Distributed coordination function: Difference between revisions

Revision as of 14:15, 21 May 2020

Revision as of 09:29, 27 May 2021

Line 5:

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/standard/802_11-2016.html|access-date=2021-05-27|website=standards.ieee.org}}</ref>

<math>\mathrm{Backoff Time} = \mathrm{random}() \times \mathrm{aSlotTime}</math>

DCF also has an optional virtual carrier sense mechanism that exchanges short Request-to-send (RTS) and Clear-to-send (CTS) frames between source and destination stations during the intervals between the data frame transmissions.

Line 13:

DCF does not solve the hidden terminal and/or exposed terminal problem completely, it only alleviates the problem through the use of RTS and CTS, and recommends the use of a larger carrier sensing range.

DCF is defined in subclause 9.3 of 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 Mbit/s, during which it conveys a single bit of relevant information.

Line 23:

* [[Point coordination function]] (PCF)

== References ==

[[Category:Media access control]]

[[Category:IEEE 802.11|D]]

<references />