CSMA,MAC

Carrier Sense Multiple Access

To minimize the chance of collision and, therefore, increase the performance, the CSMA method was developed. The chance of collision can be reduced if a station senses the medium before trying to use it. Carrier sense multiple access (CSMA) requires that each station first listen to the medium before sending. CSMA is based on the principle “sense before transmit” or “listen before talk.”

CSMA can reduce the possibility of collision, but it cannot eliminate it. The reason for a space and time model of a CSMA network. Stations are connected to a shared channel (usually a dedicated medium).

At time t1, station B senses the medium and finds it idle, so it sends a frame. At time t2 (t2 > t1), station C senses the medium and finds it idle because, at this time, the first bits from station B have not reached station C. Station C also sends a frame. The two signals collide and both frames are destroyed.

Vulnerable Time

The vulnerable time for CSMA is the propagation time Tp. This is the time needed for a signal to propagate from one end of the medium to the other. When a station sends a frame and any other station tries to send a frame during this time, a collision will result.

Persistence Methods

The 1-persistent method, the non-persistent method, and the p-persistent method. Below Figure shows the behavior of three persistence methods when a station finds a channel busy.

1-Persistent

The 1-persistent method is simple and straightforward. After the station finds the line idle, it sends its frame immediately. This method has the highest chance of collision because two or more stations may find the line idle and send their frames immediately.

Nonpersistent

In the nonpersistent method, a station that has a frame to send senses the line. If the line is idle, it sends immediately. If the line is not idle, it waits a random amount of time and then senses the line again. The nonpersistent approach reduces the chance of collision because it is unlikely that two or more stations will wait the same amount of time and retry to send simultaneously

 p-Persistent

The p-persistent method is used if the channel has time slots with a slot duration equal to or greater than the maximum propagation time. The p-persistent approach combines the advantages of the other two strategies. It reduces the chance of collision and improves efficiency. In this method, after the station finds the line idle it follows these steps:

1.      With probability p, the station sends its frame.

2.      With probability q = 1 − p, the station waits for the beginning of the next time slot and checks the line again.

a)      If the line is idle, it goes to step 1.

b)      If the line is busy, it acts as though a collision has occurred and uses the backoff procedure.

Flow Diagrams of Persistence Methods