Voice and multimedia on wireless networks have made necessary the addition of quality of service (QoS) facilities...
to the IEEE 802.11 MAC layer standard. After several years of work, the 802.11 standards committee has recently completed work on 802.11e, an extension to the original standard. 802.11e adds QOS facilities plus a set of performance enhancements.
The added facilities enable network managers to configure networks to match application requirements. Understanding the new access methods and how the parameters affect network performance is vital to ensuring excellent performance.
Access methods in 802.11
The original 802.11 standard defined two access methods for wireless networks: Distributed Coordination Function (DCF) and Point Coordination Function (PCF).
DCF uses Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA). A station waits to begin transmitting until it senses that the wireless medium is quiet. Unlike Ethernet, stations cannot always detect collisions, so the protocol specifies that the receiving station transmit an acknowledgement.
When the station senses that the wireless medium is idle, it generates a random number within bounds defined by the Contention Window parameter. The station then begins to count down from the random number. If the medium is still free when the countdown reaches zero, the station begins to transmit.
PCF defines a polled protocol. It has not been widely implemented. Neither DCF nor PCF supports any type of data prioritization.
New access methods in 802.11e: EDCA and HCCA
The 802.11e standard defines Hybrid Coordination Function (HCF). HCF replaces both DCF and PCF with new access methods to provide improved access to bandwidth and reduced latency for high-priority traffic. The access method called Enhanced Distribution Coordinate Access (EDCA) extends DCF, and the access method called Hybrid Controlled Channel Access (HCCA) is an extension of PCF.
EDCA specifies four access categories (ACs), each corresponding to a class of data. Four parameters are configured for each access category:
- CWmin -- Minimum Contention Window
- CWmax -- Maximum Contention Window
- TXOP -- Transmission Opportunity Limit
- AIFS -- Arbitration Inter-Frame Space
Configuring these parameters for each class of data enables network managers to tune networks for their mix of applications and traffic loads.
As in DCF, a station with data to send waits until the medium is idle, but with 802.11e it then waits an additional period. The length of the additional period depends on the type of data to be sent. The Arbitration Inter-Frame Space (AIFS) value configured for the access category of the data defines the additional wait period.
AIFS should be configured with a small value for the access category for voice data while AIFS for data such as e-mail and FTP should be configured with a large value. Voice requires low latency. A small AIFS means voice data will begin the next stage of the network contention process sooner than less-sensitive traffic.
After the AIFS period, the station generates a random number between the Minimum Contention Window (CWmin) and the Maximum Contention Window (CWmax). The access category for high-priority data should be configured with a low CWmin and low CWmax.
The combination of AIFS, CWmin and CWmax should be configured so that, in most cases, high-priority data gains access to the network. The sum of AIFS plus CWmax for high-priority data should be greater than AIFS plus CWmin for low-priority data so that the low-priority data is not completely blocked.
The Transmission Opportunity Limit (TXOP) for an access category defines the maximum length of a single transmission. If the data to be sent is too large to transfer within the TXOP limit, the station splits it into multiple transmissions.
TXOP for voice data should be small, since voice packets are short. For FTP, e-mail and Web data, a larger TXOP should be configured so that when data is sent, there will be less need to fragment it into multiple transmissions.
Access points (APs) can control network load through the use of transmission specifications (TSPECs). An access point can request that each station send a TSPEC request for each access category. The request specifies the station's requirements for that access category in terms of expected volume of data and how much delay can be tolerated. If an access point calculates that the total requests it receives from stations exceed network capacity, it will deny requests. If a request is denied, the requesting station cannot send data at that access category and must combine it with data at a less demanding one.
Like PCF, HCCA is a polled protocol. When used, it can always gain access to the medium because it waits a shorter time than the shortest AIFS of any EDCA user. HCCA enables individually configured QOS settings for each application. The Hybrid Coordinator (HC) located in the access point polls individual stations and grants access to the medium based on the specific QOS settings that have been configured. There is no contention, so latency for high-priority data does not suffer as total network traffic increases.
IEEE 802.11e provides QOS and network performance enhancements needed now by applications such as voice and multimedia. As new applications are developed and experience is gained with 802.11e, further work and additional protocol extensions will undoubtedly be needed.
David B. Jacobs has more than twenty years of networking industry experience. He has managed leading-edge software development projects and consulted to Fortune 500 companies as well as software start-ups.