UMTS Network Interfaces and Protocol Stack

UMTS network follows the typical communication model in telecom which defines a set of horizontal and vertical layers. The horizontal layers are physical, network, transport and application layers – as defined in the OSI model. The vertical layers are functional areas, namely control plane, user plane and data plane. Control planes are used to control a link or a connection; user planes are used to transparently transmit user data from the higher layers. The UMTS network interfaces and protocol stacks follow the same communication model. Standard transmission issues, which are independent of UTRAN requirements, are applied in the horizontal transport network layer. The UTRAN requirements are addressed in the horizontal radio network layer across different types of control and user planes. The UMTS network introduces four new key interfaces and protocol stacks: Uu, Iub, Iur, and Iu.

Iu: Radio Access Network Application Part (RANAP) [3G TS 25.413]. This interface provides UTRAN–specific signaling and control over the Iu. The following is some typical RANAP functions:

• Overall radio access bearer (RAB) management, which includes the RAB’s setup, maintenance, and release
• Management of Iu connections
• Transport of nonaccess stratum (NAS) information between the UE and the CN; for example, NAS contains the mobility management signaling and broadcast information.
• Exchanging UE location information between the RNC and CN
• Paging requests from the CN to the UE
• Overload and general error situation handling

Iur: Radio Network Sublayer Application Part (RNSAP) [3G TS 25.423]. This interface provides UTRAN–specific signaling and control for the following sample functions:

• Management of radio links, physical links, and common transport channel resources
• Paging
• SRNC relocation
• Measurements of dedicated resources

Iub: Node B Application Part (NBAP) [3G TS 25.433]. This interface provides UTRAN specific signaling and control for the following sample areas:

• Management of common channels, common resources, and radio links
• Configuration management, such as cell configuration management
• Measurement handling and control
• Synchronization (TDD)
• Reporting of error situations

Uu: Radio Resource Control (RRC) [3G TS 25.331]. This interface handles the control plane signaling over the Uu between the UE and the UTRAN. Some of the functions offered by the RRC

include the following areas:

• Broadcasting information
• Management of connections between the UE and the UTRAN, which include their establishment, maintenance, and release
• Management of the radio bearers, which include their establishment, maintenance, release, and the corresponding connection mobility
• Ciphering control
• Outer loop power control
• Message integrity protection
• Timing advance in the TDD mode
• UE measurement report evaluation
• Paging and notifying