# Assignment 1

```‫ וחדשנות‬,‫ תפעול‬,‫ ארכיטקטורה‬-‫רשתות תקשורת‬
Computer Networks by A.S. Tanenbaum
Exercise 1
Due Date: December 4th
Questions 1-5 are worth 20 points. Question 6 is optional and is worth 40 points….
1) A suitcase that measures 1 meter by 50 cm by 20 cm is full with disk on keys. Each disk on key
holds 4GB and measures 5cmx1cmx1cm. The suitcase is put in a car that travels 200 km at 100
km/hr. What is the effective bandwidth (in MB per second) that is being provided?
2) A company wishes to back up its data to a remote site that is 100 km away. Each night they have
to back up 1000 GB of data and it must be done within three hours (from 1am to 4am). The local
Telco provides guaranteed bandwidth of speeds that are multiples of 10MB per second for \$100
per month (i.e. 50MB/sec will cost \$500 per month). What will the cost be to back up this data
per month? Can you think of an alternative solution (be creative) that will cost just a few dollars
a day? Hint, the data does not need to arrive at the remote site within 3 hours; it just needs to
be out of the main site within 3 hours.
3) A regional Telco has 10 million subscribers. Each telephone is connected to the central office by
a copper twisted pair. The average length of the twisted pair is 10km. The diameter of the
strand is 1mm, the specific gravity is 9.0 grams/cm3, and copper sells for \$10 per kilogram. How
much is the copper in the local loops worth?
4) You are the lead designer for a Telco and need to make a rough estimate of the number of
circular cells that are 100m in diameter would be needed to cover San Francisco (120 sq km).
a. How many cells are needed?
b. If each base station cost \$10,000 and the average distance from each base station to the
core network was 10 km and the cost of the backhaul was \$100 per km, how much
would you need to invest in creating the network?
5) 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?
6) A group of 2**n – 1 routers are interconnected in a centralized binary tree with a router at each
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 router j. Derive an approximate equation for the
mean number of hops per message for a large n assuming that all router pairs are equally likely.
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