The 802.11e draft specification introduces two new modes of operation: EDCF and HCF. As with the original 802.11 MAC, the 802.11e enhancements are designed to work with all possible 802.11 physical
EDCF defines eight traffic classes. Various parameters
HCF is analogous to PCF, but it allows a
Many priority schemes to support QoS are currently being discussed. IEEE 802.11 Task Group E currently defines enhancements to the previously discussed 802.11 MAC that are called 802.11e. These enhancements introduce two new MAC modes-EDCF and HCF. Both these QoS-enhanced MAC protocols support up to eight priority levels of traffic that map directly to the RSVP protocol and other protocol priority levels.
Enhanced Distributed Coordination Function (EDCF)
The major enhancement provided by EDCF versus DCF is the introduction of eight distinct traffic classes. Aside from this, EDCF, as the
Figure 5-10: EDCF and HCF in 802.11 networks
Here, each traffic class starts a back-off after detecting that the channel is idle for an AIFS. The AIFS is at least as large as the DIFS and can be
Second, the minimum value of the CW for each traffic class, denoted by CWMin can be selected on a per-traffic-class basis. In DCF, a global constant CWMin is used to initialize all CW values.
Third, when a collision is
The CWMax value sets the maximum possible value for the CW on a per-traffic-class basis; however, CWMax is typically intended to
Within a station, the eight traffic classes have independent transmission queues. These behave as virtual
The QoS parameters, which are provided on a per-traffic-class basis, can be
Hybrid Coordination Function (HCF)
HCF is an extension of the polling idea in PCF. Just like in PCF, under HCF, the superframe is divided into the CFP that starts with every beacon and the CP. During the CP, access is governed by EDCF, though the HC (
During the CFP, the HC issues a QoS CF-Poll to a particular station to give it a TXOP. The HC specifies the starting time and maximum duration as part of the CF-Poll frame. During the CFP, no stations attempt to gain access to the medium, so when a CF-Poll is received, they assume a TXOP and transmit any data they have. The CFP ends after the time announced by the beacon frame or by a CF-End frame.
If a station is given a CF-Poll, it is expected to start responding with data within an SIFS period. If it does not, the HC can take over the medium after a PIFS time and allocate another CF-Poll to another station. This allows very efficient use of the medium during the CFP.
To determine which station to give the TXOP to, the HC uses perstation/per-traffic-class queue length data that it collects and maintains to reflect the current snapshot of the infrastructure BSS. The QoS Control field that has been added to the MAC frame definition enables stations implementing 802.11e to send queue lengths per traffic class to the HC.
Scheduling The MAC defines protocols and mechanisms to perform HCF and EDCF. However, a couple of opportunities exist to perform scheduling decisions that are not determined by pure random number selection as in DCF.
HC Scheduling The HC has a snapshot view of the per-traffic-class/per-station queue length information over time, including that of the AP itself. With this, it has to decide to whom to allocate TXOPs during the CFP. This involves considering at least the following:
The priority of the traffic class
The required QoS for the traffic class (low jitter, high bandwidth, low latency, and so on)
Queue lengths per traffic class
Queue lengths per station
The duration of TXOP available and to be allocated
Past QoS seen by the traffic class
The practice is to implement a simple scheme of calculating a weighted average queue length per station (weights are based on traffic class queues within a station) and allocate the maximum available TXOP within the CFP to the station with the largest average. However, various possible schemes are available to meet different goals.
Endpoint Scheduling Within TXOP
When a wireless station gets a TXOP by polling from the HC, the HC does not specify a particular traffic class for the TXOP. This
By decentralizing this decision, the protocol provides a scalable mechanism of maintaining traffic class history and
EDCF provides significant improvements for high-priority QoS traffic; however, these improvements are typically provided at the cost of
HCF, just like its predecessor PCF, provides for much more efficient use of the medium when the medium is heavily loaded. Unlike PCF, HCF does a good job of channel utilization even when the channel is operating well below capacity. Due to reduced overhead, HCF can provide better QoS support for high-priority streams while allocating reasonable bandwidth to lower-priority streams.
Both coordination functions are backward compatible with DCF and PCF. This fact, along with our results, leads us to believe that EDCF and HCF will soon see ubiquitous adaptation into mainstream WLAN technology. 
 James and Ruth LaRocca, 802.11 Demystified , (New York: McGraw-Hill, 2002), 141-142.
Priyank Garg, Rushabh Doshi, Majid Malek, Russell Greene, and Maggie Cheng, "Achieving Higher Throughput and QoS in 802.11 Wireless LANs," a white paper from Stanford University, http://milliways.