802.11k makes WLANs measure up

The IEEE 802.11 standard for wireless LANs  enables interoperability between different vendors' access points and switches, but it does not let WLAN systems assess a client's radio frequency resources. Consequently, this limits administrators' ability to efficiently manage their networks.

As a proposed standard for radio resource measurement, 802.11k aims to provide key client feedback to WLAN access points and switches. The proposed standard defines a series of measurement requests and reports that detail Layer 1 and Layer 2 client statistics. In most cases, access points or WLAN switches ask clients to report data, but in some cases clients might request data from access points.

Work began on the standard in late 2002, and the IEEE expects to ratify it next year. Because 802.11k is designed to be implemented in software, existing WLAN equipment can be upgraded to support it. For the standard to be effective, both clients (WLAN cards and adapters) and infrastructure (access points and WLAN switches) will need to support it.

Here are some of the measurements 802.11k defines:

• Roaming decisions.
  
  

• RF channel knowledge.
  
  

• Hidden nodes.
  
  

• Client statistics.
  
  

• Transmit Power Control (TCP).
  
  

To improve roaming decisions, access points or WLAN switches can provide a site report to clients. The standard defines a beacon request, in which an access point asks a client to go to a specific channel and report all the access point beacons it hears. The access point collects the data, and it or a WLAN switch will analyze the beacon information, looking at details such as what services and encryption types each access point supports and how strongly the client heard the access point. Then the switch or access point generates an ordered list of access points, from best to worst service, called the site report.

Currently, access points and clients cannot share channel information. With 802.11k, an access point could have a client build a "noise histogram," which will display all non-802.11 energy on that channel. An access point also can request data about channel load or how long the channel was used during a given time. An access point or WLAN switch then will know if there's too much interference or traffic on a channel to use it for WLAN services.

Hidden nodes are clients or access points that other clients or access points cannot hear. In 802.11, nodes listen to the airwaves before transmitting to avoid collisions. When a hidden node is present, multiple nodes can transmit simultaneously, creating interference that degrades WLAN performance. With 802.11k, clients track hidden nodes and access points query clients for those lists. This information tells access points about clients on the edge of their cells. Access points can use the information to direct clients to access points from which they would get better service.

Client statistics are limited today to statistics that access points or WLAN switches maintain. Today's WLANs track items such as retries, packets transmitted and packets received. With 802.11k, access points and WLAN switches can query all clients to get reports on their statistics. With both data sets, a WLAN system will have a more complete view of network performance.