INTERNATIONAL TRAFFIC CASES

One of the primary features of GSM is the ability to perform international roaming and to handle international call cases. In order for a mobile subscriber to be able to make calls while roaming in a different GSM network, there must be an agreement between the subscriber’s home network operator and the visited network operator. This also applies to international roaming. Although there are less significant affects on other traffic cases, the two traffic cases that are affected most are outlined here.

IMSI ATTACH

When an MS is roaming internationally, the following occurs:

1. The MS is switched on and scans all GSM frequencies within one frequency band (e.g. GSM 900). It is searching for a BCCH carrier. The MS tunes to the BCCH carrier that has the strongest signal strength and reads its system information. This includes the identity of the network operator.

2. The MS compares this network identity with the list of forbidden PLMN’s in the SIM memory. This list contains all network identities, which the subscriber’s home operator does not have an international roaming agreement with. If the network, which the MS has tuned to, is a forbidden network, the MS continues to scan for a permitted network.

3. If the MS does not find a permitted network, but has identified a forbidden network, it displays the message “Emergency Calls Only”. If the MS finds a permitted network, it tunes to it and sends an IMSI attach message.

4. The remainder of this traffic case is identical to that of the normal IMSI attach case, with the only difference being that the subscriber’s HLR is located in another country.

CALL TO AN MS

When an MS is roaming internationally and a call is made to it, the procedure used is identical to when the MS is in their home network. The only major difference is that the GMSC and HLR used are in the home network, while the MSC/VLR is in a network in another country.

DROPBACK FUNCTIONALITY

The following traffic case demonstrates the advantages of using dropback functionality. It involves two subscribers:

• Subscriber A is from France and is located in France under the control of the MSC/VLR-A.

• Subscriber B is from Sweden but is currently roaming internationally in France under the control of the same MSC/VLR-A.

1. Subscriber A calls subscriber B. The call is routed internationally from France to Sweden.

2. The Swedish network then identifies that B is in under the control of MSC/VLR-A in France and routes the call back to France. The subscribers are connected to each other and continue their call.

• Without dropback: the speech on the call goes through the GMSC in Sweden.

• With dropback: the speech on the call is switched within MSC/VLR-A, thus saving on processing and transmission costs.

Traffic case showing the advantage of using dropback