Q.2) What are MAC issues? What is hidden node and Exposed node problem? How it is resolved? [10] Ans. Suppose that there are three mobile A, B, and C. The transmission range of A reaches B, but not C. The transmission range of C reaches B, but not A. Finally the transmission range of B reaches A and C.
A starts sending signal to B, C does not receive this transmission. C also wants to send data to B and senses the medium. The medium appears to be free, the carrier sense fails. C also starts sending data. It will create a collision at B. But A cannot detect this collision at B and continues with its transmission. A is
hidden for C and vice versa.
Hidden node problem has been solved by using Request to send(RTS) and Clear to send frames also called as handshake frames.
In the below given figure shows that the RTS message from A reaches B, but not C. However, because both A and C are within the range of B, the CTS message, which contains the duration of data transmission from A to B reaches C.
Station C knows that some hidden station is using the channel and refrains from transmitting until that duration is over.
Exposed node problem
The sender mistakenly thinks the medium is in use, so that it unnecessarily delays the transmission.
Device A is transmitting the data to B. Device B wants to send to C and listens to the channel.B hears A’s transmission, and B mistakenly thinks that it cannot send to C.
