Introduction to Networking - Carrier Sense Multiple Access - Collision Detection

TABLE of Contents

1.0 : Carrier Sense Multiple Access / Collision Detection

1.1 CSMA/CD mechanisms

1.2 CSMA/CD principle

2.0 : Slotted Ring

3.0 : Control token

3.1 Modes of operation

3.1.1 Listen Mode

3.1.2 Transmit Mode

3.1.3 By-pass Mode

4.0 : Network Topology

4.1 Ring Topology

4.2 Bus Topology

5.0 : Physical and Logical Topologies

5.1 The Physical Bus Topology

5.2 The Physical Ring Topology

6.0 : References

1.0 Carrier Sense Multiple Access / Collision Detection

CSMA/CD (carrier sense multiple access/collision detection) CD (collision detection) defines what happens when two devices sense a clear channel, then attempt to transmit at the same time. A collision occurs, and both devices stop transmission, wait for a random amount of time, and then retransmit. This is the technique used to access the 802.3 Ethernet network channel. This method handles collisions as they occur, but if the bus is constantly busy, collisions can occur so often that performance drops drastically. It is estimated that network traffic must be less than 40 percent of the bus capacity for the network to operate efficiently. If distances are long, time lags occur that may result in inappropriate carrier sensing, and hence collisions. [1]

Standard Ethernet networks use CSMA/CD to physically monitor the traffic on the line at participating stations. If no transmission is taking place at the time, the particular station can transmit. If two stations attempt to transmit simultaneously, this causes a collision, which is detected by all participating stations. If collision occurs, the time intervals from which the random waiting time is selected are increased step by step. This is known as exponential back off.

CSMA/CD is a type of contention protocol. Networks using the CSMA/CD procedure are simple to implement but do not have deterministic transmission characteristics. The CSMA/CD method is internationally standardized in IEEE802.3 and ISO 8802.3. [2]

1.1 CSMA/CD mechanisms

CSMA/CD employs the following mechanisms: [3]

i. An adapter may begin to transmit at any time, i.e., no slots are used.