Question

(a) Assume a Sliding Window with Go-Back N ARQ is being used on a transmission link with a 3-bit sequence number. The transmitter has previously sent frames with sequence numbers 0, 1 and 2. Is the transmitter allowed to send a frame with sequence number 3 if the frame with sequence number 2 has not been acknowledged? Explain your answer.

(b) Assume a Sliding Window with Go-Back N ARQ is being used on a transmission link with a 3-bit sequence number. 1. If the transmitter sends frames 0, 1, 2, and 3, how many more frames can be sent without an acknowledgment returned by the receiver? 2. If the receiver sends back ACK 2, how many more frames can be sent? What are their frame numbers?

(c) Assume a Sliding Window with Selective-Reject ARQ is being used on a transmission link with a 3-bit sequence number. Host A sends frames 0, 1, and 2 to Host B and frame 0 is lost. 1. How many more frames can be received by Host B? 2. What would be the frame number in the Select Rej frame sent back by Host B?

Answer 1

Go-Back N ARQ (Automatic Repeat Request) • Go-Back N ARQ (Automatic Repeat Request) protocol uses the sliding window process. In this, the transmitter sends N number of frames to the receiver at a time. Here, N is the size of the sliding window. Each frame as a sequence number. The number of sequence numbers depends on the size of the sliding window. If the sequence number of the frames is an n-bit field, then the range of sequence numbers is 0 to 2n–1. Then, the size of the sending window is 2n. • • Here, the sequence number is a 3-bit field. Then, the range of the sequence number is 0 to 23–1=7. Then, the size of the sending window is 2n= 23 = 8. Therefore, there are 8 frames to send at a time. The sequence numbers of these 8 frames are 0, 1, 2, 3, 4, 5, 6, and 7. (a) Yes. The transmitter sends the 8 frames with sequence numbers from 0 to 7 to the receiver and waits for the acknowledgment from the receiver. So, after sending the frames 0, 1, and 2, the transmitter sends the remaining frames 3, 4, 5, 6, and 7.

If the frame with sequence number 2 is not acknowledged, then the frames 2, 3, 4, 6, and 7 will be resent again to the receiver. (b) The transmitter sends the 8 frames with sequence numbers from 0 to 7 to the receiver and waits for the acknowledgment from the receiver. If the transmitter sends frames 0, 1, 2, and 3, then the number of frames needss to send is 4 frames. The total frames are 8 frames. Out of 8, four frames (0, 1, 2, and 3) has been sent. Then, the remaining frames 8-4 = 4 frames need to send to the receiver. Therefore, 4 more frames can be sent without an acknowledgment returned by the receiver. If the receiver sends back ACK 2, then the sender must send the frames 2, 3, 4, 5, 6, and 7. Therefore, 6 more frames need to send to the receiver. The frame numbers are 2, 3, 4, 5, 6, and 7.

In Selective-Reject ARQ, only the packet which is lost need to be retransmitted. Here, the sequence number is a 3-bit field. Then, the range of the sequence number is 0 to 23-1=7. The sequence numbers of these 8 frames are 0, 1, 2, 3, 4, 5, 6, and 7. The window size must be less than or equal to the half of the sequence number Then, the size of the sending window is 24/2= 23 / 2=4. Therefore, there are 4 frames to send at a time. The sequence numbers of these 4 frames are 0, 1, 2, and 3. 1. If Host A sends frames 0, 1, and 2 to Host B and frame 0 is lost, then frame 0 need to be retransmitted along with frames 3, 4, 5, 6, and 7. Therefore, 6 more frames can be received by Host B. As the frame 0 is lost, the frame number in the Select Rej frame sent back by Host B is 0 which can be represented as RRO.