Computer Network Homework 1

《Computer Networks》：chapter 1 problems 1, 6, 10, 20, 27, 28.

 

1.    Imagine that you have trained your St. Bernard, Bernie, to carry a box ofthree 8-mm tapes instead of a flask of brandy. (When your disk fills up, youconsider that an emergency.) These tapes each contain 7 gigabytes. The dog cantravel to your side, wherever you may be, at 18 km/hour. For what range ofdistances does Bernie have a higher data rate than a transmission line whosedata rate (excluding overhead) is 150 Mbps? How does your answer change if (i)Bernie’s speed is doubled; (ii) each tape capacity is doubled; (iii) the datarate of the transmission line is doubled.

Answer:

(1)  3*7gigabytes = 21gigabytes = 168 gigabits

18km/hour = 0.005m/s

x/0.005 = 200x

So: 168gigabits/200x > 150/1024 Gbps

x > 5.73 (or x > 5.6 if you suggest that 150Mbps =150/1000 Gbps)

(2)  (i)  168gigabits/100x >150/1024 x > 11.46

       (ii)  2*168gigabits/200x>150/1024x > 11.46

       (iii) 168gigaits/200x > 300/1024 x > 2.87

 

2. A client-server system uses a satellite network,with the satellite at a height of 40,000 km. What is the best-case delay inresponse to a request?

Answer: Total Distance:40,000km * 4 = 160,000km

            C = 300,000km/s Sov = 533msec

3.A disadvantage of a broadcast subnet isthe capacity wasted when multiple hosts attempt to access the channel at thesame time. As a simplistic example, suppose that time is divided into discreteslots, with each of the n hosts attempting to use the channel with probability pduring each slot. What fraction of the slots are wasted due to collisions?

 

Answer: Every channel has three statuses.Free, used but not congestion, congestion. The probability of free is (1-p)^n,and the probability of used but not congestion equals np(1-p)^(n-1). So theanswer is 1-np(1-p)^(n-1)-(1-p)^n.

 

4.A system has an n-layer protocolhierarchy. Applications generate messages of length M bytes. At each 
of thelayers, an h-byte header is added. What fraction of the network bandwidth isfilled with headers?

Answer: According to “each of the layers,an h-byte header is added”. We can draw the conclusion that the answer is h*n/(M+h*n).

 

5.How long was a bit on the original802.3 standard in meters? Use a transmission speed of 10 Mbps and 
assume thepropagation speed in coax is 2/3 the speed of light in vacuum.

Answer: 200m/μsec  In 10Mbps, it will cost 0.1μsec fornetwork to transmit a single bit. C*2/3 = 2*10^8m/s

So the answer is 2*10^8*0.1μsec=20m

 

6.An image is 1024 x 768 pixels with 3bytes/pixel. Assume the image is uncompressed. How long does it 
take totransmit it over a 56-kbps modem channel? Over a 1-Mbps cable modem? Over a10-Mbps Ethernet? Over 100-Mbps Ethernet?

Answer: The size of image: 1024*768*3 =18874368bits

            For 56-kbps: 18874368 / 56000 = 337.042s

            For1M-kbps: 18874368 / 1000000 = 18.874s

            For 10M-kbps: 18874368 / 10000000= 1.8874s

            For 100M-kbps: 18874368 / 100000000 = 0.189s

 

《Data and computer communication》：chapter 2problems  2, 3, 4

2.2 a.

The Frenchand Chinese prime ministers need to come to an agreement by telephone, butneither speaks the other’s language. Further, neither has on hand a translatorthat can translate to the language of the other. However, both prime ministershave English translators on their staffs. Draw a diagram similar to Figure 2.12to depict the situation, and describe the interaction and each level.

b. Now supposethat the Chinese prime minister’s translator can translate only into Japaneseand that the French prime minister has a German translator available. Atranslator between German and Japanese is available in Germany. Draw a newdiagram that reflects this arrangement and describe the hypothetical phone con-versation.

2.3  List the major disadvantages with thelayered approach to protocols.

Answer: Layered protocols willcause delay or some ranges of distortion in the messages which will be transmitted.

2.4  Two blue armies are each poised onopposite hills preparing to attack a single red army in the valley. The redarmy can defeat either of the blue armies separately but will fail to defeatboth blue armies if they attack simultaneously. The blue armies com- municatevia an unreliable communications system (a foot soldier). The commander withone of the blue armies would like to attack at noon. His problem is this: If hesends a message to the other blue army, ordering the attack, he cannot be sureit will get through. He could ask for acknowledgment, but that might not getthrough. Is 
there a protocol that the two blue armies can use to avoid defeat?

Answer: No. Blue armies willnever ensure whether the information has been transmitted safely.