I am working now on a project involving the use of multicasting on a wireless network. If I send multicast packets through a Wireless LAN Access Point, at which rate will those packets go to mobile clients?

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Wireless LANs transmit unicast, broadcast, and multicast frames. Multicast throughput can be impacted by both basic rate set and power-save mode.

Basic Rate: APs beacon a bit mask identifying the set of basic rates they can support. To associate with that AP, stations must support one of those rates. According to the 802.11 standard, all broadcast and multicast frames must be transmitted at a basic rate that is supported by all stations currently associated to the WLAN. You may see 1 or 2 Mbps being used as the basic rate in a mixed b/g WLAN.

Power Save: If any associated station is using power-save mode, the AP must buffer multicast frames and send them immediately following the next Delivery Traffic Indication Message (DTIM). To learn more about multicast throughput and power-save mode, read this article by Jim Geier.

This was first published in August 2006

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