Question 1 (Delay, 18%).

As shown in the figure below, a file of size F = 1000 + S bytes is transmitted on an end-to-end connection over four links, where S is the last three digits of your student number. For example, if your student number is 490123456, then S = 456 and F = 1456 bytes.

Each link is 100 km. The signal prorogation speed is 2 × 108 m/s. Assume that a header of 40 bytes is added to each packet. The bandwidth of all links is R = 1 Mbps at the beginning. The nodes use the store-and-forward scheme. (Ignore processing delays at each node.)

(0) What is your student number? Warning: If you use another student’s number as S value to answer the question, the following sub-questions will not be marked and you will get 0 in Question 1.

(1) How long does it take to transmit the file if the whole file is transmitted as a single packet. Now assume that the bandwidth of link B − C and D − E become 0.5 Mbps. Answer (2)–(4).

(2) Repeat (1).

(3) We would like to break the file into smaller packets to decrease the overall delay in the store-and-forward scheme. Assume that each time you break the file to make a new packet, you have to add 40 bytes as the header of the new packet. Repeat (2) when we break the file into N = 4 packets.

(4) What should be the optimal size of the packets to have the minimum overall delay to deliver the whole file? Find the overall delay. Hint: Since the link B − C has a smaller bandwidth compared with A − B, packets could be queued for some time!