Editing IEEE 802.11e-2005 (section)

== Original 802.11 MAC ==
=== Distributed coordination function (DCF) ===
{{Main|Distributed coordination function}}
The basic 802.11 MAC layer uses the distributed coordination function (DCF) to share the medium between multiple stations.  (DCF) relies on [[CSMA/CA]] and optional [[802.11 RTS/CTS]] to share the medium between stations.  This has several limitations:
*if many stations attempt to communicate at the same time, many collisions will occur which will lower the available bandwidth and possibly lead to [[congestive collapse]].
*there are no quality-of-service (QoS) guarantees. In particular, there is no notion of high or low priority traffic.

=== Point coordination function (PCF) ===
{{Main|Point coordination function}}
The original 802.11 MAC defines another coordination function called the point coordination function (PCF). This is available only in "infrastructure" mode, where stations are connected to the network through an [[Wireless access point|Access Point]] (AP).  This mode is optional, and only very few APs or Wi-Fi adapters actually implement it.{{Citation needed|date=July 2010}}  APs send ''[[Electric beacon#Wi-Fi|beacon]]'' frames at regular intervals (usually every 100 [[TU (time unit)|TU]] or 0.1024 second).  Between these ''beacon'' frames, PCF defines two periods: the Contention Free Period (CFP) and the Contention Period (CP).  In the CP, DCF is used.  In the CFP, the AP sends Contention-Free-Poll (CF-Poll) packets to each station, one at a time, to give them the right to send a packet.  The AP is the coordinator.  Although this allows for a better management of QoS, PCF does not define classes of traffic as is common with other QoS systems (e.g. [[802.1p]] and [[DiffServ]]).