Statistical Time Division Multiplexing

Statistical Time Division Multiplexing uses intelligent devices that are capable of identifying when a terminal is idle. They allocate time only to lines when required. This means that more lines can be connected to a transmission medium because this device statistically compensates for normal idle time (in data communication lines). Newer STDM units provide additional capabilities: data compression, line priority, mixed speed lines, host port sharing, network port control, automatic speed detection and much more.

Pulse Code Modulation (PCM)

TDM signal although being in pulse form remains in analog samples and sample levels can have infinite possible values. For converting these analog samples to digital form, another form of modulation is applied. This is known as PCM. The process involves two stage

Quantization

It is the approximation of the level of sample by the nearest value drawn from assortment of the discrete values. For example, if the set consists of discrete levels, 0 to 7 volts and the sample level of 3.2 volts is approximated with a discrete level of 3 volt.

Coding

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This refers to the conversion of discrete level of the sample after quantization to binary code of fixed length. For example, 3 volts may be coded as 011. The number of bits in the code is determined by the total number of discrete levels. In telephony, 256 discrete levels are used, therefore number of bits per code is 8.

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Chapter-05 Hardware and Software components

Network Interface Card

A network adapter card plugs into the workstation, providing the connection to the network. Adapter cards come from many different manufacturers, and support a wide variety of cable media and bus types [ISA, MCA, EISA, PCI, and PCMCIA. New cards are software configurable, using a software program to configure the resources used by the card. Other cards are PNP [plug and Play], which automatically configure their resources when installed in the computer, simplifying installation. With an operating system like Windows 95, auto-detection of new hardware makes network connections simple and quick.

On power-up, the computer detects the new network card, assigns the correct resources to it, and then installs the networking software required for connection to the network. All the user need do is assigned the network details like computer name.

For Ethernet or 10BaseT cards, each card is identified by a twelve digit hexadecimal number. This number uniquely identifies the computer. These network card numbers are used in the Medium

Access (MAC) Layer to identify the destination for the data. When talking to another computer, the data you send to that computer is prefixed with the number of the card you are sending the data to.

This allows intermediate devices in the network to decide in which direction the data should go, in order to transport the data to its correct destination.

A typical adapter card looks like.

A PCMCIA adapter card, suitable for connecting to a portable laptop computer to a network, looks like. Peripheral cards associated with EISA and MCA are normally self configuring.

Resources Used By Peripheral Cards Essentially, there are four resources which are user configurable for peripheral cards. Some cards may only use one (a port location(s)), others may require all four.

I/O Port Address: In the PC, the port numbers used by peripheral cards range from 200h to 3FFh. The I/O port address is used by the PC to communicate with the peripheral card (issue commands, read responses, and perform data transfer).

Interrupt Request Line: The interrupt request line is used by the card to signal the processor that the card requires the processors attention..

Direct Memory Request Line: The DMA request line is used to transfer data between the peripheral card and the computers memory at high speed. DMA channel 0 cannot be used, as it is reserved for system use.

Buffer Memory Address: Some peripheral cards prefer to use memory space rather than an I/O port address to transfer data to the processor. This memory space occupied by the peripheral card appears in the main system memory RAM area available to the processor (usually between C0000h to EFFFFh).

So How Do Peripheral Cards Work? Peripheral cards require a software driver to function. This software driver provides the interface between the card and the operating system, making the services provided by the card available to the user.

The software driver is normally configured to match the resource settings of the card. This is done by a configuration utility, and stored either in the executable file, or a separate file (like .ini or .cfg).

It is obviously important for the configuration settings in the software driver to match those configured on the peripheral card.

The software driver provides the follow functions:

a. Initialization routine

b. Interrupt service routine

c. Procedures to transmit and receive data

d. Procedures for status, configuration and control

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e. The basic operation looks something like,

f. Card receives data

g. Card generates interrupt by asserting interrupt request line

h. Processor responds to interrupt request and jumps to service routine

i. Service routine instructs processor to read data from port location

j. Interrupt service routine releases processor to continue previous work

Repeaters

Repeaters EXTEND network segments. They amplify the incoming signal received from one segment and send it on to all other attached segments. This allows the distance limitations of network cabling to be extended. There are limits on the number of repeaters which can be used. The repeater counts as a single node in the maximum node count associated with the Ethernet standard [30 for thin coax].

Repeaters also allow isolation of segments in the event of failures or fault conditions. Disconnecting one side of a repeater effectively isolates the associated segments from the network.

Using repeaters simply allows you to extend your network distance limitations. It does not give you any more bandwidth or allow you to transmit data faster.

A repeater works at the Physical Layer by simply repeating all data from one segment to another.

Features of repeater

� Increase traffic on segments

� Have distance limitations

� Limitations on the number that can be used

� Propagate errors in the network

� Cannot be administered or controlled via remote access

� Cannot loop back to itself (must be unique single paths)

� No traffic isolation or filtering

Bridges

Bridges interconnect Ethernet segments. Most bridges today support filtering and forwarding, as well as Spanning Tree Algorithm. The IEEE 802.1D specification is the standard for bridges.

During initialization, the bridge learns about the network and the routes. Packets are passed onto other network segments based on the MAC layer. Each time the bridge is presented with a frame, the source address is stored. The bridge builds up a table which identifies the segment to which the device is located on. This internal table is then used to determine which segment incoming frames should be forwarded to. The size of this table is important, especially if the network has a large number of workstations/servers.

Advantages

1. Increase the number of attached workstations and network segments 2. Since bridges buffer frames, it is possible to interconnect different segments which use

different MAC protocols 3. Since bridges work at the MAC layer, they are transparent to higher level protocols 4. By subdividing the LAN into smaller segments, overall reliability is increased and the network

becomes easier to maintain 5. Used for non routable protocols like NETBEUI which must be bridged

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6. Help localize network traffic by only forwarding data onto other segments as required (unlike repeaters)

Disadvantages

1. The buffering of frames introduces network delays 2. Bridges may overload during periods of high traffic 3. Bridges which combine different MAC protocols require the frames to be modified before

transmission onto the new segment. This causes delays 4. In complex networks, data may be sent over redundant paths, and the shortest path is not

always taken 5. Bridges pass on broadcasts, giving rise to broadcast storms on the network

Bridges are ideally used in environments where there a number of well defined workgroups, each operating more or less independent of each other, with occasional access to servers outside of their localized workgroup or network segment. Bridges do not offer performance improvements when used in diverse or scattered workgroups, where the majority of access occurs outside of the local segment.

Ideally, if workstations on network segment A needed access to a server, the best place to locate that server is on the same segment as the workstations, as this minimizes traffic on the other segment, and avoids the delay incurred by the bridge.

A bridge works at the MAC Layer by looking at the destination address and forwarding the frame to the appropriate segment upon which the destination computer resides.

Features of Bridge

� Operate at the MAC layer (layer 2 of the OSI model)

� Can reduce traffic on other segments

� Broadcasts are forwarded to every segment

� Most allow remote access and configuration

� Often SNMP (Simple Network Management Protocol) enabled

� Loops can be used (redundant paths) if using spanning tree algorithm

� Small delays introduced

� Fault tolerant by isolating fault segments and reconfiguring paths in the event of failure

� Not efficient with complex networks

� Redundant paths to other networks are not used (would be useful if the major path being used was overloaded)

� Shortest path is not always chosen by spanning tree algorithm

Routers

Routers were devised in order to separate networks logically. Filtering at this level (on TCP/IP addresses, also known as level 3 switching) will take longer than that of a bridge or switch which only looks at the MAC layer.

Most routers can also perform bridging functions. A major feature of routers, because they can filter packets at a protocol level, is to act as a firewall. This is essentially a barrier, which prevents unwanted packets either entering or leaving designated areas of the network.

A router works at the Network Layer or higher, by looking at information embedded within the data field, like a TCP/IP address, then forwards the frame to the appropriate segment upon which the destination computer resides.

Router features

� use dynamic routing

� operate at the protocol level

� remote administration and configuration via SNMP

� support complex networks

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� the more filtering done, the lower the performance

� provides security

� segment networks logically

� broadcast storms can be isolated

� often provide bridge functions also

� more complex routing protocols used [such as RIP, IGRP, OSPF]

HUBS

There are many types of hubs. Passive hubs are simple splitters or combiners that group workstations into a single segment, whereas active hubs include a repeater function and are thus capable of supporting many more connections.

In standard Ethernet, all stations are connected to the same network segment in bus configuration. Traffic on the bus is controlled using the CSMA (Carrier Sense Multiple Access) protocol, and all stations share the available bandwidth.

10BaseT Hubs dedicate the entire bandwidth to each port (workstation). The workstations attach to the hub using UTP. The hub provides a number of ports, which are logically, combined using a single backplane, which often runs at a much higher data rate than that of the ports.

Ports can also be buffered, to allow packets to be held in case the hub or port is busy. And, because each workstation has it's own port, it does not contend with other workstations for access, having the entire bandwidth available for it's exclusive use.

Hub options also include an SNMP (Simple Network Management Protocol) agent. This allows the use of network management software to remotely administer and configure the hub. Detailed statistics related to port usage and bandwidths are often available, allowing informed decisions to be made concerning the state of the network.

Advantages

� Each port has exclusive access to its bandwidth (no CSMA/CD) � Hubs may be cascaded to add additional ports � SNMP managed hubs offer good management tools and statistics � Utilize existing cabling and other network components � Becoming a low cost solution

Switches

Ethernet switches increase network performance by decreasing the amount of extraneous traffic on individual network segments attached to the switch. They also filter packets a bit like a router does. In addition, Ethernet switches work and function like bridges at the MAC layer, but instead of reading the entire incoming Ethernet frame before forwarding it to the destination segment, usually only read the destination address in the frame before retransmitting it to the correct segment. In this way, switches forward frames faster than bridges, offering fewer delays through the network, hence better performance.

Nodes which inter-communicate frequently should be placed on the same segment. Switches work at the MAC layer level.

Switches divide the network into smaller collision domains [a collision domain is a group of workstations that contend for the same bandwidth]. Each segment into the switch has its own collision domain (where the bandwidth is competed for by workstations in that segment). As packets arrive at the switch, it looks at the MAC address in the header, and decides which segment to forward the packet to. Higher protocols like IPX and TCP/IP are buried deep inside the packet, so are invisible to the switch. Once the destination segment has been determined, the packet is forwarded without delay.

Advantages

� Existing cabling structure and network adapters is preserved � Switches can be used to segment overloaded networks � Switches can be used to create server farms or implement backbones � Technology is proven, Ethernet is a widely used standard

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� Improved efficiency and faster performance due to low latency switching times � Each port does not contend with other ports, each having their own full bandwidth (there is no

contention like there is on Ethernet)

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Chapter-06 Switched Network

Switched Data Sub-Network

LAN has its own boundaries and limitation with respect to performance and area coverage. To transmit data beyond a local area, it is required to take services of intermediary switching nodes.

Switching is the selection and establishment of path from the source to a destination through subnet. The job of switching node is to carry data from source end to receiver end through following nodes. The end devices involved in data communication are known as stations. They could be terminals, computers, telephone or any other communicating device. Switching devices that rend their services are known as switching node or simply nodes. And collection of such nodes is referred as a communication network. Data entering network from a station routed to a destination by being switched from node to node. Following of switching are � Flexible Topology: One access point to number of destination � Resource Sharing Switched data network is made up of an interconnected collection of node and interconnected links are known as trunks

Types of Switched Data Network

Circuit Switched Network

Communication using Circuit Switched Network means that there is a dedicated path between two stations. This path is a connected sequence of links between network nodes. A logical channel on every physical link is dedicated to the connection. The connection path is made dedicated (established) before any data transmission begins.

This was originally developed to handle voice traffic, but is now used for data traffic. The Public Telephone Network is the best example.

Circuit Switched Network consists of Circuit Switched Nodes. Circuit Switched Nodes carry out cross connection between incoming trunk circuit and outgoing circuit and create transmission path. There three phases in Circuit Switched Network

1. Connection Establishment 2. Data transfer 3. Release

Features � Destination address is fixed once � Delivery delay is minimal � Data rates are same at both ends � Data transmission is bi-directional � No error control � No data storage in sub-network

Packet Switched Network

In 1970, this technique came into existence as a efficient method for data communication over long distances. It was appreciated for its advantages: flexibility, resource sharing, robustness and responsiveness. This is a distributed collection of packet switching nodes. Circuit Switched Network suffers two main shortcomings: � Under utilization of line capacity in user host connection. � Constant rate of data transmission. This limits the utility of the network in interconnecting variety

of most computers and terminals. Advantage of packet switched network over Circuit Switched Network is if source has a longer massage, the massage is broken into a series of the packet. Each packet contains a portion (or all for a short message) of the user data plus some control information. The control information includes routing information. At each node en route packet is received, stored briefly, and passed on to the next node till destination. Advantages: � Greater line efficiency � Varying data rate � Properties can be used � Varying delivery delay � Reduced processing time at the node � Reduced end-to-end message transmission

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Routing of Data Packets

Datagram Routing

A datagram is a packet of data with complete address of destination. In this approach, each packet is treated independently with no reference to packets that have gone before. Following methodologies could be use to decide the route of datagram:

� Send datagram on one trunk circuit at random.

� Send datagram on the trunk which has the shortest queue irrespective of destination

� Brute Force Approach: Send datagram in all the outgoing trunks except in the direction from which the datagram came.

� A routing table is maintained at each node. This is used for looking up and decides the next node.

Features:

� No connection establishment and release phase

� Finite and fluctuating delivery delay

� Finite error rate due to lost and duplicate packets

� Disordering the packets

� Source and destination data rates can differ

� Destination and source addresses are specified on each packets

� Non-reliable service as there is no acknowledgement

Virtual Circuit Routing

Virtual circuit routing appear quiet similar to circuit switching. A Virtual circuit routing is requested using a call request packet which incurs a delay at each node. The Virtual circuit is accepted with a call accept packet. In contrast to circuit switching case the call acceptance also experiences node delays even through Virtual circuit route is now established; the reason is that this packet is queued at each node and must wait for its turn of re-transmission. Once the virtual circuit route is established, the message is transmitted in packets. It should be clear that this phase of operation can be no faster than circuit switching. For comparable networks, this is because circuit switching is an essentially transparent process providing a constant data rate across network. Packet switching involves delay at each node in the path; this delay could be variable and increased with the load.

Features:

� Connection establishment and release phase

� Destination and source address specified only once

� Sequences delivery of packets

� Source and destination data rate may differ

� Finite and almost constant delivery delay

� Delivery assured using acknowledgement

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Chapter-07 Network Protocols

What is a Protocol?

A protocol is used for communication coordination between two stations. For two systems to communicate successfully, they must speak the same language. What is communicated, how is it communicated and when it is communicated must abide to some acceptable norms between systems involved. These set of conventions is referred as a protocol, which may be defined as set of rules governing the exchange of data.

Elements of a Protocol

Syntax: This part of protocol is concerned with the data format and the signal level

Semantic: This part of protocol includes control information and error-handling code

Timing: This part of protocol deals with speed matching and sequencing of data transfer

TCP/IP is an example of protocol that is widely used in internet connectivity.

Characteristics of Protocols

A protocol can display any of the following characteristics

1. Direct or Indirect: If two stations are working in point-to-point or multi-point broadcast configuration, these system may communicate directly without requiring any intervening active agent. But, if systems are connected through switched network, direct protocol no longer has any meaning. Here two systems depend on the functioning of other entities to exchange information. This refers to indirect protocol.

2. Monolithic or Structured: To be completely monolithic, protocol has to be implemented as single package including all of the functions. Major disadvantage of this kind of protocol is a small change in any part will be done by designing of entire package. Moreover, debugging is not also easy-to-do task.

In structured protocol design, design refers to the hardware and software that regulates the communication function.

3. Symmetric or Asymmetric: Symmetric protocols define and control communication between peer entities, whereas asymmetric protocol controls hierarchical communication

4. Standard or Non-standard: A non-standard protocol in one that is built for a specific communication situation or a particular model of computer. Standard protocol refers to protocol architecture that enjoys world wide acceptance for communication functions.

Switched Network

Internet

Point-to-point

Multi-point Broadcast Network

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Functions of a Protocol

A protocol is concerned with exchanging stream of data between two entities. A data block that is transmitted between two entities is technically known as Protocol Data Unit (PDU). Following are the major functions that are carried out by most of protocols.

Segmentation and Reassembly

At the application level, logical unit of data is referred as a message. For better management and control, message is broken into blocks of pre-defined small-sized packets. This process is called segmentation or fragmentation.

Reassembling is just opposite of segmentation. On the receiving end it is required that the segmented data must reassembled into original.

Encapsulation

This refers to the methodology of compartmentalization of all required logic for handling data communication in a single package. Specifically it means the inclusion of control information with the data to be transmitted such as address, error-detecting code and protocol information.

Connection control

This function specifies how the communication link has to be managed during the data transmission from source to destination. A logical association or connection is established between communicating entities. Three phases occur during communication:

� Connection Establishment

� Data Transfer

� Connection termination

Ordered Delivery

The protocol must ensure that data packets should be delivered to the destination station in the same sequence in which they were accept from the sending station. This becomes more important in when two communicating entities are in different hosts connected by a network, as it induces the risk of losing data packets in the route.

Flow Control

It is a function performed by the receiving entity to limit the amount or rate of data that is sent by the transmitting entity. The simplest form of flow control is a stop-and-wait procedure, in which each data packet must be acknowledged before the next can be sent.

Error Control

This refers to the techniques that are required to prevent loss or damage data and control information. Generally all the methods involve error-detection based on frame check sequence and retransmission of the faulty frame.

Addressing

There must be a unique, standard and simple-to-use method of address assigning and address resolving method with the protocol. Addressing is an complex issue in which following point are of paramount importance:

Addressing Levels: These refer to the level in the communication architecture at which entity is named. A unique address is associated with each system and intermediary system in a communication configuration. This also known as Network level address

Addressing Scope: This refers to the applicability of the address in global space of network

Connection Identifier: In case of connection oriented communication, the connection identifier is used to uniquely identify each service access point.

Addressing Mode: It refers to implement the address onto the system. An address refers to a signal system or a port. - unicast in which address refers to single system or port. Otherwise

- broadcast in which address refers to multiple recipients in domain

- multicast in which address refers to specific subset of entities

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Chapter-08 ISO/OSI Layered Network Architecture It is not necessary that network systems used all over the world are working the same architecture. Hence to make the different type of machines communicated with each other there was a need for standardization of network architecture.

Communication Categories

Hierarchical Communication

The messages exchanged between the adjacent layers during the Hierarchical Communication are called Interface Control Information (ICI)

Peer-to-peer Communication

Peer-to-peer Communication is between the peer layers for carrying out an assigned set of functions. The messages which are exchanged between the peer layers are called Protocol Control Information (PCI)

Since there is no direct path between the peer layers Protocol Control Information is exchanged using the service by the lower layer

OSI TERMS

Connection A connection is logical association of peer entities to provide services to next higher layer

Service Access Point

(SAP)

For hierarchical communication, the adjacent layer entities interact through a service access point, which at the interface between the layers

SAP-Address (N)-SAP address or (N)-address identifies the SAP located between (N+1)-layer and (N)-layer

Connection End

Point Identifier

A SAP path can support multiple connections on its communication path. (N)-connection end point identifier uniquely identifies a connection. It consists of two parts

� (N)-address of (N)-SAP

� A suffix which is uniquely within the scope of (N)-SAP

Data Units Protocol Control Information (PCI): (N)-PCI is the protocol control information exchanged between (N)-entities to coordinated their functions

Service Data Unit (SDU): (N)-SDU is the data transferred between the (N)-connection and whose identity is preserved during transmission.

Protocol Data Unit (PDU): (N)-PDU is the combination of (N)-PCI and (N)-SDU.

Interface Control Information (ICI): (N)-ICI is the information exchanged (N+1)-entity and (N)-entity to coordinate their function.

Interface Data Unit (IDU): (N)-IDU is total data transfer across the SAP between (N+1)-entity and (N)-entity.

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ISO/OSI Reference Model

The International Standards Organization (ISO) Open Systems Interconnect (OSI) is a standard set of rules describing the transfer of data between each layer in a network operating system. Each layer has a specific function. For example, the physical layer deals with the electrical and cable specifications. The OSI Model clearly defines the interfaces between each layer. The application of the ISO/OSI model has allowed the modern multi-protocol networks that exist today.

OSI Reference Model represents generalization of concepts regarding inter-process communication. In this approach the communication has been divided into hierarchical functional layers. It modularizes a set of system interconnection rule in a particular way by defining a series of layer functions. Each layer handles certain specific predefined function.

In the OSI Reference Model, the communication function is divided into the hierarchy of seven layers

Physical Layer

Physical layer is basically concerned with the transmission of the raw bits over the transmission media. The Physical layer provides its services to the Data Link Layer and receives services of the physical interconnection channel for transmitting electrical signals. The Physical layer carries out the following functions:

Conversion of bits into electrical signals having characteristics suitable for the transmission media

a) Signal Encoding

b) Relaying of digital signals using intermediary devices such as modems

c) The rules and procedures for interaction between Physical layer entities are called Physical layer Protocols.

Physical Connection: The Physical layer at the two ends provides a transport service from Data Link layer on one end to Data Link layer on the other end over a physical connection activated by them to transmit data. Physical Connection is different from Physical transmission path in the sense that it works at bit-level where as latter works at the electrical signal level.

Services to Data Link layer

Physical

Application

Presentation

Session

Transport

Network

Data Link

Physical

Application

Presentation

Session

Transport

Network

Data Link

Physical

Medium

Peer Protocol

Relay System

(N+1)-Layer

(N)-Layer

(N)-IDU

(N+1)-PDU

(N)-ICI

(N)-ICI

(N)-SDU

(N)-SDU

(N)-ICI DATA UNITS IN THE OSI

REFERENCE MODEL

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1. Activation/Deactivation of the Physical Connection: The Physical layer activates/deactivates the physical connection on the request of Data Link layer for transmission of bits.

2. Physical Service Data Units (Ph-SDU): Ph-SDU received from Data Link layer consists of 1 bit in serial transmission or ‘n’ bits in parallel transmission.

3. Sequence Delivery: The Physical layer delivers bits in the same order in which they were submitted by the Data Link layer.

4. Fault Condition Notification: The Data Link layer is notified in the case of error detection.

Functions within the Physical layer

a) Setup and release of the physical connection between entities in the Data Link layer

b) A physical connection may use a relay at on intermediary point to regenerate the electrical signal. The process to activate and deactivate the relay is done by Physical layer

c) The physical transmission of bits may be synchronous or asynchronous

d) If the signal encoding is required, this function is carried out by the Physical Layer

Physical Layer Standards

a) Mechanical Specifications: This deals with type of connectors, physical dimensions, allocation of pins and so-on. In other words, it includes mechanical design of the connectors which are used on the equipment and the interconnecting cables and pin assignment of the connectors

b) Electrical Specifications: This is concerned with the electrical characteristics e.g., voltage level impedance.

c) Functional Specifications: These deals with the meaning of voltage levels on the certain pins of the connectors.

d) Procedural Specifications: The procedural specification define the rules applying to various functions, the sequence in which certain events may occur

Data Link layer

The Physical Layer lack certain features such as:

• It does not have any function to control error induced if the electric signal gets impaired due to disturbances encountered while transmission

• It does not supports any data flow control mechanism which could be implemented in case of error induced if the receiving device is not ready to receive the incoming signal.

These all features are implemented in Data Link layer for error-free transmission. The Data Link layer is the second layer of OSI Reference Model. This layer shields upper layers from the characteristics of the physical transmission and provides a reliable and error-free Data Link Connection. It receives the data from the higher layer and blocks them along with certain control bits. This data block along with control bits is called frames. This frame is transmitted to the physical layer which converted bits into electrical signals which are transmitted over the physical transmission media. At the receiving end, the physical layer converts the incoming electrical signals back to bits and the frame is handed over to the Data Link Layer. The Data Link Layer removes the control bits and check for errors.

The control bits includes error check bit addresses sequence numbers etc. the additional bits usually enable error control, flow control and link managements.

Services to Network Layer

The Data Link Layer receives the services from the Physical Layer and provides service to the Network Layer.

Following service are provided by the Data Link layer to the Network Layer

• Data Link Connection: Provides one or more Data Link connections between two network entities.

• Data Link Connection end point identifier: to identify the individual Data Link connection.

• Sequencing: Maintain the integrity of the data Sequence.

• Error notification: Data Link layer informs Network Layer about unrecoverable error.

• Flow Control: The Network Layer can control the rate at which it receives data from Data Link layer and Data Link layer will accepts data from Network Layer

Services within the Data Link layer

1. Setups and releases the data link connections

2. Splitting of one data link connection onto several physical connection

3. Framing function organizes bits into frames

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4. Controls the sequence of the frames transmitted over the data link

5. Error detection and error-recovery

6. Flow control

7. Identification and parameter exchange

8. Link management

Network Layer

It is the third layer of OSI Reference Model. It consumes the services provided by Data Link Layer and provides services to Transport Layer. The basic purpose of the Network Layer is provides mean to access the sub-net for routing the data to the destination end system.

The Network Layer decides how to pass the data frames over the sub-net so that they reach their destination.

Network Services

The services provided by the Network Layer to the Transport Layer are called network services. The network services proving transparent transfer of data units to the Transport entities and data units are received and delivered from Network Service Access Point (N-SAP). The network services can be categories in two classes:

Connection Mode Network Services (CONS): In CONS a network connection is first established between the communicating transport entities and then data unit received from transport layer are transported over the connection. The data units are always delivery in same sequence in which they were received. CONS are a reliable service because it has built-in error recovery procedures. And in case of network failure, transport entities are informed. CCITT X.213 specifies CONS

Connection-Less Mode Network Services (CLNS): In CLNS, each data unit carries the destination and source addresses and is delivered independently than other data units. That is the Network Layer of the nodes makes the routing decisions independently for each data packet. It result into following faults:

� Some data packets may be lost

� Some data packets may delivered out of sequence

� Duplication of some data packets

CLNS can’t be categorized as reliable because there is no guarantee that the data packets will be correctly delivered. Transport entities have to make their own effort to collect the delivery of data packets.

Services to Transport Layer

1. Unique addresses are provided to identify transport entities

2. Point-to-point Network Connection is established

3. Quality of service is provided that includes parameters such as, Residual Error rate, throughput, connection delay, transit delay, service availability and reliability

4. Un-recoverable errors are notified to the transport entities

5. Controlled flow of data

6. Sequenced delivery of data

7. Connection termination.

Function of Network Layer

Network Connection: This function provides a network connection between transport entities, making use of Data Link connection provided by Data Link

Routing and Relaying: Routing function helps in selecting appropriate route between two systems for data transmission. Network connections are provided by network entities in end systems but there may involve intermediary system that provides relaying.

Multiplexing: The network entity may multiplex several network connections on a single data link connection, in order to better utilization of data link connection

Segmenting and Blocking: This function is performed by network layer over data packets to get a data packet of required size for the purpose of data transmission.

Error Detection and Recovery: The error detection is used to check the quality of the service provided by the Network Layer over the network connection. In case of error detection, it implements error-recovery mechanism

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Sub-layering of Network Layer

This function is required due to the following reasons:

� Difference in the operational functions of various subnetworks. Network layer has to take case of the differences and provide uniform network services.

� Subnetwork may operate on different Subnetwork access protocols. Hence, emerges the need different set of protocols to be implemented in the end systems according to the type of subnet

Since the network layer has to interact with the subnet access mode and intermediary system, it results into complexity of the Network Layer function. Hence it becomes necessary to specify the internal architecture of the Network Layer so that the functioning of the entities can be simplified. The Network Layer is sub-divided into three sub-layers:

Subnetwork Independent Convergence Function Sub-layer (SNIC)

The convergence means bringing together into a common interpretation. This layer includes following functions:

� SNIC entity provides CONS or CLNS in the system as requested by the Transport entity.

� Relaying function which involves forwarding of the N-SDU by the SNIC entity of intermediary OSI system from the SNIC entity of one end system to the SNIC entity of another system.

� Routing function which decide over which of the possible many subnetworks particular information will travel

Subnetwork Dependent Convergence Function Sub-layer (SNDC)

It includes functions required to convert the subnet into a well defined service expected by the SNIC sub-layer. This protocol is used for the following functions:

� Add to, correct or mute function provided by the sub-network so that. an uniform basic network service boundary is provided.

� Relate the services provided by the subnetwork to the provisions of the Network services

Subnetwork Access Control Function Sub-layer (SNAC)

SNAC sub-layer performs all the functions and protocols with the corresponding layer of sub-networking. Eg. X.25 (Packet Switched Data Network) and X.21 (Circuit Switched Data Network)

Transport Layer

The Network Layer and other lower layers are the part of all the end systems and intermediary systems. But, the Transport Layer is only implemented at the end systems.

The Transport Layer provides the functions necessary to bridge the gap between the services available from the Network Layer and those required by the layers above. The Transport Layer provides transparent, reliable and cost effective transfer of data between user entities in the Session Layer.

Services to Session Layer

The Transport Layer uniquely each session entity by its transport addresses. The Transport Service like other layers of OSI model, ISO defines a connectionless mode and connection mode transport service. But currently, most of the applications are based on Connection mode transport service.

1. Transport connection establishment and release

2. Data Transfer

- Normal Data Transfer: Data can be transfer in any integral number of octets. This could be two way simultaneously

- Expedited Data Transfer: Number of octets is restricted to 16. It ensure data delivery before any further data transmission

Session Layer

It is fifth layer of the ISO Reference model which rends its services to its upper layer i.e., Presentation Layer using the services of Transport Layer. Allows two applications to establish, use and disconnect a connection between them called a session. The Session Layer controls and structures the interaction between different application entities. That is, the main function of this layer establishes, maintain and manage dialogue between different use entities.

Since, Session Connection are mapped onto the Transport connection on the one-to-one basis, hence the Session Layer does not supports multiplexing. In case the Transport connection fails, a new connection is established without the session use intervention or knowledge.

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Services to Presentation Layer

• Session Connection Establishment

• Session Connection Release

- User Abort

- Provider Abort

- Orderly Release

- Negotiate Release

• Normal Data Transfer

• Session Connection Synchronization: Synchronization refers to assuring the same state of dialog between the session service user at any point of time

• Resynchronization: It is the process of restoring the state of dialog to a previous state.

- Restart: Return to previous state of dialog

- Abandon: Cancel the current dialog

- Set: Re-agreeing the condition of dialog

Presentation Layer

This layer is concerned about the way information is to be presented to the end user. Up to Session Layer, data is treated as the string bits. The Presentation Layer deals only with the representation of data (syntax) and the data structures employed in the representation and not with the meaning of the data (semantics). Computer system may employ different representation of data such as ACSII or EBCDIC etc. Hence, the Presentation Layer provides common representation between applications/ system making them independent of syntax. The Presentation Layer only deals with the concepts of external data representation irrespective of the methodology employed by the end user to represent data internally.

Data type

The term data types visualize the data values to be operated upon and the subsequent result. The data types are classified into two classes:

5. Primitive Type are basic and elementary data type such as integer, character, date

6. Constructed Type are the derived data type based upon primitive type such as record

Data Syntax

Presentation Layer and Application Layer use types of data syntax:

Abstract Syntax: It is not concerned with the representation of data. As the application entities exchange the Application Layer data units between themselves using the underlying services of the Presentation Layer. This type of syntax is used to define the structure of Application Layer data unit

Local Concrete Syntax: This syntax specifies the format is used for the representation of data by an end system. These may varies from system to system.

Common Transfer Syntax: This syntax is used for encoding the information to be exchanged between two presentation entities. The sending presentation entity encodes the data values of its local concrete system into the common transfer syntax and the receiving presentation entity and performs the reverse process

Service to Application Layer

� Transformation of syntax

� Selection of syntax

Function within the Presentation Layer

� Session establishment request

� Data Transfer

� Negotiation and re-negotiation of syntax

� Transformation of syntax

� Session Termination request.

Application Layer

It is the topmost layer of the OSI Reference model. Its architecture is totally different from that of other layers. The basic purpose of this layer is to provide a interface for corresponding application processes to communicate via OSI environment. An application process can be defined as set of resources that can be used to generate some useful information for a user.

39

Thus this layer provides a platform for exchanging data units between two processes via application entities, application protocols and Presentation Layer services.

An application entity could be further partitioned into collection of several Application Service Element (ASE). ASEs are used to carry out different function. For example, an ASE may call Presentation Layer services or set communication path with other ASEs.

ASE can be classified into two classes:

Core/Common Application Service Entity (CASE)

Association Control Service Element (ACSE): This ASE controls and manages the relationship between the application entities. This is the vital ASE because every service that is enjoyed by application entity is at connection-mode. Hence ACSE establishes and releases the association between application entities

Reliable Transfer Service Element (RTSE): This ASE with association of ACSE provides a facility for recovering from the lost connection between application entities. For this purpose it makes use of Session Layer synchronization service which is mapped by the Presentation Layer for it

Remote Operation Service Element (ROSE): This ASE gives authority to application entity to associate with other application entity to which there is no direct access. This operation includes a invoker which sends request for remote operation and a performer which returns the response.

Commitment, Concurrency and Recovery (CCR): This ASE provides the capability to perform distributed processing transactions such as, updating multiple copies of a specific database. During updating, it maintains data integrity and consistency in spite of outrage in communication system.

Specific Application Service Element (SASE)

File Transfer, Access and Management (FTAM): This ASE provides a reliable file operation between two communication stations irrespective of the fact that both may be using different file management system. The OSI file service is based upon concept of a virtual file store. The virtual file store is an abstract description of real file store which exists in the local system environment.

The virtual file store technique gives the FTAM service independence from specific implementation of the local system.

Virtual Terminal (VT): An OSI virtual terminal is a service provide for accessing communication between two human terminal users, between two computer systems, or a human terminal and a computer system. In yester years, the human terminal was connected to the host via a dedicated link, where as today computer can be linked to any kind of network and access any resource on the network. This could be only possible by local installation of virtual terminal interface and protocols.

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Chapter-09 Data Link Protocols The Data Link layers should also agree on set of regulations o be adapted to exchange of control information. These specified set of rules and procedures for carrying out data link control function is called data link protocol.

A data link protocol takes care of following elements:

� Format of the frame (Location and size)

� Content of various fields

� Sequence of message to be exchanged to carry out the error control, flow control and link management

These are numerous types of data link protocols being used, but the following two types of protocols are preferred and commonly implemented:

i) Binary Synchronous Data Link Control (BISYNC)

ii) High Level Data Link Control (HDLC)

Frame Design Considerations

As earlier discussed, frame is basic unit transmitted by the Data Link layer. A frame consists of user data and control fields. Each frame is processed as a single entity for error and flow control. The general format has three components:

a) Header

b) Data

c) Trailer

Header Data Trailer

<------------FRAME------------>

Types of Frame Formats

The frame format is so designed that the receiver is always able to locate the beginning of the frame and its various fields so that it could separate data fields. To identify a frame and its various fields, field delimiters are used. The requirement of field delimiters is determined by the frame structure. A data link protocol can adopt fixed or variable format of the frame.

In the fixed format, all fields are always present in all the frames. In variable format, the presence of any field is optional. In fixed format the length of a frame may also be fixed or variable.

Variable Format – Variable Length

All the fields in this format are optional. If a field exists, its size is variable. In this format, the presence of each field is indicated by a field identifier with symbols signifying the start and end of the frame.

X A B C Y

X – Start of Frame

A – Start of First Field

B – Start of Second Field

C – Start of Third Field

Y – End of Frame

Figure: Variable Format– Variable Length Frame

Fixed Format – Fixed Length

In this type of format of frame the design of the frame is decided once for all and the field size is also fixed in all frames. It requires only one identifier at the beginning of the frame. All specification about the frame is known to the receiver in advance

X


Figure: Fixed Format – Fixed Length Frame

Fixed Format – Variable length

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In this type of format start and end identifiers are required. Except first field, other fields require separate identifier/delimiters

X B C Y


A – Start of First Field

B – Start of Second Field

C – Start of Third Field

Y – End of Frame

Figure: Fixed Format – Variable Length Frame

Type of Data Link Protocols

Bit Oriented Data Link Protocol

In this protocol, control information is coded in bit level and the length of the data field may not be a multiple of bytes. Bit level implies that the length of control symbol needs not to be one full byte e.g., HDLC protocol.

Byte Oriented Data Link Protocol

In this protocol, all control symbols should be at least one byte long. The length of data field can be a multiple of byte, e.g. BISYNC protocol.

Binary Synchronous Data Link Protocol (BISYNC)

BISYNC protocol is used for communication between IBM computers and terminals. Related ISO standards are ISO-1745, ISO-2111, ISO-2628 and ISO-2629.

Features

� Byte-oriented data protocol

� Supports three data code sets – ASCII, EBCDIC and Transcode

� Support synchronous two-way alternate communication

� Application for point-to-point and point-to-multipoint communication

Relationship between stations

There is master (originator of message) and slave (recipient of message) between stations.

Point-to-point Communication

This kind of communication exists between two hosts. These two stations contend for master status when they want to transmit a message.

Point-to-multipoint Communication

In this kind of communication, there is only one host and several tributary stations. The host decides who can send or receive message. For this two techniques are used.

Polling: Host selects a station which will act as master next to it and control further communication. After job completion, the control is returned back to the Host.

Selecting: This is the process in which the host decides about the station to which it will transmit data. The select station then takes over slave status.

Transmission Frames

BISYNC employs variable format and variable size frame. BISYNC makes use of two types of frames:

� Supervisory Frames are used for sending control information and are not protected against the content errors

� Data Frames are used to send user data and also contains error detection code.

HDLC (High Level Data Link Control)

Tributary Stations

Host

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HDLC is developed by ISO and is the most widely implemented Data Link protocol. It is characterized by high flexibility, adaptability, reliability and efficiency of operation for present and future synchronous data communication.

HDLC is a bit-oriented protocol. It satisfies following data link requirements:

� Point-to-point and point-to-multipoint communication

� Two-way simultaneously communication over full duplex circuits

� Two-way alternate communication over full duplex circuits

� Communication between equal stations and host and remote station

� Full data transparency

Type of stations

Primary Station: It has responsibility of data link management. It can be designated as host or master station

Secondary Station: It operates under the control of primary station. It is equivalent to tributary or slave station.

Combined Station: This system can operate as both primary as well as secondary station.

Frames

All the frames sent by the primary station are called commands. And the frames sent by the secondary stations are called response.

Mode of Data transfer

Normal Response Mode (NRM) provides unequal type of data transfer capabilities between logically unequal stations. In NRM, primary station controls the overall link management function. It is a synchronous mode of communication. The secondary station can send a frame only after receiving permission from the primary station. It is good for polled multipoint operation where ordered interaction between host and number of secondary station is required.

Asynchronous Response Mode (ARM) is similar to NRM. But in multipoint environment only one secondary station can be active at a time keeping other station in disconnect mode

Asynchronous Balance Mode (ABM) is applicable to point-to-point communication between two combined stations. Both stations are capable of link management

Normal Disconnect Mode (NDM): All stations are logically disconnected. In this mode secondary station is activated by mode-setting command for NRM from the primary station

Asynchronous Disconnect Mode (ADM): In ADM, the stations enter in either ARM or ABM when the corresponding mode-setting command is exchanged. A secondary in ADM can request mode-setting command from the primary station in order to establish data transfer mode.

Initialization Mode (IM): In this mode, operational parameters are exchanged. It is invoked when primary station senses the malfunctioning of secondary station and its operational parameter to be corrected. This process can also be requested by the secondary station.

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Chapter-10 Local Area Network It is a kind of broadcast network where each station is attached to a transmission media shared by other stations. That means one station broadcasts transmission and other stations receive it.. Moreover being packet switched transmission is in form of packets.

In this section you will learn about LAN architecture that is widely implemented. You will also learn medium access control (MAC) and Logical Link Control (LLC) and medium access control techniques.

LAN Attributes

� Geographic coverage local area network is limited to less that 5 KM

� Data rate exceeds 1 Mbps

� The physical interconnecting media is privately owned

� Shared physical interconnecting media

Architecture of LAN

The standardized protocol architecture includes physical, medium access and logical link control layers

Application Layer

Upper Layer

Protocols

Presentation Layer

Session Layer

Transport Layer

Network Layer

Data Link Layer

Logical Link Layer

Medium Access Control

Physical Layer Physical Layer

Medium Medium

Physical Layer

This layer defines the specification of the transmission medium and the topology. This layer includes following functions:

� On transmission, assemble data into frames with address and error-detection field

� On reception, disassemble frame, perform address recognition and error-detection

� Govern access to the LAN transmission medium

� Provide an interface to higher layer and perform flow and error control

The data link layer in the LAN is divide into two sub-layers:

MAC (Medium Access Control)

MAC sub layer provides the medium access control, error detection and station addressing. MAC sub layer provides connectionless mode services to LLC (Logical Link Control Layer).

The MAC sub layer service primitives:

- Request for transmission

- Indication of availability of service

- Confirm the transmission completion

LLC (Logical Link Control)

LLC is concern with the transmission of link level protocol data unit between two station without using intermediary switching node(s). LLC has two following characteristics that make it different from the other link layers:

� Must support multiple access, shared medium nature of the link

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� It is relieved for some details of link access by the MAC Layer

LLC Services

LLC handles methods of addressing across the medium and controlling data exchanged between two users. Three types of services are provided by LLC:

• Unacknowledged Connectionless Service

- Datagram based

- No flow control

- No Error control

- No guaranteed delivery

• Connection mode services

- Logical Connection

- Flow control

- Error control

- Guaranteed delivery

• Acknowledged Connectionless Service

- Datagram based

- Flow control

- Error control

- Guaranteed delivery

Ethernet Protocol

The IEEE-802.3 Protocol is based on the Xerox Network Standard (XNS) called Ethernet. The IEEE-802.3 Protocol is commonly called Ethernet but it is just one version.

These are the four versions of the Ethernet frame:

� Ethernet-802.2 Frame type used on Netware 3.12 & 4.01

� Ethernet-802.3 Frame type used on Netware 3.x & 2.x (raw)

� Ethernet-II Frame type used on DEC, TCP/IP

� Ethernet-SNAP Frame type used on Appletalk (SubNet Access Protocol)

The Source and Destination must have the same Ethernet Frame type in order to communicate.

CSMA / CD (Carrier Sense Multiple Access / Collision Detect)

Bus arbitration is performed on all versions of Ethernet using the CSMA / CD (Carrier Sense Multiple Access / Collision Detect) protocol. Bus arbitration is another way of discussing how to control who is allowed to talk on the medium (and when). Put simply, it is used to determine who's turn it is to talk.

In CSMA / CD, all stations, on the same segment of cable, listen for the carrier signal. If they hear the carrier, then they know that someone else it talking on the wire. If they don't hear carrier then they know that they can talk. This is called the Carrier Sense portion of CSMA / CD.

All stations share the same segment of cable, and can talk on it similar to a party line. This is the Multiple Access portion of CSMA / CD. If 2 stations should attempt to talk at the same time, a collision is detected, and both stations back off--for a random amount of time--before they try again. This is the Collision Detect portion of CSMA/CD.

IEEE 802.3 Ethernet Media Types

IEEE 802.3 defines five media types of IEEE 802.3 Ethernet Types shown below:

IEEE 802.3 10Base5 Thick Coax 10Mbps Baseband 500m

IEEE 802.3a 10Base2 Thin Coax 10Mbps Baseband 185m

IEEE803b 10Broad36 Broadband 10 Mbps Broadband 3600m

IEEE802.3e 1Base5 Star-LAN 1 Mbps Baseband 500m

IEEE 802.3i 10BaseT Twisted Pair 10Mps Baseband 100m

IEEE 802.3 10Base5 (Thick Coax) is used only as backbones to networks.

Backbones are lines that connect buildings & network equipment together (such as Bridges, Routers, B-router, Hubs, Concentrators, Gateways, etc.). 10Base5 is now being replaced by either Thin Coax or fibre optics.

Cabling Standards

� Cat 3, 4 and 5 cables

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� IBM Type 1-9 cabling standards

� EIA568A and 568B

� Ethernet cabling standards: IEEE 802.3 (10Base5), IEEE 802.3a (10Base2),IEEE 802.3i (10BaseT)

� Unshielded Twisted Pair (UTP)

� Shielded Twisted Pair (STP)

� Connectors: RJ45, RJ11, Hermaphroditic connectors, RS-232, DB-25,BNC, TEE

Hardware Devices

� Network Interface Cards (NICs)

� Repeaters

� Ethernet Hubs or multi port repeaters

� Token Ring Multi Station Access Units (MSAUs), Control Access Units (CAUs) and Lobe Access Modules (LAMs)

� Bridges, Brouters, Routers, Gateways, Print servers, File servers, Switches

LAN Protocols

� Ethernet frame types: Ethernet II, Ethernet SNAP, Ethernet 802.2, Ethernet 802.3

� Media Access Control layer (MAC layer)

� Token Ring: IBM and IEEE 802.5

� Logical Link Control Layer (LLC) IEEE 802.2

� TCP/IP, SMB, NetBIOS and NETBEUI, IPX/SPX, Fiber Distributed Data Interchange (FDDI)

� Asynchronous Transfer Mode (ATM)

Ethernet and Token Rings

Ethernet –CSMA/CD-IEEE8021.3-ISO 8802/3 & 7425

Created by Xerox and later developed in collaboration with Intel and DEC, also known as DIX (DEC-Intel and DEC). It was later modified by IEEE to be known as IEEE, 802.3 standard equivalents to the ISO 8802/3 standards. It uses a Bus structure, a single cable connecting all devices in the network. All devices are equal status and can start transmitting any instant. The device continuously senses the channel (carrier sense). If the line is idle, it starts transmitting the data bits. If another device starts transmission at the same instance (Multiple Access), there could be interference or mixing or collision of the data bits of one terminal with others. Therefore, the device must check for any such collision and by any detected (Collision detection), must want, re monitor and retransmit. Hence is named as CSMA/CD. Only one signal may travel over the channel CBaseband communication) Speed-10 Mbps. This type of access is called probabilistic on non-deterministic i.e. there is a certain measure of chances which data is being transmitted regarding the actual transmission which depends on the availability of the time. The maximum length of the connecting segment between two terminals could be 500 meters and each segment can support as much 100 terminals

IEEE 802.5 Token Ring

The LAN structure in an improvement over CSMA/CD in case of heavy load as the reduction in performance is not as low as in the former. The maximum data transmission rate would be as high as 16 Mbps. Another advantage over Ethernet is the use of twisted pair which are cheaper than Co-axial, waking it more cost effective. All terminals are connected in the ring like structure, i.e. a closed loop. A token which is a special bit sequence looping around the loop. Unlike CSMA/CD, the terminal cannot transmit derives to arrive. This concludes there could be never be any collision and therefore incase of heavy traffic. The degradation in performance would be liner function of the no. of terminals, resulting into much better length load efficiency then the IEEE 802.3 the major drawback is in case any terminal fails the entire network breakdowns. A double ring structure i.e. two concentric rings may be used to buffer the effect at some additional cost. Another limitation is the maximum distance between terminals is only 100 meter.

IEEE 802.4 (Token Bus)

A combination of the Ethernet and touching system all the terminals are connected using one co-axial cable. Broadband communication channel is used to facilitate transmission of different signals over the channel using varying frequency. The token passing makes a bus structure from a logical ring. The token is received and sent to the terminal on the network to it is addressed. There are two channel, forward and reverse. The signal is reflected from one end and is re modulated and retransmitted over the channel. The system provide better efficiency in case of high load as compare

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to Ethernet and the wiring is easier than in ring. However, the signaling routines are more complicated and therefore are more expensive.

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Chapter-11 X.25 INTERFACE X.25 is an analog packet switching network and it can be considered Slow Packet Switching. The transfer speeds are typically 56 kbps to 2.08 Mbps. There is a world-wide set of Public X.25 Networks; it's possible for an organization to have its own private X.25 network.

X.25 is an over 20 year old, established technology. There are many multi-vendor solutions: dissimilar technologies in an organization are allowed to access the X.25 network. In Canada, the main X.25 network is called Datapac, a public offering of X.25. You pay either a flat rate or by the packet.

X.25 is used to connect LANs together. Due to its slow transfer speed, it is used for the following:

� Host terminal emulations: low data

� Client/Server applications such as E-mail: small files, bandwidth

� File Server: large amount of data & real-time traffic (doesn't work well)

� Databases: usually large databases, but queries are small inbound and medium size outbound.

X.25 has a high protocol overhead compared to other networks. This reduces the Transfer speed and bandwidth utilization (i.e. it’s not as efficient).

Overhead Example

Truck A represents X.25. It has a heavy "empty" weight of 5 tons (overhead). The Bridge (medium) only allows 6 tons of weight. This means that Truck A can only carry 1 Ton of cargo (i.e. data). Truck B is a smaller truck, and weighs 3 tons empty. This means that it can carry up to 3 tons of cargo (data) across the bridge (medium). Truck B makes better use of its weight when crossing the bridge (i.e. it utilizes its bandwidth better; it is more efficient).

X.25 Architecture

OSI MODEL X.25 Model

Application Layer

Presentation Layer

Session Layer

Transport Layer

Network Layer

SNIC

SNDC

SNAC

Data Link Layer Data Link Layer

Physical Layer Physical Layer

Medium Medium

X.25 consists of the following layers:

Physical Layer

� Physical media specification

� Translation of bits to signal levels and vice versa

� Four type of specifications:

X.21 Sync Digital Interface 9.6kbps (unbalance), 64Kbps (balanced)

X.21bis Leased Line Analog Interface

V.24 RS232 Leased Lines

V.35 RS232 Duplex operation over Leased Lines

It uses four flavors of medium (similar to the multiple Ethernet flavors: 10BaseT, Thinnet and Thicknet). The X.25 packet is carried on serial data lines.

Data Link Layer

� Framing of data bits

� Link Management

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� Error-detection and recovery

� Protocols implemented

LAPB (Link Access Procedure Balanced)

HDLC (High Level Data Link Control)

It uses HDLC & LAPB for the Data Link layer; LAPB is considered a subset of HDLC. Both are similar to IEEE-802.2 LLC (Logical Link Control), and provide 2-way communications. The B in LAPB stands for balanced communications (another way of saying Full-Duplex - both sides communicating at the same time). The X.25 packet is carried within the LAPB frame's info field: this is similar to how the LLC packet is carried within the MAC frame's info field

Network Layer

� Subnet access

� Station addressing

� Address resolution

� Routing

� Protocols implemented

PLP (Packet Layer Protocol), or

SNDCF (Sub network Dependant Convergence Function)

X.25 uses IP network addresses and it's one of the reasons for the high overhead.

X.25 connects to the network using either a DCE modem or DSU/CSU (Data Service Unit/Channel Service Unit). X.25 allows 4,096 logical channels to be connected on one physical connection. The Packet Assembler/Disassembler (PAD) connects the DSU/CSU to the DTEs (user devices, which can be terminals or LANs).

The X.3 standard governs the operation of the PAD and the X.28 standard governs the operation of the PAD-to-terminal connection. The X.29 standard defines the End-to-End communications, from DTE-to-DTE through the X.25 Network.

X.25 SERVICES

X.25 has a high overhead because it provides extensive error checking. Each device in the X.25 network acknowledges every packet that's sent. This slows down the transfer of information, and uses up available bandwidth. When X.25 was first introduced, the quality of the analog phone lines required this extensive error checking.

X.25 provides a virtual circuit connection mode service by establishing end-to-end logical path using subnet. There are two types of connection:

SVC (Switched Virtual Circuit)

� Established on the request of DTE

� Terminate at the end of the call

� Resource available only for direction of call

PVC (Permanent Virtual Circuit)

� Permanent logical connection between two DTE

� No need for connection establishment

X.25 Packet Formats

There are three X.25 packet formats.

Call Request - Call connection/disconnection

Control Packet - Data control

Data Packet - Information transfer

Logical Channels

There are 4,096 Logical Channels available on a single physical connection to an X.25 network. The Logical Channels are divided into Groups and Channels. There can be 16 groups (4 bits) of 256 channels (8 bits). 16 x 256 = 4,096. The Logical Channel Numbers (LCN) is used to identify the connections to the Network.

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Chapter-12 ISDN (Integrated Services Digital Network) The Integrated Services Digital Network (ISDN) is a method used to bridge "the last mile" between the Central Office and the premise connection (home). ISDN uses the existing wiring, so no new cabling is required.

The application of digital technology to the telephone of the world has yielded significant benefits by way of increased reliability and reduced operating cost. Furthermore these digital networks has enable the development of public services which can offer end-to-end switched transmission of integrated voice, computer data and other form digitized information.

This class of telecommunication, which is known as ISDN, has been the subject of standard work by CCITT and has the commitment of major Public Telecommunication authorities worldwide.

ISDN lines can be dedicated lines that are always up and connected, or they can be dial on demand (DOD) lines. When the line is required, the connection is dialed up and made. The connection time for an ISDN line is very quick, in the order of 0.5 second or so. This can result in a substantial cost saving if used over long distance (or paying by the minute). The line charges are only for when data is being transferred, and not when it is sitting idle.

ISDN Channels

ISDN supports user call of voice, text or image on any of the user 64 kbps channels. These channels are referred as B-Channels, as available in Basic (B) or Primary (P) bundles from the major telecommunication authorities of the world. Calls over these channels can be made simultaneously to different destinations and used for the transmission voice, text, image or the mixture of these three. Types of call can be either circuit switched or packet switched. Beside B-channel, there is another type of channel, called the D-channel, which is used to set up and tear of call, to any destination, can be achieved by over the D-channel.

ISDN Access Rates

Basic Rate Interface (BRI) consists of 2B + D channels. This stands for 2 Bearer channels of 64 kbps each for data and one D channel of 16 kbps for handshaking and control. Having a separate channel for handshaking and control is called "out of band" signaling. The 2B channels can be bonded together for a single data channel with a 128 kbps transfer rate.

Primary Rate Interface (PRI) consists of 23B + D channels. This stands for 23 Bearer channels of 64 kbps each for data and one D channel of 64 kbps for handshaking and control. The Bearer channels can be bonded in any combination as required.

ISDN Services

There three types of telecommunication services are available to the ISDN user:

� Bearer Service is the network service (lower three layers), and is what the user encounters when attaching to ISDN at the plug-in-the-wall level. E.g. a 64 kbps circuit switched B-channel, with no restrictions on the traffic type.

� Tele-service is composed higher layer function (Layer 4-7) and usually operates the standard Bearer services. For example, Facsimile

� Supplementary Services are options that are available with both bearer services and Tele-services. For example, display of the calling party’s number on the incoming call.

ISDN Architecture

The architecture of ISDN follows the OSI closely, but the lower three OSI layers are applicable to ISDN. The higher layers are covered by CCITT standards for other types of telecommunication services. As a network, ISDN is not concerned with user layers 4-7. These are end-to-end layers employed by the user for exchange of information. The network access is concerned only with layer 1 to 3.

ISDN Physical Layer

Physical Layer is defined in I.430 and I.431, specifies the Physical Interface for the both basic and primary access. Because B and D channels are multiplexed over the same Physical Interface, these standards apply to both types of channels. Functions that are included in the physical layer are as follow:

� Channel management

� Frame structure

� Frame alignment

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� Bit stream control

� Bit and octet timing

� D-Channel Connection

� Power transfer

� Bit transmission

ISDN Data Link Layer

For D-Channel a new Data Link Layer standard, LAPD (Link Access Protocol D-Channel) has been defined and is specified by CCITT in Q.920 and Q.921. The standard is based on HDLC, modified to meet the ISDN requirements. All transmission on the D-Channel is form of LAPB frames that are exchanged between the subscriber equipment and an ISDN switching element. This layer performs following functions:

� Logical establishment of a link

� Detection of transmission error

� Error recovery

� Flow Control

ISDN Network Layer

The Network layer of ISDN is described in Q.930 and the layer protocol is specified in Q.931. This protocol is meant to define, establish, maintain and terminate network connections across an ISDN between communication entities. Following function are performed by this layer:

• Signaling

- identify calling party - identify called party - identify type of connection

• Connection management

- initiate connection - maintain connection - release connection

ISDN Advantages

� ISDN is a mature technology; it has been around since the late 1980s. It is has been tried, tested and works

� It is governed by a world-wide set of standards � It provides symmetrical transfer rates: the transmit rate is the same as the receive rate. � It has consistent transfer rates. If you have a 64kbps bearer channel then that's the speed that

you transfer at. � It is competitive priced compared to other technologies.

ISDN Disadvantages

LAPD (Q.921)

Control Signalling

Packet

Telemetry

Circuit

Switched

Semi-Permanent

Packet Switched

Application

Presentation

Session

Transport

Network

Data Link

End-to-end User

Signalling

Physical

Q.931 Call Control

X.25 Packet Level

For Further Study

I.430 Basic Interface – I.431 Primary Interface

Frame Relay

LAPB

X.25 Packet Level

D-Channel

B and H

Channels

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� An external power supply is required. If the power fails, the phones won't work. � Special digital phones are required or a Terminal Adapter to talk to the existing POTS devices. � It is very expensive to upgrade a central office switch.

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Chapter-13 TCP/IP Suite TCP/IP is one of the most commonly used network protocols. It was developed in 1970 by Department of Defense (DoD) as a strategy for connecting dissimilar networks. It is specified in MIL-STD-1777 document. TCP/IP has become de-facto standard protocol architecture.

TCP/IP was an outcome of an experimental project over packet switched network research work named as ARPANET

TCP/IP Approach

The TCP/IP protocol suite sees that the job of communication is too complicate and diverse to be completed as single unit. Thus divides the entire job broadly into to sections:

� First section IP deals with addressing aspect of network and station

� Second section TCP controls and provides reliable and error free transportation service.

Advantages if TCP/IP

Lower technical risk and cost in case of migration

Provided foundation stone for wide spread development of Internet.

Architecture of TCP/IP protocol suite

Although there is no official TCP/IP architecture, on the basis of services provide by it, the suite can be broken into five relative layers:

Physical Layer

This layer is concerned with the physical interface between DTE and the physical transmission media. It also defines characteristics of transmission media such as signal level, data rate etc. Network Devices are network interface cards (NIC) and their software drivers are place in this layer.

Network Layer

This layer controls the exchange of data packets between an end system and the network to which it is connected. This layer simply manages access and routing data across a network for two end systems connected to the same network

IP Layer

This layer manages data communication between two end systems attached to different networks. This layer provides the routing function across multiple networks. Its main job is to find the best route through the Internet to the destination. IP (Internet Protocol) is used for this task. This protocol is implemented not only in the end system but also in the router. It could be broken to two parts:

� ARP stands for Address Resolution Protocol and it is used to map IP addresses to MAC addresses. This is needed because the Network layer is not aware of the Data Link layer's addresses and vice versa.

� ICMP stands for Internet Control Message Protocol, and is used mainly for troubleshooting TCP/IP network connections. Two common programs, ping and traceroute, are part of ICMP.

ICMP IP ARP

Application

Presentation

Session

Transport

Network

Data Link

Application (PORT)

Physical

Host-to-Host TCP

Network Devices

Internet

Network

Physical

HTTP (80)

SNMP (161

162)

FTP (20

21)

TFTP (69)

SMTP (25

Telnet (23

NNTP (119)

UDP

OSI Reference Model and DoD TCP/IP Model

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Host-to-Host Layer

It is also known as TCP Layer. This layer ensures complete, safer and error-free data transmission between source and destination stations. Moreover, the services provided by this layer are independent of nature of the application. To accomplish this job it uses following protocols:

� TCP stands for Transmission Control Protocol, and is used to guarantee end to end delivery of segments of data, to put out of order segments in order, and to check for transmission errors. TCP is a connection-oriented service.

� UDP stands for User Datagram Protocol, and is a connectionless service. This results in a low overhead and fast transfer service (relies on the upper layer protocols to provide error checking and delivery of data).

Application Layer

This layer encapsulate the logic to support various user application such as file transfer, electronic mail etc. The Application layer includes many hundreds of network-aware programs and services such as the following:

� HTTP (80): HyperText Transport Protocol, which is used for transferring web pages.

� SNMP (161/162): Simple Network Management Protocol, which is used for managing network devices.

� FTP (20/21): File Transfer Protocol, which is used for transferring files across the network.

� TFTP (69): Trivial File Transfer Protocol, which is a low overhead fast transfer FTP protocol.

� SMTP (25): Simple Mail Transfer Protocol, which is used for transferring email across the Internet.

� Telnet (23): An application for remotely logging into a server across the network.

� NNTP (119): Network News Transfer Protocol, which is used for transferring news.

The numbers, shown in brackets next to the protocols, are called the well-known Port Numbers or Socket. TCP and UDP use these port numbers to indicate where the segments should be sent.

IP PROTOCOL

The Network Layer protocol for TCP/IP is the Internet Protocol (IP). It uses IP addresses and the subnet mask to determine if the datagram is on a local or remote network. If it is on the remote network, the datagram is forwarded to the default gateway (which is a router that links to another network). Functionally, it is similar as OSI connectionless network protocol (CLNP).

IP keeps track of the number of transverses through each router (that the datagram goes through to reach its destination). Each transverse is called a hop. If the hop count exceeds 255 hops, the datagram is removed, and the destination is considered unreachable. IP's name for the hop count is called Time to Live (TTL).

IP Services

IP provides limited set services to TCP namely send and receive.

Send primitive is use to transmit user data packets

Deliver primitive is used to notify the arrival of data packet to the user.

Both use following parameters:

� Source address: Address of the sender

� Destination Address: Address of the recipient

� Service Indicators: Flag indicating the treatment of data packet in its transmission through subnet

� Identifier: Unique identity to each data packet. It is used for reassembly and error-reporting. Not present in Deliver primitive.

� Time to Live: Network hops count. Not present in Deliver primitive.

� Data Length: Length of the data that is transmitted

� Option data: Option flags such as security, source routing, route recording, timestamping.

� Data: User data to be transmitted

IP Addresses

The source and destination address field contains IP addresses. IP address consists of a 32-bit global internet number, and is represented by the dot-decimal format. A portion of an IP address represents the network address, and the remaining portion the host (or local) computer’s address.

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The address is coded to allow variable allocation of bits to specify network and host. This encoding provided flexibility in assigning address to hosts and decides the network sizes on an internet.

There are three main class of IP address. Depending upon the class, different parts of the address show the network portion of address and the host address, as shown in following diagram.

For example, 142.110.237.1 is an IP address of a computer. There are 4 decimal digits separated by three dots. Each digit is allowed the range of 0 to 255. This range corresponds to 8 bits (one byte) of information. The network that the computer resides on is 142.110.237.0 (Note: IP addresses that end in a 0 represent network addresses). The host address of the firewall is 0.0.0.1 (Note: the network portion of the IP address is represented by 0s). Each host on the network and Internet must have a unique IP address.

Following table lists the three classes of IP addresses, number of networks and host that are available on each class

Network Class Address in First Field Number of Networks Number of Hosts

A 1-126 126 16777214

B 128-191 16384 65534

C 192-223 2097152 254

D 224-239 - -

E 240-255 - -

There are several classifications of IP addresses: they include network addresses and special purpose addresses.

Class A addresses

Class A addresses always have bit 0 set to 0; bits 1-7 are used as the network ID; bits 8-31 are used as the host ID. Class A networks are used by very large companies, such as IBM, US Dept of Defense and AT&T.

Class B addresses

Class B addresses always have bit 0 and 1 set to 10. Bits 2-15 are used as the network ID. Bits 16-31 are used as the host ID. Class B networks are assigned to large companies and universities.

Class C addresses

Class C addresses always have bits 0-2 set to 110. Bits 3-24 are used as the network ID. Bits 25-31 are used as the host ID. Class C network addresses are assigned to small companies and local Internet providers.

Class D Addresses

IP address range 224.0.0.0 to 239.0.0.0. Class D addresses always have bits 0-3 set to 1110, bits 4-31 are used as the Multicast address. Class D network addresses are used by multicasting. Multicasting is a method of reducing network traffic (rather than send a separate datagram to each host if multiple hosts require the same information).

Class E Addresses

IP addresses range from 240.0.0.0 to 255.0.0.0. It is reserved by the Internet for its own use.

Reserved IP Addresses

Name IP Address Description

Loopback 127.0.0.1 This IP address is reserved for Loopback testing. It refers to the same machine on which it is implemented. An A class address.

Subnet Mask A.B.C.1 It is the first address of the network and is typically used for the router address.

0 Network (7-bits) Host (24 bits) Class A

1 0 Network (14-bits) Host (16 bits) Class B

1 1 0 Network (21-bits) Host (8 bits) Class C

1 1 1 0 Multicast Address (31-bits) Class D

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Network Number A.B.C.0 This address is used to define the range of network to the router starting from A.B.C.0 to A.B.C.255

IP Broadcast A.B.C.255 This is used for broadcasting purposes. It is a C class address

SUBNET MASK

The subnet mask is used to specify which part of the IP address is the network address and which part is the host address. By default, the following subnet masks are applied:

Class A Class B Class C

255.0.0.0 255.255.0.0 255.255.255.0

By using 255, user selects the octet or octets to identify the network address. For example, in the Class B IP network address 192.200.102.100 with the subnet mask 255.255.0.0, 192.200 is the network address and 102.100 is the host address.

ARP

Address Resolution Protocol (ARP) resides in the bottom half of the Network layer. It can be considered a mechanism for mapping addresses between the Network logical addresses and MAC (Media Access Control) layer physical addresses. ARP provides the mechanism to map MAC addresses to IP addresses in a temporary memory space called the ARP cache. The ARP cache is a dynamic cache and the information is stored only for 120 seconds (then it is discarded).

ICMP

The job of the Internet Control Message Protocol (ICMP) is to report errors that may have occurred in processing IP datagrams. ICMP is an integral part of IP and its messages are encapsulated within an IP datagram.

TCP Protocol

The Transmission Control Protocol (TCP) is responsible for reliable end-to-end delivery (segments of information). Segment is the term that is used to describe the data that is transmitted and received at the Transport level of the OSI model (i.e. where TCP resides). TCP also redirects the data to the appropriate port (upper level service) that is required. The reliable end-to-end delivery of data is accomplished by the following:

� Connection-oriented service Segments are acknowledged to the source when received by the destination. A sliding window is used to enable unacknowledged segments on the "wire" in order to speed up transmission rates

� Sequencing of segments Data is broken up into segments that are numbered (sequenced) when transmitted. The destination TCP layer keeps track of the received segments and places them in the proper order (re-sequences).

� Requesting retransmission of lost data If a segment is lost in transmission (missing sequence number). The destination will timeout and request that all segments starting at the lost segment be retransmitted.

� Error checking Segments are checked for data integrity when received using a 32 bit CRC check.

The redirection of data to the upper level service is accomplished by using Source and Destination Port numbers. Multiple connections to the same service are allowed. For example, you may have many users (clients) connected to a single web server (http is normally port 80). Each client will have a unique Port number assigned (typically above 8000) but the web server will only use Port 80.

UDP Protocol

The User Datagram Protocol (UDP) is a connectionless host. UDP hosts service that operates at the Transport layer of the OSI model. UDP relies on the upper layer protocol for both error correction and reliable service. The protocol is transaction oriented and delivery and duplicate protection are not guaranteed. The major uses of this protocol are DNS and TFTP.

UDP has a small header and for all intents and purposes it adds Port addressing to the IP header. The IP header routes datagrams to the correct host on the network. UDP routes the datagram to the correct application.

SNMP

SNMP is not actually a protocol: it's a client server application that runs on the UDP (User Datagram Protocol) service of the TCP/IP protocol suite. It was developed to be an efficient means of sending network management information over UDP, using Ports 161(SNMP) and 162 (SNMPTRAP).

� SNMP consists of three parts: Messages, Agents and Managers.

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� SNMP Messages (such as Get and GetResponse) communicate the management information.

� SNMP Managers asks the questions (polls) and manages the Agents approximately every 15 minutes to see if anything has changed.

� SNMP Agents are resources to be managed such as hosts, servers, routers, hubs, etc....

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Chapter-14 FDDI (Fibre Distributed Data Interface) The Fibre Distributed Data Interface is the LAN based on the optical fibre as the transmission media. This standard is developed by ANSI. It operates under IEEE 802.2 LLC sublayer allowing to be integrated easily with other LAN standards.

FDDI was originally developed as back-end network interconnecting several hosts and high speed peripherals. It can also be used as a backbone network interconnecting several front-end LANs. It has ring topology and 1000 stations can be connected on the ring.

Optical fibre as transmission media in a LAN enjoys several advantages over copper media.

� Its potential bandwidth is immense.

� It is thinner, lighter in weight

� It is immune to electromagnetic interface

Type of Services

FDDI LANs provides two types of services:

Synchronous services

These type of services is for real time application in which bandwidth and response time are critical parameter and predictable. The allocation of ring bandwidth for synchronous transmissions is done mutually by all stations

Asynchronous services

This type of service provides dynamic bandwidth and is suitable for heavy traffic and interactive application. Unused synchronous bandwidth is transferred for asynchronous transmissions.

Traffic Control

FDDI controls the traffic in such a way that the token is rotated round the ring once in the Target Token Rotation Time (TTRT). In other words, total time for transmitting synchronous and some asynchronous data by all the station is less than TTRT. All the stations maintain two timers: Token Rotation Timer (TRT) and Token Holding Timer (THT).

If the token arrives earlier than TTRT at a station, the left over time is transferred to the THT. First, the synchronous data us transmitted up to the allotted time and then asynchronous data is transmitted till expiry of THT. The token is released immediately thereafter for the next station.

If the token arrives late, only the synchronous data is transmitted for the allotted time and the token is released thereafter. The maximum token rotation time can be 2 X TTRT beyond which a corrective action is required. The station send claim token frames indicating the required TTRT values. The station with the lowest TTRT wins and claims the token.

Priority Management

The synchronous data gets the top priority. For the asynchronous data, an optional priority scheme with eight levels of priorities is implemented.

Ring Management

The Ring Management function includes ring initialization, ring monitoring, and token monitoring. These functions are also implemented in FDDI ring. The ring initialization function also involves allocation of synchronous bandwidth to each station

FDDI Physical Layer Specification

The FDDI standard specifies a ring topology operating at 100 Mbps. Two media are as given below:

Transmission Medium Optical Fibre Twisted Pair

Data Rates (Mbps): 100 100

Max of No. of Repeaters: 100 100

Max length between Repeaters: 2 KM 100 M

book-data communication

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