The Elements of the UTRAN
The UTRAN is a new brand of radio access network, and thus does not re-use elements of the existing GSM radio network, although some comparisons can be usefully drawn.
In terms of architecture, the UTRAN is divided into individual Radio Network Systems, where each is controlled by a Radio Network Controller (RNC), connected to several Node B, where Node B is broadly equivalent to the BTS, and the RNC similar in architectural hierarchy to the BSC. The Interface between RNCs and Node Bs is defined, and is assigned the label Iub.
Another significant new addition is a brand new interface, the Iur, which is required to cope with soft handover, a new feature of CDMA systems which arises because adjacent cells are operating on the same carrier frequency band.
The Node B
The term Node B refers to the base station equipment which communicates with the subscriberÃ¢â‚¬â„¢s handset via the radio link (and of course with the main network via a telecoms link).
It provides radio resources for a UMTS network, and uses UMTS channel allocation to communicate with the handset. It provides all the RF processing, enabling transmission and reception information to and from the mobile terminal. This information is encoded using the W-CDMA scheme.
The RNC controls the operation of multiple Node Bs, managing resources such as allocating capacity for data calls, and providing critical signalling such as call set-up, plus switching and traffic routing functionality.
Compared to 2G systems, it is broadly equivalent to the BSC, but also includes some functionality of the MSC. In particular, it enables autonomous Radio Resource
Management by the UTRAN by allowing RNCs to directly communicate (via the Iur interface), eliminating this burden from the core network. So all handover processes, even where moving between cells controlled by different RNCs, are kept within the
UTRAN. Compare this with the situation in GSM, where handover between different BSC areas required involvement of the MSC, and hence the core network.
Review of Core Network Architecture & Elements (R.99)
The Core Network of UMTS, in its first release, is taken directly from the architecture and elements of GSM Phase 2+.
Two domains are defined, circuit switched (CS) and packet-switched (PS). The latter is provided by the implementation of GPRS within the GSM network.
These two transport and signalling domains will both interact with a number of databases, which are sometimes grouped under the Home Server Subsystem (HSS), and with service platforms such as the CAMEL Service Environment (CSE), which provide services and support for the Virtual Home Environment.
The two key elements of the Circuit-Switched domain are as follows:
- Serving MSC (MSC), the switching node at which the core network interfaces to that part of the access network in which the mobile station is located. The Serving MSC interacts with, or includes, a database called the Visitor Location Register (VLR), containing information regarding the mobile stations which it is currently serving.
- Gateway MSC (GMSC), the switching node which connects a UMTS core network with an external circuit-switched network, such as ISDN or PSTN.
The two key and equivalent elements in the Packet-Switched domain are:
- Serving GPRS Support Node (SGSN), the packet router at which the core network interfaces to the part of the access network in which the mobile station is located.
- Gateway GSN (GGSN), another packet router, connecting the UMTS core network with an external packet-switched network, such as the Internet, X.25 or similar.
In many networks, the Serving MSC/VLR and the SGSN will interact in order to ensure better integrated management and information updating between PS and CS services.
The HSS includes database such as the Home Location Register (HLR), which includes a variety of subscriber related data, enabling users to be offered the services to which they are entitled. Also included within the HSS is the database for authentication.
The interface connecting the core and radio access networks (at the Serving MSC and SGSN) is known as the Iu Interface, and is fully defined in UMTS. This interface is subdivided into the IuPS and IuCS, in order to support each domain in the core network, but within this subdivision there remains much commonality.
An important new feature of UMTS is that the core network and radio access network are kept entirely separate and independent by means of this Iu Interface, allowing any UMTS core network to be connected to any UMTS radio access network.
elements which may in turn serve one or more cells. These elements are completely new designs, although comparison can be drawn with