Roaming


Roaming is a wireless telecommunication term typically used with mobile devices, such as mobile phones. It refers to a mobile phone being used outside the range of its native network and connecting to another available cell network.

Technical definition

In more technical terms, roaming refers to the ability for a cellular customer to automatically make and receive voice calls, send and receive data, or access other services, including home data services, when travelling outside the geographical coverage area of the home network, by means of using a visited network. For example: should a subscriber travel beyond their cell phone company's transmitter range, their cell phone would automatically utilize another phone company's service, if available.
The process is supported by the Telecommunication processes of mobility management, authentication, authorization and accounting billing procedures.

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, e.g. WLAN 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 :
  • SIM-based : GSM subscriber roams onto a public WLAN operated by:
  • * their GSM operator, or
  • * 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, or
  • * another operator who has a roaming agreement with their GSM operator.
Although these user/network scenarios focus on roaming from GSM network operator's networks, 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 the case of session continuity, seamless access to these services across different access types is provided.

Roaming (electric vehicles)

The term "roaming", also known as "e-roaming", is a concept for charging battery electric vehicles at other charging stations. In practice, e-roaming allows EV drivers to achieve greater interoperability by providing access to public charging points from any owner/operator's EV charging network through a common platform and a single network subscription or contract.
There are proprietary charging networks, such as Tesla Superchargers, or providers sharing charging points through contracts and agreements. Thus, the EV can "roam" between those charging points.

Home and visited networks

"Home network" refers to the network the subscriber is registered with.
"Visitor network" refers to the network a subscriber roams temporarily and is outside the bounds of the "home network".

Roaming agreements

The legal roaming business aspects negotiated between the roaming partners for billing of the services obtained are usually stipulated in so called roaming agreements. The GSM Association broadly outlines the content of such roaming agreements in standardized form for its members. For the legal aspects of authentication, authorization and billing of the visiting subscriber, the roaming agreements typically can comprise minimal safety standards, as e.g. location update procedures or financial security or warranty procedures.

The roaming process

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

Location update

Location updating is the mechanism that is used to determine the location of an MS in the idle state.
  1. 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.
  2. The visited network contacts the home network and requests service information about the roaming device using the IMSI number.
  3. 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 cell phone is on the visited network so that any information sent to that device can be correctly routed.

    Mobile terminated call

It occurs for example when a call is made to a roaming cell phone.
Signaling process:
  1. The calling subscriber dials the mobile subscriber's MSISDN of the roaming cell phone.
  2. Based on the information contained in the MSISDN, the call is routed to the mobile network gateway MSC. It's done with an ISUP IAM message.
  3. To locate the MS, the GMSC sends to the HLR a MAP SRI message. The MAP SRI message contains the MSISDN number and with this MSISDN the HLR will obtain the IMSI.
  4. Because of past location updates, the HLR already knows the VLR that currently serves the subscriber. The HLR will send to the VLR a MAP PRN message to obtain the MSRN of the roaming cell phone. Like that the HLR will be able to route the call to the correct MSC.
  5. With the IMSI contained in the MAP PRN message, the VLR assigns a temporary number known as the mobile station roaming number to the roaming cell phone. This MSRN number is sent back to the HLR in a MAP RIA message.
  6. Now with the MSRN number, the GMSC knows how to route the call to reach the roaming cell phone. Then, the call is made using ISUP signaling between the GMSC and the visited MSC. The GMSC will generate an ISUP IAM message with the MSRN as the called party number.
  7. When the MSC of the visitor network receives the IAM, it recognizes the MSRN and knows the IMSI for which the MSRN was allocated. The MSC then returns the MSRN to the pool for future use on another call. Afterwards, the MSC sends to the VLR a MAP SI message to request information like the called MS's capabilities, services subscribed to, and so on. If the called MS is authorized and capable of taking the call, the VLR sends a MAP CC message back to the MSC.
In order that a subscriber is able to register 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 for GSM / CIBER 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.
As recently many carriers launched own retail rate plans and bundles for Roaming, TAP records are generally used for wholesale Inter-Operators settlements only

Tariffs

Roaming fees are typically charged on a per-minute basis for wireless voice service, per text message sent and received and per megabyte of data used for data service, and they are typically determined by the service provider's pricing plan.
Several carriers in both the United States and India 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.
In the UK, the main network providers generally send text alerts to advise users that they will now be charged international rates so it is clear when this will apply. UK data roaming charges abroad vary depending on the nature of the phone agreement. Some carriers, including T-Mobile and Virgin Mobile, do not allow pay as you go customers to use international roaming without pre-purchase of an international "add on" or "bolt on."
An operator intending to provide roaming services to visitors publishes the tariffs that would be charged in their 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.

Roaming within the EU

In the European Union, regulation on roaming charges began on 30 June 2007, forcing service providers to lower their roaming fees across the 28-member bloc. It later also included EEA member states. The regulation set a price cap of €0.39 per minute for outgoing calls, and €0.15 per minute for incoming calls - excluding tax. Having still found that market conditions did not justify lifting the capping of roaming within the EEA, the Commission replaced the law in 2012. Under the 2012 Regulation, retail roaming capping charges expired in 2017 and wholesale capping charges expired in 2022. In mid-2009 there was also an €0.11 maximum price for SMS text message included into this regulation.
On 11 June 2013, the European Commission voted to end mobile roaming charges for the first time.
Following a European Commission vote on 15 December 2016, roaming charges within the European Union were to be abolished by June 2017. While the European Commission believed that ending roaming charges would stimulate entrepreneurship and trade, mobile operators had their doubts about the changes.
On 15 June 2017, Regulation 2016/2286, nicknamed "Roam like at Home" and having been signed by the European Parliament and Commission in May of the same year came into force. It abolished all roaming charges within the EU, Iceland, Liechtenstein and Norway.