There are two hosts A and B in a packet-switched network, interconnected by three routers, as illustrated in the figure below. Each link is annotated with link capacity (above) and propagation delay (below). Each router follows the store-and-forward model to process packets, and has a queue that can hold 5 packets for each direction. If the queue is full and a new packet arrives at the router, the new packet is dropped. (Note: Once a packet starts being transmitted, it will be removed from the queue. Thus, there may be 6 packets in the router at any given time point). Suppose the network is empty and all the routers are idle.

a. If Host A sends a packet of length 1500 bytes to Host B, how long does it take for this packet to arrive at Host B?

b. If Host A sends three 1500 byte packets to Host B, one immediately af- ter the other. How long does it take for the last packet to reach Host B?

c. If Host A sends 20 back-to-back 1500 byte packets to Host B, which packets

will be dropped during the transmission? Assume the packets are numbered from 1 to

20 in order.

d. Calculate the worst-case end-to-end delay for (Hint: not all the queues will be full):

(a) For Host As 1500 bytes packet to reach Host B

(b) For Host Bs 1500 bytes packet to reach Host A

1Mbps 500Kbps 1Mbps 2Mbps B 2ms 20ms II 30ms 2ms