page 1computer networks fall 2002 the data link layer r framing r error control m error correction m...

Computer Networks Fall 2002

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


DATA LINK LAYER Services

Unacknowledged connectionless service Voice

Acknowledged connectionless service wireless

Acknowledged connection-oriented service no out-of-order


Data Link Protocol


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.


Data Framing

Character count Starting and ending characters with

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


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.


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


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


Physical Layer Coding Violations

V: Violation bit

1 0 0 0 1 1 0

userdata

EndingDelimiter

V V 0 V V 0 0 0

end of frame

StartingDelimiter

beginning of frame

V V 1 V V 0 0 0


Error Control

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

acknowledgment / retransmission Lost frame : Time-out mechanism Valid frame : Sequence numbering /

discarding for valid but duplicate frames


Flow Control

A technique for assuring that a transmitting station does not overwhelm a receiving station with data


Error Detection and Correction Error-correcting codes Error-detecting codes


Codeword

n-bit codewords with n =m+r and m data bits r redundant or check bits

Not all the 2n codewords are legal. In general, only 2m codewords are legal.

ASCII codes of 7 bits with 1 even parity bit ASCII code for “A”: 1000001 codeword for “A”: 10000010 or 01000001 Codeword 10000011 or 11000001 is illegal.


Hamming Distance

The number of bit positions in which two codewords differ is called the Hamming distance between these two codewords.

The Hamming distance of the complete code is the minimum Hamming distance between all pairs of legal codewords.


Codeword Space

To detect d-bit errors, a distance d +1 code is needed.

To correct d-bit errors, a distance 2d +1 code is needed.

1-bit error detection 1-bit error correction


Codeword Space : 3-bits


Error Correction with Codewords: An Example


Determine the Number of Check bits To correct single bit errors with m bits mess

age and r check bits (n=m+r) Code space : 2m n-bit legal messages(codewords) For an n-bit legal codeword, all its n neighbors at

a distance 1 from it are illegal n+1 bit patterns are dedicated to each legal mes

sage (n+1)2m <= 2n and consequently (m+r+1)2m <= 2m+r

. we have : (m+r+1) <= 2r


Hamming Code

The bits that are powers of 2 (1, 2, 4, 8, 16, etc.) are check or parity bits of some data bits.

For example, m=7 (ASCII) and r=4 (7+4+1 <= 24)

B1 : B3 B5 B7 B9 B11

B2 : B3 B6 B7 B10 B11

B4 : B5 B6 B7

B8 : B9 B10 B11

1 001 000 “H”xx1x001x0000 1 0 1 0 0 01 01 00 1001 000000110010000 codeword


Hamming Code : Example


To Correct Burst Errors

A sequence of k consecutive codewords are arranged as a matrix, one codeword per row. To correct burst errors, the data should be transmitted one column at a time, i.e., column by column.

If a burst error of length k bits occurs, at most 1 bit in each of the k consecutive codewords will have been affected, but the Hamming code can correct one error per codeword, so the entire block can be restored.


To Correct Burst Errors


When a Burst Error Occurs


To Correct Burst Errors: An Example

ASCII Codeword1001000 001100100001100001 101110010011101101 11101010101

011 001 111 110 011 000 101 010 001 000 0111 2 3 4 5 6 7 8 9 10 11 column

burst error

011 001 111 111 101 000 101 010 001 000 011

Codeword001110100001011000100111111010101

Codeword001100100001011100100111101010101

B1(011100)+B4(1101) => B5

B1(110001)+B4(1000) => B5

B4(1101) => B4

B1 : B3 B5 B7 B9 B11

B2 : B3 B6 B7 B10 B11

B4 : B5 B6 B7

B8 : B9 B10 B11


Error Detection

Parity bit low error rate

Longitudinal redundancy check (LRC) and Vertical redundancy check (VRC)

Cyclic redundancy code (CRC code) / Polynomial code Easy hardware implementation Can detect burst errors


Parity Bit

block size: 1000 bits Hamming code: 1000+10 bits (m+r+1 <=

2n) parity bit: 1000+1 bits

a message: 106 bits without no error

• Hamming code: 106+10000 bits• parity bit: 106+1000 bits

with a low error rate 10-6

• Hamming code: 106+10000 bits• parity bit: 106+1000+(1000+1) bits ( one error

occurs)retransmitted block


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


Cyclic Redundancy Code (CRC) / Polynomial Code A m-bit frame is regarded as the coefficient li

st for a polynomial, M(x), with m terms, ranging from xm-1 to x0. For example, 110001 represents a polynomial x5+x4+x0.

Addition and subtraction are identical to EXCLUSIVE OR. For example, 10011011 11110000+ 11001010 -10100110= 01010001 =01010110


Computing Cyclic Redundancy Code Both the sender and the receiver must agree

upon a generator polynomial, G(x), with the degree r in advance.

Append r zero bits into M(x) => xrM(x) Divide xrM(x) by G(x) Subtract the remainder from xrM(x) and let t

he result be T(x)


CRC Example


CRC at Receiver End

T(x) is received with no error. T(x) is divisible by G(x)

When errors represented by error bit pattern E, ( ie. E(x)), occur, (T(x) +E(x)) /G(x) has zero reminder only if E(x) is

divisible by G(x) Carefully selects G(x) such that E(x) is not divisibl

e by G(x)


CRC with Single and Double Errors Single-bit error

E(x)= xj

If G(x) contains tow or more terms, it will never divided E(x)

Two isolate single-bits errors E(x)= xi +xj = xj ( xi-j+ 1) ….. i>jAssume G(x) is not divisible by x

• Select G(x) that does not divide xk+1, where 1< k i-j


CRC with Odd-Number-bit Errors


CRC with Burst Errors


Well-Known Generator Polynomials CRC-12: x12+x11+x3+x2+x+1 CRC-16: x16+x15+x2+1 CRC-CCITT: x16+x12+x5+1 CRC-32: x32+x26+x23+x22+x16+x12+x11+x10+

x8+ x7+x5+x4+x2+x+1 CRC-16 and CRC-CCITT

Catch all single and double errors, all errors with an odd number of bits, all burst errors of length 16 or less, 99.997% of 17-bit error burst, 99.998% of 18-bit error and longer bursts


Flow Control Techniques

Stop-and-wait The receiver provide feedback to the sender The sender sends one frame and then waits

for an acknowledgement before proceeding


Dealing with the Damaged Frames Adding a timer ( on sender )

The receiver would only send an ack if the data were correctly received

After time out, the sender send the frame again


Dealing with the Lost Frames

A lost data frame can be resent by using a timer

When an acknowledgement is lost The sender’s timer eventually time-out The sender resends the same data frame After the data frame is received, the receiver

sends acknowledgement again


Duplicate Frames : Two Cases


Dealing with Duplicate Frames


When Sender’s Time-out is Too Short


Solution: Put Sequence Number in the Acknowledgement


Piggybacking


Acknowledgement

Techniques Control Frame Data Frame: Piggybacking

• The technique of temporarily delaying outgoing acknowledgements so that they can be hooked onto the next outgoing data frame is known as piggybacking.

Types Positive Acknowledgement: The received

frame has no error. Negative Acknowledgement: The received

frame has errors.


Flow Control

Flow control is a technique for assuring that a transmitting station does not overwhelm a receiving station with data.

Sliding Window Protocols: At any instant of time, the sender maintains a set of sequence numbers corresponding to frames it is permitted to send. These frames are said to fall within the sending window. Similarly, the receiver also maintain a receiving window corresponding to the set of frames it is permitted to accept.


Sliding Windows

Each outbound frame contain a sequence number ranging from 0 to 2n-1

Sending window Maintained by the sender A set of sequence numbers corresponding to

frames the sender is permitted to send Receiving window

Maintained by the receiver Corresponds the set of frames the receiver is

permitted to accept


Maintain Sliding Windows

Sending window When a frame is sent, the upper edge of the

window is advanced by one When an acknowledgement is received, the

lower edge of the window is advanced by one Receiving window

Any received frame the sequence number outside the window is discarded

When a frame whose sequence number is equal to lower edge of the window is received, the window is rotated by one


A Sliding Window Size of One


Sliding Window Protocols

One bit sliding window protocol ( Stop-and-Wait)

Go-back-N Protocol Selective Repeat Protocol


One Bit Sliding Window Protocol Stop –and-Wait

A protocol in which the sender sends one frame and then waits for an acknowledgement before proceeding.

a sliding window protocol with maximum window size = 1 Sequence number {0, 1} is sufficient

Frame header fields Seq : the sequence number of this frame Ack : the number of the last frame is received


Efficiency of Stop-and-Wait

50 kbps data rate500 msec round-trip delay1000 bits per frame

time (msec)t=0 start sendingt=20 completely sentt=250 frame arrivalt=270 completely receivedt=270 acknowledge without delayt=520 acknowledgement arrival

26000 bits could be transmitted in 520 msec.Efficiency: 1000/26000=20/520 < 4% used

Stop-and-Wait


Efficiency of Stop-and-Wait


Pipelining

50 kbps data rate500 msec round-trip delay1000 bits per frame

time (msec)t=0 start sending the first framet=20 the first completely sentt=250 frame arrivalt=270 the first completely receivedt=270 acknowledge without delayt=520 acknowledgement arrival

To pipeline frames without waiting for acknowledgements.


Pipelining


Line of Utilization for Stop-and-Wait Capacity : b bits/sec, frame size : l bits, roun

d-trip delay : R seconds Transmission time : l/b Total duration : R + l/b Utilization : (l/b) / (R + l/b) = l / (l+bR)


Go-back-N

Sender’s window size is n (n >1) At most n outstanding frames can be sent

After receiving a damaged frame Receiver discards all subsequent frames Sender retransmits the damaged frame and

all its successors after the times out


Go-back-N : An Example


Selective Repeat

Receiver’s window size is n ( n >1 ) At most n frames can be buffered

Receiver stores all the correct frames following the bad one

The sender retransmits only the bad frame not all its successors


Selective Repeat : An Example


Retransmission with Sliding Window Protocols ARQ (Automatic Repeat reQuest): When an e

rror is detected, the receiver requests that the frame be retransmitted. Stop-and-wait ARQ Go-back-N continuous ARQ Selective-repeat continuous ARQ

All of them belong to the sliding window protocols.

Both Go-back-N and Selective-repeat ARQs uses the pipelining technique.


ARQ

Instead of using positive or negative acknowledgement, the receiver sends back only the frame number of the next frame is expects

On the receipt of each frame (including damaged) It may speed up the transmission of lost or

damaged frames, if the time-out value is “loose”


Stop-and-Wait with ARQ


Window Size

The optimal sender’s windows size is 1+(round-trip delay / transmission time)

Maximum window size of Go-back-N ARQ is MaxSeq, where the sequence number are 0, 1, ... , MaxSeq.

Maximum window size of Selective-repeat ARQ is (MaxSeq+1)/2.


Protocol Specification and Verification Finite State Machine Model Petri Net Model


Finite State Machine Model

States Instants that the protocol machine is waiting

for the next event to happen Transitions

Occur when some event occurs Change form some state to another


Finite State Machine Model for Bit Destuffing

1 2 3 4 5 6

T

1/

0/100/0

1/1/ 0/1/ 1/ 1/

S E

1/

Bit destuffing

0/1110

0/11111

0/110

0/11110

bit destuffed

end offrame

error beginningof the next

frame

input/output


State Digram for Stop-and-Wait


Example Data Link Protocols

HDLC bit oriented bit stuffing

SLIP PPP

character oriented character stuffing multiprotocol framing


High-level Data Link Control

derived from SDLC (Synchronous Data Link Control) of IBM’s SNA

ADCCP (Advanced Data Communication Control Procedure): ANSI

HDLC: ISO LAP (Link Access Procedure): X.25 of CCITT LAPB: CCITT http://members.tripod.com/~vkalra/hdlc.ht

ml


HDLC Frame Format

A frame is delimited with the flag sequence 01111110. On idle point-to-point lines, flag sequences are transmitted continuously.

Address to identify one of multiple terminals for point-to-

multipoint lines to distinguish commands from responses for point-to-

point lines

bits 8 8 8 >=0 16 801111110 Address Control Data Checksum 01111110


PDN

HDLC Frame Addressing

DCEDTE

Address B

Address Aaddress=0xC0, response

address=0xC0, command

address=0x80, response

address=0x80, command


HDLC Frame Type

Type Command Response 1 2 3 4 5 6 7 8Information I 0 N(S) P N(R)Supervisory RR

REJRNRSR

RRREJRNRSR

1 0 0 01 0 0 11 0 1 01 0 1 1 *

P/FP/FP/FP/F

N(R)N(R)N(R)N(R)

UnnumberedSABMDISC

DM

UAFRMR

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

FPPFF

0 0 01 0 00 1 01 1 00 0 1

Control Field

* not defined in LAPB


HDLC P/F Bit

In Command frames: P (POLL) bit In Response frames: F (FINAL) bit Each Command frame with the POLL bit

set to 1 must receive a Response frame with the FINAL bit set to 1.


RR (Receive Ready): acknowledgement frame N(R): the next frame expected to be

received REJ (Reject): negative acknowledgement frame

N(R): the first frame in sequence not received correctly (Go-back-N protocol)

RNR (Receive not Ready): same as Type 0 and telling the sender to stop sending

SR (Selective Reject): acknowledgement frame N(R): the frame expected to be

retransmitted (Selective Repeat protocol)

HDLC Supervisory Frame


HDLC Unnumbered Frame

DISC

Link Termination

UA

Link Initiation

SABM

DM

SABM

UA

accept

reject UA Unnumbered/AcknowledgementSABM Set Asynchronous Balanced ModeDISC DisconnectDM Disconnected Mode


Serial Line IP

SLIP (Serial Line IP): to connect SUN workstations to the Internet over a dial-up line using a modem in 1984

RFC 1055 Sending IP packets over the line, with a

special flag byte at the end for framing. Using character stuffing.

Many problems: no error detection, supporting only IP, no authentication, etc.


Point-to-Point Protocol

PPP: RFC 1661, RFC 1662, RFC 1663 A multiprotocol framing mechanism suitable

for use over modems, HDLC lines, SONET , and other physical layers.

Supports error detection, option negotiation, header compression, and optionally, reliable transmission using HDLC framing.

Character-oriented


Homework

#2, #3, #6, #9,#11

page 1computer networks fall 2002 the data link layer r framing r error control m error correction m...

Documents

computer networks

bits r

characters frame

voice r

wireless r

r redundant

frame v v

error control r