GSM Mobility

Roaming with GSM is made possible through the separation of switching capability and subscription
data. A GSM subscriber has her subscription data, including CAMEL data, permanently registered
in the HLR in her HPLMN. The GSM operator is responsible for provisioning this data in the HLR.

The MSC and GMSC in a PLMN, on the other hand, are not specific for one subscriber group.
The switching capability of the MSC in a PLMN may be used by that PLMN’s own subscribers,
but also by inbound roaming subscribers; see Figure 1.2.
In Figure 1.2, the GSM user who is a subscriber of PLMN-A roams to PLMN-B. The HLR in
PLMN-A transfers the user’s subscription data to the MSC in PLMN-B. The subscriber’s subscrip-
tion data remains in the MSC/VLR as long as she is served by a BSS that is connected to that
MSC. Even when the user switches her MS off and then on again, the subscription data remains
in the MSC. After an extended period of the MS being switched off, the subscription data will
be purged from the MSC. When the subscriber switches her MS on again, the subscriber has to
re-register with the MSC, which entails the MSC asking the HLR in the HPLMN to re-send the
subscription data for that subscriber.


When the subscriber moves from one MSC service area (MSC-1) to another MSC service area
(MSC-2), the HLR will instruct MSC-1 to purge the subscription data of this subscriber and will
send the subscription data to MSC-2.