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The GSM Architecture consists of three major sub-systems. These
are Base Station Sub-System (BSS) that provides the air interface
for Mobile Stations (MS), Network Sub-System (NSS) that connects
calls between users, and Operation Sub-System (OSS) that allows
remote monitoring and management of network.

SIM
- Subscriber IdentityModule,
ME - Mobile Equipment, BTS -
Base Transceiver Station, BSC
- Base Station Controller, TCU
- Transcoder Unit, MSC - Mobile
Switching Centre, PSTN - Public
Switched Telephone Network, HLR
- Home Location Register, VLR
- Visitor Location Register, AUC
- Authentication Centre, EIR
- Equipment Identity Register, OMC
- Operations & Maintenance Centre, OMC-R
- OMC devoted to BSS, OMC-S -
OMC devoted to NSS.
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Base
Station Sub-system (BSS)
The
Base Station function is divided into two main functional
elements, the Base Station Controller (BSC) which also includes
the Transcoder Unit (TCU), and the Base Transceiver System
(BTS).
The BSC can control several BTS units. Each BTS will consist
of a number of transceivers (TRX) and will serve a cell or
a number of cells. The BSC unit also performs transcoding
functions to convert between 64Kbps channel rate used in the
Switching System and the 16Kbps channel rate for GSM traffic.
Network
Switching Sub-system (NSS)
The
switching system connects mobile subscribers to other mobile
subscribers in the same network or to other networks. To perform
this function a number of Mobile Switching Centres (MSC) are
employed. However to connect to users on another network requires
a Gateway MSC (GMSC) which provides interconnection between
different networks.
Associated with the MSC are location registers and authentication
functions that allow the network to locate and validate system
users. There is one Home Location Register (HLR) and Authentication
Centre (AUC). However, each MSC has a Visitor Location Register
(VLR) associated with it.
Operation Sub-System (OSS)
The
OMC provides remote monitoring of the network performance
and permits remote re-configuration and fault management activity
as well as alarm and event monitoring.
Mobile
Station (MS)
The
MS comprises of Mobile Equipment (ME) and a Subscriber Identity
Module (SIM). The ME contains the software and hardware to
operate as a mobile radio terminal. The SIM in conjunction
with the network Authentication Centre (AUC) validates the
MS.
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UMTS
Network Architecture
One
of the requirements for the Release 99 Architecture is to
support roaming and inter-operation with the GSM system, hence
the GSM system appears as one of the components of the UMTS
Release 99 Architecture.

USIM
- UMTS Subscriber Identity Module, UE
- User Equipment, Node
B - UMTS Base Station, SRNC
- Serving Radio Network Controller, DRNC
- Drift Radio Network Controller, SGSN
- Serving GPRS Support Node, GGSN
- Gateway GPRS Support Node, PDN
- Public Data Network, SIM
- Subscriber Identity Module, ME
- Mobile Equipment, BTS
- Base Transceiver Station, BSC
- Base Station Controller, MSC
- Mobile Switching Centre, GMSC
- Gateway Mobile Switching Centre, PSTN
- Public Switched Telephone Network, HLR
- Home Location Register, VLR
- Visitor Location Register.
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CS-Domain
This
is the Circuit Switched domain that is traditionally known
to provide services such as speech calls. The Core Network
(CN) component that implements the CS services for UMTS is
the 3G-MSC.
PS-Domain
This
is the Packet Switched domain that is known to provide services
such as IP Based traffic. The CN component that implements
the PS services for UMTS is the 3G-SGSN.
UMTS
Terrestrial Radio Access Network (UTRAN)
This consists of the Radio Network Controller (RNC) and the
Node B. The UTRAN is responsible for functions that relate
to access, radio mobility and resource utilisation. The Serving
Radio Network Controller (SRNC) is responsible for the logical
connection between the UE and the CN. The Drift Radio Network
Controller (DRNC) provides additional radio resources for
a UE that is in a dedicated connection and a soft-handover
state. The Node B that is attached to the DRNC will provide
the physical resource to the UE, and the information on the
uplink and the downlink is routed towards the SRNC.
User
Equipment (UE)
The
radio terminal that the subscriber uses to receive service
from the UTRAN is known as the UE. This will arrive in the
form of PDA terminals and Handsets similar to current GSM
mobiles. The UE's will be supporting multimode GSM, GPRS and
UMTS services. They will be supporting multi-band GSM900,
DCS1800 and PCS1900 systems. The capabilities of these User
Equipments will vary hence the UTRAN will read UE capabilities
during set-up.
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