Point Coordination Function
Encyclopedia
Point coordination function (PCF) is a Media Access Control
(MAC) technique used in IEEE 802.11 based WLANs
. It resides in a point coordinator also known as Access Point
(AP), to coordinate the communication within the network. The AP waits for PIFS
duration rather than DIFS duration to grasp the channel. PIFS is less than DIFS duration and hence the point coordinator always has the priority to access the channel.
The PCF is located directly above the Distributed Coordination Function
(DCF), in the IEEE 802.11 MAC Architecture.
Channel access in PCF mode is centralized and hence the point coordinator sends CF-Poll frame to the PCF capable station to permit it to transmit a frame. In case the polled stations does not have any frames to send, then it must transmit null frame.
Since most APs have logical bus topologies (they are shared circuits) only one message can be processed at one time (it is a contention
based system), and thus a media access control
technique is required.
Wireless networks may suffer from a hidden node problem
where some regular nodes (which communicate only with the AP) cannot see other nodes on the extreme edge of the geographical radius of the network because the wireless signal attenuates before it can reach that far. Thus having an AP in the middle allows the distance to be halved, allowing all nodes to see the AP, and consequentially, halving the maximum distance between two nodes on the extreme edges of a circle-star topology.
PCF seems to be implemented only in very few hardware devices as it is not part of the Wi-Fi Alliance
's interoperability standard.
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...
(MAC) technique used in IEEE 802.11 based WLANs
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...
. It resides in a point coordinator also known as 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...
(AP), to coordinate the communication within the network. The AP waits for PIFS
PCF Interframe Space
PCF Interframe Space is one of the interframe space used in IEEE 802.11 based Wireless LANs. PCF enabled access point wait for PIFS duration rather than DIFS to occupy the wireless medium. PIFS duration is less than DIFS and greater than SIFS...
duration rather than DIFS duration to grasp the channel. PIFS is less than DIFS duration and hence the point coordinator always has the priority to access the channel.
The PCF is located directly above the Distributed Coordination Function
Distributed Coordination Function
Distributed coordination function is the fundamental MAC technique of the IEEE 802.11 based WLAN standard. DCF employs a CSMA/CA with binary exponential backoff algorithm....
(DCF), in the IEEE 802.11 MAC Architecture.
Channel access in PCF mode is centralized and hence the point coordinator sends CF-Poll frame to the PCF capable station to permit it to transmit a frame. In case the polled stations does not have any frames to send, then it must transmit null frame.
Since most APs have logical bus topologies (they are shared circuits) only one message can be processed at one time (it is a contention
Contention (telecommunications)
In packet mode communication networks, contention is a media access method that is used to share a broadcast medium.-Collision detection and recovery:...
based system), and thus a media access control
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 is required.
Wireless networks may suffer from a hidden node problem
Hidden node problem
In wireless networking, the hidden node problem or hidden terminal problem occurs when a node is visible from a wireless access point , but not from other nodes communicating with said AP. This leads to difficulties in media access control....
where some regular nodes (which communicate only with the AP) cannot see other nodes on the extreme edge of the geographical radius of the network because the wireless signal attenuates before it can reach that far. Thus having an AP in the middle allows the distance to be halved, allowing all nodes to see the AP, and consequentially, halving the maximum distance between two nodes on the extreme edges of a circle-star topology.
PCF seems to be implemented only in very few hardware devices as it is not part of the Wi-Fi Alliance
Wi-Fi Alliance
The Wi-Fi Alliance is a trade association that promotes Wireless LAN technology and certifies products if they conform to certain standards of interoperability. Not every IEEE 802.11-compliant device is submitted for certification to the Wi-Fi Alliance, sometimes because of costs associated with...
's interoperability standard.
See also
- Contention free pollableContention free pollableContention-free pollable is a state of operation for wireless networking nodes. The condition is saying that the node is able to use the Point Coordination Function, as opposed to the Distributed Coordination Function, within a wireless LAN....
- Distributed Coordination FunctionDistributed Coordination FunctionDistributed coordination function is the fundamental MAC technique of the IEEE 802.11 based WLAN standard. DCF employs a CSMA/CA with binary exponential backoff algorithm....
- Hybrid Coordination Function