25 series - Iur interface protocol stack
The Iur interface connects different RNCs together. It is new to UMTS, and is typically (but not exclusively) used to support mobiles that are in soft handover to Node B's controlled by different RNCs.
Like the other UTRAN interfaces, its protocol stack is divided into two layers. All the UMTS-related issues are in the radio network layer, while the transport network layer contains standard technology used for the underlying transport. The stack is also divided into three planes. The user plane handles data streams that eventually reach the UE, the control plane handles Iur signalling messages and the transport network control plane manages the underlying transport.
So there are two important protocols here: the RNSAP defines the signalling messages exchanged over the Iur interface and the frame protocols define the data streams.
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