Overview of the 3GPP Release 1999 Network Architecture

Figure 4-4 shows the network architecture for 3GPP Release 1999, the
first set of specifications for UMTS.Working our way from the top left, we
see that a user device is termed the User Equipment (UE). Strictly speaking,
the UE contains the Mobile Equipment (ME) and the UMTS Subscriber
Identity Module (USIM). The USIM is a chip that contains some
subscription-related information, plus security keys. It is similar to the
SIM in GSM.

The interface between the UE and the network is termed the Uu interface.
This is the WCDMA air interface previously described. Strictly speaking,
the WCDMA interface, at least at the physical layer, is between the UE
and the BTS. In 3GPP specifications, the base station is known as Node B.
This was originally a temporary name that somehow stuck.

A Node B is connected to a single Radio Network Controller (RNC). The
RNC controls the radio resources of the Node Bs that are connected to it.
The RNC is analogous to a BSC in GSM. Combined, an RNC and the Node
Bs that are connected to it are known as a Radio Network Subsystem
(RNS). The interface between a Node B and an RNC is the Iub interface.
Unlike the equivalent Abis interface in GSM, the Iub interface is fully standardized
and open. It is possible to connect a Node B to an RNC of a different
vendor.

Unlike in GSM, where BSCs are not connected to each other, in the
UMTS RAN (officially, the UMTS Terrestrial Radio Access Network, or
UTRAN), an interface exists between the RNCs. This interface is termed
Iur. The primary purpose of this interface is to support inter-RNC mobility
and soft handover between Node Bs connected to different RNCs. The Iur
signaling in support of soft handoff is described in more detail later in
Chapter 6.

The UTRAN is connected to the core network via the Iu interface. The Iu
interface, however, has two different components. The connection from
UTRAN to the circuit-switched part of the core network is via the Iu-CS
interface, which connects an RNC to a single Mobile Switching Center
(MSC)/Visitor Location Register (VLR). The connection from UTRAN to the
packet-switched part of the core network is termed Iu-PS. This connection
is from an RNC to an SGSN.

It can be seen from Figure 4-4 that all of the interfaces in the UTRAN of
3GPP Release 1999 are based on Asynchronous Transfer Mode (ATM).ATM
was chosen because of its capability to support a range of different service
types (such as a variable bit rate for packet-based services and a constant
bit rate for circuit-switched services).

One can see from Figure 4-4 that the core network uses the same basic
architecture as that of GSM/GPRS. This was purposely done so that the
new radio access technology could be supported by an established, robust
core network technology. It should be possible for an existing core network
to be upgraded to support UTRAN, so that a given MSC, for example, could
connect to both a UTRAN RNC and a GSM BSC.

In fact, UMTS specifications include support for a hard handover from
UMTS to GSM and vice-versa. This is an important requirement, since the widespread rollout of UMTS coverage will take time to complete, and if
holes exist in UMTS coverage, it is desirable that a UMTS subscriber
should receive service from the more ubiquitous GSM coverage. If UTRAN
and the GSM BSS are supported by different MSCs, then an inter-system
handover could be achieved through an inter-MSC handover. Given that
many of the functions of the MSC/VLR are similar for UMTS and GSM,
however, it makes sense for a given MSC to be able to support both types of
access simultaneously. Similar logic suggests that a given SGSN should be
able to simultaneously support an Iu-PS connection to an RNC and a Gb
interface to a GPRS BSC.

In most vendor implementations, many of the network elements are
being upgraded to simultaneously support GSM/GPRS and UMTS. Such
network elements include the MSC/VLR, the Home Location Register
(HLR), the SGSN, and the GGSN. For some vendors, the base stations
deployed for GSM/GPRS have been designed so that they can be upgraded
to support both GSM and UMTS simultaneously. This is a major consideration
for those network operators that want to deploy a UMTS network in
parallel with an existing GSM network. For some vendors, the BSC is being
upgraded to act as both a GSM BSC and a UMTS RNC. This configuration
is rare, however. The different interfaces and functions (such as a soft handover)
required of a UMTS RNC mean that its technology is quite different
from that of a GSM BSC. Consequently, it is normal to find separate UMTS
RNCs and GSM BSCs. 153