The IEEE defines different access methods for 802.3 and 802.11 standards. On wired Ethernet LANs, Carrier Sense Multiple Access/Collision Detection (CSMA/CD) is used to detect collisions. Collision detection is used to improve performance by terminating transmission as soon as a collision is detected and reducing the probability of a second collision on retry.
Unlike the 802.3 standards, the 802.11 standards seek to avoid, rather than to detect, collisions and thus employ Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA). The basic premise behind collision avoidance is that any STA wishing to transmit a frame must first sense the medium, and if the medium is busy (e.g. because another STA is transmitting at that time), the STA must defer its transmission. However, if the medium is free, the STA is allowed to transmit the frame or frames as desired.
Collision avoidance works well when the medium is not heavily loaded (i.e. there are not a lot of STAs using the medium) because it allows STAs to transmit with minimal delay. However, there is still the possibility that collisions will occur because different STAs may sense the medium as free at the same time and decide to transmit at the same time. While such issues could be avoided by using collision detection mechanisms in conjunction with an exponential random backoff algorithm, collision detection mechanisms cannot be applied to WLANs.
This is because WLANs operate in half-duplex mode, since a single frequency is used to transmit and receive data. Because a single channel is used, it cannot always be assumed that all wireless STAs will be able to hear each other. For example, this might be because the transmitting station may have its receiver turned off when it is sending data. In CSMA/CD, the device would simply send a jam signal. However, if the receiver is off, the STA will never be able to tell when another STA is sending data at the same time as it, which would result in collisions. To overcome these issues, the IEEE 802.11 standard defines different MAC coordination functions. These are described in detail in the following sections.