The simplest approach to using a wireless lan might be to try CSMA : just listen for the other transmissions and only transmit if no one else is doing so. The trouble is, this protocol is not really appropriate because what matters is the interference at the receiver, not at the sender. To see the nature of the problem, consider four wireless machines, A, B, C, D, such that A and B are within each others radio’s range, so are B and C, C and D pairs. Thus each terminal can potentially interfere with its neighbouring terminals which are in its radio’s range but not with others.
A---------------> B <------------ C
Consider the situation when A is transmitting to B. If C senses the medium, it will not hear A, because A is out of its range and thus falsely concludes that it can transmit. If C does start transmitting, it will interfere at B wiping out the frame from A. The problem of a station not being able to detect a potential competetor for the medium because the competetor is too far is called Hidden Station Problem.
A <------------- B --------------> C
Now consider the reverse situation, B transmitting to A. If C senses the medium, it will hear an ongoing transmission and falsely conclude that it may not send to D, when in fact such a transmission would cause bad reception only in the zone between B and C, where neither of the intended receiver is located. This situation is called Exposed Station Problem.
MACA and MACAW
An early protocol designed for wirelss lans is MACA
( Multiple Access with Collision Avoidance ). I t was as the basis
for the IEEE 802.11 WLAN standard. The basic idea behind it is, for the
sender to stimulate the receiver into outputting a short frame, so that
stations nearby can detect this transmission and avoid transmitting themselves
for the duration of the upcoming data frame. MACA is illutrated in the
figure below.
To explain this protocol, consider how A sends a frame to B.
Despite these precautions, collision can still occur. For eg. B and C could both send RTS frames to A at the same timethese will collide and be lost. In the event of a collision, an unsuccessful transmitter waits a random amount of time and tries again later. The algorithm used is Binary Exponential BackOff .
MACAW is an improvised version of MACA. It was observed that without DLL ( Data Link Layer ) acknowledgements, lost frames were not re-transmitted until the transport layer noticed their absence. This problem was solved by introducing an ACK frame after each successful data frame. To avoid collision of RTS frames from two nearby stations, the carrier sensing feature of the CSMA was also included. In addition, the Backoff algorithm runs separately for each data stream rather than for each station. This change improved the fairness of the protocol. A mechanism for stations to exchange information about congestion and, a way to make the Backoff algorithm to react less violoently to temporary problems was added to improve system performance.
Of the protocols explained above, MACAW is the most feasible.
