1. Assume that a file is transferred from a node A to a node B. The file has been fragmented in 5 frames (denoted as f0, f1, f2, f3, f4). Show the flow of frames exchanged between the two nodes, and the status of the windows after the transmission/reception of each information/control frame, if the field of sequence number is three-bit-length, and the sender and receiver are using the maximum possible window size (7), for the following data link control techniques: Simplest Stop-And-Wait Stop-and-wait ARQ Go-back-N ARQ Selective- Repeat ARQ

2. AB f0 f1 f2 f3 f4 Simplest (ideal channel, no error)

3. Stop and Wait in ideal channel (no error) AB f0 (seq 0) f1 (seq 1) ACK 0 f2 (seq 0) ACK 1 ACK 0 f3 (seq 1)

4. Stop and Wait ARQ AB f0 (0) ACK 1 f1 (1) ACK 0 f1 (1) Time out Premature Time out f2 (0) f3 (1) ACK 1 f3 (1) ACK 0 accepted discarded accepted discarded fm (n): frame #m with sequence number of n Time out

5. Go-Back-N ARQ AB f0 f1 ACK 1 ACK 2 ACK 4 f1 Time out f4 f3 ACK 5 discarded 012345670 012345678 f2 discarded f0 012345678 012345670 f1 f2 012345678 discarded 012345670 f4 012345678 Time out Premature Time out 012345670 … … 012345678 ACK 3

6. Stop-and-Wait ARQ Example Assume that computer A sends 3 frames to computer B. The three data frames sent by A are denoted as A1, A2, A3. Suppose each data frame needs 1 ms to transmit and each ack frame needs no time to transmit. The time-out value is set to be 4ms. The round trip time is always 2 ms (i.g., each direction takes 1ms). Use “A1, 0” denote a frame with data A1 and sequence 0. Use “Ack 1” denote an ack frame with ack number 1.

7. Case 1: Ideal channel without any error/lost or congestion AB A1A2A3

8. Case 2: second frame (A2) from A to B is lost AB A1A2A3

9. Case 3: second frame (A2) from A to B uses 3 ms AB A1A2A3

10. Case 4: second ack frame from B to A uses 3ms AB A1A2A3

11. Duplex Communication with Stop-and-Wait ARQ Assume that computer A and computer B communicate with each other. Each has 3 frames to send to the other. The three data frames sent by A are denoted as A1, A2, A3 and the frames from B are B1, B2, B3. They use duplex Stop-and-Wait ARQ for the communication, i.e., each data frame has a sequence # field and an Ack # field. Suppose each data frame needs 1 ms to transmit and each ack-only frame needs no time to transmit. The time-out value is set to be 5ms. The round trip time is always 2 ms. Use “A1, (0,1)” denote a frame with data A1 and sequence 0, ack 1.

12. Ideal channel without any error/lost or congestion AB A1A2 B1 A3 B2B3