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Applied Telecommunications
Systems and Technologies
Professor Michael KhaderTel 973-596-6078Office: ITC 2106Office Hours:
Lecture1
Communications Essentials
Overview
Topics
Basic Communications Model Data Communication Networking Protocols and Protocol Architecture Files, text, communication The Public Telephone Network Transmission Media Data Representation and encoding
A Communications Model
Source generates data to be transmitted
Transmitter Converts data into transmittable signals
Transmission System Carries data
Receiver Converts received signal into data
Destination Takes incoming data
Simplified Communications Model - Diagram
Key Communications Tasks Transmission System Utilization Interfacing Signal Generation Synchronization Exchange Management Error detection and correction Addressing and routing Recovery Message formatting Security Network Management
Simplified Data Communications Model
Networking
Point to point communication not usually practical Devices are too far apart Large set of devices would need
impractical number of connections Solution is a communications
network
Simplified Network Model
Wide Area Networks
Large geographical area Crossing public rights of way Rely in part on common carrier
circuits Alternative technologies
Circuit switching Packet switching Frame relay Asynchronous Transfer Mode (ATM)
Circuit Switching
Dedicated communications path established for the duration of the conversation
e.g. telephone network
Packet Switching
Data sent out of sequence Small chunks (packets) of data at a
time Packets passed from node to node
between source and destination Used for terminal to computer and
computer to computer communications
Frame Relay
Packet switching systems have large overheads to compensate for errors
Modern systems are more reliable Errors can be caught in end system Most overhead for error control is
stripped out
Asynchronous Transfer Mode ATM Evolution of frame relay Little overhead for error control Fixed packet (called cell) length Anything from 10Mbps to Gbps Constant data rate using packet
switching technique
Integrated Services Digital Network ISDN Designed to replace public telecom
system Wide variety of services Entirely digital domain
Local Area Networks
Smaller scope Building or small campus
Usually owned by same organization as attached devices
Data rates much higher Usually broadcast systems Now some switched systems and
ATM are being introduced
Protocols
Used for communications between entities in a system
Must speak the same language Entities
User applications e-mail facilities terminals
Systems Computer Terminal Remote sensor
Key Elements of a Protocol
Syntax Data formats Signal levels
Semantics Control information Error handling
Timing Speed matching Sequencing
Protocol Architecture
Task of communication broken up into modules
For example file transfer could use three modules File transfer application Communication service module Network access module
Simplified File Transfer Architecture
A Three Layer Model
Network Access Layer Transport Layer Application Layer
Network Access Layer
Exchange of data between the computer and the network
Sending computer provides address of destination
May invoke levels of service Dependent on type of network
used (LAN, packet switched etc.)
Transport Layer
Reliable data exchange Independent of network being
used Independent of application
Application Layer
Support for different user applications
e.g. e-mail, file transfer
Addressing Requirements
Two levels of addressing required Each computer needs unique
network address Each application on a (multi-
tasking) computer needs a unique address within the computer The service access point or SAP
Protocol Architectures and Networks
Protocols in Simplified Architecture
Protocol Data Units (PDU)
At each layer, protocols are used to communicate
Control information is added to user data at each layer
Transport layer may fragment user data Each fragment has a transport header added
Destination SAP Sequence number Error detection code
This gives a transport protocol data unit
Network PDU
Adds network header network address for destination
computer Facilities requests
Operation of a Protocol Architecture
TCP/IP Protocol Architecture Developed by the US Defense Advanced
Research Project Agency (DARPA) for its packet switched network (ARPANET)
Used by the global Internet No official model but a working one.
Application layer Host to host or transport layer Internet layer Network access layer Physical layer
Physical Layer
Physical interface between data transmission device (e.g. computer) and transmission medium or network
Characteristics of transmission medium
Signal levels Data rates etc.
Network Access Layer
Exchange of data between end system and network
Destination address provision Invoking services like priority
Internet Layer (IP)
Systems may be attached to different networks
Routing functions across multiple networks
Implemented in end systems and routers
Transport Layer (TCP)
Reliable delivery of data Ordering of delivery
Application Layer
Support for user applications e.g. http, SMPT
TCP/IP Protocol Architecture Model
OSI Model
Open Systems Interconnection Developed by the International
Organization for Standardization (ISO)
Seven layers A theoretical system delivered too
late! TCP/IP is the de facto standard
OSI Layers
Application Presentation Session Transport Network Data Link Physical
OSI v TCP/IP
Standards Organizations
Internet Society ISO ITU-T (formally CCITT) ATM forum
Types of Communications
Based on directional flow simplex half duplex full duplex
Transmitter Receiver
Simplex: one-way
Transmitter
Receiver Transmitter
Receiver
Full duplex: two-waysimultaneously
Transmitter
Receiver Transmitter
Receiver
or
Half duplex: one at a timeeither transmitting or receiving
Figure 1.3: types of communications, based oninformation flow consideration: simplex, full
duplex, and half duplex.
Communication classification
Based on devices and links configuration point-to-point
A single link with two devices
multipoint Multiple devices on a single link, On may by a
broadcaster and others are receivers.
Point-to-point
Multipoint
(a)
Satellite
Earth Stations
(b)
Figure 1.4: Types of communications,considering the number of devices onlink. (a): point-to-point, and (b): multi-
point.
Relationship between Files, Data, and Signals Files are collection of characters from a
code set (possibly ASCII) When a file to be transmitted:
The characters are translated into 1’s and 0’s An electrical signal representing the 1’s and
0’s is placed on the transmission medium The receiver, converts the incoming
electrical signal into 1’s and 0’s, then into characters that make up the original file.
FILE HAMLET.TXT
USER DATA Yea, from the table of my memoryI'll wipe away all trivial fond records
INTERNALREPRESENTATION
OF USER DATA
y01011001
e01100101
a01100001
,00101100
f01100110
r01110010
o01101111
m01101101
b00100000
b00100000
SIGNAL1 0 0 1 1 0 1 0 1 0 1 0 0 1 1 0
LSB LSB
LSB: Least Significant Bit
Figure 1.5: Files, Data, and Signals
Synchronous & Asynchronous transmission Asynchronous
A character in a message is transmitted as an individual entity, without regard to when the previous character was transmitted
Synchronous All characters in a message are sent contiguously, framing
characters indicate the beginning and end of the entire message
y1 0 0 1 1 0 1 0
e1 0 1 0 0 1 1 0
timeFraming LSB MSB Framing
Asynchronous transmission* Timing of each bit is specified within a character* inter-caharcter time is nonuniform* Each character must be framed
y1 0 0 1 1 0 1 0
e1 0 1 0 0 1 1 0
LSB MSB
Synchronous:* Characters, within a block, aresent contiguously* Each block is framed
Figure 1.6: Asynchronous andsynchronous transmission
In asynchronous transmission: Characters are delineated by a start and a stop bit
In synchronous, framing is for the entire message. Usually a message header and trailer are used
Frame check sequence to protect from transmission errors.
Transmission
SIGNALS Analog and digital signals Analog is equated with continuous Digital is equated with discrete
A telephone set converts the sound waves into an analog signal
The analog signal is digitized when it reaches the telephone network
The digital waveform is converted back into an analog signal and subsequently the sound wave at the receiving end.
Digital Transmission
Digitization of voice signals
Sampling equally spaced discrete values extracted from the
analog signal. The discrete values follow the amplitude of the
analog waveform The result is a PAM signal
quantization Assigning numbers approximate values to the
PAM signal
Bandwidth and Passband of a Communication Channel Bandwidth
A difference between two frequencies : highest and lowest that the channel can handle
Passband The range between the highest and lowest frequencies
CHANNEL A CHANNEL B
8.15 MHz 8.25 MHz 12.8 MHz 12.9 MHz
Bandwidth = 100 KHz
Figure 1.11: PASSBAND AND BANDWIDTHOF A CHANNEL
The Sine and Cosine Waveforms
Periodic signals Characterized by three parameters
Amplitude Frequency (in Hz) Phase shift (in degree or radian)
A
-A
time
PERIOD
PERIOD
SINE WAVE
COSINE WAVE
Figure 1.8: Sine andCosine waveforms
time
A
-A
Fourier Series Expansion
States: almost any periodic signal can be expresses as a sum of sineand cosine waveforms that are harmonically related
usually a sum of DC components and n-harmonic terms first harmonic is the fundamental frequency nthe harmonic is n times the first harmonic
)cossin()(1
0 tnbtnaatf nn
n
Noise
Types of Noise Thermal: heat dissipation in electronic
devices. Crosstalk: Occurs on adjacent
channels, conversation from one channel can be heard on another
Interference: Caused by many stimulus; power lines, Auto ignition, …. etc
The Concept of signal to noise ratio
TRANSMITTED LEVEL(K) SIGNAL
NOISE
TIME
TIME
(K)
(K + 1)
* To reduce effect of noise:A. Increase separation between levelsB. Slow rate of transmissionC. Add error-correction information
Result: Decrease the effective datatransfer rate
Figure 1.12: The concept of signal-to-noise-ratio
RECEIVED SIGNAL PLUSNOISE
Transmission Media
Guided media Twisted pair Optical fibers Coaxial cables
Unguided media Microwave Wireless
Twisted Pair
One of the oldest media At one point AT&T owned 80% of world
copper Typical use: subscriber’s loop, T1 carrier,
and moderate to low speed analog transmission.
Amplification every 5 to 6 km for analog signals
Repeaters every 2 to 3 km for digital signals
Twisted Pair
Figure 1.13: Twisted Pair
Coaxial Cables
the cable Workhorse of industry Used for both digital and analog
transmission Other use: Local Area Networks (Token
rings) Less susceptible to crosstalk and
interference
Coaxial Cables
Solid Cylinder
Stiff Wire
Insulating Bead
Polythylene filler
WireBraided
outerconductor
Figure 1.14: Coaxial Cable
Optical Fibers
Significant breakthrough in telecommunication
Enormous bandwidth Billion bits/sec (Gpbs) Low attenuation rate, 1 bit in error
ever 100 billion or more bits Immune from electrical interference
Optical Fibers
Based on rules of physics Incident angle between different
media Two types:
Single mode multi-mode
Optical FibersB1 B2 B3
a1 a2 a3
AIR/SILICABOUNDARY
SILICA
(a)
Total internalreflection
Light source(b)
Figure 1.15: (a) Three examples of a light ray from inside asilica fiber impinging on the air/silica boundary at different
angles, (b) Light trapped by total internal reflection.
Terrestrial Microwave
Towers operation use line-of-sight transmission. Line-of-site refers to the geographical
arrangement of the transmitting antennas and the receiving tower such that the wave travels in straight line from the transmitter to the receiver.
Since microwaves travel in straight line, if the towers are too far apart, the earth will get in the way .
The higher the towers are, the further apart they can be. For 100-m high tower, repeaters can be spaced 80 km apart (assuming there are no large hills in between).
Radio Tower
Satellite dish
STUDIO
BRO ADCAST RADIATIO NTO A 25 TO 75 M ILE
RADIUS
A n exam p le o f a m ic row avecon figu ra tion
Terrestrial Microwave - Continue
Microwaves do not pass through buildings well. In addition, even though the beam may be well focused at
the transmitter, there is still some divergence in space. Multipath Fading is caused when some waves may be
refracted off low-lying atmospheric layers and may take slightly longer to arrive than direct waves.
The delayed wave may arrive out of phase. The most common type of microwave antenna is a rigidly
fixed parabolic dish-shaped antenna of approximately ten feet diameter, Common frequencies used for microwave transmission are in the rang of 2 GHz to 40 GHz. The higher the frequency employed, the higher the bandwidth and data rate.
Satellite Microwave
A communication satellite is essentially a microwave relay station.
It links two or more ground-based transmitters/receivers called earth stations.
The satellite receives the analog or digital signals transmitted by an earth station on one frequency band called the uplink
It then repeats or amplifies the signal on another frequency band known as the downlink.
A single satellite can operate on a number of frequency bands called a transponder.
To ensure that the satellite is in the line-of-site of earth, it is made to rotate at a period equal of that of the earth. This is possible when the satellite is at a distance of approximately 35,800 km.
Satellite
Satellite dish
Sat
ellit
e di
sh
(a)
Satellite
Transmitter
Satellite dish
Satellite dish
Sat
ellit
e di
sh
Sat
ellit
e di
sh
S ate llite M ic row ave : (a ) po in t-to -po in t, (b ) B roadcas t (m u lti-rece ive rs
(b)
The Public Telephone Network--- History and Evolution --
Alexander Graham Bell invented the telephone Around 1890
Simple networks connected telephones by manually operated switches.
In this network, as shown on next slide, the signal is analog
To call another telephone, a customer first rings the operator and provides the phone number of the party.
The operator then determines the line that goes either directly to the other party or to another operator along a path to the other party.
The Public Telephone Network -- History and Evolution --
The parties remain connected for the duration of the conversation and are disconnected by the operator at the end of the call.
Paths are established by means of circuit switching “circuit” refers to the capability of transmitting one
telephone conversation along one link. To set up a call, a set of circuits has to be connected,
joining the two telephone sets. By modifying the connection, the operators can switch
the circuits. Circuit switching occurs at the beginning of a telephone call. Operators were later replaced by mechanical switches and, eventually, by electronic
switches.
Telephone
Telephone
Telephone
Telephone
A
A
A
The telephone network as existed around1890 (the telephone however looks like
today's telephone, I'm looking for atemplate for an antique telephone
A = Analog
The Public Telephone Network-- History and Evolution --
A major development in the Public Telephone Network is the digital transmission, as shown on the next slide.
An electronic interface in the switch converts the analog signal traveling on the link from the telephone set to the switch into a digital signal, and from digital to analog in the opposite direction.
The switches themselves are computers, which makes them very flexible.
This flexibility allows the Telephone Company to modify connections by sending specific instructions to the computer.
common channel signaling (CCS) – another major development CCS is a data communication network that the switches use to
exchange control information among themselves. This “conversation” between switches serves the same function as the conversation that took place between operators in the manual network.
Telephone
Telephone
Telephone
Telephone
A
D
D
CCS
Telephone network around 1988.The transmissions are analog (A)
or digital (D). The switches areelectronic and exchange control
information by using a datanetwork called common channel
signaling (CCS).
Current Telephone Network Structure Currently, The telephone system is organized as a highly
redundant, multilevel hierarchy. The present configuration, simplified:
From each telephone comes a pair of wires that goes directly to the telephone company’s nearest end office which is also called the local central office.
The two-wire connection between each subscriber’s telephone and the end office is known as the local loop.
In the United States alone there are about 20,000 local central offices. The concentration of the area code and the first three digits of the telephone number uniquely specify a local central office, which is why the rate structure uses this information.
If a subscriber, attached to a particular end office calls a subscriber attached to the same end office, the switching mechanism within the office sets up a direct electrical connection between the two local loops. This connection remains intact for the duration of the call.
The current Telephone Network Structure
If the called telephone is attached to another end office, a different procedure has to be used.
Each end office (local switch) has a number of outgoing lines to one or more nearby switching centers, called toll offices (or if they are within the same local area, tandem switches).
These lines are called toll-connecting trunks. If both the calling party and the called party end offices happen to have a toll connecting trunk to the same toll office (a likely occurrence if they are close by), the connection may be established within the toll office.
If the calling party and the called party do not have a toll office in common, the path will have to be established somewhere higher up in the hierarchy.
Telephone
End office
Toll office
Intermediat switches
offices
Toll office
End officeTelephone
Very high bandwidth inter-toll trunks
Toll connecting trunks
Typical circuit rout for a call of a mediumdistance
OFF-HOOK
Switch connects DTMF receiver
Dial Tone
Start DialingTelephone Number Switch Starts to Collect Digits (one
by one using DTMF circuit)
Select path
Send ringing signal if called partyis not busy
Ring-backtone
OFF-HOOKSwitch disconnects ringing signal
from callee and remove ringbacktone from caller
Removering-back
Connection Path between callerand callee
ON-HOOK (HANG up)Connection path is removed andresources are freed for another
call
Stop dial tone
Ringing signal
Removeringingsignal
A local telephone call scenario
Table 1.1: The telephone system hierarchy as it exists today
Order Name Comments
Class 1 Regional center Top of the hierarchy
Class 2 Sectional Center
Class 3 Primary Center
Class 4 Toll center Now “point of presence” (POP) where local
exchange meets IEX
Class 5 End Office In the local exchange carrier (LEC) area
Standards
Required to allow for interoperability between equipment
Advantages Ensures a large market for equipment and
software Allows products from different vendors to
communicate Disadvantages
Freeze technology May be multiple standards for the same thing
Five components of data communication
Components of data communications
Message: The information to be communicated – text, sound, video, or a combination
Sender: The device that sends the message – computer, telephone, TV, and so on.
Receiver: The device that receives the message – computer, telephone, TV, and so on.
Medium: A path by which the a message travels from sender to receiver – twisted pair, coaxial cables, fiber optic cable, or radio waves (terrestrial or satellite microwave)
Protocol: Rules that governs communications among devices
Data representation Text: represented by a bit pattern of 0s and 1s, the
number of bits in a pattern depends on the number of symbols in the language – English uses 26 symbols (a, b, …) and 26 for (A, B, ..) and 10 symbols for (0, 1, 2, ..)
ASCII: American Standard Code for Information Exchange: uses 7 bit pattern (128 symbols)
Extended ASCII: 8-bit patterns – ASCII is a subset of it by adding a 0 to the left
Unicode: 65,536 symbols because it uses a 16 bit code Images: also represented by bit patterns,
however the mechanism is different.. In its simplest form an image is divided into a matrix of pixels, where each pixel is a small dot
Data Representation
Audio: is a representation of sound, it is by nature different from text, numbers, or images. It is continuous, not discrete.
Video: Can be produced as a continuous entity, (by a TV camera), or it can be a combination of images, each a discrete entity arranged to convey the idea of motion.
Encoding Techniques
Digital data, digital signal Analog data, digital signal Digital data, analog signal Analog data, analog signal
Digital Data, Digital Signal
Digital signal Discrete, discontinuous voltage
pulses Each pulse is a signal element Binary data encoded into signal
elements
Terms (1)
Unipolar All signal elements have same sign
Polar One logic state represented by positive
voltage the other by negative voltage Data rate
Rate of data transmission in bits per second Duration or length of a bit
Time taken for transmitter to emit the bit
Terms (2)
Modulation rate Rate at which the signal level
changes Measured in baud = signal elements
per second Mark and Space
Binary 1 and Binary 0 respectively
Interpreting Signals
Need to know Timing of bits - when they start and
end Signal levels
Factors affecting successful interpreting of signals Signal to noise ratio Data rate Bandwidth
Comparison of Encoding Schemes (1) Signal Spectrum
Lack of high frequencies reduces required bandwidth
Lack of dc component allows ac coupling via transformer, providing isolation
Concentrate power in the middle of the bandwidth
Clocking Synchronizing transmitter and receiver External clock Sync mechanism based on signal
Comparison of Encoding Schemes (2) Error detection
Can be built in to signal encoding Signal interference and noise immunity
Some codes are better than others Cost and complexity
Higher signal rate (& thus data rate) lead to higher costs
Some codes require signal rate greater than data rate
Encoding Schemes
Nonreturn to Zero-Level (NRZ-L) Nonreturn to Zero Inverted (NRZI) Bipolar -AMI Pseudoternary Manchester Differential Manchester B8ZS HDB3
Nonreturn to Zero-Level (NRZ-L) Two different voltages for 0 and 1 bits Voltage constant during bit interval
no transition I.e. no return to zero voltage e.g. Absence of voltage for zero,
constant positive voltage for one More often, negative voltage for one
value and positive for the other This is NRZ-L
Nonreturn to Zero Inverted
Nonreturn to zero inverted on ones Constant voltage pulse for duration of bit Data encoded as presence or absence of
signal transition at beginning of bit time Transition (low to high or high to low)
denotes a binary 1 No transition denotes binary 0 An example of differential encoding
NRZ
Differential Encoding
Data represented by changes rather than levels
More reliable detection of transition rather than level
In complex transmission layouts it is easy to lose sense of polarity
NRZ pros and cons
Pros Easy to engineer Make good use of bandwidth
Cons dc component Lack of synchronization capability
Used for magnetic recording Not often used for signal
transmission
Multilevel Binary
Use more than two levels Bipolar-AMI
zero represented by no line signal one represented by positive or negative pulse one pulses alternate in polarity No loss of sync if a long string of ones (zeros
still a problem) No net dc component Lower bandwidth Easy error detection
Pseudoternary
One represented by absence of line signal
Zero represented by alternating positive and negative
No advantage or disadvantage over bipolar-AMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary Not as efficient as NRZ
Each signal element only represents one bit In a 3 level system could represent log23 =
1.58 bits Receiver must distinguish between three
levels (+A, -A, 0)
Requires approx. 3dB more signal power for same probability of bit error
Biphase
Manchester Transition in middle of each bit period Transition serves as clock and data Low to high represents one High to low represents zero Used by IEEE 802.3
Differential Manchester Midbit transition is clocking only Transition at start of a bit period represents zero No transition at start of a bit period represents one Note: this is a differential encoding scheme Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
Con At least one transition per bit time and
possibly two Maximum modulation rate is twice NRZ Requires more bandwidth
Pros Synchronization on mid bit transition (self
clocking) No dc component Error detection
Absence of expected transition
Modulation Rate
Scrambling
Use scrambling to replace sequences that would produce constant voltage
Filling sequence Must produce enough transitions to sync Must be recognized by receiver and replace with
original Same length as original
No dc component No long sequences of zero level line signal No reduction in data rate Error detection capability
B8ZS
Bipolar With 8 Zeros Substitution Based on bipolar-AMI If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+ If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+- Causes two violations of AMI code Unlikely to occur as a result of noise Receiver detects and interprets as octet of
all zeros
HDB3
High Density Bipolar 3 Zeros Based on bipolar-AMI String of four zeros replaced with
one or two pulses
B8ZS and HDB3
Digital Data, Analog Signal
Public telephone system 300Hz to 3400Hz Use modem (modulator-demodulator)
Amplitude shift keying (ASK) Frequency shift keying (FSK) Phase shift keying (PK)
Modulation Techniques
Amplitude Shift Keying
Values represented by different amplitudes of carrier
Usually, one amplitude is zero i.e. presence and absence of carrier is
used Susceptible to sudden gain changes Inefficient Up to 1200bps on voice grade lines Used over optical fiber
Binary Frequency Shift Keying Most common form is binary FSK (BFSK) Two binary values represented by two
different frequencies (near carrier) Less susceptible to error than ASK Up to 1200bps on voice grade lines High frequency radio Even higher frequency on LANs using
co-ax
Multiple FSK
More than two frequencies used More bandwidth efficient More prone to error Each signalling element represents
more than one bit
FSK on Voice Grade Line
Phase Shift Keying
Phase of carrier signal is shifted to represent data
Binary PSK Two phases represent two binary digits
Differential PSK Phase shifted relative to previous
transmission rather than some reference signal
Differential PSK
Quadrature PSK
More efficient use by each signal element representing more than one bit e.g. shifts of /2 (90o) Each element represents two bits Can use 8 phase angles and have more than
one amplitude 9600bps modem use 12 angles , four of
which have two amplitudes Offset QPSK (orthogonal QPSK)
Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK Waveforms
Performance of Digital to Analog Modulation Schemes Bandwidth
ASK and PSK bandwidth directly related to bit rate
FSK bandwidth related to data rate for lower frequencies, but to offset of modulated frequency from carrier at high frequencies
(See Stallings for math) In the presence of noise, bit error rate of
PSK and QPSK are about 3dB superior to ASK and FSK
Quadrature Amplitude Modulation QAM used on asymmetric digital subscriber
line (ADSL) and some wireless Combination of ASK and PSK Logical extension of QPSK Send two different signals simultaneously on
same carrier frequency Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated Two independent signals over same medium Demodulate and combine for original binary
output
QAM Modulator
QAM Levels
Two level ASK Each of two streams in one of two states Four state system Essentially QPSK
Four level ASK Combined stream in one of 16 states
64 and 256 state systems have been implemented
Improved data rate for given bandwidth Increased potential error rate
Analog Data, Digital Signal
Digitization Conversion of analog data into digital data Digital data can then be transmitted using
NRZ-L Digital data can then be transmitted using code
other than NRZ-L Digital data can then be converted to analog
signal Analog to digital conversion done using a codec Pulse code modulation Delta modulation
Digitizing Analog Data
Pulse Code Modulation(PCM) (1) If a signal is sampled at regular intervals
at a rate higher than twice the highest signal frequency, the samples contain all the information of the original signal (Proof - Stallings appendix 4A)
Voice data limited to below 4000Hz Require 8000 sample per second Analog samples (Pulse Amplitude
Modulation, PAM) Each sample assigned digital value
Pulse Code Modulation(PCM) (2) 4 bit system gives 16 levels Quantized
Quantizing error or noise Approximations mean it is impossible to
recover original exactly 8 bit sample gives 256 levels Quality comparable with analog
transmission 8000 samples per second of 8 bits each
gives 64kbps
Direction of Data Flow
Simplex: the communication is Unidirectional as on a one-way street. Only one device can transmit and the other can receive. Keyboards and traditional monitors are examples
Half-Duplex: Each station can transmit and receive but not at the same time – one lane bridge with bi-directional traffic. Walkie-talkies and CB (citizen band) radio
Full Duplex: also called duplex, both stations can transmit and receive at the same time
Simplex
Half-duplex
Full-duplex
Networks
Distributed Processing --tasks are divide among multiple computers
Network Criteria most important reliability, security, and performance
Physical Structures – how devices are connected: point-to-point, and multipoint, and network topology
Categories of Networks – LAN, MAN, WAN, …
Point-to-point connection
Multipoint connection
Categories of topology
Fully connected mesh topology (for five devices)
Star topology
Bus topology
Ring topology
Categories of networks
LAN
LAN (Continued)
MAN
WAN
The InternetThe Internet
A Brief History
The Internet Today
The Internet – a Brief History
ARPANET: the advance research project agency (ARPA) in the department of defense – DOD
1973: the idea of transmission control protocol (TCP) came about which included concepts as datagrame, and encapsulations.
Shortly after TCP was split into two protocols: TCP and IP (TCP/IP)
Internet today
Terms
ISP: International Service Providers NSPs: National Service Providers Regional ISPs: smaller ISPs that are
connected to one or more NSP Local Internet Service Providers:
provide direct service to the end user. It can be connected to regional ISPs or directly to NSPs
Protocols and StandardsProtocols and Standards
Protocols key elements are syntax
semantics and timing StandardsStandards Organizations ISO. ITU-T, CCITT ANSI, and IEEE, EIA
Internet Standards
Layered Tasks
Sender, Receiver, and Carrier
Hierarchy
Services
Internet Model
Peer-to-Peer Processes
Functions of Layers
Summary of Layers
Internet layers
Peer-to-peer processes
An exchange using the Internet model
Physical layer
The physical layer is responsible for transmitting individual bits from one
node to the next.
Note:Note:
Data link layer
The data link layer is responsible for transmitting frames from
one node to the next.
Note:Note:
Node-to-node delivery
Example 1Example 1
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
Example 1
Network layer
The network layer is responsible for the delivery of packets from the
original source to the final destination.
Note:Note:
Source-to-destination delivery
Example 2Example 2
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.
Example 2
Transport layer
The transport layer is responsible for delivery of a message from one process
to another.
Note:Note:
Reliable process-to-process delivery of a message
Example 3Example 3
This is 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.
Example 3
Application layer
The application layer is responsible for providing services to the user.
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Summary of duties
OSI ModelOSI Model
A comparison
OSI model
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