6-1

Data Link Layer (cont’d)

Elementary data link protocols an unrestricted simplex protocol

– simplex

– error-free channel

– infinitely high processing capability of the receiver

1 2 3 4 5 6 7 8 9

1 2 3 4 5 6 7 8 9

...

6-2

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

6-3

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 2

ACK ACK ACKACK

time-out 1 1 1

duplicate

Example 1 Example 2

6-4

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

6-5

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

6-6

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

6-7

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

6-8

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

6-9

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.

6-10

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

6-11

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

6-12

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

6-13

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)