What are the four interfaces in the UMTS architecture, and what does each one connect?
Uu (UE↔Node B air interface), Iub (Node B↔RNC), Iur (RNC↔RNC), and Iu (RNC↔Core Network) — together they form the chain from phone to core.
* The four UMTS interfaces: Uu, Iub, Iu, and Iur (RNC-RNC). *
Walking up the stack from the phone:
| Interface | Connects | GSM equivalent |
|---|---|---|
| Uu | UE ↔ Node B (the radio air interface) | Um |
| Iub | Node B ↔ RNC | Abis |
| Iur | RNC ↔ RNC | (none — new in UMTS) |
| Iu | RNC ↔ Core Network | A (and Gb for data) |
UE ←Uu→ Node B ←Iub→ RNC ←Iu→ Core Network
↕ Iur
RNC
Why Iur is special: It has no GSM counterpart. UMTS uses CDMA, which allows soft handover — a phone can talk to two Node Bs at once during a cell change. When those Node Bs are owned by different RNCs, the two controllers must coordinate the combined connection, and Iur is the dedicated path that lets them do it.
Iu splits in the core: The single Iu interface is realised as two logical variants — Iu-CS toward the circuit-switched MSC (voice) and Iu-PS toward the packet-switched SGSN (data). The RNC routes a connection down the right one based on the service type.
Tip: Memorise the chain "Uu, Iub, Iur, Iu" bottom-up — air, backhaul, between-controllers, to-core.
Go deeper:
UMTS UTRA / UTRAN: Radio Network Subsystem (Electronics Notes) — what rides on each of Iub / Iur / Iu and why standardising them let operators buy Node Bs and RNCs from different vendors.- Handover (Wikipedia) — soft vs hard handover (make-before-break vs break-before-make) and why CDMA's shared-frequency cells are what make the Iur-coordinated soft handover possible.