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 elays 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!
Question 2 (Web Cache, 18%). Consider the following scenario where two schools of one university are installing web caches for users. (You only need to review the contents discussed in lectures in weeks 1–3 to complete this question.)
Inside each school, the one-way propagation delay from each host to the school’s gateway is 2ms. The link bandwidth is assumed to be infinity (sufficient large). The link bandwidth from each school’s gateway to the university’s gateway is 5Mbps, and one-way propagation delay is 2.5ms. The access link bandwidth from the university’s gateway to the Internet is 10Mbps, and assume that the one-way propagation delay from the gateway to any server in the public Internet is 5ms.
On average, the requests from each school to view the webpage (of the public Internet) arrive at the rate of 500 requests per second and the webpage is 1000 bytes (which fits exactly one packet). Ignore the header size. The requests themselves are very small and we assume that they do not take any bandwidth.
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