CS3502: Data and Computer Networks DATA LINK LAYER - 2 WB version.

CS3502:CS3502:

Data and Computer NetworksData and Computer Networks

DATA LINK LAYER - 2DATA LINK LAYER - 2WB versionWB version

data link layer : flow and error data link layer : flow and error control control

purpose : regulate the flow of data from sender S to receiver R, so that R is neither overwhelmed nor kept idle unnecessarily.

secondary purpose may also be used to avoid swamping the network or link with traffic.

technique : send control information between S and R, synchronizing on buffer space, transmission rates, etc.

protocols: stop-and-wait, alternating bit sliding window (go-back-N, selective repeat/reject)

performance analysis of performance analysis of networks networks

attempts to determine the efficiency of a network; that is, for various traffic loads, how well the network uses its resources to meet the needs of the traffic

examples stop and wait alternating bit

more complex networks need the use of probability and queueing theory

data link layer : stop-and -wait data link layer : stop-and -wait protocolprotocol

send 1 frame, then stop, and wait for an acknowledgment before sending the next.

RXdata

ack

data

data link layer : stop and waitdata link layer : stop and wait what happens if a message is lost?

to tolerate losses, must add timeouts (TO) and retransmissions what happens if data is lost? what happens if ack is lost? what is the obvious solution?

alternating bit protocol:add a number to data frames, to uniquely identify; enable repeated messages to be safely discarded

data link layer : stop and wait data link layer : stop and wait protocolsprotocols

what is the efficiency of this S&W protocol? i.e., of the total time spent, how much is actually spent sending the data?

variables

td, time spent transmitting the data

tp, propagation delay

tproc, processing time

tack, time spent transmitting the ack.

U, utilization or efficiency of the protocol

performance of A-B protocolperformance of A-B protocol

AB protocolU = td /( td + 2 tprop), error free

case

or U = (1-PE)td /( td + 2 tprop, )error case

td

tp

tp

link utilization of AB protocollink utilization of AB protocol

suppose we use a satellite link, tprop = 250ms; data frame is 16K bits; transmission rate is 1 Mbps. What is U ? Assume negligible error rate.

how might this be improved?

earth

sliding window protocols: no sliding window protocols: no errorerror

send a series of data frames, without waiting for acknowledgments 1 at a time

window W : the number of frames in transit between sender and receiver (max, current)

each frame numbered from 0, 1, 2, ..., w

receiver may ack 1 or more frames at a time

X sends up to max window, then waits for acks;

R uses acks to control and maximize utilization

sliding window protocol : no sliding window protocol : no errorerror

suppose w = 3: NOTE: By Convention

ack# = sequence # of next frame

expected by receiver

d0d1d2

ack3

sliding window protocolsliding window protocol

sequencing for w = 3: d0, d1, d2 wait for ack3 d3, d0, d1 wait for ack2 etc.

exercise: show all sequences possible on timing diagram for w=3. (include 3 at a time, 2 at a time, 1 at a time)

sliding window protocol : no sliding window protocol : no errorerror

what is the efficiency of the protocol? ie, what is the best utilization possible? (assume no errors in the channel)

sliding window: no channel errors

U = W td /( td + 2 tprop ), if less than 1,

U = 1, otherwise.

Why sliding window protocol?Why sliding window protocol?

for large windows, what if a message is lost? what problem do you see with this?

suppose w = 63: what if d61 lost?

d0d1

d62d61

ack61

Tw d6

1

sliding window: variables, nackssliding window: variables, nacks

standard variables NS and NR : used to keep track of sequence numbers NS : send sequence number; seq. number of the

next data frame to be sent . Increments modulo Wmax +1.

NR : receive sequence number; seq. number of the last (most recent) nack. frame received

both are local variables of the sender current window in sender is found by subtracting

the difference, NS _- NR , from maximum window size --

Wcurrent = Wmax - [NS _- NR ]

sliding window: variables, sliding window: variables, nacksnacks

go-back-N nacks: if a frame lost, it and all subsequent frames retransmitted nack: (1) acknowledges previous frames, and

(2) rejects numbered frame and all subsequent frames. Sequence numbers convention same as acks# ie. Next frame expected.

when sender gets a nack, NS must be rolled back to the value of the nack, and

NR must be rolled forward to the nack

examples

sliding window: sliding window: variables, variables, nacksnacks Initially, NS = NR = 0

NS incremented each time a frame sent

NR updated each time a nack frame received example: suppose Wmax = 5; show values

after each of following: send d0, d1, d2 receive nack1 send d1, d2, d3, d4, d5 receive nack4 send d4, d5, d0

what is current window size? Calculated modulo Wmax +1

sliding window protocolsliding window protocol Go-back-n needlessly repeats frames sliding window: selective repeat (also

called selective reject) only retransmit messages which were lost window size at most half the range of sequence

numbers (why?) timing diagrams disadvantage

more buffers, more complex algorithm, costs more advantage

higher efficiency in noisy channels

data link protocol performance data link protocol performance

go-back-N, selective repeat : no channel errors

U = W td /( td + 2 tprop ),if less than 1,

U = 1, otherwise.

performance of data link performance of data link protocolsprotocols

selective repeat: with errors U = 1 - P, for Wtd > td + 2 tprop

= (1 - P) Wtd / td + 2 tprop , otherwise

go-back-N, with errorsU = (1 - P)td/(td + 2tpP), W > 2tp/td +

1,

= W(1 - P)td/ (2tp + td)(1 - P + WP), O.W.

see Stallings Appendix 6A

HDLC: high level data link HDLC: high level data link controlcontrol

ISO standard for a data link protocol other DL standards exist, but are very

similar; e.g., PPP HDLC combines various functions of the DL

layer - flow control, error control, sequencing, framing, etc. - into a single protocol standard

HDLC standard is broad, covering several different cases

3 general classes: station type link configuration mode of operation

HDLCHDLC

station types primary P secondary S combined C types P and S are for multipoint network with

polling hub polling, etc. --> master/slave network

type C for point-to-point link

link configurations balanced : 2 combined stations on 1 direct link unbalanced : 1 P, n S’s directly connected

(e.g., bus)

HDLCHDLC frame types and formats

I-frame(information/data) S-frame (supervisory)U-frame (data)