Polytechnic University, Dept. Electrical and Computer Engineering
EE3414 Multimedia Communication System I
Spring 2006, Yao Wang
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Homework 12 (Communications Network)
(Solution)
Reading Assignment:
1) A. Leon-Garcia and I. Widjaja, Communication networks, sections 1.1, 1.2.1, 1.3.
2) William Stallings, Data and Computer Communications, 7th edition, pages 298-303, 318-326.
Written Assignment:
1. Consider a channel with bandwidth W=1 MHz and SNR =20 dB, and we want to allocate this channel
among M=10 users.
a. What bit rate is available to each user if we divide the entire channel into M channels of equal
bandwidth.
b. What bit rate is available to each user if the entire frequency band is used as a single channel and
TDM (time division multiplexing) is applied?
c. How does the comparison of (a) and (b) change if the FDM (frequency division multiplexing)
scheme in (a) requires a guard band between adjacent channels? Assume the guard band is 10%
of the channel bandwidth.
Solution:
a. Each user uses W/M=0.1MHz bandwidth. Using Shannon’s Channel Capacity formula:
bit rate = W/M* log2(1+SNR)=0.1* log2(1+100)=0.66 Mbps
b. In this case, the total bit rate afforded by the W hz is divided equally among all users:
bit rate = (W* log2(1+SNR))/M=above
c. Because of the guard band we expect that the scheme in (b) will be better since the bit rate in (a) will be
reduced. In (a), the bandwidth usable by each channel is 0.9W/M. Thus we have:
bit rate = 0.9*W/M* log2(1+SNR)=0.59 Mbps.
2. How does a broadcast communication network work? Give an example of the service provided
by broadcast communication networks.
Solution:
- Broadcast Communication Network does not have intermediate switching nodes
- Each station has a transmitter/receiver that communicates over a medium shared by other stations
- Transmission from any station is received by all other stations
- Example: radio and television broadcasting
3. Consider a network consisting of N nodes. Each node is connected to all the other N-1 nodes by
a direct link. How many links are there in the network? Explain why a typical network with a
large number of nodes is not built this way.
Solution:
N!
 N ( N  1) / 2 , i.e., the number of randomly
2!( N  2)!
choose two from N. If a network has many hosts, a huge number of links are required to have them
fully connected. This is impractical.
The number of links would be C N2 
4. Compare the circuit switching, datagram packet switching, and virtual circuit switching.
Solution:
Please see slide 38 for details.
5. What type of network the telephone system is (in terms of switching and connection type)?
Solution: The telephone system uses connection-oriented circuit switching.
6. Describe the processes involved inside the network in making a telephone call.
Solution:
Please see slide 29 for details.
7. Why are standards needed in communication networks?
Solution:
Standards are agreements, with industry-wide, national and possibly international scope, that allow
equipment manufactured by different vendors to be interoperable. Standards enable smaller companies
to enter large markets, and allow network operators to choose equipment from multiple suppliers.