25 series - Iu interface protocol stack
The Iu interface connects the radio access network to the core network.
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 Iu signalling messages and the transport network control plane manages the underlying transport.
So there are three important protocols here: the RANAP defines the signalling messages exchanged over the Iu interface, the SABP defines the data streams used by the cell broadcast service, and the frame protocols define the CS and PS data streams.
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