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CS3502:
Data and Computer Networks
DATA LINK LAYER - 2
WB version
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)
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
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
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 protocol:add a number to data
frames, to uniquely identify; enable repeated
messages to be safely discarded
data link layer : stop and wait protocols
 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 protocol
td
tp
tp
 AB
protocol
U = td /( td + 2 tprop), error free case
or U = (1-PE)td /( td + 2 tprop, )error case
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.
eart
h
 how
might this be improved?
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
sliding window protocol : no error
suppose w = 3:
frame
NOTE: By Convention
ack# = sequence # of next
d0
d1
d2
expected by receiver
ack3
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 w=3. (include 3 at a time, 2 at a time,
1 at a time)
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.
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
T
w
ack61
d6
1
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 ]
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
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
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
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 protocols
 selective
repeat: with errors
U = 1 - P,
for Wtd > td + 2 tprop
= (1 - P) Wtd / td + 2 tprop , otherwise
 go-back-N,
with errors
U = (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 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
HDLC
 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
 link
C for point-to-point link
configurations
 balanced
: 2 combined stations on 1 direct link
 unbalanced : 1 P, n S’s directly connected (e.g., bus)
HDLC
 frame
types and formats
I-frame(information/data)
 S-frame (supervisory)
U-frame (data)
