Editing Roaming (section)

=== Mobile terminated call ===
It occurs for example when a call is made to a roaming cell phone.

Signaling process:
# The calling subscriber (from within the [[PSTN|public telephone network]]) dials the mobile subscriber's [[MSISDN]] (the telephone number) of the roaming cell phone.
# Based on the information contained in the MSISDN (national destination code and the country code), the call is routed to the [[Network switching subsystem|mobile network gateway MSC (GMSC)]]. It's done with an [[ISDN User Part|ISUP]] IAM message.
# To locate the [[Mobile station|MS]], the GMSC sends to the HLR a [[Mobile Application Part|MAP]] SRI (Send Routing Information) message. The MAP SRI message contains the MSISDN number and with this MSISDN the HLR will obtain the [[International mobile subscriber identity|IMSI]].
# Because of past location updates, the HLR already knows the VLR that currently serves the subscriber. The HLR will send to the VLR a [[Mobile Application Part|MAP]] PRN (Provide Roaming Number) 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.
# With the IMSI contained in the MAP PRN message, the VLR assigns a temporary number known as the mobile station roaming number (MSRN) to the roaming cell phone. This MSRN number is sent back to the HLR in a MAP RIA (Routing Information Acknowledgement) message.
# Now with the MSRN number, the [[Network switching subsystem|GMSC]] knows how to route the call to reach the roaming cell phone. Then, the call is made using ISUP (or TUP) signaling between the GMSC and the visited MSC. The GMSC will generate an [[ISDN User Part|ISUP]] IAM message with the MSRN as the called party number (and NOT the MSISDN as the called party number).
# 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 [[Mobile Application Part|MAP]] SI (Send Information) 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 (Complete Call) 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 (International Roaming Expert Group) and [[Transferred Account Data Interchange Group|TADIG]] (Transferred Account Data Interchange Group). 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 Record) 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