Jayhawk

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:

a) ALOHA

b) Slotted ALOHA

c) p- persistent CSMA

d) CSMA-CD

e) DOCSIS

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.


Author

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)