What basic rates do 802.11b/g Cisco routers use?

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Apr 29th, 2008
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Hi, I am developing an analytic model for the operation of 802.11b/g. Thus, I need some implementaion info. For 802.11b the specified basic rates-not data rate- are {1,2,5.5,11} Mbps; and for g they are {6,9,12,18,...,54} as I know. That is the thoery. But I want to know how things work in practice. How is the implementation of Cisco routers? Do the routers choose 1Mbps for b and 6 Mbps for g by default? Is this rate changable by users? Or does basic rate also adapt like data rates? I am grateful for any kind of information of how routers implement the 802.11b/g MAC- in terms of basic rate (not DATA RATE-I am concerned with the rate that RTS/CTS/ACK's are sent)? Also it would be very well if you know the PLCP header&preamble rate implemented?

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Overall Rating: 4.5 (2 ratings)
bcolvin Tue, 04/29/2008 - 09:21
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Good question I will attempt to answer.

First Basic/Required rates for B/G 2.4Ghz systems. As you stated most bg AP's have 1,2,5.5 and 11 sset to basic/required, terminology determined by manufacturer. In a mixed environment none of the g data rates can be set to required as non of the b clients would be able to associate. for a pure g environment normally only 6 is set to basic and the b data rates are disabled. the default configuration for the Cisco AP's is for all the b data rates to be set to basic and the remainder set to enabled. for the a radio 6 is set to basic and the remainder to enabled.

I would not be woried about the RTS/CTS/ACK traffic when i sniff the wireless there is very little of this traffic as far as bandwidth is caoncerned but read the book below,

second the headers/preamble and all management and control frames are sent at the lowest basic rate.

this link is for an excelent online book that covers all this in detail for the 802.11 standard




cananozgu Tue, 04/29/2008 - 23:49
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Dear Bill,

Thanks for your answer.

What I understood is that Cisco routers use the lowest basic rate, while communicating to a 802.11b device they use basic rate 1Mbps, while comunicating to g device they use basic rate 6Mbps and does never use the basic rates {2,5.5,11,12,higher}. When there are both b ang g devices around the router, then the basic rate is set to 1Mbps. That is the implementation of Cisco routers, right?

Could you also provide me with a reference containing the implementation of basic rates of Cisco routers (so that I can give a reference in my article)? I searched through the datasheets but there is no info about the basic rate (only data rate is specified).

Best regards,


bcolvin Wed, 04/30/2008 - 09:25
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1. The default configutation for bg radios is is to sset all the b rates to basic and enable all the g rates. this requires all the devices that attempt association to support the data rates set to basic. if you had an old DSSS device that only supported 1 and 2 mb it would not associate if data rates 5.5 or 11 were set to basic. in a mixed environment the beacon will be transmitted at the lowest basic rate and will never be sent at 6 unless it is the lowest basic rate which would not any b only devices to connect as they can not decode ODFM.

to reiterate in a mixed environment all basic comunication is at the lowest b rate set to basic. the cisco default is 1,2,5.5,11 set to basic. none of the g rates are set to basic and all devices b and g will communicate at these rates for preamble, header, management and control. the data load of the packet will be transmited at the highst data rate determined by hte client capabilities and signal strength.

2 try this link


good luck



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