2. GSM (Global System for Mobile Communications) , is a standard set
developed by the European Telecommunications Standards Institute (ETSI) to
describe protocols for second generation (2G) digital cellular networks used
by mobile phones . It became the global standard for mobile communications
with over 80% market share.
The GSM standard was developed as a replacement for first generation (1G)
analog cellular networks, and originally described a digital, circuit-switched
network optimized for full duplex voice telephony
3. GSM Architecture Overview
This includes a brief explanation of the different network
subsystems and a description of the functionality of the elements
within each of the subsystems. Topics include:
General architecture overview
The Mobile Station (MS) Subsystem and Elements
The Base Station Subsystem (BSS) and Elements
The Network Subsystem (NSS) and Elements
4.
5. The mobile station consists of :
• mobile equipment (ME)
• subscriber identity module (SIM)
The SIM stores permanent and temporary data about the mobile, the
subscriber and the network, including :
• The International Mobile Subscribers Identity (IMSI)
• Authentication key (Ki) and algorithms for authentication check
The mobile equipment has a unique International Mobile Equipment
Identity (IMEI), which is used by the EIR
Mobile Station
6. The BSS comprises:
Base Station Controller (BSC)
• One or more Base Transceiver Stations (BTSs)
BTS contains:
Radio Transmitter/Receiver (TRX)
• Signal processing and control equipment
Antennas and feeder cables
The BSC:
allocates a channel for the duration of a call
maintains the call:
• controls the power transmitted by the BTS or MS
• generates a handover to another cell when required
Base Station Subsystem (BSS)
7. Network Switching System (NSS)
The NSS combines the call routing switches (MSCs and GMSC)
with database registers required to keep track of subscribers’
movements and use of the system. Call routing between MSCs
is taken via existing PSTN or ISDN networks. Signaling between
the registers uses Signaling System No. 7 protocol.
Section 1 – GSM Architecture Overview
Key elements of the NSS:
Mobile Switching Center (MSC)
Visitor Location Register (VLR)
Home Location Register (HLR)
Authentication Center (AuC)
Equipment Identity Register (EIR)
Gateway MSC (GMSC)
Section 3 – NSS Topology
8. Mobile Switching Center (MSC)
The following are typical MSC functions in a cellular system:
Provide switched connections with PSTN
Provide switched connections between mobile subscribers
Coordinate the location and handover process
Provide custom services to mobile users
Collect billing data
9. Home Location Register
HLR is a database that stores subscription and set of functions needed to
manage subscriber data in one PLMN area. Any administrative action by
the service provider or changes made by subscriber is first carried out on
the HLR and then update the VLR.
Section 3 – NSS Topology
10. Visitor Location Register
It is a subscriber database containing the information about all the MS
currently located in the MSC service area. VLR can be considered as a
distributed HLR in the case of a roaming subscriber. If MS moves into a
new service area (MSC), VLR requests the HLR to provide the relevant
data and store it, for making the calls for that MS.
It can also be viewed as a subset of a HLR.
Section 3 – NSS Topology
11. Authentication Center (AUC)
AUC is always integrated with HLR for the purpose of the
authentication. At subscription time, the Subscriber
Authentication Key (Ki) is allocated to the subscriber, together
with the IMSI.
AUC stores the following information for each subscriber
1. The IMSI number,
2. The individual authentication key Ki,
Section 3 – NSS Topology
12. Equipment Identification Register (EIR)
Purpose of this feature is to make sure that no stolen or unauthorized mobile
equipment is used in the network.
EIR is a database that stores a unique International Mobile Equipment Identity
(IMEI) number for each item of mobile equipment.
Procedure:
The MSC/VLR requests the IMEI from the MS and sends it to a
EIR.
On request of IMEI, the EIR makes use of three possible defined
lists:
- A white list: containing all number of all equipment identities
that have been allocated in the different participating countries.
- A black list: containing all equipment identities that are
considered to be barred.
- A grey list: containing (operator’s decision) faulty or non-
approved mobile equipment.
Result is sent to MSC/VLR and influences the decision about
access to the system.
13. Operation And Maintenance Center (OMC)
The OMC centralizes all operations and maintenance activities for the MSCs and
BSSs using remote software control. It provides remote testing, operations, and
maintenance capabilities for the entire system from one central location. Each
BSS, MSC, HLR, VLR, EIR, and AUC can be monitored and controlled from the
OMC.
17. BSC
Base Station Controller
The call request reaches the BSC from the BTS and
is forwarded to MSC.
After call is established, the BSC will perform
decoding of the call (in typical config.)
21. • Does the A subscriber have
money left on his/her account?
• When the call is established and
on-going the subscriber’s
account is decremented
accordingly.
PrePaid Node
26. • HLR says ’I am the home of the B
subscriber and I know where he/she is
right now’ (i.e. which VLR)
• Tells this VLR ’Give me a visitor address
for this subscriber’ (Roaming Number)
HLR
Home Location Register
34. • ’Thank you, HLR!’
• Speech connection path will set up
now towards the MSC/VLR where the
B subscriber is presently located.
MSC
Mobile Switching Centre
36. • Sets up a speech connection towards BSC.
• Delivers B-subscriber ID to BSC and asks
to page for the subscriber using that ID.
MSC
Mobile Switching Centre
38. • BSC now sends a PAGING MESSAGE to all the BTSs
that it covers/controls.
• Carrys PAGING RESPONSE from called MS back to
MSC.
• Allocates a RADIO channel through a BTS with best
available and allowable SIGNAL STRENGTH for
called MS.
• Performs SPEECH CODING when call is established.
BSC
Base Station Controller
