PCO Operation Overview

PCO Operation Overview

By CWNP On 09/06/2007 - 2 Comments

Phased Coexistence Operation (PCO) is an optional coexistence mechanism in which an AP divides time into alternating 20 MHz and 40 MHz phases.   Although PCO improves throughput in some circumstances, PCO might also introduce jitter.


Turning PCO On/Off:

A PCO-capable AP activates PCO if it decides that PCO is more appropriate than either 20/40 MHz mode or 20 MHz mode in the current circumstances.  The algorithm for making this decision is beyond the scope of the 802.11n amendment, so vendors will get to choose how they implement this feature.

The PCO-capable AP sets the Supported Channel Width Set subfield in the HT Capabilities Info field (in the Beacon) to 1 to indicate that the BSS is 20/40 MHz capable and sets the PCO Active field in the HT Information element (in the Beacon) to 1 to enable PCO operation in the BSS.  When a PCO AP detects that PCO is not providing a performance benefit, the PCO AP may deactivate PCO and switch to either 20/40 MHz mode or 20 MHz mode.  When switching to 20 MHz mode, the PCO AP first switches to 20/40 MHz mode and then to 20 MHz mode.  The algorithm for making this decision is also beyond the scope of the 802.11n amendment.

A PCO-capable AP sets the PCO Active field in the HT Information element to 0 when PCO operation is disabled.  Since the AP advertises the current mode in its Beacon and
Probe Response frames, its associated stations are informed of any mode change.  

PCO-capable Stations:

PCO APs interpret the receipt of an Association Request frame with the PCO field set to 1 in the HT Extended Capabilities field in the HT Capabilities element as an indication that the station transmitting the Association Request frame is PCO-capable.  If the PCO field in the Association Request to a PCO AP is set to 1 and the association succeeds, the station operates in PCO mode.  If an association fails when a station attempts to associate as a PCO station, the station may treat the BSS as a 20/40 MHz BSS and may attempt an association as a non-PCO-capable 20/40 station.

Changing PCO Phases:

There are two methods of changing the PCO phase from 20-to-40 MHz or 40-to-20 MHz.  The first is provided through the PCO AP broadcasting a "Set PCO Phase" management/action frame (which includes the PCO Phase Control field) to the BSS.  This frame may ONLY be sent by a PCO AP.  The second method is by setting the PCO Phase field of the HT Information element in the Beacon (or Probe Response), where 0 = 20 MHz and 1 = 40 MHz.

Contention-Free Access:

One unexpected twist to all of this is that PCO APs use the CF Parameter Set in Beacons and CF-End frames.  Thank goodness TGn found a good use for CFP frames.  Either the CFPDurationRemaining value in the CF Parameter Set of a Beacon or the duration/ID value of a Set PCO Phase action frame sent on the primary channel is interpreted as the duration of the 40 MHz PCO phase and starts the 40 MHz phase.  A CTS-to-Self frame's duration/ID value can also be used to indicate the intended duration of a 40 MHz phase.  This CTS-to-Self frame must be sent in non-HT duplicate format (same format on both 20 MHz channels at the same time) after the secondary channel has been clear for a PIFS.  The reason the primary channel doesn't have to be sensed also is that it's already reserved due to the Beacon and/or the Set PCO Phase action frame (sent on the primary channel).  An HT station may disassociate and associate or reassociate with an AP to change its PCO capabilities.

CF-End frames are sent after a Beacon, Set PCO Phase frame, or CTS-to-Self frame (whichever is used to start the 40 MHz phase) for the purpose of giving express transmission permission to PCO stations.  Beacons, Set PCO Phase frames, and CTS-to-Self frames are used to shut down transmissions of contention-based stations in the BSA for both the primary and secondary channels while PCO stations transmit.

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