Wireless local area networks operate using a shared, high bit rate transmission medium to which all devices are attached and information frames relating to all calls are transmitted. MAC sublayer defines how a user obtains a channel when he or she needs one.

MAC schemes include random access, order access, deterministic access, and mixed access. The random access MAC protocols are, ALOHA (asynchronous, slotted), carrier-sense multiple-access (CSMA) (CSMA/collision-detection (CD), CSMA/collision-avoidance (CA), non-persistent, and p-persistent). The maximum throughput of slotted ALOHA protocol is about 36% of the data rate of the channel. It is simple, but not very efficient. Most WLANs implement a random access protocol, CSMA/CA with some modification, to deal with the hidden node problem. The CSMA peaks at about 60%. When the traffic becomes heavy, it degrades badly.

IEEE 802.11 uses a modified protocol known as carrier sense multiple access with collision avoidance (CSMA/CA) or distributed coordination function (DCF).

CSMA/CA attempts to avoid collisions by using explicit packet acknowledgment (ACK), which means an ACK packet is sent by the receiving station to confirm that the data packet arrived intact.

The CSMA/CA protocol is very effective when the medium is not heavily loaded since it allows stations to transmit with minimum delay. But there is always a chance of stations simultaneously sensing the medium as being free and transmitting at the same time, causing a collision. These collisions must be identified so that the MAC layer can retransmit the packet by itself and not by the upper layers, which would cause significant delay. In the Ethernet with CSMA/CD the collision is recognized by the transmitting station, which goes into a retransmission phase based on an exponential random backoff algorithm. While these collision detection mechanisms are a good idea on a wired LAN, they cannot be used on a WLAN environment for two main reasons:

• Implementing a collision detection mechanism would require the implementation of a full duplex radio capable of transmitting and receiving at the same time, an approach that would increase the cost significantly.
• In a wireless environment we cannot assume that all stations hear each other, and the fact that a station wants to transmit and senses the medium as free does not necessarily mean that the medium is free around the receiver area.

To overcome these problems, the 802.11 uses a CA mechanism together with a positive ACK. The MAC layer of a station wishing to transmit senses the medium. If the medium is free for a specified time, called distributed inter-frame space (DIFS), then the station is able to transmit the packet; if the medium is busy the station defers using the exponential backoff algorithm.

This scheme implies that, except in cases of very high network congestion, no packets will be lost because retransmission occurs each time a packet is not acknowledged. This entails that all packets sent will reach their destination in sequence.

The following are some of the reasons it is preferable to use smaller packets in a WLAN environment:

• Due to higher BER of a radio link, the probability of a packet getting corrupted increases with packet size.
• In case of corrupted packets (either due to collision or interference), smaller packets cause less overhead.
• On an FHSS system the medium is interrupted periodically for hopping. With smaller packets the chance that the transmission will be postponed after dwell time is reduced.

A simple send-and-wait algorithm is used at the MAC sublayer. In this mechanism the transmitting station is not allowed to transmit a new packet until one of the following happens:

• Receives an ACK for the packet, or
• Decides that packet was retransmitted too many times and drops the whole frame.