The UTRAN Architecture

In most mobile communications networks, the network architecture can be
split into two main parts—the access network and the core network. The
access network is specific to the access technology being used, whereas the
core network is shielded from the vagaries of the access technology and
should ideally be able to handle multiple different access networks. This
split applies quite well to UMTS, where the access network is known as the
UMTS Terrestrial Radio Access Network (UTRAN). It is supported by a core
network that is based upon the core network used for GSM. In fact, the
GSM core network can be upgraded to simultaneously support both
UTRAN and a GSM radio access network.
The UTRAN architecture is shown in Figure 6-16 as it applies to the first
release of UMTS specification—3GPP Release 1999. The UTRAN comprises
two types of nodes—the Radio Network Controller (RNC) and the Node B,
which is the base station. The RNC is analogous to the GSM Base Station
Controller (BSC). The RNC is responsible for the control of the radio
resources within the network. It interfaces with one or more base stations,
known as Node Bs. The interface between the RNC and the Node B is the
Iub interface. Unlike the equivalent Abis interface in GSM, the Iub interface
is open, which means that a network operator could acquire Node Bs from
one vendor and RNCs from another vendor.Together an RNC and the set of
Node Bs that it supports are known as a Radio Network Subsystem (RNS).
Unlike in GSM where BSCs are not connected to each other, UTRAN
contains an interface between RNCs. This is known as the Iur interface.
The primary purpose of the Iur interface is to support inter-RNC mobility
and a soft handover between Node Bs connected to different RNCs.
The user device is the UE. It comprises the Mobile Equipment (ME) and
the UTMTS Subscriber Identity Module (USIM). UTRAN communicates
with the UE over the Uu interface. The Uu interface is none other than the
WCDMA air interface that we have already described in this chapter.
UTRAN communicates with the core network over the Iu interface. The
Iu interface has two components—the Iu-CS interface, which supports
circuit-switched services, and the Iu-PS interface, which supports packet-switched services. The Iu-CS interface connects the RNC to an MSC
and is similar to the GSM A-interface. The Iu-PS interface connects the
RNC to an SGSN and is analogous to the GPRS Gb interface.
In 3GPP Release 1999, all of the interfaces within UTRAN, as well as the
interfaces between UTRAN and the core network, use Asynchronous Transfer
Mode (ATM) as the transport mechanism.