I will try below to explain to people how a Mobile phone call takes place.
First you have to realize that on every mobile phone system their is a computer system called a HLR (Home Location Register). These computers have everything about your phone programmed into it. Such as your Phone Number, Serial Number, what types of calls you are allowed to make and which ones you are not, and what features you have purchased from the carrier.
Now let’s see what happens when you first power up the phone. The phone will first start to receive signals from all the cell sites in the area. This is all cell sites and not just the ones from your carrier you have service with. Inside your phone is a database that stores all the information on the different carriers and systems it is allowed to make calls on. It also lists all this data in a priority level as to which ones to look for first. The phone will then look by priority to see which systems it can see and lock on to.
Once the mobile phone locks onto a particular system it will transmit to the cell site its mobile number and its serial number. This information will be passed on to the switch and then on to the HLR for verification. If you are roaming there is a computer called VLR (Visitor Location Register) that will get the info from the HLR and store it for future use.
If this verification take place it will then light the in-service light on the phone and allow you to make calls. Now let’s see what happens when everything is validated and you dial a number on your phone and hit SEND.
Lets first understand some of the systems at a cell site and switch. These are the systems that will be used to translate the call. Once you understand those then everything else falls into place quickly. First, in the RF signal, in the air to your phone, is two types of signals. One is called a control signal or overhead channel and the other one is called a voice channel. The control signal is where all the data is transmitted on. This is the info such as your phone number, serial number, and the digits you have dialed. The voice cannel is where the call will be placed once it is processed and ringing starts and you need to get two way audio for the call. The transmitters at the cell site communicate with the switch via a cable or fiber provided by the phone company back to another system called the switch. This switch is where everything is actually controlled from and will be explained more later on. Then from there you have more phone lines to the land line phone company for getting the call to someone’s house. Now let’s talk about your particular call.
Let’s say you dial your house and its number is 555-234-1122. First, the phone will send the info to the cell site on the control overhead channel. The cell sites transmitter will take this info and send it on to the switch to be processed. This message is sent via the fiber cable it has connecting the cell site to the switch. You can have one switch running hundreds of cell sites. The switch will then do a few things before allowing the call. First it will check with your HLR/VLR entry and see if you are allowed that type of call. If it is a valid call, it will then check another database to see what to do with the digits you dialed. It also has to verify the digits are a valid number of digits for a phone call. With this check it will also be looking up where this number is located and who it should forward the call to, either a local telephone company or a long distance carrier. Once this is completed, it will forward on the digits to the land line phone company. Once they get it they deliver it to the appropriate place. Once the call is sent on to the land line phone company the switch will send a message to the cell site assigning it a voice channel and you will then start hearing the ringing on the other end. Once the other party answers there is a message sent back called a “OFF-HOOK” signal and that triggers things on the switch for billing to start and such. When the call ends there is a “ON-HOOK” signal sent that stops billing.
Keep in mind all this happens in just a few seconds. It’s happening during that quite time you hear on the phone before you hear any ringing. If the messaging gets slowed down for some reason, it causes the customer to hear quiet longer than they normally would.
This is a very basic overview of call processing on a Mobile Phone system. Hopefully after this you have a better understanding of what is taking place with your phone and what it means when you make a call.
After more than 20 years as a Switch Engineer and part owner for a major wireless carrier, I still find the most common question I am asked even though I am retired is “Why Don’t I Have Cell Phone Service”. This is really not a very difficult question to answer once you have the understanding of how cell phones work and the different carriers you may have service through.
First let me explain that there are two major types of Digital Phones. One is CDMA which is the main service carriers like Verizon and Sprint use and then there’s GSM which is what AT&T and T-Mobile use. There are a few others but they are not as widely used in the industry at this time. These two digital technologies operate totally different from each other and are not compatible with each other. In other words at this time you cannot use a Verizon phone on a AT&T system and vice versa.
Now let’s say you are standing next to your friend and he has phone service and you do not. Most likely one of you has a CDMA phone and the other is a GSM phone. Let’s say Friend “A” is on AT&T and Friend “B” is on Verizon. Since these digital technologies are not compatible at this time then it would require both carriers to have a Cell Site system in the general area as to where you are. Keep in mind in a perfect world (Flat with no trees) a cell site will talk on average about 20 air miles. So you will both need to be within that distance to the cell tower. As we all know it’s not a perfect world so expect anywhere from 10 to 15 miles in Rural country and up to 5 miles in Urban. That is the basics of the causes. Now let me explain why there isn’t always a tower where people think there should be one.
Below I will be listing some numbers. These numbers are for reference only and will be adjusted by each carrier based on their business model. But the numbers I use are very close to what they may use.
First thing that will determine if a carrier places a cell tower in a general area is how much revenue they can generate and what is the total payback of the site. This means how many calls will be made and how many years it will take for the site to break even between build and maintenance costs and revenue. On most systems they shoot for a 24 to 48 month payback. They also look at things such as population and road traffic. They will need to see if they can get on average about 2000 customers in the area. Thus based on the population and the number of other carriers in the area this can become quite difficult in small populated areas. The Average cost for a cell site is about $750K. This includes everything from a tower to equipment and man power to get it up and running from scratch.
Let’s say a carrier can now justify a tower in a particular area. The next biggest hurdle they run into is local governments and the citizens themselves. Everyone wants Cell Phone service but no one wants a tower in the area where they can see it and take away from their view. Most people want their cake and eat it too. Also a lot of local Governments have instituted a ban on new towers and thus this makes it even more difficult. Then of course you have things like the FAA if you are near an Airport.
If for one of the above reasons a carrier cannot put a tower up where they would like. They now are forced to co-locate on another tower in the area. This is being done more and more but this also causes other issues. The higher on the tower you place the antenna’s the better coverage you can get. Thus, the first one or owner will usually get better coverage. If one carrier has to share a tower with another, this means that the first carrier will have an advantage over the other. Thus making it hard to get customers the service they want and expect. Also, if they are competing against each other, the one carrier that owns the tower can make the lease so high that it takes the second carrier beyond their justification for being in the area to begin with. In other words carrier “A” can make it too expensive to compete against carrier “B” in an area.
Now you see why one person can have phone service in an area and another doesn’t. This is why it’s important for the consumer to look at where they want their phone to work before they sign a contract with a carrier. Most places, if Carrier “A” isn’t there and Carrier “B” is, you may have a chance to roam but then Carrier “C” must be using the same digital technology that you have for it to work.
First let me start off by saying that I do not write these articles about the Wireless Phone Business to defend the carriers and the way they operate. I would be the first to tell you that from my experience in the industry for many years running one of them, that it is a business like everything else an it’s all about maximizing rate of return on investment when it comes to customers and charges.
I get asked a lot from people when they learn who I am, as to why when they switch from one carrier to another they usually have to purchase a new phone? This is actually a very simple answer but I will need to explain in more detail how a phone or data device works for you to understand.
First thing you have to understand that until everyone goes to LTE technology, that there are basically two types of Digital systems made in the wireless industry.
1) The first is CDMA which is the technology used by such carriers such as Verizon and Sprint
2) The second is GSM/UMTS which is the technology used by Carriers such as AT&T and T-Mobile
The first reason you can’t move your phone is probably the most obvious to people, that is that if you are moving from one digital technology to another, the phone will not be designed to work on the other carriers technology.
The second reason is actually the most complicated to explain but I will try and list it all for you. This would be the case if you were moving from a CDMA carrier to a CDMA carrier or the same for GSM/UMTS Carriers.
It all comes down to the programming of the phone itself. There are so many parameters that go into the programming of a cellular phone to make it work the way customers are accustomed to. This includes things such as to how to do run applications like SMS, MMS, and WAP. Each one of the applications in the phone requires certain programming to match the carrier’s equipment. This includes things such as DNS server IP addresses, how often the phone scans for a stronger signal, the max size of a picture to be sent via MMS, and things along that line. These are all settings that are installed in the phone in what is called a PRI file.
Many of you may have heard of a PRL file, which is where the phone gets its roaming list from as to who the phone can roam on and who it cannot. But actually the more important file is called a PRI and that is what tells the phone how to operate in general. This has become even more complicated of a file since the introduction of data aps and the internet.
You will see all over the internet in forums about people unlocking or hacking a phone. 90% of the time this is just enough to allow you to change the phone number or the data address of the device, most of the time doesn’t unlock it enough to allow someone to change the entire PRI. Unlocking a phone usually just allows someone to use a tool called QPST or CDMA Toll that can go into the phone and change particular parameters that will allow you to make phone calls or surf but doesn’t actually allow you to change fields used for MMS and WAP and certain other applications.
To truly change the PRI for a particular vendor phone you have to have flashing software and tools. Almost every manufacturer of mobile devices has their own version of the QPST for doing that. To get that software you usually have to be a seller of that manufacturer or a carrier before it is totally shared with you.
As you can imagine a carrier is not going to have this software available at all sales stores you may go into to sign up for service. A company may only have 1 or 2 copies of this software and that is just for testing in there lab the different models of phones and how they operate in different scenarios.
Also not all phones are alike, some of the major carriers will actually put each phone model through extensive testing on their network to make sure it meets certain criteria. There are some models of phones that may drop more calls than others because of the way the antenna is built into the device. They won’t allow these models on their network because it just opens up the opportunity for customers to start calling and complaining about the service, when it’s actually the device they are using not the service. This is very expensive for most carriers to perform so you will find only the major carriers will have test labs..
Even with the emergence of LTE and the fact that all carriers will eventually go to LTE, This technology doesn’t support voice calls as the average consumer thinks of it. Voice calls will actually be made over a data application and in order to perform these functions there are even more detailed requirements that must be put into each phone based on the home carriers system. LTE does solve the problem of different digital technologies on different carriers but the other issues will still prevail.
So as you can see there are some technical reasons why a carrier will not let you bring a phone from one carrier to another carrier. It’s more about the phone itself and the programming of it than it is about the carrier wanting to sell you something new. I agree, it’s very frustrating as a customer to have to purchase a new phone if you are changing carriers. Again, you can hack them and unlock them to allow you to do the basics, but you still can’t expect a carrier to do that for you due to time and effort. Even the IPHONE when it is hacked into the only real data most people can get is via Wi-Fi and not 3G.