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.
Have you ever wondered why your local wireless carrier has placed, on your phone bill, so many roaming charges? In this day and age, customers want to be able to make and receive calls worldwide with little to no effort. There are many things that go into allowing this, at the same time being seamless to the end user while traveling.
Before we get into this article too deep, I feel you should have an understanding of a few basic terms I will be using. There are not many, but these are the cornerstone to how everything works together smoothly.
HLR: Home Location Register. This is a database on your home carriers system that stores everything to know about you and your phone. Some of the things it keeps track of are; Serial Number, Phone Number, and features you have paid for. It also tracks where you currently are which I will explain a little later on in this article.
VLR: Visitors Location Register. This is a database that holds the same information as the HLR but is located on a visiting carrier switch and not your home switch. It also keeps track of where you are but in more details than the HLR.
SS7: Signally System Number 7. This is a protocol that wireless carriers use to talk between each other in order to pass information. It is used for a lot of different messaging, but for this article it is used to talk between HLR’s and VLR’s.
STP: Signaling Transfer Point. These are systems that are used to transport the SS7 Protocol between HLR’s and VLR’s among all wireless carriers.
Now let us start with what happens when you first arrive into a new system by either powering on your phone or by driving. Without you even knowing it, your mobile phone will send a message to the roaming carriers system telling it that they are in the area and would like to be able to use their system. When this happens it, initiates a message from the VLR to its home HLR to gather information on this customer so that when it’s time to make a call it has the data needed to offer the service. It will store this information for a given amount of time determined by the HLR or till the customer drives to another carriers system.
Now, when you go to make a call, the VLR has all the info it needs and process the call based on your feature plan. Nothing fancy happens here. The big issue is when you want to receive a call.
When someone goes to call you, they obviously don’t know exactly where you are and who’s system you are on. If coming from your house, the call is routed by the phone company to your homes system based on your 10 digit phone number. The first thing your home switch does is it looks in the HLR to see if it knows where your phone is currently located. It will have this information, because, when the VLR requested it the HLR logged the info for just this reason. The HLR will see that the customer is being serviced at this time by another carriers system and will forward a message to its VLR stating this customer has a incoming phone call. The VLR will then check its database to verify that they do indeed have the customer on the system as it tracks where exactly the customer is located and will then assign what is called a TLDN. This is basically a temporary phone number that it assigns to the customer for the purpose of receiving the call. The TLDN is forwarded back to the HLR and it in turn forwards the incoming call to this number. That call will then route to the visiting system switch and on to the customer’s phone. Once the call is answered, the TLDN is release by the system so that other customers have it available to receive calls also.
As I was mentioning earlier in this article, HLR’s and VLR’s transfer their SS7 Protocol messaging between each other via nationwide STP’s. These STP’s are systems that third party companies maintain and operate. They operate links to all carriers worldwide. The purpose of these companies are to avoid carriers from having to have links to each company as that can get costly and messy. You hook up to these companies and they have the links in place and just route your messaging for you. There are a couple major companies that offer this service, they are VeriSign and Syniverse. Other companies exist but these are the most popular ones used. These companies also have links to each other for talking to those companies that are with the other. This makes the whole world one big happy messaging link. All carriers have to do after this is sign roaming agreements with a carrier and then notify these STP companies and have them open up the messaging between the associated HLR’s and VLR’s.
In closing: Whether your internationally roaming or just going down the street to your friends. Whenever you leave your home market you could entail roaming changes. Hopefully this article helps explain why those costs are in place. Wireless roaming is a major concern to carriers since latest price plans have started offering it free. Nothing is free in this world, therefore, each call made or received while roaming does ultimately cost the carriers just in messaging costs to STP’s.