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Module 2: Data Link Layer & MAC Sublayer

20212m

Station A needs to send a message consisting of 9 packets to Station B using a sliding window (window size 3) and go-back-n error control strategy. All packets are ready and immediately available for transmission. If every 5th packet that A transmits gets lost (but no packets from B ever get lost), then what is the number of packets that A will transmit for sending the message to B?

(i) 12
(ii) 14
(iii) 16
(iv) 18

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(iii) 16

Since Go-Back-N is used, whenever a packet is lost, every packet sent after it in that window also has to be resent (even if those later packets were themselves successfully transmitted), because the receiver discards any out-of-order arrival and only cumulatively acknowledges up to the last correctly received in-order packet.

Tracking every transmission A makes (numbering them 1st, 2nd, 3rd... in the order sent, including retransmissions), and marking every 5th one as lost:

Phase 1 (initial send, window=3):
  Send: 1, 2, 3, 4, 5(LOST— 5th transmission), 6, 7
  -> 7 transmissions so far. Timeout on packet 5.

Phase 2 (go back to 5, resend + continue):
  Send: 5, 6, 7(LOST — 10th transmission overall), 8, 9
  -> 5 more transmissions (total 12). Timeout on packet 7.

Phase 3 (go back to 7, resend + continue):
  Send: 7, 8, 9(LOST — 15th transmission overall)
  -> 3 more transmissions (total 15). Timeout on packet 9.

Phase 4 (go back to 9, resend):
  Send: 9 — succeeds (16th transmission, not a multiple of 5)
  -> 1 more transmission (total 16). All 9 packets now delivered.

Total transmissions = 7 + 5 + 3 + 1 = 16

Notice the pattern: each time, the packet landing on a multiple of 5 (5th, 10th, 15th transmission) is exactly the one lost, forcing a fresh Go-Back-N retransmission cycle from that point.

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