dBm/signal strength on GSM -- acceptable numbers?

TheGSRGuy

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Would someone care to explain how to read and compare the dBm signal strengths showed on phones? I'm trying to do a side-by-side comparison of RF performance and want to see how significant a 1 dBm difference is.


From Wikipedia (if this is even correct):
Zero dBm equals one milliwatt. A 3 dB increase represents roughly doubling the power, which means that 3 dBm equals roughly 2 mW. For a 3 dB decrease, the power is reduced by about one half, making −3 dBm equal to about 0.5 milliwatt. To express an arbitrary power P as x dBm, or go in the other direction, the following equations may be used:

x = 10 \log_{10}(1000P)\, or x = 10 \log_{10}P + 30\,

and

P = 10^{(x/10)}/1000 \,



Case in point: if one phone shows -75dBm and another right next to it shows -72dBm, is that a significant difference? What's the threshold of "noticeable difference"?

Are there even acceptable ranges that the signal strength should fall within?

I'd like to compare two phones side by side here, so any info someone can share would be helpful. Thanks!
 
The difference between -72dB and -75dB won't be noticable because they are both well above the minimum threshold of somewhere around -100dB to -105dB for GSM. If you take the 3dB difference between -99dB and -102dB then it could mean the difference between dropping the call or not. Generally, you can say, for GSM:

-105 to -100 = Bad/drop call
-99 to -90 = Getting bad/signal may break up
-89 to -80 = OK/shouldn't have problems, but maybe
-79 to -65 = Good
Over -65 = Excellent

take your 2 phones to an area where the reception is boarderline (ie: a basement) and see how they perform.

....but keep in mind, it's not the phones reception but it's uplink transmit power that limits the range of the phone.
 
The Blackberry 8310 is GSM only, while the iPhone 3G is GSM/UMTS, and was on 3G in all your screenshots. So you are comparing 2G reception on your BB and 3G reception on your iPhone 3G. You're comparing "Apples" and oranges (pun intended :hehehe: )

Anyway, I don't like iPhones and will agree with you anyway that the BB probably does have better reception. But next time switch your iPhone into 2G mode only (if possible), then re-do the test.

PS- While GSM drops at about -100 to -105, WCDMA can hang on to around -115dB because GSM uses a single narrowband signal versus WCDMA wideband and RAKE receiver and soft-handover that can combine signals from multiple basestations...

PPS- The "crazy" UMTS frequencies are actually the UARFCN, or channel numbers. You can convert the UARFCN to it's actual frequency using my on-line calculator (or download the tool to your desktop):
http://www.radioraiders.com/umts-frequency.php
 
I actually turned off 3G and repeated the test this morning. Didn't have time to post to my Tumblr page again.

The Curve was still showing better reception, though the disparity narrowed. Still sunk to -110ish on the iPhone, but never below -85 on the Curve.
 
TheGSRGuy said:
I actually turned off 3G and repeated the test this morning. Didn't have time to post to my Tumblr page again.

The Curve was still showing better reception, though the disparity narrowed. Still sunk to -110ish on the iPhone, but never below -85 on the Curve.
I think -110dBm is about as low as you can go in GSM. You may get a signal, but probably only strong enough for signaling, you probably wouldn't be able to make a call.

...do you mean in places the iPhone had -110 the BB had -85dBm? That's a 25dB difference. That's a huge difference. You're sure they were both on GSM? :confused:
 
Also, can you clarify why I would see a value of 65535? Seems like the phone is just showing an upper boundary/limit.
 
RadioRaiders said:
I think -110dBm is about as low as you can go in GSM. You may get a signal, but probably only strong enough for signaling, you probably wouldn't be able to make a call.

...do you mean in places the iPhone had -110 the BB had -85dBm? That's a 25dB difference. That's a huge difference. You're sure they were both on GSM? :confused:

Positive. The disparity is monstrous between the two.

In areas with strong signal, the meters are within a few dBm. When it's a weaker area, the disparity grows to 20dBm or more.

Specifics: http://dparm.tumblr.com/post/101866591/at-t-signal-testing-round-2
 
TheGSRGuy said:
Also, can you clarify why I would see a value of 65535? Seems like the phone is just showing an upper boundary/limit.
That value doesn't make sense in UMTS or GSM terms.

Coincidentally, in IP terms, IP addresses can have a maximum of 65535 ports. But I'm not sure why the iPhone is showing that instead of a UARFCN? Maybe some software bug...or maybe Steve Jobs just likes and understands IP numbers more than radio numbers? :hehehe: ...Apple may know compuers, but not radio, which is why I'd never buy a phone from them (I don't really like their computers either, but thats a different topic ;) )

http://en.wikipedia.org/wiki/List_of_well-known_ports_(computing)
 
Believe me, I'm in IT -- I understand the whole TCP/UDP ports idea. :)

I'm assuming that 65535 value is just a bug in the software. 65535 is 2^16.

I went ahead and ordered a Bold since I'm eligible for a device upgrade. Everyone says the RF is a lot better, especially here in Chicago. I considered the Nokia E71 but there were some other Hofo members that said the Bold was getting better signal than their E71. (also, AT&T doesn't have the E71 yet)
 
TheGSRGuy said:
Positive. The disparity is monstrous between the two.

In areas with strong signal, the meters are within a few dBm. When it's a weaker area, the disparity grows to 20dBm or more.

Specifics: http://dparm.tumblr.com/post/101866591/at-t-signal-testing-round-2
While you may have a defective iPhone, those numbers are too big to be believable. I can be believe a 3dB-6dB deifference in marginal areas but a 20dB difference. It seems that one might be parked on 1900Mhz, while the other one on 850. You did turn off 3G, right? You know of course that measurements of phones in the idle state are totally misleading, right? Just because a phone is parked in idle on a 1900MHz channel that does not necessarily mean that that is where the actual conversation is going to take palce. The measurement shown is that of the control channe, not of the channel you will end up. Are you sure that they're both on the same UARCFN?
 
dBm/signal strength on GSM

For GSM:
-105 to -100 = Bad/drop call
-99 to -90 = Getting bad/signal may break up
-89 to -80 = OK/shouldn't have problems, but maybe
-79 to -65 = Good
Over -65 = Excellent


Are these numbers in dBm??? Where can I find this info in a technical document?
 
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