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

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AB protocol. U = td /( td 2 tprop) ... link utilization of AB protocol ... exercise: show all sequences possible on timing diagram for w=3. (include 3 at a ... – PowerPoint PPT presentation

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


1
CS3502Data and Computer NetworksDATA LINK
LAYER - 2WB version

2
data link layer flow and error 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)

3
performance analysis of 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

4
data link layer stop-and -wait protocol
  • send 1 frame, then stop, and wait for an
    acknowledgment before sending the next.

X
R
data
ack
data
5
data 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 protocoladd a number to data
    frames, to uniquely identify enable repeated
    messages to be safely discarded

6
data link layer stop and wait protocols
  • what is the efficiency of this SW 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

7
performance of A-B protocol
td tp tp
  • AB protocol
  • U td /( td 2 tprop), error free case
  • or U (1-PE)td /( td 2 tprop, )error case

8
link 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
9
sliding window protocols no error
  • 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

10
sliding window protocol no error
  • suppose w 3
    NOTE By Convention
  • ack sequence of next frame
  • expected by receiver

d0
d1
d2
ack3
11
sliding 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 w3. (include 3 at a time, 2 at a
    time, 1 at a time)

12
sliding window protocol no error
  • 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.

13
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?

d0
d1
d61
d62
Tw
ack61
d61
14
sliding 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

15
sliding window variables, nacks
  • 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

16
sliding window variables, nacks
  • 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

17
sliding 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

18
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.

19
performance of data link protocols
  • selective repeat with errors
  • U 1 - P, for Wtd gt td 2 tprop
  • (1 - P) Wtd / td 2 tprop , otherwise
  • go-back-N, with errors
  • U (1 - P)td/(td 2tpP), W gt 2tp/td 1,
  • W(1 - P)td/ (2tp td)(1 - P WP),
    O.W.
  • see Stallings Appendix 6A

20
HDLC high level data link control
  • 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

21
HDLC
  • station types
  • primary P
  • secondary S
  • combined C
  • types P and S are for multipoint network with
    polling
  • hub polling, etc. --gt master/slave network
  • type C for point-to-point link
  • link configurations
  • balanced 2 combined stations on 1 direct link
  • unbalanced 1 P, n Ss directly connected
    (e.g., bus)

22
HDLC
  • frame types and formats
  • I-frame(information/data)
  • S-frame (supervisory)
  • U-frame (data)
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