Wednesday, March 2, 2011

Wireless Roaming and its Effect on Quality

Roaming occurs when a handset moves out of the range of one access point into the range of another. It gives users the mobility to move around within a local coverage area and still be connected to the network. However, roaming is one of the primary reasons why users experience problems on wireless networks. Excessive roaming times lead to poor quality for voice and video over wireless and can lead to dropped calls or data connections.
Roaming usually involves a channel change, but that depends on the type of technology deployed. If it's a multi-channel architecture, which is most likely the case, a channel change is required. When roaming occurs, the client needs to be re-authenticated and re-associated with the new access point, which takes longer than 150 milliseconds in most instances, especially when advanced features like WPA2 and WMM are in use. Most organization's wireless networks are outfitted with multiple access points (APs) and users can experience poor signal strength and performance despite proper coverage in the area if the client is connected to the "wrong" AP. Even in the most modern, centrally managed systems, the wireless client is the one who decides when to switch from one AP to another. This decision is typically determined based on the current signal strength and is executed by the underlying software controlling the wireless client radio (the "supplicant"). This software is different from manufacturer to manufacturer and from device to device, so the way the decision to roam is made varies widely. In most cases, the wireless client will wait too long and as a result the available signal strength lowers, before the client switches to an AP with greater signal strength.
New and improved standards are available that specify the conditions for "fast roaming," enabling transitions that take as little as 5 - 10 milliseconds. These specifications include:
  • 802.11i - with opportunistic key caching so there is no re-authentication step
  • 802.11r - fast BSS transition, which optimizes the hand-off as clients move from one access point to another
  • 802.11k - radio resource management of WLANs allows re- authentication to be maintained between multiple APs and has predictive capabilities
These new standards (802.11i isn't new, but it's still part of an improving situation for roaming) allow APs greater control in determining when roaming should occur and the APs are more in tune with the current performance of, and demands on, the wireless network. However, this situation is even better when the overall wireless network is under the control of a centralized manager. The issue is that adoption of 11k and 11r has been very slow, especially in wireless clients, and until adoption increases significantly users will continue to suffer slow AP transitions when roaming, leading to poor voice and video over IP performance.
In the meantime, the best approach is to carefully monitor and analyze the roaming activity on your network. Obtaining a complete and accurate view requires real-time aggregation of data from multiple channels and APs, with integrated analysis that leads to detailed reporting - who is roaming, how long each event is taking and what does the average look like for each AP. The end result is simple, yet the process is complex, demonstrating why proper network analysis tools are key to staying productive.

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