The GSM system architecture consists of four main subsystems: Mobile Station (MS), Base Station Subsystem (BSS), Network Switching Subsystem (NSS), and Operational Support Subsystem (OSS). Each subsystem has distinct components and functions, with the MS including mobile equipment and a SIM card for communication, the BSS handling traffic between mobile phones and networks, the NSS managing system switching and connections to other networks, and the OSS supporting maintenance and troubleshooting of the entire system. Together, these components facilitate effective mobile communication.

1. GSM SYSTEM ARCHITECTURE
By:
Ashutosh Kumar
Priyanshi Verma

2. WORKING OF A GSM NETWORK
The GSM architecture consists of four major
interconnected subsystems that interact with
themselves and with users through certain
network interface.
The four subsystems are :
1. Mobile System (MS)
2. Base Station Subsystem (BSS)
3. Network Subsystem (NSS)
4. Operational Support Subsystem (OSS)

3. 1.MOBLIE STATION (MS)
Mobile Equipment (ME):
 It is uniquely identified by an IMEI number.
 It is used for voice and data transmission.
 It also monitors power and signal quality of surrounding cells.
 160 characters long SMS can also be sent using Mobile Equipment.

4. Subscriber Identity Module (SIM):
 It is a smart card that contains the International Mobile
Subscriber Identity (IMSI) number.
 it is a portable memory chip that makes phone call
across the world.
It is generally used in mobile phones that operates on
GSM network.
It is portable and you can use it with any supporting
mobile phone.
.

5. 2.BASE STATION SUBSYSTEM(BSS)
BSS stands for Base Station Subsystem.
BSS handles traffic and signaling between a mobile phone
and the network switching subsystem (NSS).
BSS having two components:
1. Base Transceiver Station(BTS)
2. Base Station Controller(BSC)

6. .
Base Transceiver Station(BTS):
• It communicates with mobile stations via radio air
interface and communicates with BSC via Abis interface.
• It encodes, encrypts, multiplexes, modulates and feeds
the RF signal to the antenna.

7. Base Station Controller(BSC)
The BSC is the connection between the Mobile and
MSC.
It controls the power transmission of the BSS and MS
in its area.
The function of the BSC is to allocate the necessary
time slot between the BTS and the MSC.
It is a switching device that handles radio resources.

8. 3.NETWORK SWITCHING SUBSYSTEM (NSS)
• It manages the switching functions of the system and allows MSCs to communicate
with other networks such as PSTN and ISDN.
• It consists of :
1.Mobile Switching Centre (MSC)
2. Home Location Registers (HLR)
3. Visitor Location Registers (VLR)
4. Authentication Centre (AUC)
5. Equipment Identity Registry (EIR)

9. Mobile Switching Centre (MSC)
• It is a heart of the network. It manages the communication between GSM and other
networks.
• It manages call set up function, routing and basic switching.
• It performs mobility management including registration, location updating and inter
BSS and inter MSC call handoff.

10. .
Home Location Registers (HLR)
• Stores permanent data about subscribers (Profile, Location info).
• Subscription information of registered user is stored.
• It is a permanent database about mobile subscriber in a large service area.
Visitor Location Registers (VLR)
• It is a temporary database which updates whenever new MS enters in its area by
HLR database.
• Work in co-ordination with HLR.

11. .
Authentication Centre( AUC)
• It is a protected database.
• It stores a copy of secret key.
• It is used for authentication.
• It protects from different type of fraud.
Equipment Identity Register (EIR)
• It is a database that contain list of all valid mobile on network.
• It is a database that is used to track handset using IMEI number.
• IMEI is marked as invalid in case of stolen.

12. 4.OPERATIONAL SUPPORT SUBSYSTEM
It supports the operation and maintenance of GSM and allows system engineers to
monitor and troubleshoot all aspects of GSM system.
Purpose of OSS is to offer the customer cost-effective support for all GSM related
maintenance services.
It supports one or more Operation Maintenance Centre (OMC) which are used to
monitor the performance of each MS, BSC and MSC within a GSM system.