DCF and PCF in WiFi Networks – Understanding the Differences

ieee 802.11network access

I am studying, trying to learn how DCF (Distributed Coordination Function) and PCF (Point Coordination Function) works as access methods in WiFi, and how they relate to infrastructure vs ad hoc networks.

My teacher in this course has claimed that DCF is used in ad hoc modes, while PCF is used in infrastructure. I can find no other sources online that back this claim. Most sources online will say WiFi uses CSMA/CA – and as far as I understand, DCF is the only method that implements this protocol. Also, even the Wikipedia page for PCF claims it is "implemented only in very few hardware devices". Also, infrastructure seems to be the most used setup, so it stands to reason that infrastructure must support the DCF access method.

Can anyone explain how DCF and PCF relates to ad hoc and infrastructure?

Best Answer

I am far from expert in this, however my belief is that the choice of DCF vs PCF is not strictly about ad-hoc vs infrastructure, but that PCF inherently can only work in infrastructure mode as there has to be a coordinator, while DCF could be used ad-hoc or infrastructure; also was the only mandatory method.

DCF and PCF have been superseded by new Hybrid Coordination Function in 802.11e. (Wikipedia)

Following note says Cisco didn't implement PCF, and that 802.11e was going to effectively kill it off. You can still find academic papers though. Here's one devising distributed hybrid coordination functions, designed to be PCF-for-ad-hoc, but only done in simulation.

Nice summary presentation of QoS techniques for wifi here

CISCO

In 2002, Cisco forums included the following post from staff, (emphasis added):

From: bmcmurdo Cisco Employee

09-04-2002 09:07 AM

Re: PCF or just DCF in Cisco AP ?

PCF is not implemented on Cisco APs. PCF is not generally thought to be a very effective/useful QoS technique

The radio Access Point (and radio firmware) are "PCF aware" meaning if they detect a PCF coordinator they will "play nice" in that area and back off a little so it can do it's best effort.

IEEE 102.11e [sic] working group is currently working on a QoS standard but that has not yet been ratified.

802.11e came out in 2005, but has since been superseded.

ARUBA

This blog post from a CCNA-Wireless suggests that you have to hunt to find PCF in real life: "curious has anyone else seen this or PCF in the wild ?" https://blogs.arubanetworks.com/industries/access-point-supporting-pcf-contention-free-parameter/

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