CSMA/CD

Explain how collision is handled using CSMA/CD over a wired medium.

The Carrier Sense Multiple Access with Collision Detection (CSMA/CD) method augments CSMA algorithm to handle collisions when detected over a wired medium.

In this method, a station monitors the medium after it sends a frame

The channel is sensed before frame transmission by using one of the persistence methods. If there is a collision, the frame is sent again, otherwise successful transmission.

Transmission and collision detection is a continuous process. The station transmits and receives simultaneously.

For CSMA/CD to work, the sending station must detect a collision before sending the last bit of the frame, since sender does not keep a copy of the frame.

Frames should be long enough to allow collision detection prior to end of transmission

If a collision is detected during transmission, the station transmits a brief jamming signal to ensure that all stations know that there has been a collision and aborts transmission.

The station waits for a random amount of time and attempts retransmission. Retransmission is given after 15 attempts.

Example

At time t1, station A starts sending the bits of its frame.

At time t2, station C has not yet sensed the first bit sent by station A.

Station C starts sending bits in its frame, which propagate in both directions.

Station C detects a collision at time t3 when it receives the first bit of A's frame. Station C sends a jamming signal and immediately aborts transmission.

Station A detects collision at time t4 and aborts transmission.