Lecture Notes: Go Back N ARQ

Session Outcomes

Sliding Window Protocol vs. Stop and Wait Protocol:
- Stop and Wait: Only one frame sent at a time; waits for acknowledgment (ACK) before sending the next frame.
- Sliding Window: Multiple frames can be sent before receiving an ACK for the first frame.

'N' Definition:
- Refers to sender's window size.
- Example: Go Back 3 means 3 frames can be sent before expecting an ACK.

Protocol Pipelining:
- Sender can send multiple frames before waiting for the first frame's ACK.
Frame Numbering:
- Frames are numbered sequentially using sequence numbers for identification.
Window Size Impact:
- The number of frames that can be sent is determined by the sender window size (N).

If an ACK for a frame is not received in time:
- All frames in the current window are retransmitted (e.g., if frame 5 is sent, all frames in the current window must be resent).

The size of the sending window determines the sequence numbers of outbound frames.
Example:
- If sending window size is 4 (N=4), sequence numbers will be 0, 1, 2, 3.
- Sequence numbers repeat as needed (e.g., if there are 10 frames: 0, 1, 2, 3, 0, 1, 2, 3, ...).

Sender and Receiver Setup:
- 11 frames numbered 0 to 10.
- Sender window size = 4.
Sending Frames:
- Sender sends frames 0, 1, 2, 3.
- Await ACK for frame 0.
Receiving ACKs:
- If ACK for frame 0 is received, sender sends frame 4 (window slides).
Handling Missing ACK:
- If frame 2's ACK is lost or frame 2 is lost:
  - Sender waits for a timeout.
  - Retransmits from the starting frame in the current window (frame 2).
  - Discards subsequent frames (4, 5) at the receiver since frame 2 wasn’t received.