Wi-Fi Clients Follow the Data Rates Advertised by the AP

As a follow-up to my previous post on Wi-Fi probing behavior and configured data rates, I thought I'd round-out the discussion showing how the client ultimately follows the AP's lead when it comes to communicating it's Basic and supported data rates when it connects to the WLAN.

It's very important for an AP to ensure that the clients attempting to connect to the WLAN support the mandatory (termed 'Basic') data rates configured as part of the network policy. Doing so ensures that all clients can properly decode and receive management frames like beacons as well as broadcast and multicast traffic that is essential to successful network connectivity, which are all sent out using one of the Basic data rates. Beacons and broadcasts are typically sent at the lowest Basic data rate, while multicast may be sent out at the highest Basic data rate depending on the AP vendor.

The communication of the Basic and supported data rates must occur in both directions, so both parties can understand the capabilities of the other.

Clients that are actively scanning send out a list of supported data rates, but don't mark any as Basic since that is the responsibility of the AP. This is just a raw list of data rates the client is capable of transmitting and receiving.

Probe Request Supported Data Rates

APs advertise it out during beacons so that clients that are passively scanning can make decisions on whether or not they can connect to that AP. APs will also include their list of supported rates in the probe response. Both frame types contain the same list of Basic and supported rate information, distinguishing which rates are mandatory for operation in this BSS (basic service set). Notice that 802.11n rates are carried in the HT Capability Information Element, listed as supported MCS sets for various numbers of spatial streams. Clients will include this element in the association request as well.

Probe Response Basic and Supported Data Rates

Finally, once it comes time to connect, the client will actually adjust it's supported data rate list contained in the Association Request frame in order to align with the policy advertised by the AP; this time listing which data rates are Basic (mandatory) and which ones are supported. Differences between the client and AP may still exist for the supported data rates, but the Basic data rates must match. Using the list found in the association request, the AP makes the determination whether or not the client is allowed to connect to the WLAN.

Association Request Basic and Supported Data Rates;
"Data Rate Follow-Back" behavior of Wi-Fi Clients

I call this the "Data Rate Follow-Back" behavior of Wi-Fi clients :) You won't find that in any book as a standard term, it's just my own terminology.

Cheers,
Andrew