RAHUL DEVA

Data Link Layer

The Data Link Layer

Framing Error Control

Error Correction Error Detection

Flow Control Retransmission with Sliding Window Protocols Protocol Specification and Verification Example Data Link Protocols

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Data Link Layer Services

Unacknowledged connectionless service Voice

Acknowledged connectionless servicewireless

Acknowledged connection-oriented service no out-of-order

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Framing

To break the bit stream up into discrete frames and compute the checksum for each frame. When a frame arrives at the destination, the checksum is recomputed and validated.

One way to achieve this framing is to insert time gaps between frames. However, networks rarely make any guarantees about timing.

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Error Control

Three types of frames at a receiver’s endDamaged frame: Negative

acknowledgment / retransmissionLost frame: Time-out mechanismValid frame: Sequence numbering /

discarding for valid but duplicate frames

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Flow Control

A technique for assuring that a transmitting station

does not overwhelm a receiving station with data

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Data Framing

Character count Starting and ending characters with character stuffing Starting and ending flags with bit stuffing Physical layer coding violations

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Character Count

5 1 2 3 4 5 6 7 8 9 8 0 1 2 3 4 5 6 8 7 8 9 0 1 2 3

character count

frame 15 characters

frame 25 characters

frame 38 characters

frame 48 characters

5 1 2 3 4 7 6 7 8 9 8 0 1 2 3 4 5 6 8 7 8 9 0 1 2 3

frame 2(wrong)

frame 15 characters

one-bit error

Now a character count

Even if the checksum is incorrect so the destination knows that the frame is bad, it still has no way of telling where the next frame starts.

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Starting and Ending Characters with Character Stuffing

DLE STX user data DLE ETX

beginningof frame

end offrame

DLE(0x10): Data Link EscapeSTX(0x02): Start of TeXtETX(0x03): End of TeXt

DLE STX A DLE B DLE ETX

user data 0x10

DLE STX A DLE B DLE ETX

DLE stuffed at the sender

DLE destuffed at the receiver

DLE STX A DLE DLE B DLE ETX

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Starting and Ending Flags with Bit Stuffing

01111110 user data 01111110

beginningof frame

end offrame

flag

011011111111111111110010

011011111111111111110010

011011111011111011111010010

bits stuffed at the sender

bits destuffed at the receiver

original data

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Introduction: Error Detection and Correction

It is physically impossible for any data recording or transmission medium to be 100% perfect over its entire expected useful life.

Noise is always present If a communications line experiences too much noise, the

signal will be lost or corrupted Communication systems should check for transmission errors Once an error is detected, a system may perform some action Some systems perform no error control, but simply let the

data in error be discarded

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Noise

Also known as thermal or Gaussian noiseRelatively constant and can be reducedIf noise gets too strong, it can completely disrupt the

signal

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Noise (continued)13

Impulse Noise

One of the most disruptive forms of noise Random spikes of power that can destroy one or more bits of

information Difficult to remove from an analog signal because it may be hard to

distinguish from the original signal Impulse noise can damage more bits if the bits are closer together

(transmitted at a faster rate)

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The effect of impulse noise on a digital signal

Crosstalk

Unwanted coupling between two different signal paths For example, hearing another conversation while talking on

the telephone Relatively constant and can be reduced with proper measures

Three Telephone Circuits experiencing crosstalk

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Echo

The reflective feedback of a transmitted signal as the signal moves through a medium

Most often occurs on coaxial cable If echo bad enough, it could interfere with original signalRelatively constant, and can be significantly reduced

A signal bouncing back at the end of a cable and causing echo

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Jitter

The result of small timing irregularities during the transmission of digital signals

Occurs when a digital signal is repeated over and overIf serious enough, jitter forces systems to slow down their

transmissionSteps can be taken to reduce jitter

Original digital signal and digital signal with jitter

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Delay Distortion

Occurs because the velocity of propagation of a signal through a medium varies with the frequency of the signal Can be reduced

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Attenuation

The continuous loss of a signal’s strength as it travels through a medium

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Error Prevention (continued)

Error Detection

Despite the best prevention techniques, errors may still happen

To detect an error, something extra has to be added to the data/signal This extra is an error detection code

Three basic techniques for detecting errors: parity checking, arithmetic checksum, and cyclic redundancy checksum

Parity Checks

Simple parityIf performing even parity, add a parity bit such that

an even number of 1s are maintainedIf performing odd parity, add a parity bit such that

an odd number of 1s are maintainedFor example, send 1001010 using even parityFor example, send 1001011 using even parity

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Parity Checks (continued)

Simple parity (continued)What happens if the character 10010101 is sent

and the first two 0s accidentally become two 1s?Thus, the following character is received:

11110101Will there be a parity error?

Problem: Simple parity only detects odd numbers of bits in error

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Parity Checks (continued)

Longitudinal parity Adds a parity bit to each character then adds

a row of parity bits after a block of charactersThe row of parity bits is actually a parity bit

for each “column” of charactersThe row of parity bits plus the column parity

bits add a great amount of redundancy to a block of characters

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Vertical Redundancy Check (VRC)

Append a single bit at the end of data block such that the number of ones is even Even Parity (odd parity is similar)0110011 011001100110001 01100011

VRC is also known as Parity CheckPerformance:

Detects all odd-number errors in a data block

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Longitudinal Redundancy Check (LRC)

Organize data into a table and create a parity for each column

11100111 11011101 00111001 10101001

11100111110111010011100110101001

10101010

11100111 11011101 00111001 10101001 10101010

Original Data LRC

Longitudinal Redundancy Check and Vertical Redundancy Check

1 0 1 1 0 1 1 11 1 0 1 0 1 1 10 0 1 1 1 0 1 01 1 1 1 0 0 0 01 0 0 0 1 0 1 10 1 0 1 1 1 1 10 1 1 1 1 1 1 0

VRC

LRC

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Parity Checks (continued)

Both simple parity and longitudinal parity do not catch all errors

Simple parity only catches odd numbers of bit errorsLongitudinal parity is better at catching errors but

requires too many check bits added to a block of dataWe need a better error detection method

What about arithmetic checksum?

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Modulo 2

Modulo 2 arithmetic works like clock arithmetic. In clock arithmetic, if we add 2 hours to 11:00, we get 1:00. In modulo 2 arithmetic if we add 1 to 1, we get 0. The addition

rules couldn’t be simpler:

0 + 0 = 0 0 + 1 = 1

1 + 0 = 1 1 + 1 = 0

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Find the quotient and remainder when 1111101 is divided by 1101 in modulo 2 arithmetic.As with traditional

division, we note that the dividend is divisible once by the divisor.

We place the divisor under the dividend and perform modulo 2 subtraction.

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Error Detection and Correction

Find the quotient and remainder when 1111101 is divided by 1101 in modulo 2 arithmetic…Now we bring down the

next bit of the dividend.We see that 00101 is not

divisible by 1101. So we place a zero in the quotient.

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Error Detection and Correction

Find the quotient and remainder when 1111101 is divided by 1101 in modulo 2 arithmetic…1010 is divisible by 1101

in modulo 2.We perform the modulo 2

subtraction.

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Error Detection and Correction

Find the quotient and remainder when 1111101 is divided by 1101 in modulo 2 arithmetic…

We find the quotient is 1011, and the remainder is 0010.

This procedure is very useful to us in calculating CRC syndromes.

Note: The divisor in this example corresponds to a modulo 2 polynomial: X 3 + X 2 + 1.

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Suppose we want to transmit the information string: 1111101.

The receiver and sender decide to use the (arbitrary) polynomial pattern, 1101.

The information string is shifted left by one position less than the number of positions in the divisor.

The remainder is found through modulo 2 division (at right) and added to the information string: 1111101000 + 111 = 1111101111.

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Cyclic redundancy checking (CRC)

If no bits are lost or corrupted, dividing the received information string by the agreed upon pattern will give a remainder of zero.

We see this is so in the calculation at the right.

Real applications use longer polynomials to cover larger information strings. Some of the standard poly-

nomials are listed in the text.

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Cyclic redundancy checking (CRC)

The minimum Hamming distance for a code, D(min), determines its error detecting and error correcting capability.

For any code word, X, to be interpreted as a different valid code word, Y, at least D(min) single-bit errors must occur in X.

Thus, to detect k (or fewer) single-bit errors, the code must have a Hamming distance of D(min) = k + 1.

Hamming codes36

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Flow Control and Error Control

Flow control A set of procedures that tells the sender how much data

can be sent before waiting for acknowledgment Error control

Includes both error detection and correction Allows receiver to inform sender of lost or duplicate

frames Mostly based on Automatic Repeat Request (ARQ)

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Data Link Protocols

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Protocols for Noiseless Channel

Assuming channel is error free Not realistic…

No need for error control

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"Simplest" Mechanism

Assuming Noiseless channel Unlimited buffer and speed for the receiver

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"Simplest" : Pseudo Code

Sender

Receiver

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"Simplest": Flow Diagram

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Stop-and-Wait Mechanism

Still noiseless channel Receiver has limited buffer

Requires flow control Sender sends one frame at a time and wait for an

acknowledgment

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Stop-and-Wait: Overview

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Stop-and-Wait: Pseudo Code

Sender side

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Stop-and-Wait: Pseudo Code

Receiver side

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Stop-and-Wait: Flow Diagram

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Noisy Channel

Realistic Error can and will happen Require error control

Mechanisms: Stop-and-Wait ARQ Go-Back-N ARQ Selective Repeat ARQ

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Stop-and-Wait ARQ

Sender keeps a copy of sent frame until successful delivery is ensured

Receiver responds with an ack when it successfully receives a frame

Both data and ack frames must be numbered.

(For identification purpose, both data frames and ACK frames are numbered alternately 0 and 1.)

When sender does not receive an ack within certain time, it assumes frame is lost, then retransmits the same frame.

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Stop-and-Wait ARQ

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Flow Diagram: Normal Operation

Time

Sender Receiver

Time

Frame 0

ACK 1

Frame 1

ACK 0

S = 0

S = 1

R = 0

R = 1

R = 0

Deliver

Deliver

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Thinking Corner

Why data frames need to be numbered?

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Flow Diagram: Lost Frame

Time

Sender Receiver

Time

Frame 0

ACK 1

Frame 1

ACK 0

S = 0 R = 0

R = 1

R = 0

S = 1

Timeout

Frame 1

Deliver

Deliver

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Flow Diagram: Lost ACK

Time

Sender Receiver

Time

Frame 0

ACK 1

Frame 1

ACK 0

S = 0 R = 0

R = 1

R = 0

Frame 1S = 1

Timeout

ACK 0

S = 1

S = 0

R = 0Frame 0 expected;

discard

Deliver

Deliver

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Flow Diagram: Delayed ACK

Sender Receiver

Frame 0

ACK 1

ACK 0

S = 0 R = 0

R = 1

R = 0

Frame 0Timeout

S = 1

S = 0

R = 1

Frame 0 expected; discard

Deliver

ACK 1Frame 1

Timeout Frame 1

Deliver

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Bidirectional Transmission

Data are transferred both ways ACK are "piggybacked" with data frames

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Improving Link Utilization

Previous example demonstrates major disadvantage of Stop-and-Wait ARQ

Prefer to send more frames before waiting for ACK Example:

Recalculate the link utilization if we allow up to 15 frames to be sent before waiting for an ACK

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Go-Back-N ARQ

Allows multiple frames to be sent before waiting for ACK These frames must be numbered differently Frame numbers are called Sequence numbers

Frames must be received in the correct order If a frame is lost, the lost frame and all of the

following frames must be retransmitted

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Sequence Numbers

Frame header contains m bits for sequence number That allows up to 2m different frame numbers How big should m be?

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Sending Window

Sending more than one frame at once requires sender to buffer multiple frames Known as "sending window" Any of these frames in the window can be lost

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"Sliding" Window

Once the first frames in the window is ACKed ACKed frames are removed from the buffer More frames are transmitted Result: The window slides to the right

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Receiving Window

Receiver expects one frame at a time

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Send vs. Receive Windows

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Go-Back-N: Window Sizes

For m-bit sequence numbers Send window size: at most 2m-1

Up to 2m-1 frames can be sent without ACK Receive window size: 1

Frames must be received in order

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Go-Back-N: Normal Operation

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Go-Back-N: Lost Frame

ACKs are cumulative

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Lost ACK: Window Size < 2m

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Lost ACK: Window Size = 2m

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Selective Repeat ARQ

Go-Back-N always discards out-of-order frames Losing one frame may result in

retransmission of multiple frames Very inefficient in noisy link

Selective Repeat ARQ allows frames to be received out of order Therefore, receive window > 1

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Send and Receive Windows

Sender and receiver share window space equally

For m-bit sequence numbers Send window: up to 2m-1

Receive window: up to 2m-1

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Send Window

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Receive Window

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Negative ACK

Used by receiver to indicate missing frame

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Selective Repeat: Window Size