Jan 30thHow Phone will Travel

If a wireless phone tower only covers a few square miles, how is it, I can travel for miles and never drop my call with the person I am talking to?

This question is one that has been asked many times by new employees at my previous company.

The first thing I will need to explain to you are some basic wireless terms that will be used in this article.

1) BTS: Stands for Base Station Controller. It is a fancy name for a cell site

2) BSC: Base Station Controller. Controls all messaging back and forth to the BTS

3) MSC: Mobile Station Controller. This is the main switching computer that controls the BSC and all call traffic

4) CDMA/GSM or UMTS: These are different Digital Technologies used by wireless carriers

Now let’s start talking a little about how this all works.

First you will need to set up a call on your wireless phone. Once you are on that particular call you begin driving down the road. There is a slight difference in this process between GSM and CDMA. I will attempt to point them out as we go through this. As you are on the phone, your signal is constantly being monitored and evaluated, also each BTS on the BSC  has a database that lists all the corresponding sites around it that a customer could possibly drive into the area of.

The next step there is a difference in how it works:

CDMA / UMTS: In CDMA and UMTS, your call is actually being processed but all the surrounding sites at the same time. The BSC monitors the signal quality on the call and as you drive, the levels will get better on one site over another and the BSC just process the call on the best quality site. As you drive on, your noise level changes, and the BSC will just process the call from the new site. This is called Soft Hand Off’s. As you drive on, this process is just repeated over and over with the sites going down the road based on the BSC database listing of the surrounding sites.

GSM: In GSM it works a little different. The calls are not actually processed over multiple sites at one time, because the phone in GSM is forced to retune to a different Freq every time you move from one site to another. So how it works, is it just monitors your signal level on all surrounding sites based on the BSC database. Then once your signal level is stronger on one of the neighbor sites than the one you currently are on, the phone receives a message from the BSC telling it to retune to the channel of the neighbor site and the phone retunes and the call hands off.

Because of these differences, if you listen closely, you may hear a small click or a word of audio may be muted because of the retuning of the phone. In CDMA, all sites actually operate on the same frequency and it performs a Soft Hand Off, so it is seamless to the customer and device.

The signal levels that these processes us to compare and performed at are also set by the carrier in their BSC database.

The number one reason for a consumer to drop their call is usually a neighbor cell site is not loaded properly in the BSC as a hand off candidate or that possibly the signal levels that are set in the database may need to be adjusted slightly. This process is not an exact art for all situations so drop calls will happen that cannot be explained.

Now let me explain quickly how this whole process works when you drive on a road where one area is on one wireless carrier system and the other area is on someone else’s wireless system. Because this requires two different carriers to be involved, many times your calls will just drop because the two carriers have not gone into an agreement among themselves, most of the times this is due to costs associated in putting the process in place.

If both carriers decide they want to put this in place, it is very similar to what already happens above. The difference is, that the BSC database also has to have information on the neighbor BTS’s. However, when it needs to get info of the neighbor’s site it has to communicate over a link that is put in place connecting the two companies. If the call actually reaches levels that it hands off the call, the neighbor has to take the audio for that call also and that requires some sophisticated signaling and trunking to be in place. Also due to the time it takes for all this messaging to take place between carriers, it is not uncommon for the calls to drop if the areas terrain is hilly or the customer drives too quickly through. The costs associated with this, is the main reason carriers do not put this in place. You will usually only see this in place if the area in question is on a major interstate or where there is enough traffic to justify not dropping the calls.

I hope this all made sense to you and if not feel free to drop me a note or comment and I will try to answer your questions for you.

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