GSM network architecture

The GSM network architecture consists of three major subsystems:

1. Base Station Subsystem
2. Network Switching Subsystem
3. Public Network

MS - Mobile Station

BTS - Base Transceiver Station

BSC - Base Station Controller

OMC - Operation Maintenance Center

MSC - Mobile Switching Center

HLR - Home Location Resister

VLR - Visitor Location Resister

AUC - Authentication Center

PSTN - Public Switching Telephone Network

ISDN - Integrated Services Digital Network 

Mobile Station[MS]:

The Mobile Station is made up of two entities mention as below

Mobile Equipment (ME)

Subscriber Identity Module(SIM)

Mobile Equipment (ME):

• Portable , vehicle mounted, hand held device.
• Uniquely identified by an IMEI (International Mobile Equipment Identity)
• Voice and data transmission
• Monitoring power and signal quality of surrounding cells for optimum handover
• Power level: 0.8W - 8W

Subscriber Identity Module (SIM):

• Smart card contains the International Mobile Subscriber Identity (IMSI).
• Can be moved from phone to phone - contains key information to activate the phone.
• Allows user to send and receive other subscriber services.
• Encoded network identification details - Key Ki, Kc and A3, A5 and A8 algorithms
• Protected by a password or PIN

Base Station Subsystem (BSB):

Base Station Subsystem is composed of two parts that communicate across the standardized Abis interface operation between components made by different suppliers.

1. Base Transceiver Station (BTS)
2. Base Station Controller (BSC)

Base Transceiver Station (BTS):

• Encoded, Encrypted, Multiplexes and feeds the RF signals to the antenna.
• Frequency hopping & Power control.
• Communicates with Mobile Station and BSC
• Consists of Transceiver (TRx) units

Base Station Controller (BSC):

• Manages Radio resources for BTS
• Assigns Frequency and time slots for all MS's in its area
• Handles call set up
• Handover for each MS
• Radio Power control
• It communicates with MSC and BTS

Network Switching Subsystem (NSS):

• Mobile Switching Center (MSC):
• Heart of the network
• Manages communication between GSM and other networks
• Call setup function and basic collection
• Call routing
• Billing information and collection
• Mobility management :- Registration, Location Updating and Inter BSS and inter MSC call handoff
• MSC does gateway function while its custemer roams to other network by using HLR/VLR.

Home Location Resisters (HLR):

• Permanent database about mobile subscribers in a large service area (generally one per GSM network operator).
• Database contains IMSI, MSISDN, Prepaid/Postpaid, Roaming Restrictions, supplementary services.
• It is refenced using SS7 signalling capabilities for every incoming call to the GSM network for determining the current location of subscriber (i.e. For obtaining mobile station routing number  (MSRN) so that the call may be routed to the mobile stations serving MSC).

Visitor Location Resisters (VLR):

• Temporary database which updates whenever new MS enters its area, by HLR database.
• Controls those mobiles roaming in its area
• Reduces number of queries to HLR.
• Database contains IMSI, TMSI, MSISDN, MSRN, Location Area, Authentication key.

Authentication Center (AUC):

• Protects against intruders in air interface
• Maintains authentication keys and algorithms and provides security triplets (RAND, SRES, Kc)
• Generally associated with HLR.

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