1. Data Link Layer (cont’d)
Elementary data link protocols
an unrestricted simplex protocol
simplex
error-free channel
infinitely high processing capability of the receiver
...

2. Data Link Layer (cont’d)
Elementary data link protocols (cont’d)
a simplex stop-and-wait protocol
simplex
error-free channel
at most one outstanding frame at a time
ACK’s are used (automatic repeat request, ARQ) 1 2 3
ACK
ACK
ACK

3. Data Link Layer (cont’d)
Elementary data link protocols (cont’d)
a simplex protocol for a noisy channel
simplex
frames may be lost or damaged
ACK and time-out mechanism are used
sequence numbers are used to avoid duplicates 1
time-out 1 1 1 2
ACK
ACK
duplicate
Example 1
Example 2

4. Data Link Layer (cont’d)
Sliding window protocols
bidirectional
piggybacking is used
frames may be lost or damaged
ACK and time-out mechanism are used
sequence numbers are used

5. Data Link Layer (cont’d)
Sliding window protocols (cont’d)
sending window (SW): a set of sequence numbers for frames sent out but not yet ack’ed
receiving window (RW): a set of sequence numbers for frames the receiver is permitted to accept
Fig. 3-12, p.204

6. Data Link Layer (cont’d)
Sliding window protocols (cont’d)
one-bit sliding window protocols
one bit for sequence numbers (0 and 1)
max. SW size = max. RW size =1
stop-and-wait protocol
Fig. 3-14, p. 207

7. Data Link Layer (cont’d)
Sliding window protocols (cont’d)
a protocol using go-back-n
pipelining is used (multiple outstanding frames)
single resequencing buffer (RW size = 1)
only the in-sequence frame is accepted by the receiver
SW size is Max_Seq

8. Data Link Layer (cont’d)
Sliding window protocols (cont’d)
a protocol using go-back-n (cont’d)
effect of receiver buffer size
Fig. 3-15, p. 208

9. Data Link Layer (cont’d)
Sliding window protocols (cont’d)
a protocol using go-back-n (cont’d)
SW size is Max_Seq
example with Max_Seq = 7 and SW size = 8 but not 7
The sender sends frames 0 through 7.
The piggybacked ACK’s for frames 0 through 7 are lost.
The sender resends frames 0 through 7 after time-out.
The receiver accepts duplicate frames 0 through 7.

10. Data Link Layer (cont’d)
Sliding window protocols (cont’d)
a protocol using selective repeat
the same as go-back-n except that the receiving buffer size > 1 [= (Max_Seq+1)/2]
max. SW size = (Max_Seq+1)/2
NAK’s are used when frame errors or out-of-sequence frames are detected to request the retrans. of such frames
Fig. 3-19, p. 218

11. Data Link Layer (cont’d)
Sliding window protocols (cont’d)
remarks
NAK’s can also be used in go-back-n protocols
NAK’s can speed up retransmissions
typically SW size + RW size = Max_Seq + 1
the above three sliding window protocols differ among themselves in terms of efficiency, complexity and buffer requirement

12. Data Link Layer (cont’d)
Protocol specification and verification
finite state machines for protocol modeling
components of a finite state machine
states (including one particular initial state)
transitions (event driven)
reachability analysis for protocol correctiveness
incompleteness
deadlock
extraneous transition

13. Data Link Layer (cont’d)
Protocol specification and verification (cont’d)
example
2 states for the sender and the receiver respectively
4 states for the channel (sending 0, 1, ACK frame or idle)
10 system states [(000) as initial] & 9 kinds of transitions
(Fig. 3-20, p. 221)