What is Roaming

Roaming in cellular systems is defined as the extension of services to a user whose location is outside of the range of the home MSC (Mobile Switching Centre) to which the user originally belongs. For a common cellular phone user, roaming is always associated with the increase in call charges on their number when they travel outside their own state and have to use the services of a foreign network.

Here is a brief summary of how the whole operation takes place:-

When your cellular phone moves out of the range of your home MSC and in to a foreign MSC, it first of all comes in contact with the foreign MSC via its control channels on which the MSC repeatedly broadcasts information about itself, much like a beacon signal. The cellular phone on picking up this signal, acknowledges it by sending the MSC information about itself and thereby it register itself with the foreign MSC. In the first generation of cellular systems, this automatic registration was not possible and the registration was always initiated by the Home MSC on request of the user planning to travel outside the range. The Visitor Location Register (VLR) of the foreign MSC stores information about the cellular phone and the foreign MSC then communicates with the home MSC of the cellular phone, letting it know about the current location of its user. This communication takes place via the backbone SS9 signaling network which has been laid out to provide high speed links between all the MSC’s. In the home location register (HLR) of the home MSC, the cellular user information is therefore associated with the foreign MSCaddress in which he/she is currently present.

Now when a call originates for the above cellular phone, it is first of all routed to the Home MSC of the phone. The home MSC on finding that the cellular number is associated with a foreign MSC in the HLR, routes the call forward to the foreign MSC. The foreign MSC on finding the information about the phone in its VLR; forwards the call to the Base Station in whose range the cellular phone is currently present. The Base Station then using the proper control and voice channels, sets up the call with the cellular phone with the help of the instructions from its MSC. The routing of calls from the Home MSC to the foreign MSC is the main reason for increased roaming costs as it uses the services of the backbone networks that connect the various MSC

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