Physical

Link

Network

Transport

Session

Presentation

Application

OSI Reference Model-Seven LayersOSI Reference Model-Seven Layers

Transporting 0’s and 1’s point-to-point

Organizing flow of 0’s &1’s

Frame formatting-error detecting and correcting codes, ARQ

Routing and Flow Control from Link-to-Link

Managing path flow overseveral links

Setting up and Taking Down Connections

-Synchronization

Character Formats-ASCII, Encryption

User Functions

Data Link Level Design Issues

• Services provided to Network Layer

• Framing

• Error control

• Flow control

Services provided to Network Layer

Layer 4

Layer 3

Layer 2

Layer 1

Layer 4

Layer 3

Layer 2

Layer 1

Physical Medium

Host 1 Host 2

Virtual Data Path

ActualData Path

Most Common Services

• Unacknowledged connectionless service– Very low error rate– Voice traffic

• Acknowledged connectionless service– Frame arrived safely– Good for wireless (unreliable) channels

• Acknowledged connection oriented service– Reliable stream of ordered frames– Three phases -connection, data flow, disconnect

3 32 2 22 2

2

ROUTER

Routing Processor

Link LayerProcessor

Transmission line to a router

FramesPackets

Data link protocol

Placement of Data Link Protocol

• Character count - obsolete

• Starting and ending characters, with character stuffing (see next slide)

• Starting and ending flags, with bit stuffing(see following slide)

• Physical layer coding violations

Framing

Character Stuffing (Transparency to User)

DLE STX DLE ETXDLE ETXDLE

Frame

Beginningof frame

End offrame

Actual data sequence

Stuffed character

User data

The first of two DLE characters is always a stuff-delete

Bit Stuffing (Transparency to User)

01111110 011111010

Frame

Beginningof frame

End offrame

Actual data sequence

Stuffed bit

User data

A “0” following five “1’s” is always deleted

01111110

Error control

0 1 1 1 0 1 0 0

Violation

Physical layer coding violations

Error Control

• Automatic Repeat Request (ARQ)– Error detecting codes– Positive acknowledgement(ACK)– Negative acknowledgement(NACK)– Timeout– Delay sensitivity

• Forward Error Correction– Error correcting codes– Delay insensitive– Much more overhead required

Link-by-Link vs End-to-End Error Control

There’s a tradeoff

N links p = Probability of error on a single link

M = Single message transmission time

Link-by-line acknowledgementAverage no. link retransmissions=1/(1-p)Average message transmission time per link=M/(1-p)Average message transmission time N links = NM/(1-p)

End-to-end acknowledgementProbability no error end-to-end=(1-p)N

Probability at least one error end-to-end=q=1- (1-p)N

Average no. end-to-end retransmissions=1/(1-q)= (1-p)-N

Average transmission time, end-to-end = M (1-p)-N

Assumption of negligible propagation delay

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 110

0

101

102

103

104

Link error probability

Nor

mal

ized

tra

nsm

issi

on t

ime Link-by-link 3 links

Link-by-link 10 links

End-to-end 3 links

End-to-end 10 links

Flow control• Closed loop controls

– Sliding window

• Open loop– Leaky bucket

0 0

1 1

PE

PE

Encdr mod channel demod decdr1,0 1,0

Pe = probability of bit error

Binary Symmetric Channel

Fig. 1 Communica tion Syst em

DigitalS ink Demod

DigitalSourc e Modul ator

...0110...

Ch annel

Encode r

Decode r

...0110...

…11000...

…11000...

Redundancy Encoding

• Block codes– (n,k) code n= codeword length, k=Information bits

• ASCII (8,7) code

– Parity bits• ASCII “e” 10100110 Parity bit

– Hamming distance-number of different places

• Convolutional Codes– Difference in path lengths– Viterbi decoding

4327

3216

4315

bbbb

bbbb

bbbb

(1)

111111115

010011114

000101113

101001112

001110111

100010110

11010019

01100018

10011107

00101106

01110105

11000104

01011003

11101002

10110001

00000000

bitCodeword 7654321 bbbbbbb

(7,4) Hamming Code

76543218 bbbbbbbb

Augmented Hamming Codeextra parity bit over all other bits

1111111115

0010011114

1000101113

0101001112

0001110111

1100010110

011010019

101100018

010011107

100101106

001110105

111000104

101011003

011101002

110110001

000000000

bitCodeword 87654321 bbbbbbbb

Hamming Distance and Error Correction

Example-six bit codewords

Codeword 1- 001101

Codeword 2- 101001

HammingDistanceTwo

Number of places codewords differ

Code-Distance twofour code words

Code-distance threetwo code words

-Code point

011 111

110

101

000

(3,2) code

(3,1) code

000

111

n bit codeword - vertex on an n dimensional cube

Number of edges tranversed from one point to another=Hamming distance between codewords

Codewords

Intermediate vertices Distance = 5

Codewords

Single Error Detecting Code-Distance two

Single-error correcting or double-error detecting Distance three

Single-error correcting and double-error detecting Distance four

Vertex

Distances and error correction and detection

1D

12 C

Distance and Correcting and Detecting Capability

D=Number of detectable errorsC=Number of correctable errorsMinimum distance between codewords

Examples: (7,4) Hamming code =3D= 2 and C=1Augmented Hamming (8,4) code =4, D=3 and C =1

Received n bits not a codeword

Received n bits closest to one codeword

Transmitter Channel Receiver

Feedback Channel

NACK when error detected

Automatic Repeat Request

n,...,,,j;)P(Pj

n)nj(P jn

ej

e 2101bits in errors

Forward Error Correction

Pe=P(Single bit in error)Independence from bit to bit

n

Cj

jne

je )P(P

j

n)C(P)(P

1

1errorsbit than Moreerror decoding

Code Data Rate,k/n Word error prob-

ability for310eP

Word error prob-

ability for610eP

(7,4) Hamming .57 51012 . 111012 .(15,11) Hamming .73 41051 . 101051 .(31,26) Hamming .84 410654 . 1010654 .(23,12) Golay .52 91068 . 211068 .

Trade between data rate and error correcting power

Code Efficiency

n bit codeword - vertex on an n dimensional cubeAn efficient code packs the code “densely”

n nearestneigbors

Code point

n-1 nearestneighbors of adjacentpoint

Code point

i

nNumber of vertices at distance I from a codeword

C

i i

n

0

Number of vertices at distance C or lessVertices belong to codeword

Packing the cube efficiently

k2 Number of code points

Lower bound on number of verticesEquality for Perfect code

C

i

kn

i

n

1

22

Implementation of Coding and Decoding

In digital hardware, naturally

Cyclic codes-every codeword a cyclic shift of another

eg Hamming code codewords 1,3,4,6,8,10 and cyclic shifts3 - 0011010 13 - 1101000

Cyclic codes represented by a polynomial

Each code has a unique generator polynomial-G(x)e.g. For (7,4) Hamming, G(x)= 123 xx

Representing a binary sequence by a polynomiale.g., 010110101 -

Division:

3223 11

100001011101 :Addition

x)x()xx(

x x x x7 5 4 2 1

111

011100101011101 :t ionMult iplica345223

xxx)x()xx(

x x x x7 4 3 4 x x x x x x x x6 5 2 2 2 21 1 1( ) ( )

11010111110111110101

Divisor Quotient Remainder

124567 xxxxx123 xx467 xxx

125 xx

xxx

x

34

4 1

245 xxx

124 xxx

1

123

3

xx

xx

xx 2

Divisor

Quotient

Remainder

Forming a codeword from k information bits

1) D(x) - k-1 degree polynomial representing data2) Form xn-kD(x) (n-1 degree)3) Divide xn-kD(x) by G(x) , save remainder R(x)4) Form codeword C(x)= xn-kD(x) +R(x)

Mathematical justification

)x(R)x(G)x(Q)x(Dx kn

)x(G)x(Q)x(R)x(Dx)x(C kn

Codeword is a multiple of G(x)If not an error has occurred.

Example 1110(14) to be transmitted-D(x)= x3+ x2+ x

Multiplication by x3 gives x6+ x5+ x4

Remainder after division by x3+ x2+1 is x

C(x)=x6+ x5+ x4+ x 1110010(14)

Error in transmission channel error 1111010 Received

x6+ x5+ x4+ x3 + x /x3+ x2+1 R(x)= x3

++ T T Tc-Inputsequence

o-Outputsequence

s1 2s s3

Remainder 0Valid codewordno error detected

o s

s o s

s s

s o c

n n

n n n

n n

n n n

3 1

3 2 1

2 1 1

1

c s s s o1 2 3

1 1 0 0 0

1 1 1 0 0

0 0 1 1 0

1 0 0 0 1

0 0 0 0 0

1 1 0 0 0

1 1 1 0 0

0 0 1 1 0

1 0 0 0 1

0g 2g 3g

Divider circuit

Encoder Implementation

DividerCircuit

InputSequence

Remainder - R(x)

n-k tic delay

D(x)

D(x) x n k

x n k

D(x)+ R(x)x n k

Generating Polynomials of Cyclic Codes

Ham g x x

Ham g x x

Golay x x x x x x

min ( , )

min ( , )

( , )

7 4 1

1511 1

23 12 1

3

4

11 9 7 6 5

Standards for Error DetectionCRC -12 -

CRC 16 -

CRC 16(ANSI) -

CRC - CCITT -

CRC - 32

x x x x x x x x

x x x x x x

x x x x x x x x x

x x x x x x x x x x x

12 11 3 2 11 2

16 15 2 15

16 15 5 15 4 3 2

16 12 5 15 14 12 4 3 2

1 1 1

1 1 1

1 1 1

1 1 1

( )( )

( )( )

( )( )

( )( )

- x x x x x x x x x x x x x x32 26 23 22 16 12 11 10 8 7 5 4 2 1

Burst Error Capabilities of CRC Codes

TransmittedC(x)

Channel errorE(x)

ReceivedC(x) + E(x)

Because of error C(x) + E(x) is not divisible by G(x)E(x)/G(x)=R(x) ModG(x)

•If (x+1) a factor of G(x) odd number of errors

• All double errors

000

xx)x(G)x(E

ji),x(xxx)x(E ijiji 1

•All bursts of length < r+1

Interleaving for Burst Errors

n1

k2 codewords each (n1 k1)

n1

n1

Imaginedrearrangement

n2 -

k2

n2 -

k2

Error burst

n1

Additional Storage and Processing

n1 (n2 - k2 ) parity bits

Turbo codes

n2 -

k2

Convolutional codes

Tinput110100

Example(3,1) code

output+

T

+

111 100 010 110 011 001 000

0

0

0

0

1

2 1

3 1 2

1 2

i

i s

i s s

s sInitial state -

s1 s2

Initial state - s s1 2 0

Trellis Representations

00

01

10

11

00

01

10

11

000111

001110

011100

010101

State Input Next state

Viterbi Algorithm-shortest path

Trellis codes-Euclidian distance

shortest path-Hamming distance

convencoder

blockencoder CHAN block

decoderVA

Configurations

Correction

blockencoder CHAN

Errordetection

NACK

Detection

A B

Data Link Prototcols

Data Link LayerSource Machine

Unrestricted SimplexTransmitter

Form Frame

Physical Layer

Packet fromNetwork Layer

Elementary Data Link Protocols(3.3)

Flag Flag

Basic Link Level Frame

Info CKSMFrame Header

Kind Seq ACKUser DataFormed into aPacket and Passedto Network Layer

Frame headerThree fields

Dealing with an Unreliable Channel

• Timeout routines

• Sequence number in Frames

Unrestricted SimplexReceiver

•Wakeup•Remove frame from Physical level buffer•Form packet and Pass to Network Layer•Go back to sleep

Simplex Stop-and Wait

•Infinite processor speed or infinite buffer not needed•Inserting maximum delay too conservative

•ACK to sender is solution (Data in one direction only)•Transmitter

•Fetch packet from network level•Form frame•Frame stored in transmit buffer until ACK received•Transmitted after ACK

•Receiver•Remove frame from Physical level buffer•Form packet and Pass to Network Layer•Return ACK

Flow Control Problem

Stop-and Wait Simplex - Noisy Channel

Complicating Factor - ACK errorsSame frame received in succssionRemedy - sequence numbers 0 and 1

•Transmitter•Initialize sequence number•Fetch packet from network level•Form frame•Insert sequence number in frame•Timeout and resend frame•Frame stored in transmit buffer until ACK received•Invert sequence number

Receiver•CHECKSUM to validate received frame •Remove frame from Physical level buffer•Check sequence number•Form packet and Pass to Network Layer•Invert expect sequence number•Return ACK

Sliding Window Protocols

•Full Duplex Info and ACKs simultaneously•Piggybacking-ACK on Data Frames

-complications-ad hoc scheme• Sequence numbers - maximum 2n-1•Sending window - Number of frames allowed to send

•Receiving window - Number of frames allowed to accept

One bit Sliding Window(Stop-and-Wait)

Link Layer A

Link Layer B

Info and piggybacked ACKs

Info and piggybacked ACKs

One process as initiatorelse duplication(See Fig 3-14b)

A sends (0,1,A0)

A gets (0,0,B0)*A sends (1,0,A1)

A gets (1,1,B1) *A sends (0,1,A2)

A gets (0,0,B2) *A sends (1,0,A3)

B gets (0,1,A0)B sends (0,0,B0)

B gets (1,0,A1) *B sends (1,1,B1)

B gets (0,1,A2) *B sends (0,0,B2)

B gets (1,0,A3) *B sends (1,1,B3)

(Seq,ACK,Pkt #)

Time

A in Lead

A sends (0,1,A0)

A gets (0,1,B0) *A sends (0,0,A0)

A gets (0,0,B0)A sends (1,0,A1)

A gets (1,0,B1) *A sends (1,1,A1)

B sends (0,1,B0)

B gets (0,1,A0) *B sends (0,0,B0)

B gets (0,0,A0)B sends (1,0,B1)

B gets (1,0,A1)B sends (1,1,B1)

B gets (1,1,A1)B sends (0,1,B2)Time

No leader

(Seq,ACK,Pkt #)

Low throughput one bit window

Line FlowFrame

TransmissionTimeout interval Message

Transmission

Throughput(Line Utilization)

F T

F/(F+T)

Example: 50 kbps satellite channel, 1000 bit frame, Round trip delay 520 msec. F=20 msec

Window = w > 1 Pipelining

1 2 3 4 5 6 7 8

Eg: w = 4

Transmit

1 2 3 4 5 6 7 8Receive

ACKs

Can be all at onceACK last in batch

Error control

Go-back-n

1 2 3 4 5 2 3 4 5Transmit

1 2 3 4 5 2 3 4 5

Receive

Frame in error Ignored

Timeoutor NACK

A subtlety in sequence numbering

Sequence numbers-0,1,…,MAX_SEQ

Up to MAX_SEQ frames can be outstandingnot MAX_SEQ+1

Example: w=4, MAX_SEQ=3

•Frames 0, 1, 2, 3 received correctly but ACKs lost•Frames 0, 1, 2, 3 retransmitted receiver thinks they are newRemedy: MAX_SEQ=3, w=3, receiver waiting for 3in above scenario

Selective repeat

1 2 3 4 5 2 6 7

Transmit

1 2 3 4 5 2 6 7

Receive

Frame in error Stored

Timeoutor NACK

ACK

NACK

Careful of repeats

A sends frames0 1 2 3 4 5 6

A times out andresends frames0 1 2 3 4 5 6

A sends frames7 0 1 2 3 4 5

Window at B0 1 2 3 4 5 6B gets 0 1 2 3 4 5 6without errorB sends ACK andAdvances window to7 0 1 2 3 4 5(empty buffers)

B gets 0 1 2 3 4 5 6without errorB sends ACK for0 1 2 3 4 5 6 Bad packets sent to network level

(Seq,ACK,Pkt #)

ACKs destroyed

Problem with SR

A sends frames0 1 2 3

A times out andresends frames0 1 2 3

Window at B0 1 2 3B gets 0 1 2 3without errorB sends ACK forAdvances window to4 5 6 7(empty buffers)

B gets 0 1 2 3without errorB sends ACK

(Seq,ACK,Pkt #)

ACKs destroyed

Remedy: New window has no overlap with original windowi.e window half of max sequence number

Number of buffers=window sizeTimer for each bufferAuxiliary timer

Efficiency Comparisions

F=Frame transmission time

RT=Roundtrip delay(propagation, processing)

w=Window size (sec) > RT (simplification)

PF=Probability frame error detected= 1-(1-PB)F

=1-(1-F PB)= F PB for PB<<1

TSaw , TGBn , TSR Total frame transmission times

=F/ Total frame transmission time

Stop and Wait

1 2 2 2 3 4Transmit

1 2 2 2 3 4

Receive

Frame in error

RT RT

Tsaw=(F+RT)/(1- PF)= (1- PF)/(1+RT/F)

Go-back-n

1 2 3 4 5 6 7 8 9 10 2 3 4 5Transmit

1 2 3 4 5 6 7 8 9 10 2 3 4 5

Receive

RT

TGBn =F /(1- PF)+RT PF /(1- PF)

= (1- PF)/(1+ PF RT/F)

Selective Repeat

1 2 3 4 5 6 7 8 9 10 2 111213

1 2 3 4 5 6 7 8 9 10 2 111213

Receive

RT

TSR = F /(1- PF) = 1- PF

Tsaw=(F+RT)/(1- PF)

= (1- PF)/(1+RT/F)

TGBn =F /(1- PF)+RT PF /(1- PF)

= (1- PF)/(1+ PF RT/F)

TSR = F /(1- PF) = 1- PF

Stop and Wait

Go-Back-n

Selective Repeat

0 2 4 6 8 10 12 14 16 18 200

0.2

0.4

0.6

0.8

1

1.2

Eff

icie

ncy

Normalized round trip

SAW PF=.1SAW PF=.01

SR PF=.1

SR PF=.01

GBn PF=.1

GBn PF=.01

Homework problems

#1 Calculate the efficiency of all three protocols for the following set of parameters:1) Frame length = 1000 bits2) Line rate = 1.544 Mbps3) Line length = 1000 km4) Bit error probability=10-4

5) Assume window larger than RT6) Assume propagation of 6 sec/km

#12, #15,#20, #24,#25

Finite State Machine Models of Protocols

• S = Set of possible channel and process states

• M = Set of frames on channel

• I = Set of initial states

• T = Set of transitions between states

• Reachability analysis

• Deadlock state– No transition out of state– No forward progress possible

• SDL - Specification Description Protocol

Example: Send-and-Wait Protocol

• Two sender states, 0,1

• Two receiver states, 0,1

• Four Channel states– 0 Frame in transit– 1 Frame in transit– A Ack in transit– Channel empty

0 0 -

0 0 0

0 0 0 1 0 -1 0 1

0 1 00 1 -

1 0A

1 1 -

1 1 1

0 1 A

0 7

0

7

1

05

4 2

0

3

6

8

08 0

Tra

nsi

tion

Wh

o ru

ns?

Fra

me

acce

pte

d

Fra

me

emit

ted

To

net

wor

k la

yer

0 (Frame lost)1 R 0 A Yes2 S A 1 -3 R 1 A Yes4 S A 0 -5 R 0 A No6 R 1 A No7 S (timeout) 0 -8 S (timeout) 1 -

Stop and Wait Protocol (Sender, Receiver, Channel)

Normaloperation

Flag Flag Data CKSMAddresses Control

Bit oriented protocols SDLC, HDLC, ADCCP, LAP, LAP

Addresses - Multiple terminals lines

Control - Sequence number, ACKs, etc (See next slide.)

Data - Transparency to User

CKSM - CRC Codes

Types of FramesDifferences in Control FieldsInformation

0 Sequence P/F Next

Control Field

Piggybacked ACKType indicator 3 bit number Poll/Final

Supervisory

1 0 Type P/F Next

Control Field

0 -ACK , 1 - NACK, 2 - Receive not ready3 - Selective reject

Unnumbered - Control and Data (sometimes) - depends onparticular protocol

Control Functions

• DISC(disconnect)-Machine going down

• SNRM(Set Normal Response Mode)-Machine coming back on unbalanced

• SABM(Set Asynchronous Balanced Mode)

• FRMR(Frame Reject)-Frame looks funny

• UA(Unnumbered Acknowledgement)-ACK for control frames

• Miscellaneous - Initialization, polling, status reports

LAN

LAN

LAN

LAN

The Internet

Router

Subnet

HostsLeased Lines

MODEM

MODEM

RO

UT

ER

Dial-up Telephone Line

TCP/IP ConnectionUsing SLIP of PPP

Access

PPP Point-to-Point Protocol

• Framing similar to HDLC-Frame delineation, error detection

• LCP(Link Control Protocol)-line bringing up,testing, setting options, putting down

• NPC(Network Control Protocol)-Negotiating network layer options

PPP FrameSimilar to HDLC but Character Oriented

Flag Payload CKSM Addresses Control FlagProtocol

•Character stuffing to make frame integral number of bytes•Flag - 01111110•Address - 11111111 all stations accept the frame•Control - 00000011 default, sequence number on unreliablelines•Address and control field can be omitted by negotiation•Protocol-Kind of packet in payload field•Payload variable up to some maximum•Checksum-2 bytes with 4 bytes optional

Dead Network

Establish Authenticate

OpenTerminate

Failed

Failed

Carrier Detected Both sides agree on options Successfulauthentication

NCP configurationDoneCarrier Dropped

ATM(Asynchronous Transfer Mode)

5 octets 48 octets

CellHeader User Data

Upper layers Upper layers

Plane managementLayer management

Control plane User Plane

ATM Adaptation layer

ATM layer

Physical layer

PMD-Physical medium dependent

TC - Transmission convergencecells bits

5 octets 48 octets

CellHeader User Data

No error control

HEC - Checksum over headerusing generator polynomialControl bytes

ationsynchronizfor 01010101

128

)xxx(

Non-Data Cells in ATM

• Idle cells in synchronous media

• OAM (Operations and Maintenance)– First three header bytes all zeros– Exchanges between switches– Speed matching in SONET– Framing function

Cell ReceptionCell delineation

HUNT

PRESYNCH

SYNC

Bit-by-bitcheck

Cell-by-cellcheck

Correct HEC detected

Incorrect HEC detected

Consecutivecorrect HECs

Consecutiveincorrect HECs

Probability of random “correct” pattern = 2-8

Probability of random “correct” patterns = 2-8