Maximum RSSI / SNR on 1242AP

Unanswered Question

We are using Cisco 1242 APs to do short building-to-building bridge shots using 802.11a frequencies. Is there a Cisco-recommended maximum RSSI or SNR value for such a link? I have heard from various link installers that too hot a signal can burn out the radios. These are links of up to 300 metres if that matters.

Thank you.

I have this problem too.
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Thank you for your response fella5. After looking through the data sheet I should probably clarify my question. The different output levels, i.e. 50mw, coupled with a 21db gain antenna could put us over the FCC limit of 4000mw at the end of the chain but that's not really what I'm after.

When the remote installer set the gear up so I could configure it specifically for that store he put the two 9.5db Cisco antennas facing each other a few inches apart. When I logged in and did a show dot11 association mac it showed an RSSI of -18db, which was definitely the hottest signal I had ever seen.

My instincts, coupled with what others have told me, said that might be bad for the radios and I immediately asked him to point the antennas away from each other while I completed the configuration.

The bottom line of my question is - was I correct in worrying about the radios being damaged in this case? Is there a Cisco-recommended maximum for this number in a permanent point-to-point link?

If the answer to this question was actually in the data sheet I'm afraid I don't understand how to interpret it and would really appreciate the explanation.

Scott Fella Fri, 06/13/2008 - 08:47

I understand. the data sheet shows you rssi for a given tx and antenna gain. For 300 meters you will not get 54, 48, 36. You have to look to see if you have too much interference in that channel. There should be a channel scan test you can do to see the noise. Changing channels and or antennas might help. Also, the length and type of cable from antenna to radio will affect your db gain. The antennas must have coax seal to prevent moisture from degrading the signal and cable life.

Thank you again for your very prompt response. I still fear I'm not making myself clear enough. The link is up and the connection is solid at 54mbps (the distance in this case must be well under 300m but since I'm hundreds of miles away from the site I have no precise measurement).

My question is related to it possibly being "too good". When I told the senior installer who was overseeing the antenna erection that I was seeing a signal to noise ratio of 48dBm he suggested I lower the transmit strength because that was "too hot" and could damage the radios. I just need to know if what he is refering to is applicable in this case. I also would like to know if there is a signal strength threshold that Cisco defines as "too hot" and potentially damaging to the radios.

I would appreciate your insight.

Scott Fella Fri, 06/13/2008 - 09:45

Ususally a SNR of 25 or greater means you have a pretty good signal without too much noise. I have never heard of being too hot and damaging the radios. What type of antennas are you using.

Fella5, I had not heard of such a thing either, however I thought it might be related to the danger of powering up a radio like the 1242 without a terminator or an antenna of the correct impedance on the connectors. I actually don't know the specs on the antennas because a substitution was made in the field due to connector compatability issues with the lightning arrestors. I suspect they are a little more substantial than the 9.5 db Cisco patches I sent up.

Perhaps someone else has heard of this before?

Scott Fella Fri, 06/13/2008 - 10:51

For one, You need to make sure you have selected eith primary right or left side. Damagind of the radios is when you have an antenna connected on one side and the other side is open and you specify diversity. Verify which port the antenna is going to and make sure you set that.

dennischolmes Mon, 06/16/2008 - 07:20

To all. YES you can be too hot for the radios. That is why when testing in the lab that Cisco suggests powering the units at low power. You have several receive components in the radio that can be severly damaged if they receive to much RF radiation. As Scott said above, a good SNR is around 24-25. A little over is not going to hurt the radios but at full power and with 21db gain antennae this is not going to make anyone happy here. TAC, FCC, or anyone caught between the link for long periods of time(they might just glow blue). Power it down by half. Verify the link. Take some external readings from airmagnet or cognio just to verify each end of the link and you should be good to go.


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