1Quiz 1

Communication Networks (EE 706), Spring14

Feb. 6, 2014; Total: 10 marks; Time: 55 minutesNote:

You are allowed to use one A4 sheet with handwritten notes on both sides.

QUESTION 1 (1 + 1 = 2 MARKS)

For both parts, assume that data packets or ACKs may be delayed by arbitrary amounts,

corrupted or lost, but not re-ordered.

(a) Give an example in which a Selective Repeat system that uses window value N and M =

2N 1 distinct sequence numbers operates incorrectly.(b) Consider the Go-Back-N protocol with the change that the receiver does not send an ACK

packet for data packets with sequence numbers other than the expected sequence number.

Give an example of incorrect operation.

QUESTION 2 (1.5 + 1.5 + 1.5 = 4.5 MARKS)

(a) Consider two end systems a sender and a receiver connected via three intermediate routers.

Each router uses store-and-forward routing. The rate of each of the four links on the sender-

to-receiver path is 100 Mbps, and a packet size of 12000 bits is used. Each link has a

propagation delay of 10 s. Queuing delays and processing times are negligible at all nodes.

Find the end-to-end delay of a single packet.

(b) Find the average throughput for the scenario in part (a) if a large file is sent from the sender

to the receiver as a sequence of packets of 12000 bits each using the stop-and-wait protocol.

Assume that the transmission times of ACK packets are negligible and that packets or ACKs

are not corrupted or lost. Also, ignore header overheads.

(c) Redo part (a) with the change that each router uses cut-through routing: it begins transmitting

the packet on the outgoing link after the first 200 bits have been received on the incoming

link.

2QUESTION 3 (1.5 + 2 = 3.5 MARKS)

(a) Consider a stop-and-wait strategy in which, rather than using a sequence number for each

packet, the sender sends the number of times the given packet has been sent previously.

Thus, apart from data, a data packet contains a checksum and a number j, which is equal to

0 the first time a packet is transmitted, 1 on the first retransmission and so on. The receiver

returns an ACK if a packet is received without errors and a NAK if it is corrupted, without

including j or any sequence number in the feedback packet. Show by example that this

strategy does not work correctly no matter what rule the receiver uses for accepting (for

delivering to the layer above) packets. Assume that data packets or ACKs may be delayed

by arbitrary amounts, corrupted or lost, but not re-ordered.

(b) Redo part (a) with the following two changes: (i) the number j above is 0 on the initial

transmission of a packet and 1 on all subsequent retransmissions of the packet, and (ii)

data packets or ACKs are not lost, their delays are fixed and known, but may be corrupted.

Also, since the sender knows the round-trip time, it never times out and simply waits until

a feedback packet is received (which may be a correct ACK or NAK or a corrupt packet).