Networks and Distributed Systems CS233/333

#### Homework 1, Due at the beginning of class, Tuesday, April 15th.

Read Chapter 2, Tanenbaum, Computer Networks, 4th edition.

If you don't have your book yet, a paper copy is available outside my office.

#### Excercises

1) Television channels are 6 MHz wide, how many bits/sec can be sent if four-level digital signals are used? Assume a noiseless channel.

2) What signal-to-noise ratio is needed to put a T1 carrier on a 50 kHz line?

3) Using the data in the text, what is the maximum number of telephone numbers possible in the US?

4) You are installing a 10 km fiber and you have a flux budget of 6 dB after subtracting connectors and splices, but not fiber attenuation. Using figure 2-6 in Tanenbaum, determine which wavelengths you can use.

5) Although it looks like the Teledesic system described in the book will not get built, it has gone through several revisions, from 840 satellites, to 300 and then to 30. Assuming a period of 90 minutes (different altitudes were actually discussed at different times) what is the average interval for hand off for a stationary transmitter. Why isn't Teledesic being built?

6) What is the percentage overhead on a T-1 line, i.e. what percentage of the 1.544 Mbits/sec is not delivered to the end user.

7) For a typical hexagonal mobile phone system with 840 frequencies, how many frequencies can be used in a given cell? (Remember frequencies are not reused in adjacent cells.)

8) An alternative to a LAN is a big time sharing system with many terminals. Give three advantages of a client/servcer LAN.

9) A modem constellation diagram (like Tanenbaum Fig 2-25) has points at (0,1), (0,2), (2,3) and (0, 4). What modulation is it using and how many bits of data?

10) You have obtained a surplus Air Force Global Hawk Unmanned Aerial Vehicle (UAV) and equipped it to carry a SuperDLT tape between your company offices. The Global Hawk flies at 400 mph and has a range of about 7,500 miles. Super DLT tapes hold 160 Gigabytes of data. Up to what distance is the global Hawk faster than a 45 Mbps SMDS connection? (Assume 8 bits/byte for this problem)

11) The main disadvantage of a broadcast network is that capacity is wasted due to collisions between multiple hosts trying to use the media at the same time. A simple model of this would be to assume discrete time slots with n hosts all attempting to use the same time slot with probability p. What fraction of the slots are wasted due to collisions? Do CDMA wireless networks sufffer the same problem?

12) Imagine a protocol hiearchy which is h deep and which has messages data of M bytes. If the header size at each layer is o, what percentage of the network bandwidth is dedicated to overhead?