Subject: Mobile Communication and Computing

Topic: GSM cellular telephony-architecture and system aspects

Difficulty: High

1. GSM only knows a two-layer hierarchy.
2. The network operator stores all customer information in their HLR, all relevant information about logged-in users for a certain region are stored in the corresponding VLR.
3. The HLR has a capacity of several million users, and a VLR up to one million.
4. If a user switches form one region to another, the HLR is informed by the responsible VLR about the new location.
5. This is done regardless of a user’s activities even if no phone call or data transmission is done, the change is reported to keep the HLR up-to-date all the time.
6. If not much changes between VLRs are done, the hierarchy works well.
7. But if for example, lots of tourists are moving around very often.
8. Then the background load is caused on the core network, because the tourist’s HLRs have to be informed all the time.
9. More hierarchy layers could improve the handling of many mobile users, but also increase the system complexity.