GSM Architecture | MS, BTS, BSS, MSC, VLR, HLR, AuC, EIR, NSS, OSS


GSM is used to mean Global System for Mobile communications. GSM was designed as a circuit-switched system with the following four subsystems.

  1. Mobile Station (MS): carried by the subscriber
  2. Base Station Subsystem (BSS): radio network
  3. Network Station Subsystem (NSS): switched network
  4. Network Management System (NMS) or Operation Station Subsystem (OSS)



Portable, vehicle-mounted, handheld device. MS contains Mobile Equipment (ME) the terminal and Subscriber Identity Module (SIM). As we require mobility of a user which is the main aim of wireless technology. It is the first part.



It is the radio part; we have a controller. BSS has divided into two main parts,

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



It’s the core of the network. It manages the entire network. It contains databases that are;

  1. Mobile Switching Center (MSC)
  2. Home Location Register (HLR)
  3. Visitor Location Register (VLR)
  4. Authentication Center (AUC)
  5. Equipment Identity Register (EIR)
  6. Intelligent Network (IN)
  7. Gateway Mobile Switching Center (GMSC)
  8. Short Message Service Center (SMSC)




Any fault of any configuration relative problems, we require OSS or NMS. It is divided into two main parts.

  1. Operational Maintenance Center for Radio (OMC-R)
  2. Operation Maintenance Center for Switch (OMC-S)





1. Mobile Station (MS)

MS is connected to BTS with help of an antenna to path a signal from MS to BTS. The connection part between MS and BTS is the air interface (sometimes called Uu: User to the user interface).

2. Base Transceiver Station (BTS)

A unit that can transmit and receive is known as a transceiver. BTS has the following functions;

  1. Frequency hopping
  2. Communicates with Mobile station and BSC
  3. Mobile station signals are received by BTS


3. Base Station Controller (BSC)

  1. BSC manages all BTS in a particular area
  2. All BTS signals in a particular area are managed by BSC
  3. Handover for each MS (Mobile Station)
  4. Radio Power control 
  5. Assigns Frequency and time slots for all MSs in its area
  6. All signals from BSC will be forwarded to MSC


4. Mobile Switching Center (MSC)

It is a heart of a network (the core of a network).

  1. All BSS is controlled by MSC
  2. All switching functionality, authentications, verification as well as security purpose are performed in MSC
  3. Has several databases attached to it which are, HLR, VLR, EIR, IN, AUC, GMSC, and SMSC.


Home Location Register (HLR)

  1. It’s a permanent database. It stores the permanent location of a user.  For example, when I go to purchase SIM (Subscriber Identity Module), I require an address that proves, according to my location, to locate me to a particular cell. The cell allocated to me will be my home (the house number is not my home).
  2. All numbers are defined according to HLR.


Visitor Location Register (VLR)

  1. Temporary database which updates whenever new MS enters its area, by HLR database.
  2. Deals with change in location, i.e. Handover as you move from one place to another, all locations are stored in VLR.


Equipment Identity Register (EIR)

Made up of three sub-classes:

  1. The White List, are mobile users who communicate.
  2. The Black List, the total number of users who are blocked by a network, i.e. particular numbers are blocked by a network.
  3. The Gray List, are services denied users i.e. blocked incoming calls or blocked outcoming calls.

EIR, checks all numbers either in the white list, black list, or gray list


Authentication Center (AUC)

  1. Deals with verifications. The same key is stored in SIM; the same key is stored in the AUC database. If the key matches, the user key is authenticated.
  2. Protects against intruders in the air interface.


Intelligent Network (IN)

All bills are checked in IN. Example pre-paid bills and all added values services are recorded in IN (Intelligent Network).

Short Message Service Center (SMSC)

It works for both store and forwards short messages. SMSC has the address of a particular SMS.

Gateway Mobile Switching Center (GMSC)

GMSC is used when we require to connect several MSC. GMSC makes it possible for international calls. For example, when you want to communicate with someone outside of your country (India and Canada). i.e. Gateway of India will contact a gateway of Canada.

Public Switched Telephone Network (PSTN)

Landlines connections.

5. Network Management Subsystem or Operation Station Subsystem (NMS / OSS)

Deals with monitoring, configuration management, and performance management. (NMS / OSS) is divided into two parts;

  1. OMC-R; Operation Maintenance Center for Radio (BSS part).
  2. OMC-S; Operation Maintenance Center for Switch (NSS part).

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