This description relates to the GSM model; registration for 3G and 4G networks are similar
The SIM has integrated into it an IMSI (International Mobile Subscriber Identity). This is used to identify the SIM. When the SIM is purchased, the details are registered in the HLR. The SIM is now available to be connected to the network. Note that this registration defines the HOME network for the SIM
When the SIM is inserted into a MS and switched on, it has to be registered on the network. The MS will search for the strongest HOME network signal. Assuming that one is found; the MS is registered on that network using the IMSI. This registration will :
When a phone is roaming, not within the home country the process is similar to that above but the phone will search for the strongest signal. Once found the provider will have to negotiate with the home network to check if the SIM can be registered on the roaming network. If this is rejected, then the MS will search for another network. A Temporary IMSi number will be allocated to the SIM (TIMSI) and this will be used on the roaming network in the management of the MS / SIM. A setting on a phone can force the device to fix onto a specific network when in a specific country. This has the effect of reducing the searching and registration for a roaming device
There is no technical reason why with the home country, roaming could be allowed. Thus for example a Vodafone SIM could be allowed to roam onto a O2 network. The only reason is that the home network will not allow this. However allowing roaming in the home country would be one solution to the problem that in remote areas, one network does not get signal but another does.
Periodically an active SIM will check in with the MSC to confirm the registration. As the essence of the technology is mobility, there are procedures to allow a MS to move between MSC or networks. This is the handover technology
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