university of delaware cpeg 4191 zno class thursday 10/3 zoffice hours: tue 3:15-4 –w 12-1...

University of Delaware CPEG 419 1

No Class Thursday 10/3Office hours: Tue 3:15-4

– W 12-1 – Evans 315


Data Link Layer

Physical layer: sending signals over transmission medium.

Data link layer: responsible for error-free (reliable) communication between adjacent nodes.

Functions: flow control, error control, framing, fragmentation, and addressing (in multipoint medium).


Flow Control

What is it? Ensures that transmitter does not

overrun receiver: limited receiver buffer space.

Receiver buffers data to process before passing it up.

If no flow control, receiver buffers may fill up and data may get dropped.


Stop-and-Wait

Simplest form of flow control. Transmitter sends frame and waits. Receiver receives frame and sends ACK. Transmitter gets ACK, sends other frame, and

waits. This continues until there are no more frames to

send.

This is a good method if the propagation delay and data rate is small. Otherwise, it leads to low link utilization.


Baud rate * bits/symbol * delay = bits10M*10*0.06 = 2500000 bits (2.5Mb)!This is bandwidth * delay = bandwidth delay product.

Suppose that the baud rate is 10Mbaud/s. Suppose that there are 10bits per symbol. Suppose that the propagation delay is 25ms. How many bits can fill the wire?

Bandwidth Delay Product

Suppose that we send a frame of 1500*8 bits over this link and use stop and wait. What is the link utilization?

For a satellite, stop and wait is not a good approach.For a wireless link like 802.11, its ok (and is used)


Transmission Delay

Suppose the data rate is 100Mbps. Suppose the frame is 1500bytes. How long does it take to put the frame on the

wire? (This is transmission delay.)

How long to send a frame over this link if the propagation delay is 25ms?


Sliding Window 1

Allows multiple frames to be in transit at the same time.

Receiver allocates buffer space for n frames.

Transmitter is allowed to send n (window size) frames without receiving ACK.

Frame sequence number: labels frames.


Sliding Window 2

Receiver ack’s frame by including sequence number of next expected frame.

Cumulative ACK: ack’s multiple frames.Example: if receiver receives frames

2,3, and 4, it sends an ACK with sequence number 5, which ack’s receipt of 2, 3, and 4.


Sliding Window 3

Sender maintains sequence numbers it’s allowed to send; receiver maintains sequence number it can receive. These lists are sender and receiver windows.

Sequence numbers are bounded; if frame reserves k-bit field for sequence numbers, then they can range from 0 … 2k -1 and are modulo 2k.


Sliding Window 4

Transmission window shrinks each time frame is sent, and grows each time an ACK is received.


Example


Sliding Window (cont’d)

RR (ready to receive) n acknowledges up to frame n-1.

There is also RNR n, which ack’s up to frame n-1 but no longer accepts more frames.

RNR shuts down the receive window and consequently the transmission window.

Need subsequent RR to re-open window.


Piggybacking

When both endpoints transmit, each keeps 2 windows, transmitter and receiver windows.

Each send data and need to send ACKs.

When sending data, transmitter can “piggyback” the acknowledgment information.

When no data, send just the ACK.


Duplicate ACKs

When no data, must re-send last ACK.

Duplicate ACKs: report potential errors.


Error Detection

Transmission impairments lead to transmission errors: change of 1 or more bits in transmitted frame.

Transmission errors defined using probabilities: transmission medium modeled as a statistical system.


Error Probabilities 1Definitions:

Pb probability of single bit error (bit error rate); constant and independent for each bit.

P1 probability frame received with no errors.

P2 probability frame received with 1 or more undetected errors.

P3 probability frame received with 1 or more detected bit errors, but no undetected ones.


Error Probabilities 2

If no error detection mechanism, P3 = 0.

P1 = (1 - Pb)F and P2 = (1- P1), where F is size of frame in bits.

P1 decreases as Pb increases.

P1 decreases as F increases.


Example64-kbps ISDN channel’s bit error rate is

less than 10-6. User requirement of on average 1 frame with undetected bit error per day. Frame is 1000 bits. In a day, 5.529 x 106 frames transmitted. Required frame error rate of 1/ 5.529 x

106, or P2 = 0.18 x 10-6. But Pb = 10-6, so P1 = (1-Pb)F = 0.999 and P2

= 1 - P1 = 10-3, which is >>> required P2


Error Detection Schemes Transmitter adds additional bits for error

detection.Transmitter computes error detection bits

as function of original data.Receiver performs same calculation and

compares results. If mismatch, then error.P3 probability error detection scheme

detects error; P2 residual error rate or probability error goes undetected.


Parity

Simplest error detection scheme.Append parity bit to data block.Example: ASCII transmission

1 parity bit appended to each 7-bit ASCII character.

Even parity: 8-bit code has even number of 1’s.

Odd parity: 8-bit code has odd number of 1’s.


Parity Check

Example: transmitting ASCII “G” (1110001) using odd parity. Code transmitted is 11100011. Receiver checks received code and if odd

number of 1’s, assumes no error. Suppose it receives 11000011, then

detects error. NOTE: If more than 2 bits in error, may

not be detected.


ISDN Example - Continued

BER = 10-6.Frame errors might occur when two or

more bit errors occur. Or, no frame error if no bit errors or one bit error. 1 – ((1-Pb)1000 + 1000*(1-Pb)999*Pb)= 5e-7 With no parity, the frame error prob. = 10-3


Cyclic Redundancy Check

CRC is one of the most effective and common error detecting schemes.

Represent frames as polynomials M= 1010001101 -> x9+x7+x3+x2+x0

Both source and destination use a generator polynomial G – r bits/r-1 degree polynomial

Idea: Transmit extended frame [M|V], such that the polynomial representation is exactly divisible by G. Thus, if G does not exactly divide the received frame, there is an error.


CRC

How to find V such that G exactly divides [M|V]? Divide xrM by G. Set V = the remainder. [M|V] = xrM + V [M|V]/G = (xrM + V)/G = xrM/G + V/G =

S + V/G + V/G = S + 2V/G. But in binary 2 is the same as zero. So [M|V] / G = S.


CRC Example

Frame M 1010001101 = x9+x7+x3+x2+x0.

Pattern G 110101.Dividing (frame*25) by pattern results

in 01110.Thus T 101000110101110.Receiver can detect errors unless

received message Tr is divisible by G.


CRC Performance and Generators Instead of [M|V] we receiver [M|V] + E (E is the error). Burst errors – CRC detects bursts of length r or less.

a string of errors, with E = 0 …0 1 X X X X 1 0 … E(x) = xi(xk-1 + … + 1) where the burst length is k. If G is degree k or greater and G has a 0th degree term, then G

will not divide E.

dividemight

1

122

x

xxx

xG

xE i

dividenever will

12

12

xx

xx

xG

xE ie.g.

Will CRC detect single bit errors?Suppose that you use 4 bits (as in Ethernet), and BER is 106 what is the frame error probability?


CRC Performance and Generators

Burst Errors Continued If the burst has length r+1, then the

errors will not be detected if the burst is the same as G. If an error of length r+1 occurs and all combinations of errors are equally likely, then the burst will match G with prob. ½r-1. (So prob is BERr+1 x ½r-1)

In general, for longer bursts, the prob. is ½r


CRC

Lastly, if G contains x+1 as a factor, and E has an odd number of bit, then G will not divide E.

In IEEE 802x32 + x26 + x23 + x22 + x16 + x12 + x11 + x10 + x8 + x7

+ x5 + x4 + x2 + x + 1

CRC can be implemented in hardware with registers.

Problem: errors can occur in some pattern, so the probabilities are not quite right.


Error Correcting Codes

Hamming distance between bit strings: The number of bits where the string

differ XOR them and count the 1’s 1 0 1 0 0

1 0 0 1 00 0 1 1 0 = 2 – the distance is two.


Error Correcting codes

Take an n bit string and encode it as n+k bits in such a way that each encoded n bit string is at least d distance from any other encoded n bit string. Then you can detect errors of length d.

Do the same thing, but make it so the distance between any encoded n bit string is 2d+1 bits. Then the an error of d bits or less can be corrected by assigning the string to its closest legal value.


Error Correcting Code

Legal strings – these are the encoded versions of n bit strings.

Illegal strings – no n bit string is encode to these



Suppose that this code is 0 1 0 1 0 But this code 0 1 1 0 1 0 is received

Then you know an error occurred, and you figure that what was meant to be sent is the code 0 1 0 1 0 (the nearest legal neighbor).



Suppose that this code is 0 1 0 1 0 But this code 0 1 1 0 1 1 is received

Then you know an error occurred. But you are sure which code was actually sent.


Error ControlMechanisms to detect and correct

transmission errors.Consider 2 types of errors:

Lost frame: frame is sent but never arrives. Damaged frame: frame arrives but in error.

Error control: combination of error detection, feedback (ACK or NACK) from receiver, and retransmission by source.

Coupled with flow control feedback.


ARQ

ARQ: automatic repeat request.Works by creating a reliable data link

from an unreliable one.3 versions:

Stop-and-wait ARQ. Go-back-N ARQ. Selective-reject ARQ.


Stop-and-Wait ARQSingle outstanding frame at any time.Simple but inefficient.Use of timers to trigger retransmission of

data or ACKs.2 types of errors:

Damaged or lost frame. Damaged or lost ACK.

Sequence numbers alternate between 0 and 1.


Stop-and-Wait ARQ: ExampleSender ReceiverFrame 0

ACK1

Frame 1ACK 0

Frame 0

Timeout

Frame 0

ACK 1

Timeout

Frame 0

ACK 1 B discards duplicate.


Go-Back-N ARQVariation of sliding window for error

control.Allows a window’s worth of frames to be in

transit at any time.RR: ack’s receipt of frame.REJ: negative acknowledgment indicating

the frame in error.Destination discards frame in error plus

subsequent frames.


Go-Back-N ARQ ExampleS R S Rf0

f1f2

rr3f3

f4

f5rr4

Errorf6

rej5f7

f5f6

rr6 f7

5, 6, 7rexm.

f7

f0

f1rr0

rr(P bit=1)

rr2

f2

Timeout

Discarded


Go-Back-N ARQ Issues

For k-bit sequence number, maximum window size is (2k-1). If window size is too large, ACKs may be

ambiguous: not clear if ACK is a duplicate ACK (errors occurred).

Example: 3-bit sequence number and 8 -frame window.Source transmits f0, gets back rr1, then

sends f1--f0, and gets back another rr1. ???


Selective-Reject ARQ

Only frames transmitted are the ones that are NACK’ed (SREJ) or that timeout.

More efficient than Go-Back-N regarding amount of retransmissions.

But, receiver must buffer out-of-order frames.

More restriction on maximum window size; for k-bit sequence #’s, 2k-1 window.


Example Data Link Layer Protocol

High-Level Data Link Control (HDLC) Widely-used (ISO standard). Single frame format. Synchronous transmission.


HDLC: Frame Format

Flag: frame delimiters (01111110). Address field for multipoint links. 16-bit or 32-bit CRC. Refer to book (pages 176-185) for more

details.

8bits

8ext.

8 or16

variable 16 or32

8

flag address control data FCS flag


Other DLL Protocols 1

LAPB: Link Access Procedure, Balanced. Part of the X.25 standard. Subset of HDLC. Link between user system and switch. Same frame format as HDLC.

LAPD: Link Access Procedure, D-Channel. Part of the ISDN standard.



LLC: Logical Link Control. Part of the 802 protocol family for LANs. Link control functions divided between

the MAC layer and the LLC layer. LLC layer operates on top of MAC layer.

Dst.MACaddr

Src.MACaddr

FCSDst.LLCaddr

Src.LLCaddr

LLCctl. DataMAC

control



SLIP: Serial Line IP Dial-up protocol. No error control. Not standardized.

PPP: Point-to-Point Protocol Internet standard for dial-up

connections. Provides framing similar to HDLC.


Multiplexing

Sharing a link/channel among multiple source-destination pairs.

Example: high-capacity long-distance trunks (fiber, microwave links) carry multiple connections at the same time.

MU

X

...

DE

MU

X ...


Multiplexing Techniques

3 basic types: Frequency-Division Multiplexing (FDM). Time-Division Multiplexing (TDM). Statistical Time-Division Multiplexing

(STDM).


FDM 1

High bandwidth medium when compared to signals to be transmitted.

Widely used (e.g., TV, radio).Various signals carried simultaneously

where each one modulated onto different carrier frequency, or channel.

Channels separated by guard bands (unused) to prevent interference.


FDM 2

Time

Frequency

1 2 N


TDM 1

TDM or synchronous TDM.High data rate medium when

compared to signals to be transmitted.

Time

Frequency

12

N


TDM 2

Time divided into time slots.Frame consists of cycle of time slots.In each frame, 1 or more slots

assigned to a data source.

1 2 N... 1 2 ... N

frame Time

U1 U2 ... UN


TDM 3

No control info at this level.Flow and error control?

To be provided on a per-channel basis. Use DLL protocol such as HDLC.

Examples: SONET (Synchronous Optical Network) for optical fiber.

+’s: simple, fair.-’s: inefficient.


Statistical TDM 1Or asynchronous TDM.Dynamically allocates time slots on

demand.N input lines in statistical multiplexer, but

only k slots on TDM frame, where k < n.Multiplexer scans input lines collecting

data until frame is filled.Demultiplexer receives frame and

distributes data accordingly.


STDM 2

Data rate on mux’ed line < sum of data rates from all input lines.

Can support more devices than TDM using same link.

Problem: peak periods. Solution: multiplexers have some buffering

capacity to hold excess data. Tradeoff data rate and buffer size (response

time).

university of delaware cpeg 4191 zno class thursday 10/3 zoffice hours: tue 3:15-4 –w 12-1...

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