Cisco Support Community
Showing results for 
Search instead for 
Did you mean: 
New Member

Yet another data rate question...

I am a wireless systems engineer (voice & data), tasked with designing an appropriate antenna system configuration for a Cisco partner to resolve a particularly difficult situation. To generate an appropriate response to our client, I need to better understand the data rate fallback mechanism and algorithms. The general question is, "are the fallback data rates discrete or continuous?"

The system in question is point-to-point, using Aironet 350 bridges. Assume that the bridges are programmed to accept all data rates, and are operating at exactly -85dBm, with an 11Mbps data rate. If the input signal level slowly drops to the -89dBm level specified for a 5.5Mbps rate, does the data rate decrease gradually (i.e., ~8Mbps at -87dBm), or is a new rate negotiated only when it reaches the -85dBm level?

Associated with that, is a new data rate negotiated as soon as the input level drops below -85dBm, or does the rate remain at 11Mbps between -85dBm and -88.99...dBm ?

Finally, how long does the level have to remain at a lower or higher level before a new rate is negotiated?

My thanks for any information you can provide.

Best Regards,

Jon "Buddy" McManus

Mgr., Systems Analysis & Design

Wireless Communications, Inc.

New Member

Re: Yet another data rate question...

Receive Sensitivity is a minimum value. At -85dBm you get 11Mbps. At 84.5 you get 5.5Mbps.

HOWEVER, you will always have to deal with variables in the environment - that's what your SOM or Fade Margin is for, so don't count on a consistent, gradual drop from 11 to 5.5 to 2 to 1. I had one installation where the data rate drop from 11 to 2 took about 3 feet in a straight line.

If you need specific data rates in specific locations, there are a few options for forcing pseudo-QoS to get the kind of results you are talking about.

Matthew Wheeler

Chief Wireless Architect

CreatePlease to create content