6/10/2015 unit-1 : data communications 1 cs 1302 computer networks — unit - 1 — — data...

04/18/23 Unit-1 : Data Communications 1

CS 1302Computer Networks

— Unit - 1 —

— Data Communications —

Text Book Behrouz .A. Forouzan, “Data communication and Networking”, Tata McGrawHill, 2004

Overview of Overview of Data Communications Data Communications

and and NetworkingNetworking

04/18/23 2Unit-1 : Data Communications

Overview

Introduction

1.1 Data Communication

Components

Data Representation

Direction of Data Flow

Figure 1.1 Five components of data communication

Figure 1.2 Simplex

Figure 1.3 Half-duplex

Figure 1.4 Full-duplex

1.2 Networks

Distributed Processing

Network Criteria

Physical Structures

Categories of Networks04/18/23 10Unit-1 : Data Communications

Figure 1.5 Point-to-point connection

Figure 1.6 Multipoint connection

Figure 1.7 Categories of topology

Figure 1.8 Fully connected mesh topology (for five devices)

Figure 1.9 Star topology

Figure 1.10 Bus topology

Figure 1.11 Ring topology

Figure 1.12 Categories of networks

Figure 1.13 LAN

Figure 1.13 LAN (Continued)

Figure 1.14 MAN

Figure 1.15 WAN

1.3 The Internet1.3 The Internet

A Brief History

The Internet Today

Figure 1.16 Internet today

1.4 Protocols and Standards1.4 Protocols and Standards

Protocols

Standards

Standards Organizations

Internet Standards04/18/23 25Unit-1 : Data Communications

NetworkModels

2.1 Layered Tasks

Sender, Receiver, and Carrier

Hierarchy

Services04/18/23 27Unit-1 : Data Communications

Figure 2.1 Sending a letter

2.2 Internet Model

Peer-to-Peer Processes

Functions of Layers

Summary of Layers

Figure 2.2 Internet layers

Figure 2.3 Peer-to-peer processes

Figure 2.4 An exchange using the Internet model

Figure 2.5 Physical layer

The physical layer is responsible for transmitting individual bits from one

node to the next.

Note:Note:

Figure 2.6 Data link layer

The data link layer is responsible for transmitting frames from

one node to the next.

Note:Note:

Figure 2.7 Node-to-node delivery

Example 1Example 1

In Figure 2.8 a node with physical address 10 sends a frame to a node with physical address 87. The two nodes are connected by a link. At the data link level this frame contains physical addresses in the header. These are the only addresses needed. The rest of the header contains other information needed at this level. The trailer usually contains extra bits needed for error detection

Figure 2.8 Example 1

Figure 2.9 Network layer

The network layer is responsible for the delivery of packets from the

original source to the final destination.

Note:Note:

Figure 2.10 Source-to-destination delivery

Example 2Example 2

In Figure 2.11 we want to send data from a node with network address A and physical address 10, located on one LAN, to a node with a network address P and physical address 95, located on another LAN. Because the two devices are located on different networks, we cannot use physical addresses only; the physical addresses only have local jurisdiction. What we need here are universal addresses that can pass through the LAN boundaries. The network (logical) addresses have this characteristic.

Figure 2.12 Transport layer

The transport layer is responsible for delivery of a message from one process

to another.

Note:Note:

Figure 2.12 Reliable process-to-process delivery of a message

Example 3Example 3

Figure 2.14 shows an example of transport layer communication. Data coming from the upper layers have port addresses j and k (j is the address of the sending process, and k is the address of the receiving process). Since the data size is larger than the network layer can handle, the data are split into two packets, each packet retaining the port addresses (j and k). Then in the network layer, network addresses (A and P) are added to each packet.

Figure 2.15 Application layer

The application layer is responsible for providing services to the user.

Note:Note:

Figure 2.16 Summary of duties

2.3 OSI Model2.3 OSI Model

A comparison

Figure 2.17 OSI model

DigitalTransmission

4.1 Line Coding

Some Characteristics

Line Coding Schemes

Some Other Schemes

Figure 4.1 Line coding

Figure 4.2 Signal level versus data level

Figure 4.3 DC component

Example 1Example 1

A signal has two data levels with a pulse duration of 1 ms. We calculate the pulse rate and bit rate as follows:

Pulse Rate = 1/ 10Pulse Rate = 1/ 10-3-3= 1000 pulses/s= 1000 pulses/s

Bit Rate = Pulse Rate x logBit Rate = Pulse Rate x log22 L = 1000 x log L = 1000 x log22 2 = 1000 bps 2 = 1000 bps

Example 2Example 2

A signal has four data levels with a pulse duration of 1 ms. We calculate the pulse rate and bit rate as follows:

Pulse Rate = = 1000 pulses/sPulse Rate = = 1000 pulses/s

Bit Rate = PulseRate x logBit Rate = PulseRate x log22 L = 1000 x log L = 1000 x log22 4 = 2000 bps 4 = 2000 bps

Figure 4.4 Lack of synchronization

Example 3Example 3

In a digital transmission, the receiver clock is 0.1 percent faster than the sender clock. How many extra bits per second does the receiver receive if the data rate is 1 Kbps? How many if the data rate is 1 Mbps?

SolutionSolution

At 1 Kbps:1000 bits sent 1001 bits received1 extra bpsAt 1 Mbps: 1,000,000 bits sent 1,001,000 bits received1000 extra bps

Figure 4.5 Line coding schemes

Unipolar encoding uses only one voltage level.

Note:Note:

Figure 4.6 Unipolar encoding

Polar encoding uses two voltage levels Polar encoding uses two voltage levels (positive and negative).(positive and negative).

Note:Note:

Figure 4.7 Types of polar encoding

In NRZ-L the level of the signal is In NRZ-L the level of the signal is dependent upon the state of the bit.dependent upon the state of the bit.

Note:Note:

In NRZ-I the signal is inverted if a 1 is In NRZ-I the signal is inverted if a 1 is encountered.encountered.

Note:Note:

Figure 4.8 NRZ-L and NRZ-I encoding

Figure 4.9 RZ encoding

A good encoded digital signal must A good encoded digital signal must contain a provision for contain a provision for

synchronization.synchronization.

Note:Note:

Figure 4.10 Manchester encoding

In Manchester encoding, the In Manchester encoding, the transition at the middle of the bit is transition at the middle of the bit is

used for both synchronization and bit used for both synchronization and bit representation.representation.

Note:Note:

Figure 4.11 Differential Manchester encoding

In differential Manchester encoding, In differential Manchester encoding, the transition at the middle of the bit is the transition at the middle of the bit is

used only for synchronization. used only for synchronization. The bit representation is defined by the The bit representation is defined by the

inversion or noninversion at the inversion or noninversion at the beginning of the bit.beginning of the bit.

Note:Note:

In bipolar encoding, we use three In bipolar encoding, we use three levels: positive, zero, levels: positive, zero,

and negative.and negative.

Note:Note:

Figure 4.12 Bipolar AMI encoding

Figure 4.13 2B1Q

Figure 4.14 MLT-3 signal

4.2 Block Coding

Steps in Transformation

Some Common Block Codes

Figure 4.15 Block coding

Figure 4.16 Substitution in block coding

Table 4.1 4B/5B encodingTable 4.1 4B/5B encoding

Data Code Data Code

0000 1111011110 1000 1001010010

0001 0100101001 1001 1001110011

0010 1010010100 1010 1011010110

0011 1010110101 1011 1011110111

0100 0101001010 1100 1101011010

0101 0101101011 1101 1101111011

0110 0111001110 1110 1110011100

0111 0111101111 1111 111011110104/18/23 85Unit-1 : Data Communications

Table 4.1 4B/5B encoding (Continued)Table 4.1 4B/5B encoding (Continued)

Data Code

Q (Quiet) 0000000000

I (Idle) 1111111111

H (Halt) 0010000100

J (start delimiter) 1100011000

K (start delimiter) 1000110001

T (end delimiter) 0110101101

S (Set) 1100111001

R (Reset) 001110011104/18/23 86Unit-1 : Data Communications

Figure 4.17 Example of 8B/6T encoding

4.3 Sampling4.3 Sampling

Pulse Amplitude ModulationPulse Code ModulationSampling Rate: Nyquist TheoremHow Many Bits per Sample?Bit Rate

Figure 4.18 PAM

Pulse amplitude modulation has some Pulse amplitude modulation has some applications, but it is not used by itself applications, but it is not used by itself in data communication. However, it is in data communication. However, it is the first step in another very popular the first step in another very popular

conversion method called conversion method called pulse code modulation.pulse code modulation.

Note:Note:

Figure 4.19 Quantized PAM signal

Figure 4.20 Quantizing by using sign and magnitude

Figure 4.21 PCM

Figure 4.22 From analog signal to PCM digital code

According to the Nyquist theorem, the According to the Nyquist theorem, the sampling rate must be at least 2 times sampling rate must be at least 2 times

the highest frequency.the highest frequency.

Note:Note:

Figure 4.23 Nyquist theorem

Example 4Example 4

What sampling rate is needed for a signal with a bandwidth of 10,000 Hz (1000 to 11,000 Hz)?

SolutionSolution

The sampling rate must be twice the highest frequency in the signal:

Sampling rate = 2 x (11,000) = 22,000 samples/sSampling rate = 2 x (11,000) = 22,000 samples/s

Example 5Example 5

A signal is sampled. Each sample requires at least 12 levels of precision (+0 to +5 and -0 to -5). How many bits should be sent for each sample?

SolutionSolution

We need 4 bits; 1 bit for the sign and 3 bits for the value. A 3-bit value can represent 23 = 8 levels (000 to 111), which is more than what we need. A 2-bit value is not enough since 22 = 4. A 4-bit value is too much because 24 = 16.

Example 6Example 6

We want to digitize the human voice. What is the bit rate, assuming 8 bits per sample?

SolutionSolution

The human voice normally contains frequencies from 0 to 4000 Hz. Sampling rate = 4000 x 2 = 8000 samples/sSampling rate = 4000 x 2 = 8000 samples/s

Bit rate = sampling rate x number of bits per sample Bit rate = sampling rate x number of bits per sample = 8000 x 8 = 64,000 bps = 64 Kbps= 8000 x 8 = 64,000 bps = 64 Kbps

Note that we can always change a Note that we can always change a band-pass signal to a low-pass signal band-pass signal to a low-pass signal

before sampling. In this case, the before sampling. In this case, the sampling rate is twice the bandwidth.sampling rate is twice the bandwidth.

Note:Note:

4.4 Transmission Mode4.4 Transmission Mode

Parallel Transmission

Serial Transmission

Figure 4.24 Data transmission

Figure 4.25 Parallel transmission

Figure 4.26 Serial transmission

In asynchronous transmission, we In asynchronous transmission, we send 1 start bit (0) at the beginning send 1 start bit (0) at the beginning

and 1 or more stop bits (1s) at the end and 1 or more stop bits (1s) at the end of each byte. There may be a gap of each byte. There may be a gap

between each byte.between each byte.

Note:Note:

Asynchronous here means Asynchronous here means “asynchronous at the byte level,” but “asynchronous at the byte level,” but the bits are still synchronized; their the bits are still synchronized; their

durations are the same.durations are the same.

Note:Note:

Figure 4.27 Asynchronous transmission

In synchronous transmission, In synchronous transmission, we send bits one after another without we send bits one after another without

start/stop bits or gaps. start/stop bits or gaps. It is the responsibility of the receiver to It is the responsibility of the receiver to

group the bits.group the bits.

Note:Note:

Figure 4.28 Synchronous transmission

5.2 Telephone Modems

Modem Standards

A telephone line has a bandwidth of almost 2400 Hz for data transmission.

Note:Note:

Figure 5.18 Telephone line bandwidth

Modem stands for modulator/demodulator.

Note:Note:

Figure 5.19 Modulation/demodulation

Figure 5.20 The V.32 constellation and bandwidth

Figure 5.21 The V.32bis constellation and bandwidth

Figure 5.22 Traditional modems

Figure 5.23 56K modems

TransmissionMedia

Figure 7.1 Transmission medium and physical layer

Figure 7.2 Classes of transmission media

7.1 Guided Media7.1 Guided Media

Twisted-Pair Cable

Coaxial Cable

Fiber-Optic Cable

Figure 7.3 Twisted-pair cable

Figure 7.4 UTP and STP

Table 7.1 Categories of unshielded twisted-pair cablesTable 7.1 Categories of unshielded twisted-pair cables

Category Bandwidth Data Rate Digital/Analog Use

1 very low < 100 kbps Analog Telephone

2 < 2 MHz 2 Mbps Analog/digital T-1 lines

3 16 MHz 10 Mbps Digital LANs

6 (draft) 200 MHz 200 Mbps Digital LANs

7 (draft) 600 MHz 600 Mbps Digital LANs

Figure 7.5 UTP connector

Figure 7.6 UTP performance

Figure 7.7 Coaxial cable

Table 7.2 Categories of coaxial cablesTable 7.2 Categories of coaxial cables

Category Impedance Use

RG-59RG-59 75 Cable TV

RG-58RG-58 50 Thin Ethernet

RG-11RG-11 50 Thick Ethernet

Figure 7.8 BNC connectors

Figure 7.9 Coaxial cable performance

Figure 7.10 Bending of light ray

Figure 7.11 Optical fiber

Figure 7.12 Propagation modes

Figure 7.13 Modes

Table 7.3 Fiber typesTable 7.3 Fiber types

Type Core Cladding Mode

50/12550/125 50 125 Multimode, graded-index

62.5/12562.5/125 62.5 125 Multimode, graded-index

100/125100/125 100 125 Multimode, graded-index

7/1257/125 7 125 Single-mode

Figure 7.14 Fiber construction

Figure 7.15 Fiber-optic cable connectors

Figure 7.16 Optical fiber performance

7.2 Unguided Media: Wireless7.2 Unguided Media: Wireless

Radio Waves

Microwaves

Infrared

Figure 7.17 Electromagnetic spectrum for wireless communication

Figure 7.18 Propagation methods

Table 7.4 BandsTable 7.4 Bands

BandBand RangeRange PropagationPropagation ApplicationApplication

VLFVLF 3–30 KHz Ground Long-range radio navigation

LFLF 30–300 KHz GroundRadio beacons and

navigational locators

MFMF 300 KHz–3 MHz Sky AM radio

HF HF 3–30 MHz SkyCitizens band (CB),

ship/aircraft communication

VHF VHF 30–300 MHzSky and

line-of-sightVHF TV, FM radio

UHF UHF 300 MHz–3 GHz Line-of-sightUHF TV, cellular phones,

paging, satellite

SHF SHF 3–30 GHz Line-of-sight Satellite communication

EHFEHF 30–300 GHz Line-of-sight Long-range radio navigation

Figure 7.19 Wireless transmission waves

Figure 7.20 Omnidirectional antennas

Radio waves are used for multicast communications, such as radio and

television, and paging systems.

NoteNote::

Figure 7.21 Unidirectional antennas

Microwaves are used for unicast communication such as cellular

telephones, satellite networks, and wireless LANs.

NoteNote::

Infrared signals can be used for short-range communication in a closed area

using line-of-sight propagation.

NoteNote::

6/10/2015 unit-1 : data communications 1 cs 1302 computer networks — unit - 1 — — data...

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