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6-1 Data Link Layer (contd) l Elementary data link protocols n an unrestricted simplex protocol –simplex –error-free channel –infinitely high processing.

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Presentation on theme: "6-1 Data Link Layer (contd) l Elementary data link protocols n an unrestricted simplex protocol –simplex –error-free channel –infinitely high processing."— Presentation transcript:

1 6-1 Data Link Layer (contd) l Elementary data link protocols n an unrestricted simplex protocol –simplex –error-free channel –infinitely high processing capability of the receiver

2 6-2 Data Link Layer (contd) l Elementary data link protocols (contd) n a simplex stop-and-wait protocol –simplex –error-free channel –at most one outstanding frame at a time –ACKs are used (automatic repeat request, ARQ) 123 ACK

3 6-3 Data Link Layer (contd) l Elementary data link protocols (contd) n 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 12 ACK time-out 111 duplicate Example 1 Example 2

4 6-4 Data Link Layer (contd) l Sliding window protocols n bidirectional n piggybacking is used n frames may be lost or damaged n ACK and time-out mechanism are used n sequence numbers are used

5 6-5 Data Link Layer (contd) l Sliding window protocols (contd) n sending window (SW): a set of sequence numbers for frames sent out but not yet acked n receiving window (RW): a set of sequence numbers for frames the receiver is permitted to accept Fig. 3-12, p.204

6 6-6 Data Link Layer (contd) l Sliding window protocols (contd) n 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 6-7 Data Link Layer (contd) l Sliding window protocols (contd) n 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 6-8 Data Link Layer (contd) l Sliding window protocols (contd) n a protocol using go-back-n (contd) –effect of receiver buffer size Fig. 3-15, p. 208

9 6-9 Data Link Layer (contd) l Sliding window protocols (contd) n a protocol using go-back-n (contd) –SW size is Max_Seq –example with Max_Seq = 7 and SW size = 8 but not 7 l The sender sends frames 0 through 7. l The piggybacked ACKs for frames 0 through 7 are lost. l The sender resends frames 0 through 7 after time-out. l The receiver accepts duplicate frames 0 through 7.

10 6-10 Data Link Layer (contd) l Sliding window protocols (contd) n 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 –NAKs 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 6-11 Data Link Layer (contd) l Sliding window protocols (contd) n remarks –NAKs can also be used in go-back-n protocols –NAKs 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 6-12 Data Link Layer (contd) l Protocol specification and verification n finite state machines for protocol modeling n components of a finite state machine –states (including one particular initial state) –transitions (event driven) n reachability analysis for protocol correctiveness –incompleteness –deadlock –extraneous transition

13 6-13 Data Link Layer (contd) l Protocol specification and verification (contd) n 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)


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