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

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

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

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

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

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

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

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

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.

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

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

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

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)