Mobile phone network registration

- a summary or tutorial about the way in which a mobile phone achieves registration onto a cellular telecommunications system or network.

On any cellular telecommunications system the way in which registration and call set-up occur needs to be carefully managed. Not only does the cellular telecommunications network need to provide quick and efficient service for its rightful customers, but it also needs to be able to offer high levels of security for the user and the network.

There are many different cellular telecommunications systems in use around the globe. Older ones are being phased out, and newer cellular systems are being introduced. Accordingly there is no single way in which registration and call set up are managed. However there are some general principles that are used, and these are illustrated here.

Basic requirements
When the mobile phone is turned on it needs to be able to communicate with the cellular telecommunications network. However the phone does not have an allocated channel, time slot or chip code (dependent upon the type of access method used). It is therefore necessary for there to be some methods or allocated means within the cellular telecommunications network, whereby a newly switched on mobile can communicate with the network and set up the standard communication.

Even if a call is not to be made instantly, the network needs to be able to communicate with the mobile to know where it is. In this way the network can route any calls through the relevant base station as the network would be soon overloaded if the notification of an incoming call had to be sent via several base stations.

Registration
There are a variety of tasks that need to be undertaken when a phone is turned on. This can eb seen by the fact that it takes a few seconds from switching the phone on before it is ready for use. Part of this process is the software start-up for the phone, but the majority comes from the registration process with the cellular network. There are several aspects to the regristration. The first is to make contact with the base station, and next the mobile has to register to allow it to have access to and use the network.

In order to make contact with the base station the mobile uses a paging or control channel. The name of this channel, and the exact way in which it works will vary from one cellular standard to the next, but it is a channel that is used that the mobile can access to indicate its presence. The message sent is often called the "attach" message. Once this has been achieved it is necessary for the mobile to register with the cellular network, and to be accepted onto it.

Network elements
It is necessary to have a register or database of users allowed to register with a given network. With mobiles often being able to access the all the channels available in a country, methods of ensuring the mobile registers with the correct network, and to ensure the account is valid are required. Additionally it is required for billing purposes. To achieve this, an entity on the network often known as the Authentication Centre (AuC) is used. The network and the mobile communicate and numbers giving the identity of the subscriber. Here the user information is checked to provide authentication and encryption parameters that verify the user's identity and ensure the confidentiality of each call protecting users and network operators from fraud.

Once accepted onto the network two further registers are normally required. These are the Home Location Register (HLR) and the Visitors Location Register (VLR). These two registers are required to keep track of the mobile so that the network knows where it is at any time so that calls can be routed to the correct base station or general area of the network. These registers are used to store the last known location of the mobile. Thus at registration the register is updated and then periodically the mobile updates its position. Even when the mobile is in what is termed its idle mode it will periodically communicate with the network to update its position and status.

When the mobile is switched off it sends a detach message. This informs the network that it is switching off, and enables the network to update the last known position for the mobile.

Home and abroad
The two registers are required, one for mobiles for which the network is the home network, i.e. the one with whom the contract exists, and the other for visitors. If there was only one register then every time the mobile sent any message to the foreign network, this would need to be relayed back to the home network and this would require international signalling. The approach which is adopted is to send a message back to the HLR when the mobile first enters the new country saying that the mobile is in a different network and that any calls for that mobile should be forwarded to the foreign visited network.

Summary
By undergoing a registration procedure when the mobile is turned on, the cellular network is able to communicate correctly with it, provide access for outgoing calls, and also route any incoming calls to it in the most efficient manner. Registration also only allows those mobiles that have permission to access the network to communicate with it.