Distributed coordination function: Difference between revisions

Revision as of 15:12, 23 February 2024

Latest revision as of 03:27, 31 July 2024

(One intermediate revision by one other user not shown)

Line 6:

DCF requires a station wishing to transmit to listen for the channel status for a [[DIFS]] interval. If the channel is found busy during the DIFS interval, the station defers its transmission. In a network where a number of stations contend for the wireless medium, if multiple stations sense the channel busy and defer their access, they will also virtually simultaneously find that the channel is released and then try to seize the channel. As a result, collisions may occur. In order to avoid such collisions, DCF also specifies random backoff, which forces a station to defer its access to the channel for an extra period. The length of the backoff period is determined by the following equation:

<math>\mathrm{Backoff Time} = \mathrm{random}() \times \mathrm{aSlotTime}</math><ref>{{Cite web|title=IEEE 802.11-2016 - IEEE Standard for Information technology--Telecommunications and information exchange between systems Local and metropolitan area networks--Specific requirements - Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications|url=https://standards.ieee.org/standardieee/802_11-2016802.html11/5536/|access-date=2021-05-27|website=standards.ieee.org[[IEEE]]}}</ref>

A few features about DCF:

Line 15:

* it is defined in the IEEE 802.11 standard and is the de facto default setting for Wi-Fi hardware.

802.11 DCF consumes a significant amount of airtime, 802.11 control messages usually convey very little information. For example, an ACK message can take up to 60 μs to transmit completely, which includes an amount of airtime sufficient to transmit 3240 bits at 54 &nbsp;Mbit/s, during which it conveys a single bit of relevant information.

The IEEE 802.11 standard also defines an optional access method using a [[point coordination function]] (PCF). PCF allows the [[Wireless access point|access point]] acting as the network coordinator to manage channel access. The [[IEEE 802.11e]] amendment to the standard enhances the DCF and the PCF, through a new coordination function called Hybrid Coordination Function (HCF).