Forum

I have been searching around google for some time looking for information on rssi as it relates to wireless networks.

I think there was a post on this forum before all the december ones disappeared.

Can anyone point me in the direction of a good resource that explains rssi in detail.
I am interested in whether I can tie up a particular rssi level with the performance of a connection, and what minimum rssi level will give a working connection.

Thanks in advance

Here you go...

Receiver sensitivity is the minimum signal level allowable for a receiver to be able to decode received RF. For radios, 0 is equal to 1mW. A negative dBm reading such as -10dBm gives a value of .1mW. Different modulation types have different receive sensitivity requirements, i.e. receive sensitivity is data-rate dependent. So, while a radio may provide a receive sensitivity of -85dBm at 11Mbps, it could also successfully demodulate a weaker signal, -94dBm, at 1Mbps.

RSSI is received signal strength indicator and is a value proportional to the strength of the received RF signal. Sometimes this is viewed as a percentage (which can be very confusing); usually you'd want to display it in dBm.

If your receiver noise floor (the amount of RF interference in an environment) is -95dBm ( -100dBm is less noise than -95dBm) and the signal strength is -85dBm then the Signal to Noise Ratio (SNR) is 10dB. This would be your RSSI. Typical decent RSSIs in a WLAN environment are commonly found between 10 to 20dB.

Take a look at the receive sensitivity ratings for a Cisco Aironet PCM352 wireless PC card:

1 Mbps: -94 dBm
2 Mbps: -91 dBm
5.5 Mbps: -89 dBm
11 Mbps: -85 dBm

Compare that to a Lucent/Agere/Proxim Orinoco Gold card that has:

1 Mbps: -94 dBm
2 Mbps: -91 dBm
5.5 Mbps: -87 dBm
11 Mbps: -82 dBm

Not a lot of difference in those numbers however dBm math can be deceiving. The 3dBm difference between the 11Mbps rates means the Orinoco will have half the power of the Cisco card. Plus, the Orinoco can only go up to 32mW while the Cisco can do up to 100mW. This allows for much greater range and throughput at the higher datarates.

Hope that helps. Some of this information was gleaned from the book, "802.11 (Wi-Fi) Networking Handbook" by my good friends Neil Reid and Ron Seide.

Joel

Joel

thanks for your quick and clear reply. Just what I wanted. I will check out your friends book!

PS congratulations on your double promotion I noticed on another thread!

Regards

Andrew Hollister
CWNA

Glad to assist and thanks as well.

Joel

I don't think it has much impact, in the
real world, but only in relative terms - what you're
saying.

e.g. if all you need (to get the wireless signal
working in your house) is a small antenna
inside a pc-card, and it's made by Orinoco,
it doesn't matter to me whether Best Buy or
Fry's Electronics is giving away 20 dB gain
external antennas free of charge or not.

If something is not needed - a high gain antenna -
then it doesn't matter to most people what that
something does or is capable of doing.

The point I was trying to make was to clarify what RSSI was and how it is used in wireless networks and then try to put that into layman's terms. The only way I knew to do that was to compare actual, real world values a wireless engineer might see in a corporate environment. We weren't talking about what "most" people might experience. We were talking about what a CWNA or CWAP might experience. ;^)

And, as a matter of fact, there are folks out there that care about range and RSSI values, especially in enterprise environments. I know I do.

Joel