The UMTS open interfaces that are important to understand from an IP networking and services perspective are Iu (Cs and Ps), Iub, and Iur.
9.6.1 Iu Interface
The Iu interface connects the UTRAN/GERAN radio network to the core network. Iu is further subdivided as follows :
The Iu interface has a control plane and a user plane component associated with it. ATM is the link-layer protocol that Iu is currently specified over. However, the Iu interface could be run over IP in the near future.
The IuCs and IuPs control plane consists of radio access network application protocol (RANAP), which is run on top of SS7. AAL5 is the adaptation layer used in the control plane. The IuCs user plane resides directly over AAL2. Since voice is the primary service provided over the IuCs interface, AAL2 has been selected as the adaptation layer for IuCs. GTP-U (GPRS Tunneling Protocol ”User Plane) over UDP/IP/AAL5 is the user plane for IuPs. The protocol stacks for Iu are shown in Figure 9-4.
Figure 9-4. Control and user plane stacks for Iu.
RANAP is the signaling protocol whose functionality includes the following:
The user plane protocol in the case of IuCs is the frame protocol. For IuPs, if the payload is of type IP, it is carried via GTP-U.
The Iub interface connects the Node Bs to the RNC. The Iub interface consists of a control plane and a user plane. Again ATM is used as the underlying protocol for the Iub interaface. The control plane of the Iub interface is called NBAP (Node B application part). NBAP is further divided into common NBAP and dedicated NBAP depending on the signaling link used. The user plane Iub protocol is defined via the frame protocol. It defines the structures of the frames and inband control procedures for every type of transport channel.
The Iur interface is defined for inter-RNC communication. The control plane protocol is refered to as RNSAP (radio network system application part). RNSAP operates over SS7, which is carried over an AAL5-based ATM interface. The user plane consists of the the frame protocol. Two user plane protocols are defined, namely the dedicated channel (DCH) frame protocol and the common channel (CCH) frame protocol. The user plane is carried directly over AAL2. The interface was originally intended to support inter-RNC soft handovers. However, with the development of this interface, new functionality has been added, including support for basic inter-RNC mobility, support for dedicated and common channel traffic, and support for global resource management.