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PRICIPLES OF TELECOMMUNICATIONAND PACKET NETWORKS MSc MODULE

EEM.ptn 2007-08

Lecture Component:

The Link Layer

Lectures 6-8

Prof. George Pavlou

Centre for Communication Systems Researchhttp://www.ee.surrey.ac.uk/CCSR/Networks/

[email protected]

Tel: 01483 689480

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A3.2

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A3.3

DATA TRANSMISSION TECHNIQUES

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A3.4

LINK LEVEL FRAMING

The Physical Layer provides a digital serial transmission

capability i.e. a raw bit stream.

Link-level protocols need to exchange data link messages called

frames.

We need an unambiguous way of delimiting frames transmitted

using the bit stream physical layer service.

The process is called Framing and it is an important aspect of

the Data Link Layer.

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A3.5

COMMUNICATION MODES

Simplex: Data transmitted in one direction only e.g. temperature

sensor output in an industrial plant. Half duplex: Data transmitted between communicating parties in both

directions but not at the same time.

(Full) Duplex: Both communicating parties may transmit and receive

at the same time. In all cases, the destination computer needs to detect the following:

start of each bit.

start and end of each byte (character) .

start and end of each data link level frame (message). A frame is ablock of data comprising several characters.

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A3.6

CHARACTER CODING

There are two types of messages exchanged between communication parties:

control: special data to overcome transmission errors. information: user data.

Alphanumeric characters need to be translated into a machine code i.e. a

binary representation.

There are two major standards for character encoding :

Extended Binary Coded Decimal Interchange Code(EBCDIC).

American Standards Committee for Information Interchange (ASCII), similar

to International Alphabet Number 5 (IA5) which is an ITU standard.

Both encoding standards have:

control characters such as STX (Start of Text), ETX (End of Text), SYN

(Synchronisation), etc.

printable characters such as numbers and alphabets.

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A3.7

ASCII AND EBCDIC Examples

Printable characters ASCII (7 bits) EBCDIC (8 bits)

A 41 C1

B 42 C2

a 61 81

b 62 82

Control characters ASCII (7 bits) EBCDIC (8 bits)

STX (Start Of Text) 02 02

ETX (End Of Text) 03 03DLE (Data Link Escape) 10 10

SYN 16 32

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A3.8

ASYNCHRONOUS FRAMING

Suitable for data generated at random such as a user typing from a

keyboard; transmission line is idle for long periods and the two endsoperate asynchronously.

Each byte is treated independently. Receiver tries to keep

synchronisation per character.

Character synchronisation is achieved using one start and one ormore stop bits, which delimit the 8 character bits.

The receiver samples the middle of each bit, the transmitter must

transmit at the same rate the receiver samples.

Not good for large frames due to the inefficiency of the per-charactersynchronisation overhead: 8 / (8+2) = 80% at best.

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A3.9

ASYNCHRONOUS FRAMING (contd)

8-bit character

Start bit Stop bitbit

sampling

Asynchronous

transmission basics1 2 2 4 5 6 7 8

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A3.10

SYNCHRONOUS FRAMING

Suitable for high speed data transmissions.

Complete frames of several characters are transmitted as a continuous

string. Receiver tries to keep synchronisation per frame.

It is synchronous because some pattern is always being transmitted

between successive frames or before the start of each frame so that

the receiver gets synchronised.

There are two types of framing: Byte-oriented and Bit-oriented.

Byte-oriented framing is the older of the two methods, imposes more

overheads and is gradually fading from use in favour of bit-oriented

framing.

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A3.11

BYTE-ORIENTED FRAMING

The SYN (Synchronisation) character is sent continuously between

frames.

Frame synchronisation for byte-oriented framing:

for text frames: Using control characters such as STX (Start Of

Text) and ETX (End Of Text).

for binary frames: insert DLE (Data Link Escape) before STX and

ETX.

character stuffing is achieved by substituting every DLE pattern in

the frame with DLE-DLE at the transmitter and removing the extra

DLE at the receiver.

Closely tied to 8-bit characters, not general enough.

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A3.12

BYTE-ORIENTED FRAMING (contd)

SYN STX ETX SYN SYN

DLE

DLE STX DLE ETX SYNSYN DLE DLE SYN

Data to transmit

Frame

Data to transmit

Byte-stuffed Frame

transmission order

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A3.13

BIT-ORIENTED FRAMING

In idle periods between frames, the receiver searches for the flag

bit pattern (01111110). This is typically preceded by a number of

idle bytes (01111111) for the receiver to gain synchronisation.

Frames are delimited by the flag pattern, which denotes both the

start and end of frame.

Bit stuffing is achieved by inserting a 0 after 5 consecutive 1s at

the transmitter and removing this 0 at the receiver:

if after 5 1s there is a 0, it is the result of bit-stuffing and it is

discarded by the receiver.

if it is a 1 followed by a 0, then the last 8 bits constitute a flag.

Allows arbitrary number of bits in a frame (not just multiple of 8)

and supports character sets with any number of bits per character

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A3.14

SYNCHRONISATION OVERHEADS: EXAMPLE

Find the additional number of bits required to transmit a message (frame)

consisting of 1000 characters (8-bits each) over a data link using the following

transmission modes:

a) Asynchronous: With one start bit and one stop bit per character and a

single start-of frame and end-of-frame characters per message.

b) Synchronous byte-oriented: With two synchronisation characters and a singlestart-of-frame / end-of-frame character per message.

c) Synchronous bit-oriented

Answer:

a) Extra number of bits per character = 1+1 = 2 bits

Total additional bits required = (2*1000) + 2*(2+8) = 2020 bits

b) 4*8=32 bits (DLE/STX and DLE/ETX)

c) 2*8=16 bits (only two FLAG bytes)

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A3.15

ERROR DETECTION (1)

Transmission is error prone:

Bit Error Rate (BER): the probability that a single bit iscorrupted e.g. 10-3 means 1 bit in 103.

Forward Error Control: redundant information is transmitted sothat the receiver can both detect and correct the error.

This is better for high BER links (e.g. wireless) and exhibits highframe overhead.

Feedback Error Control: less redundant information is transmitted,receiver can detect the error and discard / request retransmission.

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A3.16

ERROR DETECTION (2)

In character-based asynchronous transmission, the parity bit method

is used (add an extra bit per character for even / odd parity). can only detect odd number of bit errors.

For block-oriented synchronous transmission, a checksum is used -

known as Frame Check Sequence (FCS) orCyclic Redundancy

Check (CRC). Based on polynomial codes, a generator polynomial of R bits

(typically 16 or 32) will detect:

all single-bit, double-bit and odd number of bit errors.

All error bursts < R and most error bursts >= R. Easy to implement in hardware.

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A3.17

DATA LINK PROTOCOL BASICS

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A3.18

THE DATA LINK LAYER IN THE OSI

REFERENCE MODEL

Application

Session

Presentation

Physical

Data Link

Network

Transport

Application Process

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A3.19

INTRODUCTION

Transmission modes:

connection oriented, reliable (virtual circuit like).

connectionless, unreliable.

Link layer functions for the reliable, connection-oriented service:

error control, i.e. error detection and correction. flow control, so that the transmitter does not flood the

receiver with information the latter cannot handle.

Similar functions may be also found in the Network and

Transport layers.

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A3.20

TYPICAL DATA LINK FRAME FORMAT

Data Link frames contain a header, data and a checksum at the

end in order to enable the receiver to detect transmission errors.

The header contains various pieces of Protocol Control

Information (PCI): frame type, sequence numbers, etc.

The data part carries the payload i.e. the information the data link

service user wishes to transfer.

The length of the header and checksum reduce the effectivebandwidth, so they should be as small as possible.

There exist frames with no data part, used entirely for control

purposes e.g. acknowledgment frames.

Header ChecksumData

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A3.21

AUTOMATIC REPEAT REQUEST

Receiver checks received frame for errors and provides apositive or (in some modes) negative acknowledgment.

This form of automatic error error control is known as Automatic

Repeat reQuest (ARQ). There are two ARQ types:

idle (or stop-and-wait) RQ, used mostly for character-oriented transmissions (e.g. modem protocols).

continuous RQ, used for bit-oriented transmissions which

characterise more general data link protocols.

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A3.22

IDLE (STOP-AND-WAIT) RQ

Terminology:

information frames are called I-frames and acknowledgment

frames are called ACK-frames.

data source is called transmitter or primary and data

destination is called receiver or secondary.

Operation:

transmitter sends I-frames and waits for acknowledgment

from receiver.

timers are used to check on transmission failure for I-frames.

wait time equals the time I-frame takes to be received and

processed by the receiver and acknowledgment to be sent

back - time-out should be slightly larger than this time.

sequence numbers are used to mark both I-frames and

acknowledgments.

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A3.23

IDLE (STOP-AND-WAIT) RQ (contd)

Two potential schemes of implementing retransmissions:

implicit retransmissions: the receiver sends an ACK(acknowledgment) message only for correct I-frames and

does nothing for I-frames received in error.

explicit retransmission: in addition, the receiver sends

NAK (negative acknowledgment) for I-frames in error. In the implicit retransmission scheme, the transmitter will

eventually resend after a time out.

In the Idle RQ scheme, the transmitter can only have one I-

frame outstanding, hence the terms idle / stop-and-wait.

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A3.24

IMPLICIT RETRANSMISSION

I(N) I(N+1) I(N+2)

I(N) I(N+1)

Transmitter

Receiver

Timer started Timer stopped

I(N) ACK(N) I(N+1)ACK(N+1)

I(N) I(N) I(N+1)

I(N)

Transmitter

Receiver

I(N) I(N)ACK(N)

I(N)

Timer expiredTimer started

Correct N I-Frame

Corrupted N I-Frame

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A3.25

EXPLICIT RETRANSMISSION

I(N) I(N) I(N+1)

I(N)

Transmitter

Receiver

Timer started Timer restarted

I(N) I(N) ACK(N)NAK(N)

I(N)

Timer stopped

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A3.26

IDLE RQ FRAME (PDU) FORMAT

SOH

N(S)

STX

I-frame

content

ETX

BCC

ACK

N(R)

BCC

NAK

N(R)

BCC

SOH: Start of Header

N(S) send sequence number

N(R) receive sequence number

ACK-frame format

NAK-frame format

STX: Start Of Text

ETX: End Of Text

BCC: Block Check Character

I-frame format

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A3.27

LINK UTILISATION

Efficiency of utilisation U can be defined as U= Tx/Tt

Tx= (Number of bits in a frame) / (Bit rate), i.e. the frame transmissiontime.

Tt is equal to Tx plus any time the transmitter spends waiting for

acknowledgments.

In practice processing and acknowledgment transmission times arenegligible compared with propagation delay, so:

Tt = Tx + 2Tp

where 2Tp represents I-frame and ACK-frame propagation delays.

Therefore,U = Tx / (Tx + 2Tp) => U = 1/(1+2a) where a = Tp / Tx

The coupling ratio a is typically:

a > 1 for WANs

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A3.28

LINK UTILISATION: EXAMPLE 11000-bit frame to be transmitted using idle RQ protocol. Assuming the velocity

of propagation is 2 *108 m/s in cable and 3 *108 m/s in vacuum, determine the

link utilisation for a data rate of 1 Mbps for the following transmission media:i) A twisted pair cable 1 km in length.

ii) A leased line 200 km in length.

iii) A satellite link of 36 000 km.Answer:

Tx= (Number of bits in a frame) / (Bit rate) = 103 / 106 = 10-3 s

Tp = Distance/ Velocity, a = Tp / Tx and U = 1 / (1+2a)

i) Tp = 5 * 10-6 , a = 5 * 10-3, hence U = 1

ii) Tp = 10-3 , a = 1, hence U = 0.33

iii) Tp = 0.24, a = 240, hence U = 0.002

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A3.29

LINK UTILISATION: EXAMPLE 2

1000-bit frame to be transmitted using idle RQ protocol. Assuming the

velocity of propagation is 2 *108 m/s, determine the link utilisation for a

twisted pair cable 1 km in length:

i) For a data rate of 1 Kbps.

ii) For a data rate of 100 Mbps.

Answer:

Tx= (Number of bits in a frame) / (Bit rate)

Tp = Distance/ Velocity, a = Tp / Tx and U = 1 / (1+2a)

i) Tx = 1, Tp = 5 * 10-6 , a = 5 * 10-6 , hence U = 1

ii) Tx = 10-5, Tp = 5 * 10

-6, a = 0.5, hence U = 0.5

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A3.30

LINK UTILISATION WITH ERRORS

Assuming a Bit Error Rate (BER) of the link P, the probability to receive a

frame of Ni bits in error is:

Pf= 1-(1-P)Ni = Ni P (approx.) if Ni P U = (1- Pf) / (1 + 2a)

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A3.31

LINK UTILISATION (IDLE RQ CONCLUSIONS)

Link efficiency is affected by the:

link delay (propagation velocity and distance).

line speed i.e. bit data rate.

Bit Error Rate (BER).

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A3.32

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A3.33

CONTINUOUS RQ

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A3.34

CONTINUOUS RQ(1)

Link utilisation is better than idle RQ.

Operation:

transmitter sends I-frames continuously without waiting for

acknowledgment from receiver.

transmitter keeps a copy of transmitted I-frames for

retransmission in case of failure.

In duplex links, a commonly used technique is piggybacking where

acknowledgments are tagged to the end of I-frames.

There are two types of continuous RQ:

Selective-repeat.

Go-back-N.

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A3.35

CONTINUOUS RQ(2)

I(N) I(N+1) I(N+2) I(N+3) I(N+4)Primary I(N) I(N+1) I(N+2) I(N+3) I(N+4)Transmitter

I(N) I(N+1) I(N+2) I(N+3) I(N+4)I(N) I(N+1) I(N+2) I(N+3) I(N+4)Receiver

I(N)

ACK(N)

ACK(N+1) ACK(N+2)ACK(N+3)

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A3.36

SELECTIVE-REPEAT (1)

Selective repeat has better utilisation than Go-back-N and can be

used in links with high BER e.g. radio links. It can operate with implicit or explicit negative acknowledgments -

in the notes we cover only the implicit acknowledgment case.

The transmitter discovers missing ACK-frames from subsequentones and retransmits the corresponding I-frames.

Possibility for receiving duplicate frames when an ACK-frame iscorrupted.

The loss of an I-frame is detected only after the next I-frame iscorrectly received, which requires a continuous stream of I-frames.

The transmitter needs to set a timer for an I-frame so that incase it is corrupted and there are no more I-frames to send, itis retransmitted after the time-out.

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A3.37

SELECTIVE-REPEAT (2): CORRUPTED

I-FRAME


I(N) I(N+1) I(N+2) I(N+3) I(N+4)I(N) I(N+1)

I(N+5)

I(N+5)

I(N+1)

Receiver

I(N)ACK(N) ACK(N+2)

ACK(N+1)

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A3.38

SELECTIVE-REPEAT(3): CORRUPTED

ACK-FRAME

I(N) I(N+1) I(N+2) I(N+3) I(N+4)Primary I(N) I(N+1) I(N+2) I(N+3) I(N)Transmitter

I(N) I(N+1) I(N+2) I(N+3) I(N)I(N) I(N+4)

I(N+5)

I(N+5)

I(N+4)

Receiver

I(N)ACK(N) ACK(N+2)

ACK(N+3)

ACK(N)ACK(N+4)ACK(N+1)

Duplicate,

ignored

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A3.39

GO-BACK-N (1)

In case of error, the transmitter retransmits all frames already sent

starting from the one in error.

In comparison with selective repeat, it is simpler and reduces the

buffer size on the receiver side but it is less efficient.

trades simplicity for efficiency in error recovery.

Operation: Assume that I-frame N+1 is corrupted.

Receiver will return NAK-frame for N+1 I-frame. A NAK-frame

is also known as Selective Reject.

Transmitter will know about corruption after N+X frames. It will then retransmit these frames, starting from N+1.

The receiver uses a timer so that it retransmits a NAK-frame in

case it is corrupted.

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A3.40

GO-BACK-N (2): CORRUPTED I-FRAME


I(N) I(N+1) I(N+2) I(N+3) I(N+4)

I(N+3)

I(N+1)

I(N+2)

I(N+2)

I(N+1)

Receiver

I(N)ACK(N) NAK(N+1)

ACK(N+1)

Frames discarded

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A3.41

GO-BACK-N (3): CORRUPTED ACK-FRAME


I(N) I(N+1) I(N+2) I(N+3) I(N+4)I(N) I(N+5)

I(N+6)

I(N+6)

I(N+5)

Receiver

I(N)ACK(N) ACK(N+2)

ACK(N+3)

ACK(N+4)ACK(N+5)ACK(N+1)

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A3.42

FLOW CONTROL

Flow control is needed to regulate traffic if the transmitter is faster

then the receiver.

An agreed window size determines the maximum number of

unacknowledged frames.

Sliding window:

receiver stops acknowledging received messages. sender notices the built up of unacknowledged messages so it

stops.

the window slides forward as acknowledgments are received

and more frames are transmitted. Window size impacts efficiency and relates also to the available

buffer storage in both sender and receiver.

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A3.43

FLOW CONTROL USING SLIDING WINDOW


Send window size = 3

I(N+3) I(N+5)

Frames waiting to

be acknowledged

Frames already

acknowledged

Frames waiting to

be sent

I(N+6)

Protocol Send Window Receive window

Idle RQ 1 1Selective repeat K K

Go-back-N K 1

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A3.44

LINK MANAGEMENTSource application

L_CONNECT.request

L_CONNECT.confirm

L_DATA.request

L_DISCONNECT.request

L_DISCONNECT.confirm

Destination application

L_CONNECT.indication

L_DATA.indication

L_DISCONNECT.indication

Source

link layer

Destination

link layer

SETUP-frame

UA-frame

I-frame

ACK-frame

DISC-frame

UA-frame

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A3.45

LINK UTILISATION

Recall that for Idle RQ utilisation U can be defined as U= Tx/Tt =>

U = Tx / (Tx + 2Tp) => U = 1/(1+2a) where a = Tp / Tx

For Continuous RQ with sliding window size K:

U = K / (1+2a) if K < 1+2a, or

U = 1 if K >= (1+2a)

In the latter case, we fill the pipeline.

LINK UTILISATION

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A3.46

LINK UTILISATION:

(CONTINUOUS RQ EXAMPLE)

Q)1000-bit frame to be transmitted using continuous RQ protocol. Assuming

the velocity of propagation is 2*108 m/s in cable and 3 *108 m/s in vacuum,

determine the link utilisation for the following transmission media:i) A 10 km link of 200 Mbps with a send window of K=7.

ii) A satellite link of 36 000 km of 1 Mbps with a send window of K = 127.

Answer:

Tp = S / V, a = Tx / Tp and U = K/(1+2a)

i) Tp = 5*10-5 , Tx = 1000 / (200*10

+6) = 5*10-6, a = 10,

For K = 7 (K< 1+2a=21), U = 7 / (1+20) = 0.33

ii) Tp = 0.24, Tx = 10-3 , a = 240

For K = 127 (K< 1+2a=481), U = 127 / (1+480) = 0.26


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A3.47

LINK UTILISATION WITH ERRORS:

SELECTIVE REPEAT

In case of the Selective Repeat scheme, utilisation in case of errors is

reduced by the number of retransmissions required per frame.

It is reminded that the number of retransmissions per frame are

Nr= 1 / (1- Pf) where Pf is the probability of receiving a frame in error.

As such, the link utilisation is:U = (K / (1+2a)) / (1 / (1- Pf)) = K (1- Pf) / (1+2a) if K < 1+2a, or

U = 1 - Pf if K >= 1+2a

Note: when K >= 1+2a we substitute K = 1+2a since bigger values of K do notincrease utilisation i.e. the pipeline is full.


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A3.48

LINK UTILISATION WITH ERRORS:

GO-BACK-N

In case of the Go-Back-N scheme, each error means that K frames

should be retransmitted.

It can be proved that the number of retransmissions per frame in this

scheme are:

Nr= ( 1 + Pf(K-1) ) / (1- Pf)As such, the link utilisation is:

U = K (1- Pf) / ( (1+2a)(1 + Pf(K-1)) ) if K < 1+2a, or

U = (1 - Pf) / ( 1 + Pf(K-1) ) if K >= (1+2a)

USES OF ARQ PROTOCOLS IN HIGHER

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A3.49

USES OF ARQ PROTOCOLS IN HIGHER

LAYERS ADAPTIVE WINDOW SIZE

Similar protocols are also used end-to-end, i.e. in the computers/hosts,in the Network and Transport layers for connection-oriented reliableprotocols

In the Network layer, ARQ-based flow control through sliding windowis used per Virtual Circuit (VC see later) e.g X.25

In the Transport layer, ARQ-based error and flow control is used perTransport Connection (e.g. TCP, TP4)

In these cases, the window size may be adaptive for congestion control

reasons, e.g. as in TCP In this case, the sender starts from a window size of 1 packet and

increases it exponentially with positive acks (2, 4, 8, 16, ) until themaximum size (i.e. receivers capacity is reached) slow start

Packet loss across the network is mainly due to congestion (bufferoverflow), in which case the maximum window size is set to thecurrent size and a slow start phase begins again From the current maximum size (threshold), there is a linear only

increase until the absolute maximum size i.e. the receivers capacity

In this way, the sending entity helps de-congesting the network

TCP ADAPTIVE WINDOW MECHANISM

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A3.50

0

4

8

12

16

20

24

28

32

36

40

44

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Transmiss ion number

C

ongestion

window(

kbytes)

Timeout

Threshold

New threshold

TCP ADAPTIVE WINDOW MECHANISM -

SLOW START ALGORITHM

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A3.51

DATA LINK LAYER PROTOCOLS

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A3.52

LINK LAYER PROTOCOL IMPLEMENTATIONS

Character-oriented protocols with idle RQ are used for low bit rates e.g.

Kermit and X-modem.

Bit oriented protocols with continuous RQ are used for high bit rates and

long distance communications.

The key protocol is the ISO High-level Data Link Control (HDLC).

Variations of HDLC are used in:

X.25 WANs use a version called LAPB.

LANs use a version called LLC2.

ISDN signalling uses a version called LAPD.

GSM signalling uses a modified LAPD called LAPDm. Modems use a version called LAPM.

Internet point-to-point lines use PPP.

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A3.53

HDLC VARIATIONS

HDLC

LAPB

(X.25)

LAPD

(ISDN)

LAPM

(V.42)

LAPDm

(GSM)

LLC2

(LANs)

PPP

(many)

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A3.54

HDLC (1)

High-level Data Link Control (HDLC) is the most common bit-

oriented link protocol in use. It has its origins in IBMs SynchronousData Link Control (SDLC).

Terminology:

primary (transmitter) frames are called commands.

secondary (receiver) frames are called responses.

unbalanced configuration where one machine will be primary and

the other will be secondary.

balanced configuration where both machines are both primary

and secondary.

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A3.55

HDLC (2)

HDLC Modes of operation:

Normal Response Mode (NRM): secondary transmits only if polledby the primary (unbalanced configuration).

Asynchronous Response Mode (ARM): secondary is allowed to

transmit asynchronously to the primary (unbalanced configuration)

Asynchronous Balanced Mode (ABM): used for duplex links andmainly in PSDN networks. Each station performs both primary and

secondary functions.

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A3.56

HDLC FRAME FORMAT

Flag FlagAddress Control Information FCS

Start-of-

frame-delimiter

End-of-

frame-delimiterframe header

FrameCheck

Se uence

8 8/16 88/16 16/320 to N

Flag = 01111110FCS = X16+X12+X5+1

Address: single, group or broadcast

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A3.57

HDLC FRAME TYPES (1)

The control field in the frame header determines the frame type.

Unnumbered frames (i.e. no sequence number): Used to setup and releaseconnections. Also, used to set NRM, ARM and ABM modes of operation.

Information Frames: Used to carry user information and piggy-backed

acknowledgments in ABM and ARM modes. The sequence number range is

0-7 for normal frames and 0-127 for extended frames. Supervisory Frames: Used for error control such as Reject (REJ) for Go-back

-N and Selective Reject (SREJ) for selective-repeat operation. Also, used for

flow control such as Receiver Ready (RR) and Receiver Not Ready (RNR).

Data transparency is achieved by using the flag field and bit stuffing.

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A3.58

HDLC FRAME TYPES (2) Information frames.

Supervisory frames - RR, RNR, REJ, SREJ.

Unnumbered commands: Set Asynchronous Response Mode (Extended) - SARM(E).

Set Normal Response Mode (Extended) - SNRM(E).

Set Asynchronous Balanced Mode (Extended) - SABM(E).

Reset - RSET.

Frame Reject - FRMR.

Disconnect - DISC.

Unnumbered responses: Unnumbered Acknowledgment - UA.

Command Reject - CDMR.

Frame Reject - FRMR.

Disconnect Mode - DM.

RQ 3.pdf

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