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View Full Version : The Truth about Signal Strength "The bars"




Galen
Sep 1, 2009, 05:23 AM
Lately I have seen a few threads in regards to signal strength, either through the ”how many bars” or using the field test in the Iphone in regards to throughput, drop calls or really the QoS you as a subscriber are receiving or in many cases not receiving.

First off one needs to understand that WCDMA (3G) is not the same thing as GSM (2G) and the bars or even the signal strength can not be compared in the same way, you are not comparing apples to apples. The RSCP values or the signal strength in WCDMA is not the most important value when dealing to the quality of the call from a radio point of view, it's actually the signal quality (or the parameter Ec/No) that needs also to be taken into account.

Not that the average subscriber cares about raw numbers, they do care that they can set up a voice call or data session, that their connection does not drop or they get the throughput promised.

You can easily carry a voice call or have “decent” throughput in even low signal strength (-100dBm to -105dBm for example). I have seen a few threads where users say that signal strength of -95dBm is considered bad, -95dBm is fine and does not increase the chance of a drop call. But if the noise level in the system increases then that will increase the chance for low throughput or dropped calls. I have seen voice calls carried at -110 dBm in good quality single cells environment (ok it was poor voice quality but still no drop.)

To show what I mean you can see from the screen shot I have an RSCP of -105dBm but in the second screen shot I have a DL speed of 3.2 Mbit/s (sorry it’s in Swedish but I think you get the point). Actually the field test in the iPhone is a bit lame so I did have a RF scanner and it was more of an average -102dBm. I did three test during th time, the lowest being around 1.5 Mbit/s while the image is from the best.

http://img17.imageshack.us/img17/9932/img0081.png http://img90.imageshack.us/img90/3348/img0082.png

There are so many factors in getting good service and while the signal strength is important (without coverage there can be no service) there are other important parameters such as radio quality (or a “clean network”) that need to be taken into account. The air interface is just a small piece of the network and many times is not the bottle neck which has been pointed out a few times.

Maybe I will add the truth about 3G throughput (what does 3.6, 7.2 14.4 or 21 Mbit/s or really mean to the end user?), maybe even the truth about MMS or LTE (4G).



jav6454
Sep 1, 2009, 08:53 AM
Yes, I understand that. However, the problem in the US is not in iPhone to Cell Site connection. Those connections are fine. The problem arises in the backhaul system. That's where all the congestion is centered and that system is the one that connects the cell site to the Internet/Voice services.

Due to AT&T not updating that accordingly with their towers, we get well, great bars, but no transfers or slow transfers as the backhaul is choking to death in traffic.

In other countries it may be different. But at least for US cases, this is the cause.

-aggie-
Sep 1, 2009, 08:56 AM
Yes, I understand that. However, the problem in the US is not in iPhone to Cell Site connection. Those connections are fine. The problem arises in the backhaul system. That's where all the congestion is centered and that system is the one that connects the cell site to the Internet/Voice services.

Due to AT&T not updating that accordingly with their towers, we get well, great bars, but no transfers or slow transfers as the backhaul is choking to death in traffic.

In other countries it may be different. But at least for US cases, this is the cause.

And the backhaul is what's currently being upgraded, correct? The stuff they say they did in Atlanta and SF?

alent1234
Sep 1, 2009, 09:08 AM
and i thought apple just coded the OS to almost always display full bars no matter how weak the signal is

jmmo20
Sep 1, 2009, 09:46 AM
and i thought apple just coded the OS to almost always display full bars no matter how weak the signal is

I greatly disagree..
I've seen my signar bars fluctuate a lot and it always work.
So far in 1 yr I've owned the 3G i've had maybe 5-10 dropped calls.

I can't believe the cell network is so bad in the US.
We have congestions here from time to time, but mostly during new year's eve when everyone uses their phone or at top touristic resorts during the summer.

thelatinist
Sep 1, 2009, 10:18 AM
OP: I have not had your experience with signal strength and call quality/call drops. Anywhere below about -100 I am at risk of a dropped call, and I've rarely been able to maintain a call below -103. You have to understand that most of us do not live or work in "good quality single cell environments"; there is constant RF interference everywhere.

jav6454: Why the obsession with backhaul? We all know that a data connection is only as fast as its slowest link, and we don't need to be reminded in every thread that AT&T's backhaul sucks.

and i thought apple just coded the OS to almost always display full bars no matter how weak the signal is

I find that I have full bars up to about -90dB; above that it drops off fairly quickly.

TMar
Sep 1, 2009, 10:20 AM
Yes, I understand that. However, the problem in the US is not in iPhone to Cell Site connection. Those connections are fine. The problem arises in the backhaul system. That's where all the congestion is centered and that system is the one that connects the cell site to the Internet/Voice services.

Due to AT&T not updating that accordingly with their towers, we get well, great bars, but no transfers or slow transfers as the backhaul is choking to death in traffic.

In other countries it may be different. But at least for US cases, this is the cause.

You do realize that AT&T leases a lot of their backends. It's a failing in US broadband standers not just in AT&T.

fehhkk
Sep 1, 2009, 10:25 AM
From my experience with the 3G, the signal bars is just an RSSI meter. I initially thought it was a more complex algorithm that displayed signal bars relying on other parameters besides just received signal strength (signal over noise).

I've been to a baseball stadium (where you have around 40,000 people in a small area), and in that situation, you can see full bars, but no network connectivity due to congestion.

In theory, it is possible to have just 1 bar of RSSI, (maybe around -90 to -100dBm), and have good downlink speeds, but not in a urban environment.

jav6454
Sep 1, 2009, 10:54 AM
OP: I have not had your experience with signal strength and call quality/call drops. Anywhere below about -100 I am at risk of a dropped call, and I've rarely been able to maintain a call below -103. You have to understand that most of us do not live or work in "good quality single cell environments"; there is constant RF interference everywhere.


jav6454: Why the obsession with backhaul? We all know that a data connection is only as fast as its slowest link, and we don't need to be reminded in every thread that AT&T's backhaul sucks.

I find that I have full bars up to about -90dB; above that it drops off fairly quickly.

Because it takes repetition to understand that the connection to the cell site is great. It's the backhaul thats wrong. You see all the threads saying "OMG poor ATT service!!" Well, people have to understand it's not the cell sites.


You do realize that AT&T leases a lot of their backends. It's a failing in US broadband standers not just in AT&T.

Part in due because the government doesn't do a good job at monitoring broadband companies to keep their backhaul's up to date.

And the backhaul is what's currently being upgraded, correct? The stuff they say they did in Atlanta and SF?

No that was a signal frequency change. They are allocating more wireless spectrum. SF and NY had 1900MHz spectrum for all services. Now AT&T is using the 850MHz spectrum they have to carry the 3G network. Meaning a less cramped airwave high way. Think of it as 2 Interstates going north. One is clogged up and the other is empty. AT&T is shifting traffic to the empty one and thus alleviating everyone.

TheSpaz
Sep 1, 2009, 12:30 PM
I've only ever had one dropped call... but I noticed something.

I have to make or receive calls for them to be dropped.

Galen
Sep 1, 2009, 01:15 PM
OP: I have not had your experience with signal strength and call quality/call drops. Anywhere below about -100 I am at risk of a dropped call, and I've rarely been able to maintain a call below -103. You have to understand that most of us do not live or work in "good quality single cell environments"; there is constant RF interference everywhere.

I find that I have full bars up to about -90dB; above that it drops off fairly quickly.

But there are lots of single cell environments and sorry for not being clear (and my poor english), maybe I should have written “dominate cells”. Take for example an indoor solution, yes it sits in a urban areas but you can have weak coverage in areas within the indoor solution. You would be surprised how the networks are built up, there are many suburban areas that are only covered by one dominate cell. In a sense the better the radio optimization is the better the quality. Not to be picky but it’s not dB, it’s dBm.

The point I was making is that signal strength is not is most important part and well the bars in the phone don’t mean all that much in the end. Even a signal strength figure can be miss leading to subscribers. Yes you can drop a call at -90dBm, you can even drop a call at -60dBm as well but you can also carry a call at -110dBm, WCDMA is a very robust system. You need to compare the Ec/No (quality) and the RSCP (signal strength) to know the radio quality, even then there are other reason the for drop calls; such as soft hand over failures, uplink interference etc etc etc which you are not able to see on your phone. Well the uplink interference is measured at the base station (or I guess what people call towers here) and sends that info to the phone so it knows how much power to use and it tries to access the network.

At least from the press reports when it comes to AT&T it seems they are still upgrading the transmission network (backhaul you call it) and it seems that is what the bottle neck is on their network. Leased lines are not all that cheap for an operator. They have just upgraded the radio network in Stockholm to 21Mbit/s but only a few areas have the transmission to handle it..... then again their are only a few USB modems out there that support it.

ThirdRail
Sep 1, 2009, 02:03 PM
What is being said makes sense, but when I upgraded to 3GS with AT&T, I was having a constant problem with dropped calls and calls not even making to my phone (just to voicemail). I had 2 different tech support people ask me to run a field test. The results were -90 and -95. Both then told me that if I was within 30 days of purchase, to return the phone.

How is it that some can make calls at -95 to -103, but anything from -85 on for me results in call failure? (I even have an AT&T tower 3 blocks away, and noticed the signal in my neighborhood hovers around -75 to -95, unless the tower is in clear view- then the signal is -55)

Gatteau
Sep 1, 2009, 11:54 PM
I have to make or receive calls for them to be dropped.


What do you mean by that, really, I don't get it?

The Californian
Sep 2, 2009, 01:13 AM
What do you mean by that, really, I don't get it?

It's more of an inside joke I believe. If you know TheSpaz well enough, you would get his humor. He pokes fun at himself at times.

stevetim
Sep 2, 2009, 05:34 AM
Omitted is the fact 3G utilizes more than one tower at a time when it can which greatly affects DL/UL speeds.

thelatinist
Sep 2, 2009, 07:39 AM
The point I was making is that signal strength is not is most important part and well the bars in the phone don’t mean all that much in the end. Even a signal strength figure can be miss leading to subscribers. Yes you can drop a call at -90dBm, you can even drop a call at -60dBm as well but you can also carry a call at -110dBm, WCDMA is a very robust system.

I don't disagree with you in theory. I'm just telling you that in the real world, at least where I live, it is not possible to carry on a call anywhere near -110dB. I've never known anyone to do it.