CSMA/CD

Overview

Carrier Sense Multiple Access networks use contention based TDM to multiplex nodes on a network. Specifically, Ethernet is a form of CSMA that uses collision detection and a bus topology to provide a very effective and inexpensive local area network alternative. In order to maintain compatibility there have been two major additions to the standards in recent years.

Objectives

• Understand the basics of the IEEE 802.3 Ethernet standard
• Understand contention TDM using CSMA/CD.
• Be familiar with the topologies of Ethernet and Fast Ethernet.

Preparation

Study sections 2.6 in the book and the notes.

Knowledge, Problem Solving & Comprehension

Terms

2-tau time period
Binary exponential backoff
Carrier Sense Multiple Access
Collision detection
Ethernet
Ethernet address (MAC address)
IEEE 802.3
IEEE 802.3u
Hub
Network Contention
Repeater
Switch
Thick LAN/Thin LAN
Transceiver

Questions

1. Show why Ethernet packets have to be at least 512 bits long.
2. Explain why IEEE 802.3 Ethernet needs a repeater every 500 meters?
3. What is the length of time represented by 2-tau?
4. How does a node on an Ethernet know that its message has been involved in a collision?
5. What is the difference between a switch and a hub?

Analysis, Synthesis & Evaluation

Suppose the maximum length of an Ethernet cable is changed to 5000 meters: what is the new minimum frame length? If the bit rate is also changed to 25 Mbps, what is the new minimum frame length. Assume that the propogation of EM is 2.0 x 10⁸ m/s.

Is starvation a problem with an Ethernet network?

What is the purpose of the Ethernet 64-bit preamble?

How does an IEEE 802.3 frame avoid having any obvious method for the receiver to determine the frame length?

An Ethernet network has N = 5 nodes and a probability of sending for each node of 0.20. Message length, L=1024 bits. What are P(0), P(1) and P(>1)?

What is the expected utilization if the collision cost is 1.1 slots.

What is the expected number of failures an Ethernet node will experience if there are 20 nodes and the probability of any one node sending is 0.01? 6.05

Consider an Ethernet network with a sending probability for each station of 0.10, an average message length of 1024 bits and an average cost of a collision of 2.5 slot times. Calculate the utilization for N = 5, 10, 15, 20, 25 and 30. What does this indicate?