GSM Interfaces and Their Uses
| Interface | Between | Purpose |
|---|
| Um | Mobile Station ↔ Base Transceiver Station (BTS) | Wireless air interface used for communication between the mobile device and the network. |
| Abis | BTS ↔ Base Station Controller (BSC) | Carries voice, data, and control signals; allows the BSC to manage BTS operations. |
| A | BSC ↔ Mobile Switching Centre (MSC) | Handles call control, handovers, and mobility management. |
| B | MSC ↔ Visitor Location Register (VLR) | Transfers temporary subscriber information for session and call handling. |
| C | MSC ↔ Home Location Register (HLR) | Retrieves permanent subscriber data used for authentication and service provisioning. |
| D | HLR ↔ VLR | Transfers subscriber information when a user roams into a new location. |
| E | MSC ↔ Other MSCs | Supports inter-MSC handovers and routing of calls. |
| F | MSC ↔ Equipment Identity Register (EIR) | Verifies the IMEI to prevent the use of stolen or unauthorized devices. |
| G | VLR ↔ Other VLRs | Transfers user data during inter-VLR handovers. |