Archived from groups: alt.internet.wireless (
More info?)
The 802.11 standards do not prescribe how the fallback scheme works. Here's
clause 9.6 ("Multirate Support") from the 1999 802.11 standard:
"Some PHYs have multiple data transfer rate capabilities that allow
implementations to perform dynamic rate switching with the objective of
improving performance. The algorithm for performing rate switching is beyond
the scope of this standard, but in order to ensure coexistence and
interoperability on multirate-capable PHYs, this standard defines a set of
rules that shall be followed by all STAs."
The rules merely state that the transmit rate must be among the basic or
mandatory set of rates, and must also be in the set of rates known to be
offered by the receiving station. Other than that, the transmitter is free
to hop around among supported rates however it pleases. In practice, all
vendors implement an orderly fallback scheme, but it may differ somewhat
from vendor to vendor, or be affected by configuration settings. For
example, it is usually possible to configure an AP router to support "g"
only, which usually means it will not connect clients using 802.11b rates or
encodings, the purpose being to avoid hybrid nets which necessarily have
degraded performance.
I may have misled a bit when I referred to 11, 5.5, 2, and 1 as 802.11b
rates. Support for these rates and the associated encoding schemes is
mandatory in 802.11g. The set of required rates is defined in clause 19.1.1
of 802.11g:
"... transmission and reception capability for 1, 2, 5.5, 11, 6, 12, and 24
Mbit/s data rates is mandatory."
Obviously most vendors implement all of the optional OFDM rates up to 54
Mbps. But the support requirement does not dictate which transmit rate must
be used in a given situation.
"Phil Thompson" <cynical_observer@hotmail.com> wrote in message
news:0nlk70d08gks8gdssb66c6lca23olcv833@4ax.com...
> On Sun, 11 Apr 2004 19:32:40 GMT, "gary" <pleasenospam@sbcglobal.net>
> wrote:
>
> >Right. 802.11g falls back (eventually) through 12, 9, then 6 Mbps.
802.11b
> >uses a completely different encoding scheme, and the bitrate sequence 11,
> >5.5, 2 and 1 Mbps.
>
> g includes this latter encoding scheme and drops back to it once the
> signal is too weak for 6 Mbps. The spec sheets I have read for 802.11g
> devices all quote the same encoding and sensitivity at the ragged edge
> as they quote for b, as g uses OFDM then drops back to DSSS
>
> Phil