The UMTS
(Universal Mobile Telecommunications System) is the third generation mobile
telecommunication system by using the WCDMA air interface technology, usually
called the WCDMA telecommunication system. It adopts a structure similar to the
second generation mobile telecommunication system, including the RAN (Radio
Access Network) and the CN (Core Network). The RAN is used to process all the
radio-related functions, while the CN is used to process all voice calls and
data connections within the UMTS system, and implements the function of
external network switching and routing. Logically, the CN is divided into the
CS (Circuit Switched) Domain and the PS (Packet Switched) Domain. UTRAN, CN and
UE (User Equipment) together constitute the whole UMTS system.
From the point of view of the 3GPP R99
standard, the UE and the UTRAN (UMTS Terrestrial Radio Access Network) are
composed of new protocols, and the design is based on WCDMA radio technologies.
However, the CN adopts the definition of GSM/GPRS, so it not only can implement
smooth transition of the network, but also can implement global roaming at the
initial phase of 3G network construction.
3.1.1 Composition of the UMTS Network System
The UMTS network system includes the
following parts:
1. UE (User Equipment)
As the user
terminal equipment, the UE exchanges data with network equipment through the Uu
interface, and provides such kinds of services within CS and PS domains as
common voice, data communication, mobile multi-media and Internet application
(For example, E-mail, WWW browse and FTP).
UE includes
the two parts below:
l ME (Mobile Equipment): Providing application and
services.
l USIM (UMTS Subscriber Module): Providing
subscriber identification.
2. UTRAN (UMTS Terrestrial Radio Access Network)
UTRAN is divided into Node B and RNC (Radio Network Controller).
l Node B
Node B is the
base station of the WCDMA system (i.e. radio transceiver), and it interconnects
with RNC via the standard Iub interface and processes the physical layer
protocols of the Uu interface. Its main functions include
spreading/de-spreading, modulation/demodulation, channel coding/decoding, and
conversion between baseband signals and RF signals.
l RNC (Radio Network Controller)
RNC (Radio Network Controller)
implements such functions as connection establishment and release, handover,
macro diversity and the management and control of radio resources. The details
are given as follows:
1)
Provides the system information
broadcast and system access control functions
2)
Provides such mobility
management functions as handover and RNC transition
3)
Provides radio resource
management and control functions such as macro diversity combination, power
control and radio bearer allocation
3. CN (Core Network)
CN (Core
Network) is responsible for connecting other networks as well as communicating
and managing UEs. The core network equipment of different protocol versions in
the WCDMA system differ. Generally, the R99 core network is divided into the CS
domain and the PS domain. The R4 core network is the same as the R99 core
network, but in the R4 core network, the MSC function of R99 CS is implemented
by the two separate entities: MSC Server and MGW. The R5 core network is the
same as the R4 core network except that R5 has been added with an IP
multi-media domain.
The R99 core
network has the following function entities:
1) MSC/VLR
MSC/VLR is a
functional node of the CS domain in the WCDMA core network. It connects with
UTRAN via the Iu-CS interface, with external networks (such as PSTN and ISDN)
via the PSTN/ISDN interface, with HLR/AUC via the C/D interface, with MSC/VLR,
GMSC or SMC via the E interface, with SCP via the CAP interface, and with SGSN
via the Gs interface. Its main functions are call control, mobility management,
authentication and ciphering of the CS domain.
2) GMSC
GMSC is the
gateway node between the CS domain of the WCDMA mobile network and external
networks, and it is an optional functional node. It connects with external
networks (PSTN, ISDN and other PLMN) through the PSTN/ISDN interface, connects
with HLR through the C interface and connects with SCP through the CAP
interface. It implements the routing function of incoming calls in the VMSC
function and inter-network settlement function of such external networks as
fixed networks.
3) SGSN
SGSN (Serving
GPRS Support Node) is a functional node of the PS domain in the WCDMA core
network. It connects with UTRAN through the Iu-PS interface, with GGSN through
the Gn/Gp interface, with HLR/AUC through the Gr interface, with MSC/VLR
through the Gs interface, with SCP through the CAP interface, with SMC through
the Gd interface, with CG through the Ga interface and with SGSN interface
through the Gn/Gp interface. And its main functions are route forwarding,
mobility management, authentication and ciphering of the PS domain.
4) GGSN
GGSN (Gateway
GPRS Supporting Node) is a functional node of the PS domain in the WCDMA core
network. It connects with SGSN through the Gn/Gp interface and with external
data networks (Internet/Intranet) through the Gi interface. It provides the
routing and encapsulation of data packets between the WCDMA mobile network and
the external data networks. Its major functions are to provide interfaces to external
IP packet networks. It needs to provide the gateway function for UE to access
external packet networks. From the point of view of external networks, GGSN
looks as if it were a router of all user IP networks in the addressable WCDMA
mobile network, and it needs to exchange routing information with external
networks.
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