What are the three named interfaces that link the elements of a GSM network, and what does each one connect?
Um (the air interface) joins the phone to the BTS, Abis joins the BTS to its BSC, and the A-interface joins the BSC to the MSC — one wireless hop at the bottom, fixed links above it.
GSM gives every hop between its boxes a name, which lets you reason precisely about where a signal is and what could go wrong there:
| Interface | Connects | Nature |
|---|---|---|
| Um (GSM Radio Air Interface) | MS ↔ BTS | The wireless link — TDMA over the 200 kHz radio carriers; the only over-the-air hop |
| Abis | BTS ↔ BSC | Wired link, carried as several 16 kbps and 64 kbps connections |
| A | BSC ↔ MSC | Wired trunk into the core network, where traffic from many BTS is multiplexed |
Read bottom to top: MS —Um→ BTS —Abis→ BSC —A→ MSC. Only Um is wireless; everything from the BTS upward is fixed cabling. That is exactly why encrypting the Um interface matters so much — it is the one link an attacker can reach over the air, without any physical access to the operator's equipment.
Tip: "Um for the User's air link, A for the core's Anchor." Um is the phone's radio hop, the A-interface is the door into the MSC core, and Abis sits between them.
Go deeper:
Um interface (Wikipedia) — the GSM air interface between the handset and the BTS, the one hop carried over radio.