Last Post: June 22, 2004:
If I have clients in a BSS operating with the standard one 802.11g to different velocities -36, 48 and 54 Mbps-:
-It should send the AP the information to 36 Mbps, to 48 Mbps and to 54 Mbps to cause to arrive the broadcast to all the clients?
-If the previous answer is affirmative, some order exists?
-How does it detect that the wireless medium is occupied -always, considering clients operating to different velocities-?
Thank you for help me.
All Beacons (which hold the parameter information for the BSS) are sent at the lowest basic rate (which is configured by the administrator). The PLCP preamble and headers of the Beacons are always sent at 6 Mbps in a pure 802.11g environment. The speed of the clients do not affect the speed at which Beacons are sent.
To detect if the medium is busy, the PHY layer does a Clear Channel Assessment (CCA). There are multiple ways the CCA can be performed, but a common way is called "Energy Detect" where it looks for a threshold of RF signal strength on the channel. It knows by this whether or not the channel is currently in use.
How does it function in an environment mixed 802.11g and 802.11b?
Questions like these are why we created the CWAP training course. The answer to your question has many detailed parts.
All Beacons (which hold the parameter information for the BSS) are sent at the lowest basic rate (which is configured by the administrator).
Does the 802.11 standard require that preambles and headers always be sent at 1Mbps, or is this configurable too? Can 2Mbps be configured as the lowest basic rate in 802.11?
You're confusing layer 1 and layer 2 topics. IF we're talking about 802.11 or 802.11b with long preambles, the PLCP Preamble and header are both sent at 1 Mbps all the time (you can't change this). The PSDU (physical layer frame payload) will be sent at the rate configured on the AP for the specific frame type. For example, if it's a Beacon, it's required to go at the lowest Basic (required) rate, which can be set to whatever you want (1, 2, 5.5, or 11 Mbps for 802.11b or 1 & 2 Mbps for 802.11).
IF we're talking about 802.11b with short preambles (remember, 802.11 doesn't support short preambles), then the PLCP preamble is sent at 1 Mbps, and the header is sent at 2 Mbps. These cannot be changed. The PSDU is then sent at the rate configured on the AP for the specific frame type. For example, if it's a data frame, it will be sent at the highest rate configured unless environmental conditions dictate otherwise.