Internet Working

UTTAM K. ROY

Dept. of Information Technology,

Jadavpur University, Kolkata

INTERNETWORKINGINTERNETWORKING

Advanced Java Programming

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© Copyright 2007, U. K. Roy, IT, JU

Internetworking

• Andrew S. Tanenbaum, “Computer Networks”, Prentice Hall of India publication

• Alberto Leon Garcia & Indra Widjaja, “Communication Networks”, Tata McGraw-Hill publication

• William Stallings, “Data & Computer Communications”, Prentice Hall of India publication

• Krouse, “Computer Networks”, Pearson publication

BooksBooks

• Farouzan, “Computer Networks”

• Farouzan, “TCP/IP Protocol Suit”


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IntroductionIntroduction Definition

Collection of (autonomous) computers connected by some fashion Example


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Advantages of Computer NetworksAdvantages of Computer Networks Resource Sharing:

Expensive resources (Database, high-speed laser printer, array processors) can be shared among users of different sites as they are connected to one another. Example

• World Wide Web(WWW) • File Transfer Protocol(FTP)• Domain Name System• Network File System• Database• Remote login

Computation/Productivity speedup: Concurrent execution of independent tasks using cluster Example

• Parallel Binary search • Matrix multiplication • Load balancing


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Advantages of Computer NetworksAdvantages of Computer Networks Reliability/Availability/Fault Tolerant:

Entire system does not go down if a part becomes faulty Example

• Introduction of secondary DNS• Distributed database

(Inter-process)Communication: Processes running at different computers can communicate Example

• Remote Procedure Call• Socket

Incremental growth: Avoid huge initial setup cost

Improved Control/Flexibility: Facility to control the system remotely

Responsiveness:


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Communications ModelCommunications Model Source

generates data to be transmitted—telephones, PC

Transmitter Converts data into transmittable signals—Modem

Transmission System Carries data

Receiver Converts received signal into data Example

• Modem

Destination Takes incoming data


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Key Communications TasksKey Communications Tasks Transmission System Utilization

Efficient use of shared medium Example

• Multiplexing—FDM, TDM, WDM• Congestion control

Interfacing Interfacing with transmission medium—electromagnetic signal propagation

Signal Generation Form and intensity such that

• Capable of being propagated • Interpretable at receiver

Synchronization Determining beginning and ending of signals and its duration


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Key Communications TasksKey Communications Tasks Exchange Management

Cooperation(Convention) between communicating parties• Format and direction(simplex, duplex)

Error detection and correction Signals are distorted before they reach at the destination Some tasks can not tolerate error—FTP

Recovery Next step to the error detection

Flow Control Fast transmitter/slow receiver problem

Addressing and routing Identifying network devices


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Key Communications TasksKey Communications Tasks Message formatting

Agreement between two parties—binary code for characters

Security Sender wish to be assured that intended receiver gets data Receiver must be sure that the data have not been altered in transit Receiver must be sure that the data come from purported sender not from intruder

Network Management Configuration Monitor its status Responds on failure and overloads


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Simplified Data Communications ModelSimplified Data Communications Model

Protocol Architecture

Protocol Architecture


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ProtocolsProtocols High degree of cooperation is needed between two computer systems 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


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ProtocolsProtocols Key elements of a Protocol

Syntax• Data formats• Signal levels

Semantics• Control information• Error handling

Timing• Speed matching• Sequencing

Standardized protocol Needed to promote interoperability among vendor equipment


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OSI ModelOSI Model Open Systems Interconnection Developed by the International Organization for Standardization (ISO) in 1977 Seven layers

Application Presentation Session Transport Network Data Link Physical

A theoretical system delivered too late! TCP/IP is the de facto standard


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OSI ModelOSI Model Physical Layer

Physical interface between data transmission device (e.g. computer) and transmission medium or network Characteristics of transmission medium

• Mechanical—connector type• Electrical—signal levels • Functional—function of individual cuircits• Procedural—sequence of events,data rates etc.

Data Link Layer Error detection and correction Flow control

Network Access Layer Exchange of data between end system and network Destination address provision Routing functions across multiple networks


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OSI ModelOSI Model Transport Layer

Connection oriented and connectionless service Reliable delivery of data Sequencing/Ordering of delivery Avoid duplication

Session Layer Dialog Discipline—full duplex or half duplex Recovery—check pointing mechanisms

Presentation Later Format/presentation/syntax of data

Application Layer Provides user interface such as file transfer (FTP), electronic mail(SMTP), remote login(Telnet/SSH/rlogin), WWW(http) etc.


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TCP/IP Protocol ArchitectureTCP/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


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OSI v TCP/IPOSI v TCP/IP


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TCP/IP Protocol Architecture ModelTCP/IP Protocol Architecture Model

Data Transmission

Data Transmission


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Transmission TerminologyTransmission Terminology Transmitter Receiver Medium

Guided medium• e.g. twisted pair, optical fiber

Unguided medium• e.g. air, water, vacuum

Direct link No intermediate devices Point-to-point

• Only 2 devices share link Multi-point

• More than two devices share the link


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Simplex One direction

• e.g. Television

Half duplex Either direction, but only one way at a time

• e.g. police radio

Full duplex Both directions at the same time

• e.g. telephone

Transmission TerminologyTransmission Terminology


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Frequency, Spectrum and BandwidthFrequency, Spectrum and Bandwidth Time domain concepts

Continuous signal• Various in a smooth way over time

Discrete signal• Maintains a constant level then changes to another constant level

Periodic signal• Pattern repeated over time

Aperiodic signal• Pattern not repeated over time


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Continuous & Discrete SignalsContinuous & Discrete Signals


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Periodic SignalsPeriodic Signals


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Sine WaveSine Wave Peak Amplitude (A)

maximum strength of signal volts

Frequency (f) Rate of change of signal Hertz (Hz) or cycles per second Period = time for one repetition (T) T = 1/f

Phase () Relative position in time


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Varying Sine WavesVarying Sine Waves


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WavelengthWavelength

Distance occupied by one cycle Distance between two points of corresponding phase in two consecutive cycles Assuming signal velocity v

= vT f = v c = 3*108 ms-1 (speed of light in free space)


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Frequency Domain ConceptsFrequency Domain Concepts Signal usually made up of many frequencies Components are sine and/or cosine waves Can be shown (Fourier analysis) that any signal is made up of component sine and/or cosine waves Can plot frequency domain functions


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Addition of Frequency ComponentsAddition of Frequency Components


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Frequency DomainFrequency Domain


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Spectrum & BandwidthSpectrum & Bandwidth Spectrum

range of frequencies contained in signal

Absolute bandwidth width of spectrum

Effective bandwidth Often just bandwidth Narrow band of frequencies containing most of the energy

DC Component Component of zero frequency


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Signal with DC ComponentSignal with DC Component


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Data Rate and BandwidthData Rate and Bandwidth

Any transmission system has a limited band of frequencies

This limits the data rate that can be carried


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Analog and Digital Data TransmissionAnalog and Digital Data Transmission Data

Entities that convey meaning

Signals Electric or electromagnetic representations of data

Transmission Communication of data by propagation and processing of signals


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DataData

Analog Continuous values within some interval e.g. sound, video

Digital Discrete values e.g. text, integers


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Acoustic Spectrum (Analog)Acoustic Spectrum (Analog)


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SignalsSignals Means by which data are propagated Analog

Continuously variable Various media

• wire, fiber optic, space Speech bandwidth 100Hz to 7kHz Telephone bandwidth 300Hz to 3400Hz Video bandwidth 4MHz

Digital Use two DC components


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Data and SignalsData and Signals Usually use digital signals for digital data and analog signals for analog data Can use analog signal to carry digital data

Modem

Can use digital signal to carry analog data Compact Disc audio


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Analog Signals Carrying Analog and Digital DataAnalog Signals Carrying Analog and Digital Data


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DataDigital Signals Carrying Analog and Digital

Data


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Analog TransmissionAnalog Transmission Analog signal transmitted without regard to content May be analog or digital data Attenuated over distance Use amplifiers to boost signal Also amplifies noise


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Digital TransmissionDigital Transmission Concerned with content Integrity endangered by noise, attenuation etc. Repeaters used Repeater receives signal Extracts bit pattern Retransmits Attenuation is overcome Noise is not amplified


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Advantages of Digital TransmissionAdvantages of Digital Transmission

Digital technology Low cost LSI/VLSI technology

Data integrity Longer distances over lower quality lines

Capacity utilization High bandwidth links economical High degree of multiplexing easier with digital techniques

Security & Privacy Encryption

Integration Can treat analog and digital data similarly


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Transmission ImpairmentsTransmission Impairments Signal received may differ from signal transmitted Analog - degradation of signal quality Digital - bit errors Caused by

Attenuation and attenuation distortion Delay distortion Noise


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AttenuationAttenuation Signal strength falls off with distance Depends on medium Exponential in nature Issues

Received signal strength:• must be enough to be detected• must be sufficiently higher than noise to be received without error

Attenuation is an increasing function of frequency• Use equalization


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Delay DistortionDelay Distortion

Only in guided media Propagation velocity varies with frequency


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Noise Noise Additional signals inserted between transmitter and receiver Thermal (Also called white noise)

Due to thermal agitation of electrons Uniformly distributed

Intermodulation Signals that are the sum and difference of original frequencies sharing a medium

Crosstalk A signal from one line is picked up by another

Impulse Irregular pulses or spikes e.g. External electromagnetic interference Short duration High amplitude


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Channel CapacityChannel Capacity Data rate

In bits per second Rate at which data can be communicated

Bandwidth In cycles per second of Hertz Constrained by transmitter and medium

Nyquist Bandwidth Establishes data rate and bandwidth for noise free channel Given M signal levels and B bandwidth, maximum data rate C that can be achieved is

C = 2Blog2M Shannon’s Capacity

Given signal to noise ratio SNR, maximum data rate

C = B log2(1 + SNR)

Transmission MediaTransmission Media


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OverviewOverview Guided - wire Unguided - wireless Characteristics and quality determined by medium and signal For guided, the medium is more important For unguided, the bandwidth produced by the antenna is more important Key concerns are data rate and distance


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Design FactorsDesign Factors Bandwidth

Higher bandwidth gives higher data rate

Transmission impairments Attenuation

Interference Number of receivers

In guided media More receivers (multi-point) introduce more attenuation


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Electromagnetic SpectrumElectromagnetic Spectrum


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Guided Transmission MediaGuided Transmission Media

Twisted Pair Coaxial cable Optical fiber


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Twisted PairTwisted Pair


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Twisted Pair - ApplicationsTwisted Pair - Applications Most common medium Telephone network

Between house and local exchange (subscriber loop)

Within buildings To private branch exchange (PBX)

For local area networks (LAN) 10Mbps or 100Mbps


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Twisted Pair - Pros and ConsTwisted Pair - Pros and Cons Cheap Easy to work with Low data rate Short range


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Twisted Pair - Transmission CharacteristicsTwisted Pair - Transmission Characteristics

Analog Amplifiers every 5km to 6km

Digital Use either analog or digital signals repeater every 2km or 3km

Limited distance Limited bandwidth (1MHz) Limited data rate (100MHz) Susceptible to interference and noise


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Unshielded and Shielded TPUnshielded and Shielded TP Unshielded Twisted Pair (UTP)

Ordinary telephone wire Cheapest Easiest to install Suffers from external EM interference

Shielded Twisted Pair (STP) Metal braid or sheathing that reduces interference More expensive Harder to handle (thick, heavy)


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UTP CategoriesUTP Categories Cat 3

up to 16MHz Voice grade found in most offices Twist length of 7.5 cm to 10 cm

Cat 4 up to 20 MHz

Cat 5 up to 100MHz Commonly pre-installed in new office buildings Twist length 0.6 cm to 0.85 cm


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Near End CrosstalkNear End Crosstalk Coupling of signal from one pair to another Coupling takes place when transmit signal entering the link couples back to

receiving pair i.e. near transmitted signal is picked up by near receiving pair


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Coaxial CableCoaxial Cable


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Coaxial Cable ApplicationsCoaxial Cable Applications

Most versatile medium Television distribution

Ariel to TV Cable TV

Long distance telephone transmission Can carry 10,000 voice calls simultaneously Being replaced by fiber optic

Short distance computer systems links Local area networks


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Coaxial Cable - Transmission CharacteristicsCoaxial Cable - Transmission Characteristics

Analog Amplifiers every few km Closer if higher frequency Up to 500MHz

Digital Repeater every 1km Closer for higher data rates


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Optical FiberOptical Fiber


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Optical Fiber - BenefitsOptical Fiber - Benefits

Greater capacity Data rates of hundreds of Gbps

Smaller size & weight Lower attenuation Electromagnetic isolation Greater repeater spacing

10s of km at least


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Optical Fiber - ApplicationsOptical Fiber - Applications Long-haul trunks Metropolitan trunks Rural exchange trunks Subscriber loops LANs


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Optical Fiber - Transmission CharacteristicsOptical Fiber - Transmission Characteristics Act as wave guide for 1014 to 1015 Hz

Portions of infrared and visible spectrum

Light Emitting Diode (LED) Cheaper Wider operating temp range Last longer

Injection Laser Diode (ILD) More efficient Greater data rate

Wavelength Division Multiplexing


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Optical Fiber Transmission ModesOptical Fiber Transmission Modes


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Wireless TransmissionWireless Transmission Unguided media Transmission and reception via antenna Directional

Focused beam Careful alignment required

Omnidirectional Signal spreads in all directions Can be received by many antennae


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FrequenciesFrequencies

2GHz to 40GHz Microwave Highly directional Point to point Satellite

30MHz to 1GHz Omnidirectional Broadcast radio

3 x 1011 to 2 x 1014

Infrared Local


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Terrestrial MicrowaveTerrestrial Microwave Parabolic dish Focused beam Line of sight Long haul telecommunications Higher frequencies give higher data rates


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Satellite MicrowaveSatellite Microwave Satellite is relay station Satellite receives on one frequency, amplifies or repeats signal and transmits on another frequency Requires geo-stationary orbit

Height of 35,784km

Television Long distance telephone Private business networks


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Broadcast RadioBroadcast Radio Omnidirectional FM radio UHF and VHF television Line of sight Suffers from multipath interference

Reflections


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InfraredInfrared Modulate noncoherent infrared light Line of sight (or reflection) Blocked by walls e.g. TV remote control, IRD port

Data EncodingData Encoding


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Encoding TechniquesEncoding Techniques Data are not generally transmitted as they are due to:

Framing Error detection and correction

Data are converted into transmittable signals There are two possibilities

Analog communication• Analog data, analog signal

• Telephone system• Digital data, analog signal

• Computer to computer communication using telephone line Digital Communication

• Digital data, digital signal• Analog data, digital signal


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Digital Data, Digital SignalDigital Data, Digital Signal Digital signal

Discrete, discontinuous voltage pulses Each pulse is a signal element Binary data encoded into signal elements


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TerminologyTerminology 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

Bit duration or length of a bit Time taken for transmitter to emit the bit

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


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Interpreting SignalsInterpreting 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


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Comparison of Encoding SchemesComparison of Encoding Schemes 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

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


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Encoding SchemesEncoding Schemes Non-Return to Zero

Non-Return to Zero-Level (NRZ-L) Non-Return to Zero Inverted (NRZ-I)

Multilevel Binary Bipolar -AMI Pseudoternary

Biphase Manchester Differential Manchester

Scrambling B8ZS HDB3


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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 Example

Non-Return to Zero Level (NRZ-L)Non-Return to Zero Level (NRZ-L)

0 1 0 0 1 1 0 0 0 1 1


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Differential EncodingDifferential 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


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Non-Return to Zero Inverted(NRZ-I)Non-Return to Zero Inverted(NRZ-I) Non-return 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

0 1 0 0 1 1 0 0 0 1 1


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NRZ pros and consNRZ pros and cons Pros

Easy to engineer Make good use of bandwidth

Cons dc component Lack of synchronization capability Lack of error detection/correction facility

Used for magnetic recording Not often used for signal transmission


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Multilevel BinaryMultilevel Binary Use more than two levels Bipolar-AMI

zero represented by no line signal one represented by positive or negative pulse alternatively one pulses alternate in polarity

Pros No loss of sync if a long string of ones (zeros still a problem) No net dc component Lower bandwidth Easy error detection

0 1 0 0 1 1 0 0 0 1 1


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PseudoternaryPseudoternary One represented by absence of line signal Zero represented by alternating positive and negative No advantage or disadvantage over bipolar-AMI

0 1 0 0 1 1 0 0 0 1 1


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Bipolar-AMI and PseudoternaryBipolar-AMI and Pseudoternary


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Trade Off for Multilevel BinaryTrade 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


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BiphaseBiphase 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

0 1 0 0 1 1 0 0 0 1 1


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BiphaseBiphase 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

0 1 0 0 1 1 0 0 0 1 1


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Biphase Pros and ConsBiphase Pros and Cons Pros

Synchronization on mid bit transition (self clocking) No dc component Error detection

• Absence of expected transition

Con At least one transition per bit time and possibly two Maximum modulation rate is twice NRZ Requires more bandwidth


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ScramblingScrambling 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


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B8ZSB8ZS 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


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HDB3HDB3 High Density Bipolar 3 Zeros Based on bipolar-AMI String of four zeros replaced with one or two pulses

Number of Bipolar pulses(ones) since last substitution

Polarity of preceding pulse

Odd Even

- 000- +00+

+ 000+ -000-


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B8ZS and HDB3B8ZS and HDB3


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Bandwidth ComparisonBandwidth Comparison


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Modulation RateModulation Rate


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Modulation RateModulation Rate

Minimum 10101010… Maximum

NRZ-L 0 (all 0s or 1s) 1.0 1.0

NRZ-I 0 (all 0s) 0.5 1.0 (all 1s)

Bipolar AMI 0 (all 0s) 1.0 1.0

Pseudoternary 0 (all 1s) 1.0 1.0

Manchester 1.0 (10101…) 1.0 2.0 (all 0s or 1s)

Differential manchester

1.0 (all 1s) 1.5 2.0 (all 0s)

Data Link LayerData Link Layer


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Services provided to the Network Layer Unacknowledged connectionless service Acknowledged connectionless service Acknowledged connection-oriented service

Accessing of underlying medium (MAC sub-layer) Data Link Control(Data Link Control Sub-layer) Framing

Identify beginning and ending of a frame

Error Detection & Correction Identifying transmission errors and if possible correction

Error Control What to do if frames/acknowledgements are damaged/lost?

Flow Control Transmitter and Receiver must be synchronized

Data Link Layer Design IssuesData Link Layer Design Issues

Medium Access Control Sub-layer

Medium Access Control Sub-layer


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IntroductionIntroduction

LLC sub-layer

MAC sub-layerData Link Layer

Physical Layer

Network Layer


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IntroductionIntroduction Channel

Unicast—single user

Broadcast—sometimes called multi-access/random access channels

Key issue in a broadcast network determine who gets to use the channel when there is

competition for it

the protocols used to determine who goes next on a multiaccess channel belong to a sublayer of the data link layer called the MAC (Medium Access Control) sublayer.


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Two schemes exist to solve the problem of allocation

for a single broadcast channel among competitive

users: static channel allocation

dynamic channel allocation

The traditional way of allocating a single channel Frequency Division Multiplexing Access(FDMA)

• Each station is allocated its own frequency

• Uneven usage of bandwidth if number of senders is large and

continuously varying

Time Division Multiplexing Access (TDMA)

• Each station is allocated its own time slot for transmission

Statistical Time Division Multiplexing Access (STDMA)

Combination of FDMA and TDMA

Channel Allocation ProblemChannel Allocation Problem


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Problems of Static AllocationProblems of Static Allocation FDMA, TDMA are inherently inefficient

C=channel capacity in bps

=frame arrival rate in frames/sec

1/=mean frame size (random/Poison distribution)

Mean time delay from queueing theory

)()/

1

NNC C

N TFDMA = = = NT

The mean delay for FDMA or TDMA is N times worse

C

1T =

For FDMA

For TDMA

)()/

1

NNC C

N TTDMA = = = NT


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Five key assumptions:1. station model

1. N number of independent stations.

2. Frame generation rate is proportional to t is t

3. Blocks after a frame generation until it is transmitted

2. single channel 1. Only one channel is available for all stations

3. collision assumption1. Collision can occur. Every station can detect collision

4a. continuous time

4b. slotted time

5a. carrier sense Sense the carrier before transmission

5b. no carrier sense transmit without sensing the carrier

Dynamic Channel AllocationDynamic Channel Allocation


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ALOHAALOHA Pure ALOHA

Slotted ALOHA


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Pure ALOHAPure ALOHA

If collision occurs, detect it

wait a random amount of time and

retransmit

A station transmits a frame whenever it has

one


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Efficiency of Pure ALOHAEfficiency of Pure ALOHA Frame time—time required to transmit a single

frame

S=mean number of frame generated per frame time (Poision)

G=mean number of frames (old and new combined) transmitted

P0=probability that a frame does not suffer a collision

Throughput S=GP0

Probability the k frames are generated during a given frame time by Poisson distribution:

!K

eGP

Gk

k


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Efficiency of Pure ALOHAEfficiency of Pure ALOHA

GeP 0

GGeS 2 Throughpu

t 5.02

1max Gwith

eS Maximum

throughput Out of 100 frames, maximum of 18 frames reach

their destination without collision


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Slotted ALOHASlotted ALOHA Time is divided into discrete intervals

Transmission of a frame is only allowed at

the beginning of a slot

If a frame is generated in the middle of a

slot, the station must wait for the next slot

GGeS Throughpu

t 1

1max Gwith

eS Maximum

throughput


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Throughput of ALOHAThroughput of ALOHA


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Throughput of ALOHAThroughput of ALOHA

GkGG

k kk eeekkPE

1

1 1

)1(

Probability that a frame will avoid collision is e-G

Probability that a frame will suffer a collision is (1-e-

G)

Probability of a transmission requires exactly k

attempts is

Pk= e-G (1-e-G)k-1

Expected number of transmission


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• Protocols in which stations listen for a carrier and act accordingly are called carrier sense protocols

• Persistent CSMA• If the channel is busy, station waits until it becomes idle (it

continually senses the carrier for the purpose of seizing it)

• Non-Persistent CSMA• If the channel is busy, station waits random amount of time

and repeats the algorithm

• 1-persistent CSMA• If the channel is idle, the station transmits with a probability

of 1• If collision occurs, it waits a random amount of time, and

starts all over again

• p-persistent• applies to slotted channels, when the station becomes

ready to send, it senses the channel, if it is idle, it transmits with a probability p

CSMA ProtocolsCSMA Protocols


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• If collision occurs, stop transmission immediately

CSMA/CD ProtocolCSMA/CD Protocol


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CollisionCollision


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Collision DetectionCollision Detection


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CSMA/CD ProtocolCSMA/CD Protocol


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Performance ComparisonPerformance Comparison


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Collision Free ProtocolsCollision Free Protocols Collision adversely affect system performance,

when cable is long and frames short and number of

stations is large

Collision free protocols is the solution Bit-Map Protocol

Binary Countdown

Basic Assumptions N number of stations

Each station has a unique address from 0 to N-1

Which station gets the channel next?

Reservation protocols.


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Bit-Map ProtocolBit-Map Protocol Basic Steps

Contention period Frame transmission

Contention slot has exactly N slots one for each station

Station j is allowed to transmit (either 1 or 0) during slot j

No other station is allowed to transmit during that slot

Station j sends a 1 if it has frame to transmit, 0 otherwise

After N time slots, each station has complete knowledge of which stations wish to transmit

Since every station agrees on who goes next, there will never be collision


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Bit-Map ProtocolBit-Map Protocol

If a station is ready, after its bit slot, it must wait for next bit map


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Performance of Bit-Map ProtocolPerformance of Bit-Map Protocol Assumptions

Duration of each contention bit slot is unity Duration of a data frame is d time units

High numbered stations must wait on the average 0.5N slots

Low numbered stations must wait on the average 1.5N slots

mean for all stations is N slots

For low load, the overhead per frame is N bits, and the amount of data is d bits, for an efficiency of d/(N+d).

For high load, the overhead per frame is 1 bit, and the amount of data is Nd bits, for an efficiency of d/(d+1)

Average waiting time per frame is N/2+Nd/2=N(d+1)/2


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Binary Countdown ProtocolBinary Countdown Protocol 1 bit overhead per station in Bit-Map protocol

Solution Use binary station address

Basic idea A station with highest address will be allowed to

transmit All addresses are assumed to be same length Stations broadcast their address bits in each slot,

stating with high order bit The bits are then BOOLEAN ORed If a station sending a 0, gets 1 (this means that

there is at least one station with higher address), it gives up


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Binary Countdown ProtocolBinary Countdown Protocol


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• The channel efficiency of this method, under low load is d/(d+ lnN).

• The channel efficiency of this method, under high load is Nd/(Nd+ lnN).

Efficiency of Binary Countdown ProtocolEfficiency of Binary Countdown Protocol


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Starvation may occur

Solution proposed by Mok and Ward Station having virtual address will be permuted

circularly

Example C, H, D, A, G, B, E, F have priorities 7, 6, 5, 4, 3, 2, 1,

0 If D transmits, it will removed from this order and

put at the end giving a priority order C, H, D, A, G, B, E, F, D

Problems of Binary Countdown ProtocolProblems of Binary Countdown Protocol


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Performance ComparisonPerformance Comparison

Two basic protocols Contention Contention free

Two performance measures Delay Channel efficiency

Contention protocols are preferable at low load

Contention free protocols are preferable at high load

Solution is Limited Contention Protocol


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Symmetric ProtocolSymmetric Protocol k number of stations

Each has a probability p of transmitting during each slot

Probability that some station will successfully acquire the channel during a given slot is A=kp(1-p)k-1

pmax= 1/k

Amax=(1-1/k)k-1


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Symmetric ProtocolSymmetric Protocol


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Limited Contention ProtocolLimited Contention Protocol Probability of channel acquisition can be increased

by reducing competition Basic idea

Divide the station into groups At slot 0, members of group 0 is permitted to

transmit If one of them succeeds, it transmits If slot 0 is empty or there is collision, members of

group 1 contend for slot 1, etc. By making appropriate division, amount of

contention can be reduced. Choice

Each group has one member—contention free protocols

All station are in single group—ALOHA/Slotted ALOHA


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Adaptive Tree Walk ProtocolAdaptive Tree Walk Protocol Number of members in a group a dynamically

changed Example

All station are allowed to transmit at slot 0 If there is no collision, next slot 1 will be used by all

stations again Otherwise, stations will be broken into two groups

each having half number of stations. Slot 1 will be used by members of first half etc.


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ExampleExample


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• Each node at level I has a fraction 2-i of the stations below it

• if the q ready stations are uniformly distributed, the expected number of them below a specific node at level i is just 2-iq

• the optimum level to begin searching the tree is at i=log2q

ImprovementImprovement


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Further ImprovementFurther Improvement G and H want to transmit At slot 0, collision will occur, slot 1 will be idle It is pointless to probe node 3 (because collision will

be the obvious result)

Data Link ControlSub-layer

Data Link ControlSub-layer


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FramingFraming

message

message

message

12

132

12

3

3

TotalDelay TD

S A B D

Time

Propagation delay

S A B D

Time

TotalDelay

TD

Transmission delay

Breaks the message into number of segments called frames

This is done to reduce total transmission time

reduce amount of retransmission due to error


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Optimal Number of frames?Optimal Number of frames?

inedmterdebetopacketsofnumberp

delaynpropagatioptratedataR

ndestinatioandsourcebetweenhopsofnumberklengthheaderh

messagetheinbitsofnumberx

R

hpx

ft

R

hpx

p

t f

1

S A B D

Time

TD

tp

2

31

2

3

1

2

3tf

tm

R

hpx

ptm

pD ktR

hpx

kR

hpx

p

T

)1(

hkx

pdpdTD )1(

0


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Expected amount of transmission?Expected amount of transmission?

xb)1( xb)1(1

xix bb )1(])1(1[ )1(

xrx

i

i

xixrx

bN

bbiN

)1(

1

)1(])1(1[1

)1(

x bit message b=bit error

rate Probability that the message will not be in error is

Probability that the message will be in error is

Probability that the message requires exactly i transmission for successful transmission [i.e. (i-1) unsuccessful transmission followed by successful transmission] is Expected number of transmission

Total number of bits transmitted without framing

xb

x

)1(


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Expected amount of transmission?Expected amount of transmission?

)/(

/

)1(

1

)1(

)1(

pxx

x

px

b

b

b

ingframwithdtransmittebitsofnumber

ingframwithoutdtransmittebitsofnumber

pxpx b

x

b

pxp

// )1()1(

)/(

p=number of frames

Expected number of transmission

Number of bits transmitted with framing

pxrp bN

/)1(

1

x=10,000 bits p=10 b=0.001 (1 out of 1000) ω= 8139


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four methods for framing are: character count

starting and ending characters

starting and ending flags

physical layer coding violations

FramingFraming


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Character CountCharacter Count Assumes character oriented data transmission This method uses a field in the header to specify the

number of characters in the frame When data link layer at the destination sees this, it

knows how many characters follow


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Starting and ending charactersStarting and ending characters Gets around the problem of resynchronization Each frame starts with special ASCII character sequence DLE

STX and ends with the sequence DLE ETX DLE is Data Link Escape, STX is Start of TeXt and ETX is End of

TeXt If destination ever loses track of frame boundaries, all it has to

do is look for DLE STX or DLE ETX to figure out where it is

DLE STX A DLEB D ETX DLE STX D B DLE ETX


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Starting and ending charactersStarting and ending characters Problem occurs when data contains DLE STX or DLE ETX

DLE STX A DLEDLE D ETX Data sent by network layer

DLE STX A DLEDLE D ETXDLEData after being character stuffed by data link layer

DLE STX A DLEDLE D ETXData passed to network layer on the receiving node

Solution Sender’s data link layer inserts an ASCII DLE character just

before an “accidental” DLE character in the data This technique is called character stuffing

Data link layer at receiving end removes the DLE character Thus framing DLE STX or DLE ETX can be distinguished by

absence or presence of a single DLE as DLEs in the data are always doubled


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Bit Stuffing

Starting and ending flagsStarting and ending flags


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The data link layer breaks the bit stream up into

discrete frames and then computes the checksum for

each frame

when a frame arrives at the destination the checksum is

recomputed, and if it is different from the one contained

in the frame, the data link layer knows that an error has

occurred

Error DetectionError Detection


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Error DetectionError Detection

Additional bits added by transmitter for error detection


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Error Detection TechniquesError Detection Techniques Parity

Value of parity bit is such that character has even (even parity) or odd (odd parity) number of ones

Even number of bit errors goes undetected


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Two-dimensional parityTwo-dimensional parity

1 0 1 0 1 1

1 1 0 1 1 0

0 1 1 1 0 1

0 0 1 0 1 0

Undetected

error

Detected error

DetectableDouble-bit-

error


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Cyclic Redundancy Check(CRC)Cyclic Redundancy Check(CRC) For a block of d bits transmitter generates r bit

sequence Transmit d+r bits which is exactly divisible by some

predetermined number G Receiver divides frame by G

If no remainder, no error


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Cyclic Redundancy Check (CRC)Cyclic Redundancy Check (CRC)

QG

RRQ

G

R

G

RQ

G

RD

G

T

RDTr

r

2.

2.

D=d-bit data G=(r+1)-bit predetermined divisor F=r-bit FCS to be determined T=(d+r)-bit message to be transmitted

G

RQ

G

D r

2.


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ExampleExampleD=1010001101

G=110101

R=? 1 1 0 1 0 1|1 0 1 0 0 0 1 1 0 1 0 0 0 0 0

1 1 0 1 0 1 1 1 1 0 1 1 1 1 0 1 0 1 1 1 1 0 1 0 1 1 0 1 0 1 1 1 1 1 1 0 1 1 0 1 0 1

1 0 1 1 0 0 1 1 0 1 0 1 1 1 0 0 1 0 1 1 0 1 0 1 0 1 1 1

0

G

R

T=101000110101110


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D=101110

G=1001

R=?

ExampleExample


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Error Correcting codesError Correcting codes

m-bit message k redundant bits are added T=(m+k)-bit message to be transmitted No of possible code words with (m+k)-bit is 2m+k

Out of 2m+k possible code words, only 2m code words

are valid Remaining code words are invalid Example

M=2, K=2 No. of possible codewords is 24 =16 No. of valid code words is 4

• 0000, 0011, 1100, 1111

No. of invalid code words is 12

0000 0100


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•Hamming codes can correct single errors

•by arranging them into matrix we can correct burst errors

Error CorrectionError Correction


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The polynomial code (cyclic redundancy code or CRC

code) are used for error detecting

when the polynomial code method is employed, the

sender and receiver must agree upon a generator

polynomial, G(x), in advance.

Both high and low bits of the generator must be 1

to compute the checksum for some frame with m bits,

corresponding to the polynomial M(x), the frame must

be longer than the generator polynomial


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The algorithm for computing the checksum is as follows:

let r be the degree of G(x). Append r zero bits to the

low-order end of the frame, so it now contains m+r

bits and corresponds to the polynomial xrM(x)

divide the bit string corresponding to G(x) into the bit

string corresponding to xrM(x) using modulo 2 division

subtract the remainder (which is always r or fewer

bits) from the bit string corresponding to xrM(x) using

modulo 2 subtraction. The result is the checksummed

frame to be transmitted. Call its polynomial T(x).


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Flow ControlFlow Control Receiver may be slower than transmitter Transmitter must not transmit frames at a rate faster than the receiver can receive Ensuring the sending entity does not overwhelm the receiving entity

Preventing buffer overflow

Transmission time Time taken to emit all bits into medium

Propagation time Time for a bit to traverse the link

timeontransmissitimenpropagatio

a

If transmission time is normalized to 1, propagation time is `a`


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Model of Frame TransmissionModel of Frame Transmission


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Stop and WaitStop and Wait Source transmits frame Destination receives frame and replies with acknowledgement Source waits for ACK before sending next frame Destination can stop flow by not sending ACK Works well for a few large frames


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FragmentationFragmentation Large block of data may be split into small frames

Limited buffer size Errors detected sooner (when whole frame received) On error, retransmission of smaller frames is needed Prevents one station occupying medium for long periods

Stop and wait becomes inadequate


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frame

prop

frame

prop

propframe

frame

t

tawhere

a

t

t

tt

t

timenpropagatiotimeontransmissi

211

21

1

2

TD=tframe+tprop+tproc+tack+tprop+tpr

oc

TD≈tframe+2tprop

ack1

1

ack1

2

ack2

tframe

tprop

tproc

tack

Stop and Wait Link UtilizationStop and Wait Link Utilization


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tprop=271 ms 1 kb frame 1 Mbps data rate Frame transmission time tframe=1 ms a=271 η= 1/(1+2x271)=0.00184



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Sliding Window Flow ControlSliding Window Flow Control Allow multiple frames to be in transit at a time Receiver has buffer W long Transmitter can send up to W frames without ACK Each frame is numbered ACK includes number of next frame expected Sequence number bounded by size of field (k)

Frames are numbered modulo 2k


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Sliding Window DiagramSliding Window Diagram


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Example Sliding WindowExample Sliding Window


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Sliding Window EnhancementsSliding Window Enhancements Receiver can acknowledge frames without permitting further transmission (Receive Not Ready) Must send a normal acknowledge to resume If duplex, use piggybacking

If no data to send, use acknowledgement frame If data but no acknowledgement to send, send last acknowledgement number again, or have ACK valid flag (TCP)


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Error ControlError Control Detection and correction of errors Lost frames Damaged frames Automatic repeat request

Error detection Positive acknowledgment Retransmission after timeout Negative acknowledgement and retransmission


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Automatic Repeat Request (ARQ)Automatic Repeat Request (ARQ) Stop and wait Go back N Selective reject (selective retransmission)


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Stop and WaitStop and Wait Source transmits single frame Wait for ACK If received frame damaged, discard it

Transmitter has timeout If no ACK within timeout, retransmit

If ACK damaged,transmitter will not recognize it Transmitter will retransmit Receive gets two copies of frame Use ACK0 and ACK1


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Stop and Wait -DiagramStop and Wait -Diagram


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Stop and Wait - Pros and ConsStop and Wait - Pros and Cons Simple Inefficient


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Go Back N Go Back N Based on sliding window If no error, ACK as usual with next frame expected Use window to control number of outstanding frames If error, reply with rejection

Discard that frame and all future frames until error frame received correctly Transmitter must go back and retransmit that frame and all subsequent frames


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Go Back N - Damaged FrameGo Back N - Damaged Frame Receiver detects error in frame i Receiver sends rejection-i Transmitter gets rejection-i Transmitter retransmits frame i and all subsequent


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Go Back N - Lost Frame (1)Go Back N - Lost Frame (1) Frame i lost Transmitter sends i+1 Receiver gets frame i+1 out of sequence Receiver send reject i Transmitter goes back to frame i and retransmits


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Go Back N - Lost Frame (2)Go Back N - Lost Frame (2) Frame i lost and no additional frame sent Receiver gets nothing and returns neither acknowledgement nor rejection Transmitter times out and sends acknowledgement frame with P bit set to 1 Receiver interprets this as command which it acknowledges with the number of the next frame it expects (frame i ) Transmitter then retransmits frame i


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Go Back N – Damaged AcknowledgementGo Back N – Damaged Acknowledgement Receiver gets frame i and send acknowledgement (i+1) which is lost Acknowledgements are cumulative, so next acknowledgement (i+n) may arrive before transmitter

times out on frame i If transmitter times out, it sends acknowledgement with P bit set as before This can be repeated a number of times before a reset procedure is initiated


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Go Back N - Damaged RejectionGo Back N - Damaged Rejection

As for lost frame (2)


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Go Back N - DiagramGo Back N - Diagram


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Maximum window size?Maximum window size? Consider a piggybacking acknowledgement scheme is used 3-bit sequence number Transmitter sends frame 0 and gets RR1 Transmitter then sends 1, 2, 3, 4, 5, 6, 7, 0 Transmitter gets RR1 What does it mean?

All eight frames are received properly All are lost and receiver is repeating its previous RR1

For n bit sequence number maximum window size can be 2n-1


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Selective Reject/RetransmissionSelective Reject/Retransmission Only rejected frames are retransmitted Subsequent frames are accepted by the receiver and buffered Minimizes retransmission Receiver must maintain large enough buffer More complex login in transmitter


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Selective Reject -DiagramSelective Reject -Diagram


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Maximum window size?Maximum window size? Transmitter sends frame 0 through 6 All 7 frames are received properly. Receiver advances its window to accept frames 7 through 5 and sends RR7 RR7 is lost in transit. Transmitter times out and retransmit frame 0 Receiver accepts this frame as new frame

For n bit sequence number maximum window size can be 2n-1


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Performance?Performance?


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aNa

N

aNU

2121

211


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)2( pfr

f

TTN

TU

t

f

T

TU Channel

Utilization Where Tf=time to transmit a single frame

Tt=total time that line is engaged in the transmission of a single frame

For error free operation using Stop-and-Wait protocol

pf

f

TT

TU

2

In the presence of error equation must be modified as

Where Nr=expected number of transmission for a frame


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p

11

p=probability that a single frame is in error

Assume ACKs and NAKs are never in error

To transmit a frame successfully, it requires exactly i attempts

That means i-1 times unsuccessful (with error) transmission followed by 1 successful (without error) transmission

Probability that a frame requires exactly i transmission is pi-1(1-p)

Nr=E[transmission]=

1

1 )1(i

i pip


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Internetworking


ap

U21

1 Stop-and-Wait ARQ

aNapN

aNpU

2121

)1(

211 Selective Reject ARQ


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Go-back-N?Go-back-N? Nr=E[number of transmitted frame to transmit one frame

successfully]

f(i)=total number of transmission if original frame must be transmitted i times

K=total number of frames retransmitted (including the original frame) for each error

f(i)=1+(i-1)K =(1-K)+Ki

pKpp

pK

K

pipKppK

ppKiKN

i

i

i

i

i

ir

11

1)1(

)1()1()1(

)1(])1[(

1

1

1

1

1

1


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Go-back-N?Go-back-N?

NKaN

NppapN

aKaNapp

U,21

)1)(21()1(

21,2121

1


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High Level Data Link ControlHigh Level Data Link Control

Widely used data link control protocol

Basic Characteristics Three types of stations Two link configurations Three data transfer mode


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HDLC Station TypesHDLC Station Types Primary station

Controls operation of link Maintains separate logical link to each secondary station Frames issued are called commands

Secondary station Operates under control of primary station Frames issued called responses

Combined station May issue commands and responses


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HDLC Link ConfigurationsHDLC Link Configurations Unbalanced

One primary and one or more secondary stations Supports full duplex and half duplex

Balanced Two combined stations Supports full duplex and half duplex


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HDLC Transfer ModesHDLC Transfer Modes Normal Response Mode (NRM)

• Unbalanced configuration• Host computer as primary, Terminals as secondary • Primary initiates transfer to secondary• Secondary may only transmit data in response to command

from primary• Used on multi-drop lines

Asynchronous Balanced Mode (ABM)• Balanced configuration• Either station may initiate transmission without receiving

permission• Most widely used• No polling overhead

Asynchronous Response Mode (ARM)• Unbalanced configuration• Secondary may initiate transmission without permission form

primary• Primary responsible for line• rarely used


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Frame StructureFrame Structure Uses synchronous transmission All transmissions in frames Single frame format for all data and control exchanges


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Flag FieldsFlag Fields Delimit frame at both ends 01111110 May close one frame and open another Receiver hunts for flag sequence to synchronize Bit stuffing used to avoid confusion with data containing 01111110

0 inserted after every sequence of five 1s If receiver detects five 1s it checks next bit If 0, it is deleted If 1 and seventh bit is 0, accept as flag If sixth and seventh bits 1, sender is indicating abort


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Bit StuffingBit Stuffing

Example with possible errors


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Address FieldAddress Field Identifies secondary station that will send or will receive frame Usually 8 bits long May be extended to multiples of 7 bits

LSB of each octet indicates that it is the last octet (1) or not (0)

All ones (11111111) is broadcast


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Control FieldControl Field Different for different frame type

Information - data to be transmitted to user (next layer up)• Flow and error control piggybacked on information frames

Supervisory - ARQ when piggyback not used Unnumbered - supplementary link control

First one or two bits of control filed identify frame type Remaining bits explained later


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Control Field DiagramControl Field Diagram


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Supervisory framesSupervisory frames


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Unnumbered framesUnnumbered frames


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Poll/Final BitPoll/Final Bit Use depends on context Command frame

P bit 1 to solicit (poll) response from peer

Response frame F bit 1 indicates response to soliciting command


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Information FieldInformation Field Only in information and some unnumbered frames Must contain integral number of octets Variable length


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Frame Check Sequence FieldFrame Check Sequence Field FCS Error detection 16 bit CRC Optional 32 bit CRC


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HDLC OperationHDLC Operation Exchange of information, supervisory and unnumbered frames Three phases

Initialization Data transfer Disconnect


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Examples of Operation (1)Examples of Operation (1)


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Examples of Operation (2)Examples of Operation (2)


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Other DLC Protocols (LAPB,LAPD)Other DLC Protocols (LAPB,LAPD) Link Access Procedure, Balanced (LAPB)

Part of X.25 (ITU-T) Subset of HDLC - ABM Point to point link between system and packet switching network node

Link Access Procedure, D-Channel ISDN (ITU-D) ABM Always 7-bit sequence numbers (no 3-bit) 16 bit address field contains two sub-addresses

• One for device and one for user (next layer up)


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Other DLC Protocols (LLC)Other DLC Protocols (LLC) Logical Link Control (LLC)

IEEE 802 Different frame format Link control split between medium access layer (MAC) and LLC (on top of MAC) No primary and secondary - all stations are peers Two addresses needed

• Sender and receiver Error detection at MAC layer

• 32 bit CRC Destination and source access points (DSAP, SSAP)


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Other DLC Protocols (Frame Relay)Other DLC Protocols (Frame Relay) Streamlined capability over high speed packet witched networks Used in place of X.25 Uses Link Access Procedure for Frame-Mode Bearer Services (LAPF) Two protocols

Control - similar to HDLC Core - subset of control

ABM 7-bit sequence numbers 16 bit CRC 2, 3 or 4 octet address field

Data link connection identifier (DLCI) Identifies logical connection

More on frame relay later


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Other DLC Protocols (ATM)Other DLC Protocols (ATM) Asynchronous Transfer Mode Streamlined capability across high speed networks Not HDLC based Frame format called “cell” Fixed 53 octet (424 bit) Details later


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Required ReadingRequired Reading Stallings chapter 7

Web sites on HDLC, frame relay, Ethernet and ATM

Local Area Networks(LANs)

Local Area Networks(LANs)

IEEE802.3 (Ethernet) LAN

IEEE802.3 (Ethernet) LAN


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Ethernet CablingEthernet Cabling


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Ethernet TopologyEthernet Topology


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EncodingEncoding


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Ethernet Frame formatEthernet Frame format


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Collision detectionCollision detection


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Ethernet EfficiencyEthernet Efficiency


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Switched Ethernet LANSwitched Ethernet LAN

IEEE802.4 (Token Bus) LAN

IEEE802.4 (Token Bus) LAN


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IEEE Standard 802.4

Token BusToken Bus


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Token Bus Frame FormatToken Bus Frame Format


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Token Bus Control FramesToken Bus Control Frames

IEEE802.5 (Token Ring) LAN

IEEE802.5 (Token Ring) LAN


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IEEE Standard 802.5: Token Ring


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Token Ring Frame FormatToken Ring Frame Format


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Token Ring Control FramesToken Ring Control Frames

Wide Area Networks(WANs)

Wide Area Networks(WANs)

IEEE802.6 (DQDB) LAN

IEEE802.6 (DQDB) LAN


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Network LayerNetwork Layer


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The network layer is concerned with getting packets from all the way to the destination.

The network layer is the lowest layer that deals with end-to-end transmission

the network layer provides services to the transport layer at the network layer/transport layer interface.

The network layer services have been designed with the following goals in mind: The services should be independent of the subnet

technology the transport layer should be shielded from the

number, the type and the topology of the subnets present

the network addresses made available to the transport layer should use a uniform numbering plan, even across LANs and WANs.

Network Layer Design IssuesNetwork Layer Design Issues


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In most subnets packets have to take multiple hops

the algorithms that choose the routes and the data structures that they use are a major area of network layer design

the routing algorithm that is a part of the network layer software responsible for deciding which output line an incoming packet should be transmitted on

In session routing route remains in force for an entire user session

Routing AlgorithmsRouting Algorithms


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Routing AlgorithmsRouting Algorithms Characteristics required

Correctness Simplicity Robustness Stability Fairness Optimality Efficiency

Performance Criteria Used for selection of route Minimum hop Least cost

Decision Time and Place Time

• Packet or virtual circuit basis

Place• Distributed

• Made by each node• Centralized• Source

Routing Strategies Non-adaptive

• Flooding• Random

Adaptive


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FloodingFlooding No network info required Packets are sent by source node to every neighbor Incoming packets are retransmitted on every link except incoming link Eventually a number of copies will arrive at destination


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Flooding ExampleFlooding Example


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Properties of FloodingProperties of Flooding All possible routes are tried

Very robust

At least one packet will have taken minimum hop count route Can be used to set up virtual circuit

All nodes are visited Useful to distribute information (e.g. routing)


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Disadvantage of FloodingDisadvantage of Flooding Huge number of packets will be generated Solution

Each packet is uniquely numbered so duplicates can be discarded Nodes can remember packets already forwarded to keep network load in bounds Can include a hop count in packets


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Random RoutingRandom Routing Node selects one outgoing path for retransmission of incoming packet Selection can be random or round robin Can select outgoing path based on probability calculation No network info needed Route is typically not least cost nor minimum hop


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Adaptive RoutingAdaptive Routing Used by almost all packet switching networks Routing decisions change as conditions on the network change

Failure Congestion

Requires info about network Decisions more complex Tradeoff between quality of network info and overhead Reacting too quickly can cause oscillation Too slowly to be relevant


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Adaptive Routing - AdvantagesAdaptive Routing - Advantages Improved performance Aid congestion control (See chapter 12) Complex system

May not realize theoretical benefits


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ClassificationClassification Based on information sources

Local (isolated)• Route to outgoing link with shortest queue• Can include bias for each destination• Rarely used - do not make use of easily available info

Adjacent nodes All nodes


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Isolated Adaptive RoutingIsolated Adaptive Routing


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ARPANET Routing Strategies(1)ARPANET Routing Strategies(1) First Generation

1969 Distributed adaptive Estimated delay as performance criterion Bellman-Ford algorithm (appendix 10a) Node exchanges delay vector with neighbors Update routing table based on incoming info Doesn't consider line speed, just queue length Queue length not a good measurement of delay Responds slowly to congestion


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ARPANET Routing Strategies(2)ARPANET Routing Strategies(2) Second Generation

1979 Uses delay as performance criterion Delay measured directly Uses Dijkstra’s algorithm (appendix 10a) Good under light and medium loads Under heavy loads, little correlation between reported delays and those experienced


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ARPANET Routing Strategies(3)ARPANET Routing Strategies(3) Third Generation

1987 Link cost calculations changed Measure average delay over last 10 seconds Normalize based on current value and previous results


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Mean delay, T= C

1


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Distance Vector RoutingDistance Vector Routing


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Count to Infinity ProblemCount to Infinity Problem


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Link State RoutingLink State Routing


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Hierarchical RoutingHierarchical Routing


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when too many packets are present in (a part of the) subnet, performance degrades, and the situation is called congestion

as traffic increases too far, the routers are no longer able to cope, and they begin losing packets

at very high traffic the performance collapses completely, and almost no packets are delivered

congestion can be brought about by insufficient memory to hold the packets, slow processors, low-bandwidth lines

Congestion tends to feed upon itself and become worse

congestion control has to do with making sure the subnet is able to carry the offered traffic, flow control, in contrast relates to point-to-point traffic between a given sender and a receiver

Congestion ControlCongestion Control


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CongestionCongestion


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control theory approach divides the solutions into two groups: open loop and closed loop

open loop solutions tend to solve the problem by good design, in essence, to make sure that it does not occur in the first place

closed loop solutions are based on the concept of feedback loop

closed loop handles congestion control by: monitoring the system to detect when and where

congestion occurs passing this information to place where action can be

taken adjusting system operations to correct the problems

open loop manages congestion by a technique called traffic shaping - forcing the packets to be transmitted at a more predictable rate

General PrinciplesGeneral Principles


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•Traffic shaping is used to regulate the average rate of data transmission which is implemented using the leaky bucket algorithm and the token bucket algorithm

Leaky Bucket AlgorithmLeaky Bucket Algorithm


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Token Bucket AlgorithmToken Bucket Algorithm


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Congestion Control in Virtual CircuitCongestion Control in Virtual Circuit


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Waiting Fair QueueWaiting Fair Queue


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Chock PacketChock Packet


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IP AddressingIP Addressing

IP Address Requirements What is an IP Address? Network IDs and Host IDs What is a Physical Segment? IP Addressing Rules Classfull IP Addressing Address Classes Class A Addresses Class B Addresses Class C Addresses Class D & E Addresses Address Class Summary


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Each Device that uses TCP/IP needs at least one!

Computer/Host (each Network Interface Card) Routers (each port or connection) Printers Other Devices

Each Device needs a Unique IP Address An Example:

206.77.105.9

Configured in TCP/IP Software

IP Addressing RequirementsIP Addressing Requirements


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What is an IP Address?What is an IP Address?

32-bit Binary Number (Address) 11000000101010000111000100010011 Divided into 4, 8-bit Octets 11000000.10101000.01110001.00010011 Converted to Decimal Numbers

See: Binary Math 192.168.113.19 Decimal range of an Octet: 0-255

It contains the device’s: Network ID and Host ID


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Network ID and Host IDNetwork ID and Host ID

Network ID Shared or Common to all

computers on the same physical segment

Unique on the Entire Network

“Area Code”

Host ID Identifies a specific device

(Host) within a physical segment

Unique on the physical segment

“Phone Number”

192.176.11.201


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IP AddressingIP Addressing

What is a Physical Segment?

A Broadcast Domain The portion of the network that you can retrieve

information from by using a broadcast packet!

Ignore Repeaters, Bridges, or Switches Forward Broadcasts

Everything (all devices) -- Out a port of a router Between two routers Routers Don’t Forward Broadcasts

IP Addressing Rules All Devices on the Same Physical Segment Share a

Common Network ID Each Physical Segment Has a Unique Network IDs


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IP AddressingIP AddressingIP Addressing Rules

Each Device (Host) Needs at Least One Unique IP Address All Devices on the Same Physical Segment Share a Common Network ID (Subnet Mask) Each Physical Segment Has a Unique Network ID (Subnet Mask)


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IP AddressingIP AddressingClassfull IP Addressing

Traditional Manner of Addressing Class A Class B Class C

Address Classes Specify Which Octets of the IP Address are the Network-ID and Which are the Host-ID

Address Classes Specify Network Sizes (Number of Hosts)


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IP AddressingIP AddressingAddress Classes

Class A Network . Host . Host . Host

Class B Network . Network . Host . Host

Class C Network . Network . Network . Host

Class D & E


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IP AddressingIP AddressingClass A Networks: The Definition

Per Specification: 1st Octet is the Network ID 2nd, 3rd, 4th Octets are the Host ID

In Binary – Any address that starts with a “0” in the first bit! First Class A Network Address:

00000001.00000000.0000000.00000000 (Binary) 1.0.0.0 (Decimal)

Last Class A Network Address: 01111111.00000000.00000000.00000000 (Binary) 127.0.0.0 (Decimal) (Loopback Address)

Class A Networks: The Definition

Per Specification: 1st Octet is the Network ID 2nd, 3rd, 4th Octets are the Host ID

In Binary – Any address that starts with a “0” in the first bit! First Class A Network Address:

00000001.00000000.0000000.00000000 (Binary) 1.0.0.0 (Decimal)

Last Class A Network Address: 01111111.00000000.00000000.00000000 (Binary) 127.0.0.0 (Decimal) (Loopback Address)


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IP AddressingIP AddressingClass A Networks: Network IDs

1st Octet is the Network ID 0.0.0.0 (Invalid) 1.0.0.0 2.0.0.0 3.0.0.0 ~~~~ 127.0.0.0 (Loop back)

2nd, 3rd, 4th Octets are the Host IDs An Assigned Class A Network Address:

33.0.0.0 (Specifies the Network) 2nd, 3rd, 4th Octets are the Host IDs

Specified by Network Administrators


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IP AddressingIP AddressingClass A Networks: The Number of Networks

1st Octet is the Network ID 1-126 = 126 Possible Class A Network IDs

2nd, 3rd, 4th Octets are the Host IDs Each of the three Octets has a possible 256 Host IDs Number of Host IDs from three Octets:

256 * 256 * 256 = 16,777,216 (minus 2) = 16,777,214 Always Subtract 2 from the number of Host IDs

Host IDs cannot be all 1’s (reserved for broadcast address)

Host IDs cannot be all 0’s (reserved for “this network only” address)


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IP AddressingIP AddressingClass A Networks: Host ID Addresses

33.0.0.0 (An Assigned Class A Address) All devices would share the 33 network ID. The Administrator would number the IP devices:

33.0.0.1 – 33.0.0.255 (255 Addresses) 33.0.1.0 – 33.0.1.255 (256 Addresses) ~~~~ 33.0.255.0 -- 33.0.255.255 (256 Addresses)

(A Total of 65,535 Addresses) 33.1.0.0 -- 33.1.255.255 (65,536 Addresses) 33.2.0.0 -- 33.2.255.255 (65,536 Addresses) ~~~~ 33.255.0.0 -- 33.255.255.254 (65,535 Addresses)

( Total Addresses: 16.7 Million)


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IP AddressingIP AddressingClass B Networks: The Definition

Per Specification: 1st and 2nd Octets are the Network ID 3rd, 4th Octets are the Host IDs

In Binary – Any address that starts with a “10” in the first two

bits of the first octet! First Class B Network Address:

10000000.00000000.0000000.00000000 (Binary) 128.0.0.0 (Decimal)

Last Class B Network Address: 10111111.11111111.00000000.00000000 (Binary) 191.255.0.0 (Decimal)


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IP AddressingIP AddressingClass B Networks: Network IDs

1st and 2nd Octets are the Network IDs 128.0.0.0 128.1.0.0 ~~~~ 128.255.0.0 129.0.0.0 129.1.0.0 ~~~~ 191.255.0.0

3rd, 4th Octets are the Host IDs An Assigned Class B Network Addresses

153.11.0.0 3rd, 4th Octets are the Host IDs



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IP AddressingIP AddressingClass B Networks: The Number of Networks

1st and 2nd Octets are the Network IDs 1st Octet 128 -- 191 = 64 Possible Network IDs 2nd Octet 0 – 255 = 256 Possible Network IDs Total Class B Network IDs 64 * 256 = 16,384

3rd, 4th Octets are the Host IDs Each of the Two Octets has a possible 256 Host IDs Number of Host IDs from Two Octets:

256 * 256 = 65,536 (minus 2) = 65,534 Always Subtract 2 from the number of Host IDs

Host ID cannot be all 1’s (reserved for broadcast address) Host ID cannot be all 0’s (reserved for “this network only”

address)


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IP AddressingIP AddressingClass B Networks: Host ID Addresses

An Assigned Class B Address 153.11.0.0

All devices would share the 153.11 Network ID. The Administrator would number the IP devices:

153.11.0.1 -- 153.11.0.255 (255 Addresses) 153.11.1.0 -- 153.11.1.255 (256 Addresses) 153.11.2.0 -- 153.11.2.255 (256 Addresses) ~~~~ 153.11.255.0 -- 153.11.255.254 (255 Addresses) Total Addresses: 65,534


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IP AddressingIP AddressingClass C Networks: The Definition

Per Specification: 1st, 2nd, 3rd Octets are the Network ID 4th Octet is the Host ID

In Binary – Any address that starts with a “110” in the first three

bits of the first octet! First Class C Network Address:

11000000.00000000.0000000.00000000 (Binary) 192.0.0.0 (Decimal)

Last Class C Network Address: 11011111.11111111.11111111.00000000 (Binary) 223.255.255.0 (Decimal)


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IP AddressingIP AddressingClass C Networks:

Network IDs

1st, 2nd, 3rd Octets are the Network IDs 192.0.0.0 – 192.0.255. 0 192.1.0.0 – 192.1.255.0 ~~~~ 192.255.0.0 – 192.255.255.0 193.0.0.0 – 193.255.255.0 ~~~~ 223.0.0.0 – 223.255.255.0 4th Octet is the Host IDs

An Assigned Class C Network Address 201.11.206.0 4th Octet is the Host IDs



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IP AddressingIP AddressingClass C Networks: The Number of Networks

1st, 2nd, 3rd Octets are the Network IDs 1st Octet 192 -- 223 = 31 Possible IDs 2nd Octet 0 – 255 = 256 Possible IDs 3nd Octet 0 – 255 = 256 Possible IDs Total Class C Network IDs 32 * 256 *256 = 2,097,152

4th Octet is the Host ID An Octet has a possible 256 IDs Number of Host IDs an Octet:

256 (minus 2) = 254 Always Subtract 2 from the number of Host IDs

Host ID cannot be all 1’s (reserved for broadcast address) Host ID cannot be all 0’s (reserved for “this network only”

address)


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IP AddressingIP AddressingClass C Networks: Host ID Addresses

An Assigned Class C Address 201.11.206.0

All devices would share the 201.11.206.0 Network ID.

The Administrator would number the IP devices: 201.11.206.1, 201.11.206.2, ~~~~ 201.11.206.254


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IP AddressingIP AddressingClass D & E

Class D Used by Multicast Applications Shared Addresses 224.0.0.0 – 239.255.255.255

Class E Experimental 240.0.0.0 +


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IP AddressingIP AddressingAddress Classes: Network IDs and Host IDs

Class A (1st Octet 1-127) Network.Host.Host.Host

Class B (1st Octet 128-191) Network.Network.Host.Host

Class C (1st Octet 192-223) Network.Network.Network.Host


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IP AddressingIP AddressingAddress Class Summary

1st Networks Hosts IDs Octet IDs /Network

Class A 1-127 12616,777,214

Class B 128-191 16,384 65,534 Class C 192-223 2,097,152 254


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IP AddressingIP AddressingIP Address: What is It?

32-bit Binary Number (Address) 11000000101010001110000100010011 Divided into 4, 8-bit Octets 11000000.10101000.11100001.00010011 Converted to Decimal Numbers

See: Binary Math 192.168.225.19 Decimal range of an Octet: 0-255

It contains the device’s: Network ID and Host ID


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Internetworking

IP AddressingIP AddressingIP Addressing Rules

Each Device (Host) Needs at Least One Unique IP Address

All Devices on the Same Physical Segment Share a Common Network ID (Subnet Mask)

Each Physical Segment Has a Unique Network ID (Subnet Mask)


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Internetworking

IP AddressingIP AddressingAddress Classes: Network IDs and Host IDs

Class A (1st Octet 1-127) Network.Host.Host.Host

Class B (1st Octet 128-191) Network.Network.Host.Host

Class C (1st Octet 192-223) Network.Network.Network.Host

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Internet Working