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Roaming refers to the ability of a mobile network subscriber to automatically receive or make calls, send and receive data or have access to other mobile network services on a network other than their home network. The name is synonymous with mobile phone use abroad, where your own home network is not available. This works both in the  GSM standard, where the term was introduced, and in the UMTS and LTE standard. The readiness of the terminals outside the own radio network is technically supported by authentication, authorization and billing procedures.


Conceptual distinction

Handovers and cell changes (also known as micromobility) must be distinguished from roaming (also known as macromobility). These terms denote the change of the radio cell during a running call or a running data connection. If the network (PLMN) is changed, it is called InterPLMN-handover or InterPLMN-cell-change.

Both functions are used, for example, when sister companies of internationally operating mobile phone companies interconnect their directly neighbouring national networks in such a way that the services can be used without interruption across borders.


Uutbound roaming

refers to the use of a foreign network (example: use of a foreign mobile network during holidays).


Inbound roaming

refers to the use of one's own network by another subscriber (example: foreign mobile phone subscriber visiting).

Note: Outbound and inbound always depend on the view of the respective network operator. A subscriber of network A, who is in network B, is an outbound roamer from the point of view of his home network A. From the point of view of the visited network B, he is an inbound roamer.


Roaming in general

Roaming is differentiated between "SIM-based roaming" and "user name/password-based roaming", whereby roaming in networks of different network standards, such as WLAN (Wireless Local Area Network) and GSM, also falls under the technical term roaming. Equipment and functionality of the device, such as SIM card capability, antenna and network interfaces and energy management, determine the possibilities of access.


The following scenarios are distinguished using WLAN / GSM roaming as an example (see GSM Association Permanent Reference Document AA.39):

  • SIM-based roaming: A GSM user moves (slang: "roams") into a public WLAN that is routed from:
    • its own GSM network operator, or
    • is operated by another network operator and has a roaming agreement with the home network operator.
  • User name/password based roaming: A GSM user connects to a public WLAN provided by:
    • its own GSM network operator, or
    • is operated by another network operator and has a corresponding agreement with the home network operator.


The roaming scenarios mentioned above are bidirectional, i.e. they cover roaming from a GSM network to a WLAN as well as from a WLAN to a GSM network. Traditional roaming in networks of the same standards, e.g. from a WLAN to a WLAN or GSM network to a GSM network, has already been described above and is also defined by the strangeness of the network, based on the type of subscriber entry in the home user directory.


Regarding the possible scenarios for the authorization of a subscriber, the mobile phone providers often strive for the subscriber to have access to the same services and services regardless of the access mode, network standard or network. However, other scenarios with a differentiation of the released services according to networks or network standards also exist, e.g. with prepaid procedures. Access scenarios may cover a wide range of services, in particular:

  • Access to shared intranet services;
  • Access to specific services of the service provider (so-called Operator Walled Garden Services) and
  • Access to the public Internet.

Seamless access to these services regardless of the type of access and the network standard is one of the goals when merging the different network standards. Session continuity (also referred to as "persistent session" or "persistent/permanent connection").


Source: https://de.wikipedia.org/wiki/Roaming

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