Site Survey S/N Ratio

[mchalej]

When using the ACU site survey function what are acceptable S/N ratios for the different transmisstion rates. I have heard in the past that a S/N ratio of 12dB is adequate for 11MB transmission. Is there any Cisco documentation to support this.

[ED CARMODY]

The answer is "it depends." Cisco AIR-PCM352 cards have about a -3db better receive sensitivity at thet 11MB rate than most cards. Cisco 352 cards tend to rate shift from 11 to 5.5 somewhere around 20db, depending on other factors.

If you do a site survey, you have to know what clients you're going to have to support. I'd bet that at some point, you're going to have to support non-Cisco cards.

My recommendation is that when you use a Cisco card to do a survey, give yourself an extra 3db of wiggle-room. So, to support 11MB data rates, I survey for a 23dB s/n ratio.

But, if you have to support voice....

[mchalej]

Tha number seems high. The Aironet site survey I class I attended, that was supposed to be supported by Cisco, stated the minimum S/N ratio for 11MB was 12dB. Was this class incorrect.

[ED CARMODY]

Technically, no, they are not incorrect. A Cisco card *can* maintain an 11Mb datarate at 12db, but your packet loss can start to be an issue.

Many device manufacturers state that their products work satisfactorily with an SNR of 20 dB or better.

[kschung]

20dB is a very good figure. For whatever its worth, that's my benchmark also. This is from experience.

kevin