Overview

Universal Terrestrial Radio Access Network (UTRAN) is a group term for network and equipment that connects mobile to the public telephone network/Internet. It contains the base stations (Node B’s) and Radio Network Controllers (RNCs) and this communications network, commonly referred to as 3G can carry many traffic types from real-time Circuit Switched to IP based Packet Switched. The UTRAN also allows connectivity between the UE and the core network.

In UMTS, the interface between RAN and CN is the Iu. For uniformity, 3GPP specifies a single protocol at Radio Network Layer for the Iu-CS and Iu-PS interfaces both i.e., RANAP for the control plane and Iu for the user plane.

The Radio Access Network Application Protocol (RANAP) is the Radio Network Layer protocol for the Iu interface. The peer entities of RANAP reside in 3G MSC/SGSN and the SRNC. All the RANAP functions are elaborated in 3GPP TS 25.413. The Control Plane includes the application protocols and the signaling bearers which transport the control information.

The application protocols used at different UTRAN interfaces are:

• Iu-CS: Radio access network application protocol (RANAP)
• Iu-PS: Radio access network application protocol (RANAP)
• Iub: Node B application protocol (NBAP)
• Iur: Radio network system application protocol (RNSAP)

IuPS interface works between the UMTS RNC (Radio Network Controller) and the SGSN (Serving GPRS Support Node) supporting UMTS integrated services such as multimedia and global roaming to mobile users. IuCS interface in UMTS links the RNC (Radio Network Controller) with a 3G MSC (3G Mobile Switching Centre).