Roaming

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Roaming is a general term in wireless telecommunications that refers to the extending of connectivity service in a location that is different from the home location where the service was registered. Roaming occurs when a subscriber of one wireless service provider uses the facilities of another wireless service provider. This second provider has no direct pre-existing financial or service agreement with this subscriber to send or receive information. A device will usually indicate when it is roaming.

The quintessential example of "roaming" is the case of cellular phones when a phone is in a location where its wireless service provider does not provide coverage (for example, another country). In some cases, roaming occurs in a phone's designated home area when it transmits via a different provider's tower (sometimes at a higher price). This is likely to occur when the service provider's signal is too weak or if the volume of callers is too high. In order for a mobile device to use a different carrier's service, the phone's service provider must have a roaming agreement with that carrier.

In 802.11 roaming can also mean subscriber mobility or handover within the same network.

1 The roaming process
2 Tariffs
3 Additional notions and types of roaming
4 See also
5 External links

[edit] 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 its 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.

[edit] Tariffs

Roaming fees are traditionally charged on a per-minute basis and they are typically determined by the service provider's pricing plan. Several carriers in the United States have eliminated these fees in their nationwide pricing plans. All of the major carriers now offer pricing plans that allow consumers to purchase nationwide roaming-free minutes. However, carriers define "nationwide" in different ways. For example, some carriers define "nationwide" as anywhere in the U.S., whereas others define it as anywhere within the carrier's network.

An operator intending to provide roaming services to visitors publishes the tariffs that would be charged in his network at least sixty days prior to its implementation under normal situations. The visited operator tariffs may include tax,discounts etc and would be based on duration in case of voice calls. For data calls, the charging may be based on the data volume sent and received. Some operators also charge a separate fee for call setup i.e. for the establishment of a call. This charge is called a Flagfall charge.

[edit] Additional notions and types of roaming

Regional roaming:

This type of roaming refers to the ability of moving from one region to another region inside national coverage of the mobile operator. Initially, operators often made commercial offers restricted to a region (sometimes to a town). Due to the success of GSM and the decrease in cost, regional roaming is rarely offered to clients except in nations with wide geographic areas like the USA, Russia, India, etc., in which there are a number of regional operators.

National roaming:

This type of roaming refers to the ability to move from one mobile operator to another in the same country. For example, a subscriber of T-Mobile USA who is allowed to roam on Cingular Wireless's service would have national roaming rights. For commercial and license reasons, this type of roaming is not allowed unless under very specific circumstances and under regulatory scrutiny. This has often taken place when a new company is assigned a mobile telephony license, to create a more competitive market by allowing the new entrant to offer coverage comparable to that of established operators (by requiring the existing operators to allow roaming while the new entrant has time to build up its own network).

International roaming:

This type of roaming refers to the ability to move to a foreign service provider's network. It is, consequently, of particular interest to international tourists and business travellers.

Broadly speaking, international roaming is easiest using the GSM standard, as it is used by over 80% of the world's mobile operators. However, even then, there may be problems, since countries have allocated different frequency bands for GSM communications (there are two groups of countries: most GSM countries use 900/1800 MHz, but the United States and some other countries in the Americas have allocated 850/1900 MHz): for a phone to work in a country with a different frequency allocation, it must support one or both of that country's frequencies, and thus be tri- or quad-band.