UMTS system domains
UMTS separates User Equipment, the UTRAN radio access network, and the Core Network. Reference interfaces define the boundaries among these domains. The Core Network contains circuit-switched and packet-switched service domains, while radio and core functions can be distributed across multiple hardware instances.
- UE terminates the radio interface and represents the subscriber.
- UTRAN manages WCDMA transmission and radio resources.
- CN handles mobility, services, subscriber data, and interconnection.
UTRAN structure
UTRAN consists of Radio Network Subsystems. Each RNS contains one RNC and one or more Node B stations. Node B provides radio transmission in one or more cells, while the RNC controls radio resources and connection procedures.
WCDMA can maintain links with several cells during . The RNC coordinates signal combining or selection and updates the UE active set.
Radio-side interfaces
- Uu is the radio interface between UE and UTRAN.
- Iub connects Node B to its controlling RNC and carries user and control traffic.
- Iur connects RNCs and can support mobility across their areas.
- Iu connects UTRAN to the Core Network in service-domain variants.
These names identify logical interfaces and protocol sets. Physical transport can vary when it preserves the required functions and quality.
Circuit-switched domain
The CS domain primarily serves real-time services that receive dedicated resources from connection setup to release in the conventional architecture. UTRAN sends this traffic through Iu-CS to the part of the core associated with MSC functions.
UE → Uu → Node B → Iub → RNC → Iu-CS → CS domain
Call control, mobility, and media transport are functionally distinct. Later core implementations can use an MSC server to control a separate media gateway.
Packet-switched domain
The PS domain forwards packets independently without reserving a fixed path for the complete session. UTRAN sends traffic through Iu-PS to SGSN functions and onward to a GGSN gateway that reaches an external packet network.
UE → Uu → Node B → Iub → RNC → Iu-PS → PS domain
The network maintains mobility and packet-session contexts. When bearer and mobility procedures support continuity, a cell change can preserve the address used by a service.
Control and user planes
The control plane carries registration, authentication, bearer establishment, and mobility signaling. The user plane carries speech or application packets. Both traverse the same architectural domains with different protocols and requirements.
The separation lets the same architecture carry registration, mobility, voice, and application traffic through domain-specific procedures. UE, Node B, RNC, MSC, SGSN, and subscriber databases contribute distinct functions to these paths.