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.
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