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IP
Gn
SGSN GGSN

Figure 6.1 GPRS general architecture. Note that there are connections from the MSC and SGSN
to the home location, visitor location and equipment identity registers (HLR/VLR/EIR)


BSS
Circuit Core

PSTN/ISDN/
CSPDN
MS 3G MSC/VLR GMSC
BSC
BTS

RAN HLR AuC EIR

Data Network
IP
e.g. Internet
Backbone
SGSN GGSN
UE
Packet Core
RNC
WBTS

Figure 6.2 UMTS release 99 network

network remains relatively unchanged, with primarily software upgrades. However, the IP
pushes further into the network, with the radio network controller (RNC) now transferring
data with the 3G SGSN using IP.
The roles of the key new components in the network are described in the following
subsections.


6.1.1 WCDMA base station (WBTS)
The Third Generation Partnership Project (3GPP) speci¬cations refer to the base station
as a Node B. However, it is more common to see this referred to as a WBTS, BTS or
even BS. Throughout this book, the BTS notation will be used. Of¬cially, a Node B is a
network entity that serves a single cell. However, sectorized sites are much more ef¬cient
and economical so a commercial outdoor BTS would generally be expected to support
6.1 UMTS NETWORK ARCHITECTURE 267


multiple cells across the full spectrum of the required operating frequency. A typical BTS
con¬guration is support of up to six sectors, with two carriers per sector.
The BTS is the termination point between the air interface and the transmission network
of the RAN. It is therefore required to support both WCDMA and ATM, connecting
through a plesiochronous or synchronous digital hierarchy (PDH or SDH) interface. The
BTS must provide all the necessary signal processing functions to support the WCDMA
air interface and this is where most of the complexity arises. In addition, the provision of
interfaces to microwave PDH or SDH radio solutions is also desirable. Some solutions
offer ATM cross-connection equipment, and ATM circuit emulation services to support
combined transport of 2G and 3G traf¬c. It is also common that some manufacturers have
multipurpose BTS solutions, which support multiple technologies on the one hardware
platform, such as transceivers for GSM, enhanced data rates for global evolution (EDGE)
and WCDMA. A BTS solution should also provide antenna diversity in both the uplink
and the downlink.


6.1.2 Radio network controller (RNC)
The RNC is the heart of the new access network. All decisions of the network operation
are made here, and at its centre is a high-speed packet switch to support a reasonable
throughput of traf¬c. An RNC is responsible for control of all the BTSs that are con-
nected to it, and maintains the link to the packet and circuit core network, that is the
mobile switching centre (MSC) and the SGSN. It also needs to be capable of supporting
interconnections to other RNCs, a new feature of UMTS. Most of the decision-making
process is software based, so a high processing capacity is required. This chapter deals
with much of the functionality of the RNC, such as radio resource management (RRM).



6.1.3 3G mobile switching centre (3G MSC)
For UMTS R99, the changes to the core network side are minimal, and these should be
mostly in the form of software upgrades to support the new access network. The role
played by the 3G MSC is exactly the same here as in GSM. However, a 2G MSC is a
narrowband device and connects to the access network via the A interface. The traf¬c is
expected to be in 64 kbps, and for voice this should be 64 kbps pulse code modulation
(PCM). The RAN, on the other hand, presents the circuit core network with an interface
which is transporting speech across ATM, and uses the adaptive multirate (AMR; refer to
Section 6.13), which codes speech to a range from 4.75 kbps to 12.2 kbps. Therefore, an
interworking function (IWF) is needed between the RAN and the MSC. The role of the
IWF is twofold: ¬rst, for user traf¬c it is responsible for transcoding of speech to and from
64 kbps PCM. If the traf¬c is circuit switched data, then it is responsible for transferring
to and from the narrowband time division multiplexing (TDM) time slots. Second, for
control information, it is responsible for mapping between the MSC signalling messages
and the signalling messages to the RAN (RANAP protocol, see later). The combination
of this IWF and a 2G MSC is considered a 3G MSC. For many manufacturers, this IWF
268 UNIVERSAL MOBILE TELECOMMUNICATIONS SYSTEM


is a separate functional unit, which enables them to retain the existing hardware of a GSM
network. It is generally known as a media gateway (MGW) as this hardware platform
can be reused in subsequent UMTS releases to switch traf¬c between TDM, ATM and IP
technologies.




6.2 NETWORK EVOLUTION

The next evolution step is the release 4 (R4) architecture (Figure 6.3). Here, the GSM
core is replaced with an IP network infrastructure based around voice over IP (VoIP)
technology.
The MSC evolves into two separate components: an MGW and an MSC server (MSS).
This essentially breaks apart the roles of connection and connection control. An MSS can
handle multiple MGWs, making the network more scalable.
Since there are now a number of IP clouds in the 3G network, it makes sense to merge
these together into one IP or IP/ATM backbone (it is likely both options will be available to
operators.) This extends IP right across the whole network, all the way to the BTS. This is
referred to as the all-IP network, or the release 5 (R5) architecture, as shown in Figure 6.4.
The HLR/VLR/EIR are generalized and referred to as the HLR subsystem (HSS).
Now the last remnants of traditional telecommunications switching are removed, leaving
a network operating completely on the IP protocol, and generalized for the transport of
many service types. Real-time services are supported through the introduction of a new
network domain, the IP multimedia subsystem (IMS). The architecture of R4 and R5 is
discussed further in Chapters 8 and 9.
Currently the 3GPP are working on release 6, which purports to cover all aspects not
addressed in frozen releases. Some call UMTS release 6 4G and it includes such issues
as interworking of hotspot radio access technologies such as wireless LAN.


BSS Circuit Core


MSS
PSTN
MS
BSC
BTS MGW MGW

RAN
HLR VLR EIR

IP
Gn
UE SGSN GGSN
Packet Core
RNC
WBTS

Figure 6.3 UMTS release 4 architecture
6.3 UMTS FDD AND TDD 269



PSTN/
ISDN/
CSPDN
RAN

HSS

IP
IMS
Backbone
UE
SGSN GGSN
RNC
WBTS
Packet Core
Uu Iu
IP Network




Figure 6.4 UMTS release 5 architecture


6.3 UMTS FDD AND TDD

Like any CDMA system, UMTS needs a wide frequency band in which to operate to effec-
tively spread signals. The de¬ning characteristic of the system is the chip rate, where a chip
is the width of one symbol of the CDMA code. UMTS uses a chip rate of 3.84 Mchips/s
and this converts to a required spectrum carrier of 5 MHz wide. Since this is wider than
the 1.25 MHz needed for the existing cdmaOne system, the UMTS air interface is termed
wideband CDMA.
There are actually two radio technologies under the UMTS umbrella: UMTS FDD
and TDD. FDD stands for frequency division duplex, and, like GSM, separates traf¬c in
the uplink and downlink by placing them at different frequency channels. Therefore an
operator must have a pair of frequencies allocated to allow it to run a network, hence the
term ˜paired spectrum™. TDD or time division duplex requires only one frequency channel,
and uplink and downlink traf¬c are separated by sending them at different times. The
ITU-T spectrum usage, as shown in Figure 6.5, for FDD is 1920“1980 MHz for uplink
traf¬c, and 2110“2170 MHz for downlink. The minimum allocation an operator needs
is two paired 5 MHz channels, one for uplink and one for downlink, at a separation of
190 MHz. However, to provide comprehensive coverage and services, it is recommended
that an operator be given three channels. Considering the spectrum allocation, there are 12
paired channels available, and many countries have now completed the licensing process
for this spectrum, allocating between two and four channels per licence. This has tended to
work out a costly process for operators, since the regulatory authorities in some countries,
notably in Europe, have auctioned these licences to the highest bidder. This has resulted
in spectrum fees as high as tens of billions of dollars in some countries.
The TDD system, which needs only one 5 MHz band in which to operate, is often
referred to as unpaired spectrum. The differences between UMTS FDD and TDD are
only evident at the lower layers, particularly on the radio interface. At higher layers, the
bulk of the operation of the two systems is the same. As the name suggests, the TDD
system separates uplink and downlink traf¬c by placing them in different time slots. As
270 UNIVERSAL MOBILE TELECOMMUNICATIONS SYSTEM



TDD FDD-UL TDD FDD-DL


1900 1920 1980 2010 2025 2110 2170 MHz

Figure 6.5 UMTS frequency allocation


will be seen later, UMTS uses a 10 ms frame structure which is divided into 15 equal
time slots. TDD can allocate these to be either uplink or downlink, with one or more
breakpoints between the two in a frame de¬ned. In this way, it is well suited to packet
traf¬c, since this allows great ¬‚exibility in dynamically dimensioning for asymmetry in
traf¬c ¬‚ow.
The TDD system should not really be considered as an independent network, but rather
as a supplement for an FDD system to provide hotspot coverage at higher data rates. It
is rather unsuitable for large-scale deployment due to interference between sites, since a
BTS may be trying to detect a weak signal from a user equipment (UE), which is blocked
out by a relatively strong signal at the same frequency from a nearby BTS. TDD is ideal
for indoor coverage over small areas.
Since FDD is the main access technology being developed currently, the explanations
presented here will focus purely on this system.



6.4 UMTS BEARER MODEL

The procedures of a mobile device connecting to a UMTS network can be split into two
areas: the access stratum (AS) and the non-access stratum (NAS). The AS involves all the
layers and subsystems that offer general services to the NAS. In UMTS, the AS consists
of all of the elements in the RAN, including the underlying ATM transport network, and
the various mechanisms such as those to provide reliable information exchange. All of the
NAS functions are those between the mobile device and the core network, for example
mobility management. Figure 6.6 shows the architecture model. The AS interacts with
the NAS through the use of service access points (SAPs).
The UMTS terrestrial radio access network (UTRAN) provides this separation of NAS
and AS functions, and allows for AS functions to be fully controlled and implemented
within the UTRAN. The two major UTRAN interfaces are the Uu, which is the interface
between the mobile device, or UE, and the UTRAN, and the Iu, which is the interface
between the UTRAN and the core network. Both of these interfaces can be divided into
control and user planes, each with appropriate protocol functions.
A bearer service is a link between two points, which is de¬ned by a certain set of char-
acteristics. In the case of UMTS, the bearer service is delivered using radio access bearers
(RABs).
A RAB is de¬ned as the service that the AS (i.e. UTRAN) provides to the NAS
for transfer of user data between the UE and core network. A RAB can consist of a
number of sub¬‚ows, which are data streams to the core network within the RAB that
6.4 UMTS BEARER MODEL 271




Non Access Stratum

SAP SAP




Radio Radio Iu Iu
Protocols Protocols Protocols Protocols




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