DC 4

Question 1

Sliding Window Flow Control

Answer 1

Key Idea: Multiple frames can be in transit, improving efficiency over stop-and-wait.
Features:
Uses sequence numbers for frames (modulo 2k2 k).
Sender can send up to W frames without waiting for acknowledgment.
Receiver acknowledges with the next expected frame number.
Benefits: Treats the transmission link as a pipeline, allowing continuous data flow.

Question 2

Stop-and-Wait ARQ

Answer 2

Key Idea: Source sends one frame, waits for acknowledgment before sending the next.
Error Handling:
Timer used for retransmission if no acknowledgment.
Frames are labeled alternately (e.g., 0, 1) to prevent duplication errors.
Advantage: Simplicity.
Disadvantage: Inefficient, as the line is underutilized.

Question 3

Go-Back-N ARQ

Answer 3

Key Idea: Sender can send multiple frames but retransmits from the error frame upon a negative acknowledgment (REJ).
Mechanism:
Receiver discards all frames after the error until retransmission.
Sender buffers unacknowledged frames.
Benefit: Better line utilization than stop-and-wait.

Question 4

Selective Reject ARQ

Answer 4

Key Idea: Only frames with errors (or those timed out) are retransmitted.
Mechanism:
Receiver buffers out-of-order frames.
Requires complex logic for proper sequence insertion.
Benefit: Reduces retransmissions.
Drawback: Less common due to complexity.

Question 5

Go-Back-N vs. Selective Reject

Answer 5

Go-Back-N:
Retransmits all frames after an error.
Simple logic but less efficient.
Selective Reject:
Retransmits only the errored frame.
Efficient but requires buffering and complex logic.

Question 6

HDLC Protocol Basics

Answer 6

Station Types:
Primary: Controls link, sends commands.
Secondary: Responds to primary.
Combined: Acts as both.
Link Configurations:
Unbalanced: One primary, multiple secondaries.
Balanced: Two combined stations.

Question 7

HDLC Transfer Modes

Answer 7

NRM (Normal Response Mode): Primary initiates, secondary responds.
ABM (Asynchronous Balanced Mode): Either station can initiate; efficient.
ARM (Asynchronous Response Mode): Secondary can initiate, but rarely used.

Question 8

Flag Fields and Bit Stuffing

Answer 8

Flag Field: Delimits frames with the unique pattern 01111110.
Bit Stuffing: Inserts a 0 after five consecutive 1s to prevent flag misinterpretation.
Benefit: Ensures data transparency.