（10）[Computer Networks]Problem Solutions for Chapter I（计算机网络第四版练习答案）

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Computer Networks Fourth Edition
Andrew S. Tanenbaum
计算机网络（第四版）课后练习#作业
Date：2009-3-9
Chapter 1：Introduction
Date：2009-3-9
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 18km/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?
Answer:
Bernie can carry 21gigabytes, means 21*8=168gigabits;
The speed 18km/hour means 18*1000/3600=0.005km/sec or 5m/sec;
Set up the max distance is km, The time of Bernie to travel is :
So the data rate is :
Compare the two data rate and , We find out:
For ，Bernie has a higher data rate than a transmission line with 150Mbps data rate.


2 Q: An alternative to LAN is simply a big timesharing system with terminals for all users. Give two advantages of a client-server system using a LAN.
Answer: LAN has many advantages compared to the “Big timesharing system.
2.1 LAN is cheaper
The C/S system based-on LAN just need a special server, all other host or users can become a Client. Don’t need the Mainframe like Big timesharing system.
2.2 Good expansibility
If some host need to add into the system, just need a network interface, or a node. C/S system based-on LAN provides more computing power, better interactive.
2.3 Easy to construction and manage
The LAN model has Open standards for construction and management. All thing, like Hardware, Network Equipment, Protocols, Addresses are Global Unification.


3 Q: The performance of a Client-Server system is influenced by two network factors: the bandwidth of the network (how many bits/sec it can transport) and the latency (how many seconds it takes for the first bit to get from the client to the server). Give an example of a network that exhibits high bandwidth and high latency. Then give an example of one with low bandwidth and low latency.
A: There many example about this.
3.1 Connect to a Web server like Google.com through a Gbps link such as a fiber link.
It has bandwidth (or data rate) of gigabits/sec, but has a high latency, so we need lots of Repeater (may be transporter, Switch, Router and others) in the network.
3.2 The telephone network has low bandwidth and low latency.
The Voice-oriented network such as the Telephone network has the low bandwidth, but it has the low latency also. So when we use a 56(kbps) modem or a ADSL Modem calling a Server in the near Telephone Exchange Center. We have low latency but low bandwidth.


4 Q: Besides bandwidth and latency, what other parameter is needed to give a good characterization of the quality of service offered by a network use for digitized voice traffic?
A: There are many parameters to describe the QoS of voice traffic. Such as :
4.3 Error ratio
The probability of the data packets made mistakes, loss, or retransmission.
4.4 Connection set up delay
The time interval from “requesting set up a connection” to “the connection setup was confirmed”.
4.5 Frames (or packets) loss ratio
The rate of loss frames (or packets) when transmission.


5 Q: A factor in the delay of a store-and-forward packet-switching system is how long it takes to store and forward a packet through a switch. If switching time is 10µsec, is this likely to be a major factor in the response of a client-server system where the client is in New York and the server is in California? Assume the propagation speed in copper and fiber to be 2/3 the speed of light in vacuum.
A: I think it is NOT.
The distance from New York to California is about 4 500km=4 500 000m.
The propagation speed in copper and fiber is:
If it is a straight in fiber of copper link, it needs: or 22500µsec.
Assume there are 100 switches in the travel path, per switch has 10µsec delay, there are 1000µsec total.
1000µsec delay in switches and 22500µsec in fiber or copper line, it just 4%.
So, switches delay is NOT a major factor under these circumstances.


6 Q: A client-server system uses a satellite network, with the satellite at a height of 40,000km. What is the best-case delay in response to a request?
A: The process was shown in Figure 1.
Figure 1 The process of C/S model through the satellite
The Request need go up to the satellite and down to another Earth Station, and the Response too. The total path length is meters.
The speed of light in air or vacuum is , and the signals too.
So, the propagation delay is: .


7 Q: In the future, when everyone has a home terminal connected to a computer network, instant public referendums on important pending legislation will become possible. Ultimately, existing legislatures could be eliminated, to let the will of the people be expressed directly. The positive aspects of such a direct democracy are fairly obvious; discuss some of the negative aspects.
A: It is a good prospect. But there are some negative aspects. Such as:
7.1 The first issue is security.
If there are no legislatures, some people with ulterior motives can hack the system to falsifying the referendums results, or destroy the Voting system.
7.2 The issue of citizen’s quality.
Though a terminal connect at home, but some people can not the proper using the Voting System, or someone’s terminal have to maintenance before every important referendums. So need a lots of technology works to do it.
7.3 Radical point may be spread quickly.
Through the network, specially the Internet, some radical point (may be no adapt to the status quo) may be spread quickly, thus affecting the fair ruling.
7.4 And so on.
8 Q: A collection of five routers is to be connected in a point-to-point subnet. Between each pair of routers, the designers may put a high-speed line, a medium-speed line, a low-speed line, or no line. If it takes 100 ms of computer time to generate and inspect each topology, how long will it take to inspect all of them?
A: The topology of a point-to-point subnet with 5 routers was shown in Figure 2.
Figure 2 The topology of a point-to-point subnet with 5 routers
There are 5 routers named R1, R2, R3, R4, and R5; there are 10 potential lines: R1R2, R1R2, R1R4, R1R5, R2R3, R2R4, R3R5, R3R4, R3R5, and R4R5; each line has 4 possibilities: HS, MS, LS, and NL.
Thus, the total number of possibilities topologies is . Inspect each of them will take 100ms. So, it will take to inspect all of them.
9 Q: A group of routers are interconnected in a centralized binary tree, with a router at each tree node. Router communicates with router by sending a message to the root of the tree. The root then sends the message back down to . Derive an approximate expression for the mean numbers of hops per message for large , assuming that all routers pairs are equally likely.
A: The topology was shown in Figure 3.
Figure 3 The topology of a network with routers
We know:
If, , Ri=>R2, Rj=>R3, the path of a message from Ri to Rj: R2àR1àR3, there are 1 hop from R2 to root(R1), and 1 hop from root(R1) to R3.
If, , there are 2 hops from Ri to root(R1), and 2 hops from root(R1) to Rj.
……
If, , there are hops from Ri to root(R1), and hops from root(R1) to Rj.
By another hand:
If , there are 3 routers total, R2 can sends message to 2 routers, R1 , R3, and there is 1 router at level 2.
If , there are 7 routers total, Ri can sends message to 6 routers, and there are 3 routers at level 3.
……
If , there are routers total, Ri can sends message to routers, and there are routers at level n.
And, now, the path from root to level has routers, and the length of hops; from root to level has routers, and hops. And so on.
Thus, there is a expression, assume the path that from Ri to the root(R1) (or from the root(R1) to Rj) is .
It means:
When n tends to infinity: , and the path from Ri to Rj(each router pairs) is .
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: For each time slots of be used successfully, there are two possibilities:
First: there is a host is sending message and there are no other hosts attempt to access the channel, and then, have no collision. The probability of this happening can be expressed as:
, and there are n hosts, so the total probabilities is .
Second: there are no hosts attempt to access the channel, so has no collision. The probability of this happening can be expressed as:

The other time slots was wasted by collisions, its probability is: .
11 Q: What are two reasons for using layered protocols?
A: There many layered protocols stacks, for example OSI/RM model, TCP/IP model, and others. There are many reasons for using layered protocols.
11.1 Facilitate to study and research.
Divided the protocols into many layers, we can study and research them easily, learn every protocol’s detail.
11.2 Make the network easy to be built and managed.
The network equipment, like switch, router, the host, and so on were defined to every layer, for corresponding protocols, when we design, build and administrate a network, we just need to consider less layer or less protocols.
11.3 有利于异构互联(I do not know how to express this in English)
The layered protocols structures make Ethernet, ATM, X.25, ISDN, and other networks with different structures can be connected, to be a bigger network. The different address, different equipments, different hosts can connect into a network.
12 Q: The president of the Specialty Paint Corp. gets the idea to work with a local beer brewer to produce an invisible beer can (as anti-litter measure). The president tells her legal department to look into it, and they in turn ask engineering for help. As a result, the chief engineer calls his counterpart at the other company to discuss the technical aspects of the project. The engineers then report back to their respective legal departments, which then confer by telephone to arrange the legal aspects. Finally, the two corporate presidents discuss the financial side of the deal. Is this an example of a multilayer protocol in the sense of the OSI model?
A: The process be shown in Figure 4.
Figure 4 The process between 2 company
Through the figure, we can know the message is not exchanged in the same layer, the transmission is not by layer-to-layer, it is not like OSI/RM: the lower layer provide services for upper layer. And the physical communication just takes place in the Physical Layer, not in every layer.
13 Q: What is the principal different between connectionless communication and connection-oriented communication?
13.4 Like a telephone system, connection-oriented communication needs three steps:
1. Set up a connection: like you need to dial up a number when you use a telephone;
2. Transfer data: like you speaking to your friend on telephone;
3. Release connection: like you (or/and your friend) hang up the telephone;
13.5 Like the post system, connectionless communication just sends the data:
Connectionless communication does not have there steps like above, the sender just sends the data, and do not care with how the network transfer (routing) the message to Receiver.