Georgi’s Blog

Computer Networks - Chapter 1 Q&A

Here are my answers to the first chapter of Computer Networks by Andrew Tanenbaum 4th edition with ISBN 0130661023.

Problem №1:
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?

Answer:
Bernie carries 3*7 = 21GB’s total. The idea is to calculate how long it will take 21GB data to be transfered over 150Mbps line. Here we go:
21GB = 21*8*1024Mb = 172032Mb Divide by 150 to get the needed time to transfer the data and we get approx. 1147 seconds needed. So we divide by 60 to get number of minutes and that is approx. 19 minutes. So about 19 minutes are needed (theoretically) to transfer 21GB data over 150Mbps data line. With 18km/s Bernie will travel 5.7 kilometers. So Bernie will have higher data rate for distance smaller than 5.7 kilometers (in one direction).

Problem №2:
An alternative to a LAN is simply a big timesharing system with terminals for all users. Give two advantages of a client-server system using a LAN.

Answer:
First advantage is that in client-server system using a LAN the clients can be far apart from the server where in a big timesharing system terminals can be in the range of the same building.
Second advantage is the client is not limited to using the big timesharing system. It can use different servers (eg. the VPN server for remote workigna and the WWW server for web).

Problem №3:
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.

Answer:
High bandwidth and high latency networks are the WAN networks.(Like satelittte)(Altough newer WAN networks are not that high latency)
low bandwith and low lattency networks would be the older lan netwoks(as all new networks are with prety high bandwith).

Problem №4:
Besides bandwidth and latency, what other parameter is needed to give a good characterization of the quality of service offered by a network used for digitized voice traffic?

Answer:
What is the type of the service. Byte streams or message sequences. With the second there could be possible delays caused by acknowledgements.

Problem №5:
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 10 μsec, 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.

Answer:
A packet will need about 15msec to go from California to New York(about 2900 miles). If switching time for the entire path is 10μsec that is 1/1500 delay, which is nothing.

Problem №6:
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?

Answer:
A satellite at a height of 40,000km is considered GEO satellite. The speed of the signal from and to the sitellite travels at the speed of light (approx 300,000km/s) and in the best case there will be about 133ms delay, but in reality GEO satellites have between 250-300ms delay.
tching time for the entire path is 10μsec that is 1/1500 delay, which is nothing.

Problem №7:
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

Answer:
I’m not going into politics…

Problem №8:
A collection of five routers is to be connected in a point-to-point subnet. Between each pair of routers, he 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?

Answer:
Each router will need to generate and inspect data for two technologies so that is 10 lines total which is 1000ms second (aka 1 second).

Problem №9:
A group of 2^n - 1 routers are interconnected in a centralized binary tree, with a router at each tree node. Router i communicates with router j by sending a message to the root of the tree. The root then sends the message back down to j. Derive an approximate expression for the mean number of hops per message for large n, assuming that all router pairs are equally likely.

Answer:
The answer is too long, it envolves sums… Here is the final part:
the mean number of hops is 2n-4
…
There are too many questions in this chapter and the last one got me tired so I’m moving to the next one!