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Networking

3.4 Questions

1. Bit Stuffing/De-stuffing

1.1 Question 01

The bit string has been received by a receiver while flag byte with bit stuffing is used for framing, with 01111110 as the flag. Destuff the data and identify the boundaries between different frames, if there exists any.

The de-stuffed data should be:

2. Error Detection

2.1 Question 01

Using CRC method for bit string , with the generator polynomial ,

  1. Derive the codeword, with the calculation steps shown clearly.
  2. Assuming the received message contains an error on the 6th bit from the right (i.e., the error polynomial is ), can the error be detected? Show your answer with detailed steps.
CRC question part 02. CRC question part 02.

3. ARQ

In Go-Back-N ARQ with a window size of 6, if the receiver has just received Frames 10, 11, 13, 12, 14, 15 in an out-of-order sequence and has just acknowledged Frame 11:

  • Which frame(s) would be discarded by the receiver?

In Go-Back-N ARQ, the receiver only accepts frames in order. It maintains an expected frame number, and any frame that arrives out of order is discarded, not buffered. Only the expected frame is accepted and acknowledged.

Given:

  • Window size = 6 (relevant for the sender, not directly for the receiver discards)
  • Receiver has acknowledged Frame 11, which means it has successfully received Frame 10 and Frame 11 in order.
  • Then the receiver gets Frames 13, 12, 14, 15 (in that order).

Step-by-step:

  • After ACKing 11, the receiver expects Frame 12.
  • It gets Frame 13 → discarded (not expected)
  • It gets Frame 12 → accepted, ACKed
  • It gets Frame 14 → discarded (because Frame 13 wasn’t accepted before)
  • It gets Frame 15 → discarded (same reason)

✅ Final Answer:

Frames discarded by the receiver:
13, 14, and 15