UMTS system domains
UMTS separates User Equipment, the UTRAN radio access network, and the Core Network. Reference interfaces define the boundaries among these domains. Radio and core functions can therefore evolve without requiring one hardware implementation.
- 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 its Node B stations. Node B provides radio transmission in one or more cells, while the RNC makes radio-resource and connection-control decisions.
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.
- Iur connects RNCs and supports 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 a switched path 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 without reserving a fixed path for the complete session. UTRAN sends traffic through Iu-PS to SGSN functions and onward to a 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. A cell change does not automatically change 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.
This article describes domain relationships and traffic paths. The individual duties of UE, Node B, RNC, MSC, SGSN, and databases are covered separately.