# Exercises

```1. It is desired to send a sequence of computer screen images over an optical fiber. The screen is
480 x 640 pixels, each pixel being 24 bits. There are 60 screen images per second. Calculate
the Throughput needed.
2. Radio antennas often work best when the diameter of the antenna is equal to the wavelength
of the radio wave. Reasonable antennas range from 1 cm to 5 meters in diameter. What
frequency range does this cover?
3. How many end office codes were there pre-1984, when each end office was named by its
three-digit area code and the first three digits of the local number? Area codes started with a
digit in the range 2?, had a 0 or 1 as the second digit, and ended with any digit. The first two
digits of a local number were always in the range 2?. The third digit could be any digit.
4. Three packet-switching networks each contain n nodes. The first network has a star topology
with a central switch, the second is a (bidirectional) ring, and the third is fully interconnected,
with a wire from every node to every other node. What are the best-, average-, and-worst case
transmission paths in hops?
5. An 8-bit byte with binary value 10101111 is to be encoded using an even-parity Hamming
code. What is the binary value after encoding?
6. A group of N stations share a 56-kbps pure ALOHA channel. Each station outputs a 1000-bit
frame on an average of once every 100 sec, even if the previous one has not yet been sent (e.g.,
the stations can buffer outgoing frames). What is the maximum value of N?
7. Consider the delay of pure ALOHA versus slotted ALOHA at low load. Which one is less?
8. Compare the ways of single channel allocation, such as Frequency Division Multiplexing
(FDM) and Transmitting at the full capacity as in datagram network. Which is better? And
what kinds of situations they fit for respectively?
9. Briefly describe the difference between switches and hubs.
10. Give two example computer applications for which connection-oriented service is appropriate.
Now give two examples for which connectionless service is best.
11. Datagram subnets route each packet as a separate unit, independent of all others.
Virtual-circuit subnets do not have to do this, since each data packet follows a predetermined
route. Does this observation mean that virtual-circuit subnets do not need the capability to
route isolated packets from an arbitrary source to an arbitrary destination? Explain your
12. What are the differences between virtual circuit network and datagram network?
13. For hierarchical routing with 4800 routers, what region and cluster sizes should be chosen to
minimize the size of the routing table for a three-layer hierarchy? A good starting place is the
hypothesis that a solution with k clusters of k regions of k routers is close to optimal, which
means that k is about the cube root of 4800 (around 16). Use trial and error to check out
combinations where all three parameters are in the general vicinity of 16.
14. Use Short Path Routing Algorithm to compute the shortest path from A to E in the network
topology illustrated in following figure, and give the chosen path.
B
3
C
1
A
3
2
1
3
D
4
E
15. Consider the subnet in the following figure. Distance vector routing is used, and the following
vectors have just come in to router C: from B: (5, 0, 8, 12, 6, 2); from D: (16, 12, 6, 0, 9, 10);
and from E: (7, 6, 3, 9, 0, 4). The measured delays to B, D, and E, are 6, 3, and 5, respectively.
What is C's new routing table? Give both the outgoing line to use and the expected delay.
```