Roaming in general

Roaming is divided into "SIM-based roaming" and "Username/password-based roaming", whereby the technical term "roaming" also encompasses roaming between networks of different network standards, such as e.g. WLAN (Wireless Local Area Network) or GSM. Device equipment and functionality, such as SIM card capability, antenna and network interfaces, and power management, determine the access possibilities.
Using the example of WLAN/GSM roaming, the following scenarios can be differentiated (cf. GSM Association Permanent Reference Document AA.39):

SIM-based roaming: GSM subscriber roams onto a Public WLAN operated by their GSM Operator.
SIM-based roaming: GSM subscriber roams onto a Public WLAN operated by another Operator who has a roaming agreement with their GSM Operator.
Username/password based roaming: GSM subscriber roams onto a Public WLAN operated by their GSM Operator.
Username/password based roaming: GSM subscriber roams onto a Public WLAN operated by another Operator who has a roaming agreement with their GSM Operator.
Although these user/network scenarios focus on roaming from GSM Network Operator's network(s), clearly roaming can be bi-directional, i.e. from Public WLAN Operators to GSM Networks. Traditional roaming in networks of the same standard, e.g. from a WLAN to a WLAN or a GSM network to a GSM network, has already been described above and is likewise defined by the foreignness of the network based on the type of subscriber entry in the home subscriber register.
In terms of user service scenarios, the user can have access to the same set of services, irrespective of access type. However, differentiation also exists. Service scenarios may include access to a range of different services, including:
Access to corporate Intranet services;
Access to operator walled garden services; and
Access to public Internet.
In the case of session continuity, seamless access to these services across different access types is provided.

Home and visited networks
The differentiation between home network and visited network is technically given by the type of subscriber entry in a specific network. If a subscriber has no entry in the home subscriber register of the network (e.g. Home Location Register (HLR) in GSM networks or local customer database in WLANs), the required subscriber data must first be requested by the visited network e.g. from the subscriber's home network in order that the subscriber can be authenticated and any authorization for using the network services can be checked. The "visiting" subscriber acquires an entry in a user database of the visited network (e.g. Visited Location Register (VLR)) and the authorized network services are enabled. For the roaming procedure in practice, the possibility of assigning the subscriber data is always indispensable in order that authentication, authorization and billing of the subscriber can be performed in the corresponding network. Thus, the term roaming is not linked to a specific network standard, but rather to the type of subscriber entry in the home subscriber register of the mobile radio network. If a subscriber can use his personal service profile, which he uses in the home network, in the visited network as well, this is also referred to as Global Service Roaming Capability.

The roaming process

The details of the roaming process differ among types of cellular networks, but in general, the process resembles the following:

When the mobile device is turned on or is transferred via a handover to the network, this new "visited" network sees the device, notices that it is not registered with its own system, and attempts to identify its home network. If there is no roaming agreement between the two networks, maintenance of service is impossible, and service is denied by the visited network.
The visited network contacts the home network and requests service information (including whether or not the mobile should be allowed to roam) about the roaming device using the IMSI number.
If successful, the visited network begins to maintain a temporary subscriber record for the device. Likewise, the home network updates its information to indicate that the mobile is on the host network so that any information sent to that device can be correctly routed.
If a call is made to a roaming mobile, the public telephone network routes the call to the phone's registered service provider, who then must route it to the visited network. That network must then provide an internal temporary phone number to the mobile. Once this number is defined, the home network forwards the incoming call to the temporary phone number, which terminates at the host network and is forwarded to the mobile.
In order that a subscriber is able to "latch" on to a visited network, a roaming agreement needs to be in place between the visited network and the home network. This agreement is established after a series of testing processes called IREG and TADIG. While the IREG testing is to test the proper functioning of the established communication links, the TADIG testing is to check the billability of the calls.
The usage by a subscriber in a visited network is captured in a file called the TAP (Transferred Account Procedure) for GSM / CIBER (Cellular Intercarrier Billing Exchange Roamer) for CDMA, AMPS etc... file and is transferred to the home network. A TAP/CIBER file contains details of the calls made by the subscriber viz. location, calling party, called party, time of call and duration, etc. The TAP/CIBER files are rated as per the tariffs charged by the visited operator. The home operator then bills these calls to its subscribers and may charge a mark-up/tax applicable locally.