Computer Network Homework 1

Computer Network

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< Data and computer communication >
chapter 2 problems 2, 3, 4

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2.2(a)

Diagram

China		France
Chinese prime minister		French prime ministers
Chinese translator		French translator
Chinese telephone	<--Transfer-->	French telephone

Describe

Prime minister <--> Translator

• Prime minister speak to his translator in their own
languege.
• The translator tells prime minister the words he translated to their own language.

Translator <--> Telephone

• Speak to telephone in English about prime minister's words.
• Listen to telephone about the words in English from the other.

Between 2 telephones

• transfer the data.

2.2(b)

Diagram

China		France
Chinese prime minister		French prime ministers
Chinese translator		French translator
Chinese telephone	Japanese <--> German	French telephone

Describe

• The airchitecture doesn't change.However, some layers' work need to adjust.

Chinese translator

• Translate prime minister's words to Japanese.
• Translate Japanese to Chinese.

German and Japanese translator

• Translate German to Japanese.
• Translate Japanese to German.

French translator

• Translate prime minister's words to German.
• Translate German to French.

2.3

The major disadvantages with the layered approach to protocols.

1. At each layer of a protocol architecture, one or more common protocols are implemented in communicating systems. Each protocol provides a set of rules for the exchange of data between systems.It makes a simple network task consume great energy.

2. Each layer of a protocol architecture just can communicate with adjacent layer. It's not flexible enough.

3. Only physical layer can communicate directly.

2.4

A:

It is a nice example for Transmission Control Protocol.There are 3 steps to solve this problem.
1. The commander A tell the commander B the battle plan.(SYN)
2. The commander B tell The commander B the bettle pan have been acknowledged.(SYN-ACK)
3. The commander A tell the commander B the acknowledgement have been received.(ACK)

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< Computer Network >
chapter 1 problems 1, 6, 10, 20, 27, 28.

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1

Q:

Imagine that you have trained your St. Bernard, Bernie, to carry a box of three 8mm tapes instead of a flask of brandy. (When your disk fills up, you consider that an emergency.) These tapes each contain 7 gigabytes. The dog can travel to your side, wherever you may be, at 18 km/hour. For what range of distances does Bernie have a higher data rate than a transmission line whose data rate (excluding overhead) is 150 Mbps?

A:

$$total = 3 * 7GB = 3 * 7 * 1024 * 8 Mbit = 172032 Mbit$$
$$18 km/h = 5 m/s$$
$$time = \frac{distance}{5} = 0.2 * distance$$
$$total = time * 150$$
$$time = \frac{total}{150} = 0.2 * distance$$
$$distance = 5734.4 m$$
So, if distance less than 5734.4m , Bernie have a higher data rate than a transmission line whose data rate (excluding overhead) is 150 Mbps.

6

Q:

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

A:

$$\frac{40000 km * 4} {300000 km/s} = 0.533333333s$$

10

Q:

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

A:

$$P = 1 - np(1-p)^{n-1}- (1-p)^n$$

20

Q:

A system has an n-layer protocol hierarchy. Applications generate messages of length M bytes. At each
of the layers, an h-byte header is added. What fraction of the network bandwidth is filled with headers?

A:

$$P=\frac{nh}{nh + M}$$

27

Q:

How long was a bit on the original 802.3 standard in meters? Use a transmission speed of 10 Mbps and
assume the propagation speed in coax is 2/3 the speed of light in vacuum.

A:

$$Length = \frac{2*10^8m/s}{10*10^6bit/s} = 20 m/bit$$

28

Q:

An image is 1024 x 768 pixels with 3 bytes/pixel. Assume the image is uncompressed. How long does it take to transmit it over a 56-kbps modem channel? Over a 1-Mbps cable modem? Over a 10-Mbps Ethernet? Over 100-Mbps Ethernet?

A:

1. $$time =\frac{1024*768*3*8bit}{56000bit/s}=337.04s$$
2. $$time =\frac{1024*768*3*8bit}{10^6bit/s}=18.87$$
3. $$time =\frac{1024*768*3*8bit}{10^7bit/s}=1.89$$
4. $$time =\frac{1024*768*3*8bit}{10^8bit/s}=0.19$$