Understand how 802.11 multicast traffic can degrade battery life.

If there is significant 802.11 multicast traffic on a wireless LAN, then you may experience relatively poor battery life of your wireless clients. The problem is that the combination of multicast traffic and 802.11 power-save mode makes clients stay awake longer than if there were no multicast traffic. When any single 802.11 radio associated with an access point has power-save mode enabled, the access point buffers the multicast frames and sends them just after the next DTIM beacon (which may be every 1, 2, or 3 beacons – set in the access point). With power-save enabled, a client radio will wake up each beacon and stay awake long enough to receive all of the multicast traffic (occurring after the DTIM beacon). I’ve found that some radios will receive the last multicast frame and then stay awake until after the next beacon (i.e., stay awake for an entire beacon interval, 100ms, after every DTIM beacon). Thus, a flow of multicast frames less than 100ms apart (the typical beacon interval) with DTIM period = 1 (i.e., DTIM occurs every beacon) may cause the radios to stay awake indefinitely. This would draw battery power as if power-save mode was not enabled.

Jim Geier is an independent consultant and founder of Wireless-Nets, Ltd (www.wireless-nets.com), a consulting firm assisting municipalities, enterprises, hospitals, airports, and equipment providers with the development and deployment of wireless networks.