Supported rates

Supported Rates Element

This element is included in the following frames sent from AP.

1. Beacon
2. Probe Response
3. Association Response
4. Reassociation Response 

AP mandates that stations joining the BSS support certain rates.  The rates required by the AP are called basic rates. In other words, basic rate contains the rates that all devices in the cell must support.  All management frames, multicast, and broadcast packets are  transmitted using one of the Basic Rates.

The Supported Rates element specifies up to eight rates. The information field is encoded as 1 to 8 octets, where each octet describes a single Supported Rate.

IEEE 802.11 specification provides mapping value for each data rate. For example,

value for few of the rates: 

02 = 1 Mb/s
   03 = 1.5 Mb/s
04 = 2 Mb/s
   05 = 2.5 Mb/s
 06 = 3 Mb/s
    09 = 4.5 Mb/s
   11 = 5.5 Mb/s

                            Figure 1: BSSBasicRateSet parameter encoding

For the basic rate an encoding is used, which sets bit 7 to 1. Following is a part of beacon frame. As per figure 1, value for 1 Mb/s is 02 (0000 0010) . Notice the hex value highlighted in the following packet. it is 82 (1000 0010) for 1 Mb/s.

This is applicable only for the basic rates (rates marked with (B)). You can notice that the other values after 11(B) which is 0x96 begin with 0.Remaining values are supported rates. (Support for these rates is not required to join a BSS, but a station may choose to transmit at any supported rate that the receiving station supports.)

Extended Supported Rates element

For STAs supporting eight or fewer data rates, this element is optional for
inclusion. For STAs supporting more than eight data rates, this element shall
be included in all of the frame types that include the supported rates element.
The information field is encoded as 1 to 255 octets where each octet describes a single supported rate.

From beacon frame: