Distributed Coordination Function
Encyclopedia
Distributed coordination function (DCF) is the fundamental MAC
technique of the IEEE 802.11
based WLAN
standard. DCF employs a CSMA/CA with binary exponential backoff algorithm.
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:
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.
DCF includes a positive acknowledge scheme, which means that if a frame is successfully received by the destination it is addressed to, the destination needs to send an ACK frame to notify the source of the successful reception.
DCF is defined in subclause 9.2 of the IEEE 802.11 standard and is the de-facto default setting for Wi-Fi hardware.
The IEEE 802.11 standard also defines an optional access method using a Point Coordination Function
(PCF). PCF allows the 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).
Media Access Control
The media access control data communication protocol sub-layer, also known as the medium access control, is a sublayer of the data link layer specified in the seven-layer OSI model , and in the four-layer TCP/IP model...
technique of the IEEE 802.11
IEEE 802.11
IEEE 802.11 is a set of standards for implementing wireless local area network computer communication in the 2.4, 3.6 and 5 GHz frequency bands. They are created and maintained by the IEEE LAN/MAN Standards Committee . The base version of the standard IEEE 802.11-2007 has had subsequent...
based WLAN
Wireless LAN
A wireless local area network links two or more devices using some wireless distribution method , and usually providing a connection through an access point to the wider internet. This gives users the mobility to move around within a local coverage area and still be connected to the network...
standard. DCF employs a CSMA/CA with binary exponential backoff algorithm.
DCF requires a station wishing to transmit to listen for the channel status for a DIFS
DIFS
In the DCF protocol, a station has to sense the status of the wireless medium before transmitting. If the medium is continuously idle for DCF Interframe Space duration, only then it is supposed to transmit a frame...
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:
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.
DCF includes a positive acknowledge scheme, which means that if a frame is successfully received by the destination it is addressed to, the destination needs to send an ACK frame to notify the source of the successful reception.
DCF is defined in subclause 9.2 of the IEEE 802.11 standard and is the de-facto default setting for Wi-Fi hardware.
The IEEE 802.11 standard also defines an optional access method using a Point Coordination Function
Point Coordination Function
Point coordination function is a Media Access Control technique used in IEEE 802.11 based WLANs. It resides in a point coordinator also known as Access Point , to coordinate the communication within the network. The AP waits for PIFS duration rather than DIFS duration to grasp the channel...
(PCF). PCF allows the access point
Wireless access point
In computer networking, a wireless access point is a device that allows wireless devices to connect to a wired network using Wi-Fi, Bluetooth or related standards...
acting as the network coordinator to manage channel access. The IEEE 802.11e
IEEE 802.11e
IEEE 802.11e-2005 or 802.11e is an approved amendment to the IEEE 802.11 standard that defines a set of Quality of Service enhancements for wireless LAN applications through modifications to the Media Access Control layer. The standard is considered of critical importance for delay-sensitive...
amendment to the standard enhances the DCF and the PCF, through a new coordination function called Hybrid Coordination Function (HCF).
See also
- Point Coordination FunctionPoint Coordination FunctionPoint coordination function is a Media Access Control technique used in IEEE 802.11 based WLANs. It resides in a point coordinator also known as Access Point , to coordinate the communication within the network. The AP waits for PIFS duration rather than DIFS duration to grasp the channel...
(PCF) - Hybrid Coordination Function (HCF)
- Best effort deliveryBest effort deliveryBest effort delivery describes a network service in which the network does not provide any guarantees that data is delivered or that a user is given a guaranteed quality of service level or a certain priority...