Why one video stream or a room full of chatty devices can slow a whole Wi-Fi network. See how much of the channel multicast takes at a slow basic rate, when sending each client a unicast transmission is more efficient, who loses frames, and what sleeping clients do to the timing.
A frame sent to a group (multicast or broadcast) has to reach every client at once, so the AP sends it at one of the network's basic rates, usually the lowest. It is sent once, with no acknowledgement and no retry, so a client that misses it just loses it. And if any client is asleep, the AP holds group frames until the next DTIM beacon. A client that would get 500 Mb/s on its own receives the stream at 6 Mb/s, or 1 Mb/s in 2.4 GHz with 802.11b rates on.
Multicast costs the same however many clients receive it; unicast grows with every client. Click or drag on the chart to change the number of clients.
Multicast is sent once with no retry, so a client with a weak signal simply loses frames. A unicast frame is acknowledged and resent until it arrives. Try a 24 Mb/s basic rate with one client far away.
Its own rate: the rate the AP would use for unicast to this client. Lost as multicast: frames it misses when one frame goes to everyone at the basic rate, never resent.
Only multicast waits for the DTIM. Unicast frames aren't held for it: a sleeping client collects its own when it wakes.
↗ opens a term's full entry in the Wi-Fi Glossary, which explains over 400 Wi-Fi terms with their IEEE 802.11 references.