WLAN - PCF                                                                                            Home : www.sharetechnote.com

 

 

 

 

PCF(Point Coordination Function)

 

At least as of now(May 2017), in most of WLAN communication there is no specific centeral controller for data scheduling (i.e, data transmission and reception). Most of the scheduling is based on DCF(Distributed Coordination Function). As another way of scheduling algorithm, a new algorithm called PCF which works (at least in part) with a centeralized controller is introduced. However, PCF is not the mandatory algorithm in 802.11 specification. It is Optional even thought the specification itself was finalized many years ago. DCF is basically Contention based algorithm and PCF is more like Contention-Free algorithm.

 

Overall scheduling algorithm of PCF is illustrated below. As shown here, even in PCF not all data communication is done in CF(Contention Free) method. It happens in alternating period of CFP(Contention Free Period) and CP(Contention Period). In this page, I will focus mostly on what's happening in CFP. Regarding what's going on in CP, I would suggest you to refer to DCF page.

In PCF Operation, a speciall scheduler called PC(Point Coordinator in Access Point) take control over the wireless channel and control the data scheduling (transmission and reception) of all the stations (WLAN user devices). Actually in PCF, the Access Point hijack the radio channel by transmission Beacon during the IFS(Inter Frame Space) in which nobody should transmit anything.

 

Following is an example of PCF operation. (In reality, there can be many variations in how the PC controlls each Stations, but the fundamental logic would be similar to the diagram shown here)

 

 

Now let's look into the above diagram in more detail.

  • Step (1) : AP sends a Beacon right after PIFS (PIFS is shorter than DIFS. It means that the Beacon is transmitted within an IFS of conventional scheduling method). Since this Beacon is transmitted onto the medium (radio channel) while nobody is transmitted, this message will reach every station(WLAN devices) without any risk of collision. This Beacon carries the information on PCF capability of the AP(See Beacon frame section for the details). By this step, AP would take the control of the wireless channel.
  • Step (2) : (If AP has data for Station A), the AP sends [Data] and [CF-Poll (a kind of Grant)] to Station A.
  • Step (3) : Since Station A get the CF-Poll(Grant) from the AP, (if it has any data for AP) it can transmit the [data and ACK] to the AP.
  • Step (4) : (Assuming that the AP has no data for Station B), now the AP sends [Ack] for Station A and [CF-Poll] to Station B.
  • Step (5) : Since Station B get the CF-Poll(Grant) from the AP, (if it has any data for AP) it can transmit the [data] to the AP
  • Step (6) : After the AP has given CF-Poll(Grant) to all the devices, it sends [ACK] for the last Station(in this case, Station B) and transmit [CF-End] in order to notify all the devices(stations) that this is the end of CFP.

 

 

Reference :

 

[1] PCF scheme 

[2] MAC 802.11 Point Coordinator Function

 

 

 

 

 s