2. GSM Network Components
• Network and Switching SubSystem
(NSS)
• Base Station SubSystem (BSS)
• Operation and Support SubSystem
(OSS)
3. The BSS Consists of Three Major
Hardware Components
• The Base Transceiver Station (BTS)
• The Base Station Controller (BSC)
• The Transcoder (TCU / TRAU)
4. Traffic
Speech
Data Short messages
Transmission
Reception
Features of the radio interface
Signal processing
Coding Ciphering
D
1
D
2
D
3
D
4
D
5
D
6
D
7
D
8
D
1
D
2
D
3
D
4
D
5
D
6
D
7
D
8
1 2 3 4 5 6 7 8
Interleaving
Modulation
Coupling system
Frequency
hopping
Power
Control
Handover
Call clearing
Measurement
preprocessing
Q
0
I
3Π/4
Π/4
Π/2
Π
5Π/4
3Π/2
7Π/4
GMSK and/or 8-PSK
0
Functions of a BTS
multiplexing
5. Data +
Software
OMC-R Interface Management
X.25 or
Ethernet
Routing
Radio Call Processing
MSC
BTS
BTS
BTS
BTS
Radio
Resources
Management
BSC Functionality
6. TCU Functions
BSC
BTS
BSS
MSC
Converts the GSM speech frames into
PSTN / ISDN A-Law or µ-Law speech.
Adapts the user’s data frames from BSS
to V110 ISDN 64 kbps ISDN format.1 2
A
Interface
TCU is the Nortel Networks name for the Transcoder Rate Adapter Unit
Ater
Interface
TCU
16. Gateway MSC
• A MSC with an interface to other networks is called a
gateway MSC.
• Network operators may opt to equip all of their MSCs
with gateway functionality or only a few.
• The call has to enter the PLMN via a gateway MSC,
which queries the HLR and then forwards the call to the
MSC where the called party is currently located.
17. TAX
ND
GMSC
MSC/VLR
1. ND PSTN Subscriber Dials ND Mobile Subscriber
in ND.
ND
LE TANDEM
HLR
ND
MS
9410098100ND
SRI
PRN to
MSC/VLR
ND
ACK from
MSC/VLR
ND
(MSRN)
MSRN
MSISDN
MSISDN
MSISDN
Page(TMSI)
18. TAX
ND
TAX
CA
GMSC
MSC/VLR
GMSC
MSC/VLR
2. ND PSTN Subscriber Dials CA Mobile Subscriber
in CA.
ND
CA
LE
HLR
HLR
CA
MS
9881098300
ND
CA
SRI
PRN to
MSC/VLR
ACK from
MSC/VLR
CA
(MSRN)
MSRN
MSISDN
Page(TMSI)
TANDEM
MSISDN
MSISDNMSISDN
19. TAX
CA
TAX
ND
GMSC
MSC/VLR
GMSC
MSC/VLR
3. ND PSTN Subscriber Dials ND Mobile Subscriber
in CA.
CA
ND
LE TANDEM
HLR
HLR
ND
MS
9410098200CA
ND
SRI
PRN to
MSC/VLR
CA
ACK from
MSC/VLR
CA
(MSRN)
MSRN
MSRN
MSRN
MSRN
MSISDN
MSISDN
MSISDN
Page(TMSI)
20. TAX
CA
TAX
ND
GMSC
MSC/VLR
GMSC
MSC/VLR
4. CA PSTN Subscriber Dials ND Mobile Subscriber in CA.
CA
ND
LE
HLR
HLR
ND
MS
9410098400
CA
ND
SRI
PRN to
MSC/VLR
ACK from
MSC/VLR
CA
(MSRN)
MSRN
MSISDN
Page(TMSI)
TANDEM
MSISDN
MSISDNMSISDN
MSRN
MSRN
MSRN
Editor's Notes
Information transmission: Speech - Data - Short messages.
Features at the radio interface:
Signal processing:
Modulation/demodulation, equalization.
Ciphering/deciphering.
Coding/decoding, interleaving/de-interleaving.
Frequency hopping.
Layer 1 management:
Radio measurements preprocessing
Handover.
Power control.
Call clearing.
Coupling system: Space diversity (reception diversity).
Remark concerning the modulation/demodulation : There is only one modulation type (either GMSK or 8-PSK) at a time. But, for EDGE, the signal processing unit must support both GMSK and 8-PSK modulations There are three types of PAGING REQUEST message:
The following functions are performed by the BSC:
Radio call processing
Radio channel allocation (traffic and signaling)
Switching between Abis and Ater interfaces
Radio procedures management (handover, …).
Radio resources management
Configuration of the TDMA
Radio channel operational state monitoring.
OMC-R interface management
X.25 link management (BSC2G) or IP Connection (Bsce3)
Providing services requested by the OMC-R
BSS configuration updating
Software storage.
TCU (TransCoder Units) are designed to reduce the number of PCM links needed to convey radio speech and data channels between BTS, BSC and MSC.
The concept of remote transcoders enables the use of 4 multiplexed channels of 16 kbps onto a single 64 kbps PCM channel.
Multiplexing is implemented within the BTS, thus the number of PCM links needed on the Abis interface is reduced.
The TCU enables code conversion of 16 kbps channels from the BSC into 64 kbps channels for MSC, in both directions.
The TCU is the product designation of Nortel Networks for the TRAU (Transcoder and Rate Adapter Unit) specified in the GSM recommendations.
External functions provide OMC-R services across the interfaces described above.
Man-Machine Interface: Allows users to access Operation and Maintenance functions.
Communication Management: Provides a set of services used by other OMC-R functions: OSI model layers, BSS interface, internal mail, communication service with a manager, FTAM File Transfer Service (FTS) and MIT (Management Information Tree) storage Management.
Configuration Management: Controls and synchronizes BSS resources.
Fault Management: Treats system malfunctions and failures to supply the elements needed for maintenance actions.
Performance Management: Provides users with all the information needed for measuring BSS and OMC-R performances.
Security Management: Defines users and terminal profiles, assign access rights to OMC-R users, controls access rights and limits them to a predefined set of objects called the user terminal zone of interest.
Common Functions: Regroup all the OMC-R’s operations and maintenance functions such as archive/restore, tape management, EFD management.
RACE Management function: The RACE PC serves as an intelligent al with its own menu-driven interface and selection buttons. RACEs are connected to the OMC-R across the PSTN., database integrity, OMC-R checking and mirroring management.
OMC-R databases: Manages the OMC-R databases.
GPRS Management Function: controls and manages the PCU (Packet Control Unit) used to support the GPRS service.
Each type of display of the logical mode shows different objects:
The first logical view (Full Network) shows all the Network Elements, from the MSC down to the sites.
The BSC level includes the BSS objects (Signaling Point, Signaling Link) for one BSC + (Controle node and Interface node) for a Bsce3
The Site level describes the BTSs belonging to one site as well as the TDMA frames.
The TCU level displays the LAPD Link and the TCBs belonging to one TCU2G. For Tcue3, no more TCB boards but cem, lsa-rc and tmu boards.This level is the only way to access the A Interface level.
The A Interface level mainly shows the XTPs used for MSC-BSC exchanges.
The Home Location Register (HLR) is a database that holds information concerning the subscribers. It performs the following functions:
Handling of permanent subscriber data:
Identification: IMSI, MSISDN.
Subscription information: related services options (Teleservices, Bearer Services and Supplementary Services).
Service limitations (e.g. roaming limitation).
Handling of temporary subscribers data:
Current VLR address where the subscriber is roaming.
Provide VLR with 5 ciphering items.
Dialogue with the AUC database (see next slide).
When a mobile station enters the LA borders, it signals its arrival, via a BSS to an MSC that stores its identity in a Visitor Location Register (VLR).
The information necessary to manage the MS is contained in the HLR and is transferred to the VLR so that it can be easily retrieved if so required.
The Location Registration procedure allows the subscriber data to follow the movements of the MS. For such reason the data contained in the VLR and in the HLR are more or less the same. Nevertheless, the data are present in the VLR only as long as the MS is registered in the area related to that VLR.
The VLR supports a mobile paging, and tracking subsystem in the local area where the mobile is presently roaming.
The detailed functions of the VLR are as follows:
Works with the HLR and AUC on authentication.
Relays cipher keys from the HLR to the BSS for encryption and decryption.
Controls allocation of the new TMSI numbers that can be periodically changed to secure a subscriber's identity.
Supports paging (incoming calls).
Tracks the state of all mobile in its area.
The Authentication Center (AUC) is a database that contains the secret authentication key Ki of each subscriber and generates security-related parameters to protect the network operator and subscribers against fraud.
The same Ki is to be found in the subscribers SIM card and is used to generate these ciphering items named triplets:
a RANDom number RAND,
a Signature RESponse SRES, using the A3 algorithm,
a ciphering Key Kc, using the A8 algorithm and computed each time authentication is performed.
Software keys Kc and SRES are never passed over the air interface.
The two algorithms A3 and A8 are operator dependent.
For security reasons, the AUC often has an internal interface with the HLR. However this is a choice of implementation. It is up to the HLR to start security algorithms located in the AUC.
Any MSC that does not possess gateway functionality has to route calls to external networks via a gateway MSC.
