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New Member

RSSI Delta between Phones


If I have a reference point (same place, AP fix settings etc.) and check the RSSI on some IP Phones (same typ 7921), wich delta (RSSI) must I accept?

I checked 4 phones.

Three phones shows me -42dBi - -50dBi and an other one shows me -64dBi.

The area around my reference point is static. No passengers, no traffic, no production equipment. Indoor.

I can't thinking, that a delta of 18dB is Ok.



Re: RSSI Delta between Phones

For my experience, A fade margin around 5 - 10 dB should be OK; the last phone seems to be broken because the different of the RSSI is too big.

The others seem to be into this margin.

Best regards.

New Member

Re: RSSI Delta between Phones

Hello Raul,

thanks for you answer.

See my comment to Serge.


Cisco Employee

Re: RSSI Delta between Phones

Hi Sven,

I suppose the phones are next to each other and you have LOS right?

Yes that -64 is way below the delta diff to me as well.

This can happen sometimes when you have multipath and no LOS.

You need to check the rssi via the site survey tool which i suppose you are doing.



New Member

Re: RSSI Delta between Phones

Thanks for your answer.

Here an additional info.

I declair a fix point as a reference point in the middle of a midsize hall (40m x 10m).
On both ends there are the two access-points.
Fist access-point works on channel 1 with power setting 3
Second access point on channel 6 with power setting 3.

I place the first 7921 phone, start the function SiteSurvey and stop it after 1,5 minutes.
After that I read the sitesurvey result from the phone over IE.
(The delta between min. RX and max. RX is ~8dBi)

I do this 5 times (  prevent for a temporary outbrake) and calculate the results (add up and divide it with 5)


AP1 42/38 43/35 46/41 43/35 43/40
AP2 43/42 44/42 45/43 44/42 45/42

Calculated  AP1= 40,6dBi  AP2= 43,2dBi

Then comes the next phone on the the same place.

And here is my problem.
Like Raul told, I think too, that a different of 5-10dBi (realy*) is OK between phones on same typ (7921).

But, which is the delta between phones where I can say "it's not ok , the phone is faulty"??????

* Think
Cisco told for Voip: do sitesurvey with the phone.
If you have a good phone you mark your cell border with -67dBi.
With the next phone you have -77dBi on the same border, only (10dBi delta between phones)

That's not good and could not be.

Regard Sven
(sorry about my bad english)

Cisco Employee

Re: RSSI Delta between Phones

Hi, if this is bound to only one phone then yes there could be something wrong on the phone.

One test would be to put the 2 phones next to eachother on a given location wihtout moving and checking the rssi of the AP through the site survey tool.

You still have very low RSSI on the phone and a strong one on the other although they are in the same position?

If yes, then the phone has something wrong and you can request an RMA for it from TAC?

Note that Human body and the way the phone is held can also cause attenutation on the signal from the AP.


Re: RSSI Delta between Phones

Only wanted to make a small comment.

We have over 2000 7921 / 7925 phones. If they get dropped a few times the antenna connector disconnects inside the phone. In fact, we just went though an entire hospital replacing bad phones ... Out of 300 phones 50 were replaced...

Check for little chips or cracks on the antenna of the phone with the bad signal...

__________________________________________________________________________________________ "Satisfaction does not come from knowing the solution, it comes from knowing why." - Rosalind Franklin ___________________________________________________________
New Member

Re: RSSI Delta between Phones

Hello George,

thanks for your message.

That's what I suspect.

The phones loose his qualtity by physical influence (dropping, eruption etc in a typical business enviroment).


New Member

Re: RSSI Delta between Phones

As I told before,

complete the same condition. (especialy place and phone direction)

If I use both phones close together at the same time, I have the same delta.

BUT, the question is not "is this phone faulty"

The question is "which delta is acceptable and when can I told, this delta is to high, this phone must be faulty"