UMTS consists of a core network and a radio access network. The core network uses optical landlines and its function basically is no different from a conventional phone network. The radio access network is known as UTRAN (for UMTS Terrestrial Radio Access Network), and this is where most of the complexity of the system resides. UTRAN is responsible for covering an operator's area with hundreds and thousands of radio cells which pick up and deliver all the traffic. A typical cell covers an area of anywhere between 500m and 2000m radius, which is considerably less than the area covered by a GSM cell. UMTS/UTRAN is designed to offer its services to highly mobile subscribers which travel at speeds up to 500km/h. This implies that a subscriber cannot be expected to rest within a cell for a long time and that calls very often need to be transferred to neighbouring cells.

A UTRAN network consists of a basic pattern that is repeated as often as required to cover a given area. The heart of the system is the Radio Network Controller or RNC in short. All traffic is routed through at least one RNC. The RNC is connected via broadband lines to several types of other network nodes, which are:

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  Node B¹. Node Bs are the workhorses of UMTS as they will be present everywhere when a network has been fully installed. A Node B features anywhere between one and six antennas which provide a number of radio cells. The Node B establishes the radio link to the UE but the RNC is the owner and manager of all resources involved.
  

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  The MSC (Mobile Switching Center) and SGSN (...) both serve as entry points from UTRAN into the core network. The difference is that the MSC handles circuit switched traffic (ie: voice calls) and SGSN handles packet oriented traffic (ie: anything other than voice calls, particularly everything that has to do with internet access).
  

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  The HLR (Home Location Register) basically is a database which holds information about which UEs are registered with an operator, who their owner is, whether last month's bill has been payed, whether they are logged in somewhere in the network and if so, which cell it is where the UE is camped on.
  

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  The VLR (Visitor Location Register) serves a similar purpose, the only difference being that it holds information about UEs that are contracted with some other operator but are currently present within the own coverage area.
  

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  The OMC is the Operations and Maintenance Center. This is where the operator controls the UTRAN network, receives alarm messages in case something breaks down, and manages the resources of the network.
  

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  The NMC (National Maintenance Centre) is the operators main base where the whole UMTS system is being managed.