| Google for "802.11 basic rate" The basic rate is the rate used when one station (any device with a TX/RX) broadcasts a frame (we call them frames at layer 1 (PHY) and 2 (MAC)).
The transmit rate is the rate used when a station transmits a frame to another specific station. BTW, this has nothing to do with IP, in fact, this is true even if IP is not in use. The low bit of the 1st octet of a MAC address determines whether a frame is multicast or not (technically the basic rate is used for multicast frames, not just broadcast ones). Why is the "transmit" rate so much higher than the "basic" rate? It all has to do with negotiation. If it turns out that when you decide to send a frame, someone else was using the medium, you'll only know this if the other side has sent you an ACK. This means you can do your backoff and try again after a short interval (usually measured in microseconds). It becomes infeasible to expect an ACK from all of the recipients for a broadcast frame however. So instead we simply send broadcasts at a much lower rate. Also note that the transmit rate is constantly changing, based on how well ACKs have been received. This is adaptive rate control. Another thing to note is that when you are using infrastructure mode, stations (like your phone, PC, etc) send multicast frames to the AP and then the AP turns around and blasts it out broadcast style. So in this case, the frame is initially sent quickly as unicast to the AP via the transmit rate, but then the AP sends the same frame out broadcast at the basic rate. Just by having stations continually send multicast frames you can lower the channel to the basic rate for everyone on the same channel. Nothing really sneaky here either. You don't even need to be on the same BSSID since this is a PHY issue. |