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.
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