Carrier sense multiple access-technicalfundaezy

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                Carrier sense multiple access

                1.To minimize the chance of collision and hence increases the performance .
                2.carrier sense multiple access requires that each station first listen to the medium before                     sending.

             consider one example :-At time tl, station B senses the medium and finds it idle, so it sends a               frame. At time t2 (t2 > tl), 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.

Persistent Method

    What should a station do if the channel is busy? What should a station do if the channel is idle?      Three methods have been devised to answer these questions: the I-persistent method, the           nonpersistent method, and the p-persistent method. Figure 12.9 shows the behavior of three    persistence methods when a station finds a channel busy.

Types of persistent method

1.1-Persistent Method
1.Non-Persistent Method
1.P-Persistent Method

1.Persistent Method

1-Persistent method is simple and straight forward .In this method after the station is find line is idle it immediately sends the frame

         This method is highest chance of collision be

cause two or more station may find the line idle               and sends these frame immediately

Non-Persistent

A station has a frame is send senses to 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 sense the line again.
The non persistent process is reduce 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 .

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