Frame exchange protocol:-

. Source station transmits data

. Destination responds with acknowledgments (ACK)

.If sourced doesn't receive ACK, it re-transmits frame

Four frame exchange:

. Source issues request to send (RTS)

. Destination responds with clear to send (CTS)

. Source transmits data

. Destination responds with ACK

IEEE 802.11 considered 2 types of MAC algorithm:

1. Distributed Access protocols
2. Centralized Access protocols

End result for 802.11 MAC algorithm called DFWMAC

Distributed Co-ordination Function:

DCF makes use of simple CSMA algorithm

DCF does not include a collision detection function

T ensure smooth & fair functioning of this algorithm, DCF includes a set of delays that amounts to priority scheme.

If a station has MAC frame to transmit, it listens to the medium.

If the medium is idle station may transmit

Otherwise it must wait until current transmission is complete

• One of the main problem in wireless networks is the hidden station problem. The problem can be solved by using RTS-CTS mechanism.

• RTS-CTS avoids the problem of hidden terminals: A and C want to send to B

  a. A sends RTS first

  b. C waits after receiving CTS from B

• RTS-CTS avoids the problem of exposed terminals:

  a. B wants to send to A, C to another terminal

  b. now C does not have to wait for it cannot receive CTS