Mobile Phone Technology

Roaming

Mobile roaming

image ©huawei

When a customer purchases a SIM, their details and the SIM details are entered into the home network. This includes the customer's and billing details, as well as what is permitted with this SIM.

Each time a mobile device with that SIM is switched on, it must be registered with the network. The device searches for the strongest home network signal. If one is found, the device will register on that network using the IMSI (International Mobile Subscriber Identity).

    This registration will:
  • Check that the equipment is not blacklisted.
  • Set up authentication, which is carried out using public–private key authentication.
  • Add the IMSI and other details to the Unified Data Management system (5G).

The MS is now on the home network and connected to, and managed by, that network.

The process is similar when a phone is roaming outside of its home country, but the phone will not find the home network. Instead, it searches for the strongest signal. Once it has found one, it will ask if it can join that network. The roaming network will then negotiate with the home network to check whether the SIM can be registered on the roaming network. If this is rejected, the MS will search for another network. Once an agreement has been reached with the home network, a Temporary IMSI (TMSI) will be allocated to the SIM, which will then be used by the roaming network to manage the SIM. Settings on a phone can prioritise certain networks while roaming. Additionally, in most countries, the visiting network will also check that the mobile device is not stolen or otherwise blacklisted.

There is no technical reason why roaming within the home country should not be allowed. For example, a Vodafone SIM could be allowed to roam on an O2 or EE network. The only reason for prohibiting intra-country roaming is if the home network (Vodafone, for example) does not allow it for commercial reasons. However, intra-country roaming could solve some of the 'not-spots' in the UK.

In the UK, a Vodafone customer may not have the strongest possible signal; the O2 or EE signal may be better. When abroad — in France, for example — Vodafone customers will always have the strongest signal, as Vodafone allows them to roam on any French network, and roaming picks the strongest, whether that is Orange, SFR, Bouygues Telecom or Free.

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