Assume that a file is transferred from a node A to a node B 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

Simplest (ideal channel, no error) B f0 f1 f2 f3 f4

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

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

Go-Back-N ARQ A B 1 2 3 4 5 6 7 f0 1 2 3 4 5 6 7 8 f1 Time out f2 1 2 3 4 5 6 7 f0 1 2 3 4 5 6 7 8 f1 Time out f2 discarded f0 f1 discarded Time out ACK 1 1 2 3 4 5 6 7 8 f2 1 2 3 4 5 6 7 ACK 2 1 2 3 4 5 6 7 8 ACK 3 f1 f2 discarded f3 discarded ACK 4 1 2 3 4 5 6 7 8 … 1 2 3 4 5 6 7 f4 ACK 5 1 2 3 4 5 6 7 8 Premature Time out f4 … 1 2 3 4 5 6 7 discarded ACK 5

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.

Case 1: Ideal channel without any error/lost or congestion B A1 A2 A3

Case 2: second frame (A2) from A to B is lost

Case 3: second frame (A2) from A to B uses 3 ms

Case 4: second ack frame from B to A uses 3ms

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.

Ideal channel without any error/lost or congestion B B1 B2 B3 A1 A2 A3