University of Kansas
Electrical Engineering & Computer Science
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, firstname.lastname@example.org and some problems modified from Communication Networks: Fundamentals Concepts and
Key Architectures, A. Leon-Garcia and I. Widjaja (Second edition)