University of Kansas

Electrical Engineering & Computer Science

EECS 563 - Homework Assignment

Homework 9

1. As the bit rate of a random access network increases Smax decreases; explain why.

2. As the number of users increases on a token based network Smax decreases; explain why.

3. A Slotted ALOHA protocol is used to share a 1 Mb/s radio channel. The system uses fixed length frames which are 1000 bits long.

a) Find the maximum packet arrival rate in packets/sec?
b) Would you operate the network using the rate found in part a)? Justify your answer

4. Timers are used in some token ring networks to provide priority access.
When a token arrives at a station, a timer is reset to 0. The stations then starts to transmit priority 4 frames until the timer reaches T4. Then the station is allowed to transmit priority 3 frames until the timer reaches T3. Following the transmission of priority 3 frames the station transmits priority 2 frames and then priority 1 frames using the same algorithm. What is the fraction of the total capacity reserved for each priority class if all stations have timer values of T4=30ms, T3=70ms, T2= 90ms, and T1=100 ms.

5. With all other factors fixed, does the efficiency of CSMA-CD increase as the network size (in km) increases? Explain. See Maximum Normalized Throughput for CSMA-CD Networks

6. Suppose that a LAN is used to interconnect a set of formation flying UAVs. Discuss the appropriateness of using a token ring, ALOHA network and CSMA-CD network for this application.

7. How are collision detected for the following protocols. Relative to a packet transmission time what is time scale that collisions are detected for the following random access protocols:


b) Slotted ALOHA

c) p- persistent CSMA



8. Use the result for Maximum Throughput for CSMA-CD given in Maximum Normalized Throughput for CSMA-CD Networks for this problem. Assume nodes are connected by coax cable. The propagation speed in coax is 0.88*(3X10*8) m/s.

a) Find the range of date rates which will result in a maximum throughput of greater that 0.5 for a packet size of 5000 bits and a distance of 250 m.

b) How much does the maximum throughput change for rate of 200 Mb/s as the packet size changes from 1000 bits to 5000 bits for a distance of 250 m?

c) Find the range of packet lengths which will result in a maximum throughput of greater than 0.6 for a distance of 500m and a rate of 1.0 Gb/s.

d) Consider the result in part c), as the packet length increases the maximum throughput increases, then why not use a packet length of 100,000 bits?

9. Describe how RTS/CTS in IEEE 802.11 addresses the hidden terminal problem.

10. A reservation system has frame transmission time of 20 ms and uses a 4 ms minislot for reservations. What is the maximum throughput for the following cases:

a) One minislot used to make one frame reservation

b) One minislot used to make o10 frame reservation

c) Nodes contend for reservation minislots using Slotted ALOHA.


Victor S. Frost, frost@eecs.ku.edu and some problems modified from Communication Networks: Fundamentals Concepts and
Key Architectures, A. Leon-Garcia and I. Widjaja (Second edition)