Wednesday, October 9, 2019


In this article, we will briefly go through the Architecture of GSM. The GSM architecture consists of four major interconnected elements which are
  1. Mobile Station (MS)
  2. Base Station Subsystem (BSS)
  3. Network Subsystem (NSS)
  4. Operation and Maintenance Center (OMC)

1. Mobile Station - Ms

This part of the Gsm network deals directly with users and allow them to initiate their desire request, like making a voice call or sending a short message. Ms consists of two entities Mobile Equipment (ME) and the Subscriber Identity Module (SIM).

Mobile Equipment (ME)

MS is a device used to access the network. It holds a unique number that is permanently attached to it and known as International Mobile Equipment Identity (IMEI). IMEI allows a network to locate its user on the system, which is later used in HLR and VLR databases.

Subscriber Identity Module (SIM)

A small chip also is known as "sim card". Sim card can be plugged in the Mobiel Equipment or also come along with ME. A unique number is known as International Mobile Subscriber Identity (IMSI), also attached to a sim card. Mobile equipment is only an electronic device without a sim card, and it can perform only a few essential functions.

2. Base Station Subsystem - BSS

The BSS is also known as Radio Network because BSS includes all essential nodes and functionalities that connect Mobile Station to the network using air interface. Base Station Subsystem includes Base Transceiver Station (BTS), Base Station Controller (BSC), and Transcoder Sub‐Multiplexer (TCSM).

Base Station Subsystem (BTS)

BTS is responsible for managing the interface between the mobile station and the network. That's why BTS acts as a hub for the entire Network. Transmission of sending and receiving signals are usually done with antennas having 120° sectors. Some of the vital function of the Base Station Subsystem (BTS) are
  • Communicate with both Mobile Station (MS) and Base Station Controller (BSC)
  • Encrypting, Modulating, Encoding, Multiplexing, and feeding RF signals to the antenna.
  • Measures quality and power of the received signal
  • Decrypting, Decoding and equalizing received signals
  • Synchronization of both Time and Frequency
  • Frequency Hoping
  • Transcoding and Rate adaptation

Base Station Controller (BSC)

The primary function of BSC is to control radio resources, especially the base stations (BTS). It also acts as an interface between a base station subsystem (BSC) and a Mobile Switching Centre (MSC). BSC has three primary functions, which include performing handovers or handoffs, controls frequency hopping, and manages radio resources for BTS. A few other tasks are listed below.
  • Provide an interface for MSC to communicate with the mobile station
  • Responsible for the control of transmitted power.
  • Allocating time slot and frequency to MS in its area
  • Determinations of time-delay of signals received from the Mobile station
  • Initiating and terminating calls setups
  • Synchronization of both Time and Frequency
  • Provide an interface for the operations and maintenance center (OMC)
  • Security, alarms, etc

The Transcoders

Basically, Transcoders are part of BSS but sometimes physically located in MSC. Although they are installed in the close of range of  MSC. The primary purpose of transcoders is to save bandwidth and reduce the network in terms of cost. Transcoders compact data to reduce the rate at which data is transmitted. It compresses 64kbps data into 16kbps and vise Versa.

3. Network Subsystem - NSS

Network Switching Subsystem is considered the core of the GSM Network. NSS performs all switching functions and allows other telecommunication networks like Public Switch Network (PSTN), international networks to communicate with GSM users. NSS contains large databases that help in performing switching functions.
NSS include
  • Mobile Switching Center (MSC)
  • Home location register (HLR)
  • Visitor location register (VLR)
  • Authentication Center (AUC)
  • Equipment identity register (EIR)

Mobile Switching Centre (MSC)

MSC is the central component of NSS and is known as the brain of the GSM network. A network usually contains more than once MSC. The primary purpose of MSC is call-switching with other mobile/fixed networks. It also has some additional functions like registration, authentication, handoffs, routing roaming calls, and providing different interfaces.
The summarize important essential function carried out by MSC are

Call processing: controls data/voice call setup, Handoffs, and subscriber validation and location.
Operations and maintenance support: Includes the management of radio resources and database, registration, location updating, traffic metering, and measurement.
Interworking: Provide the interface for PSTN networks, Fix networks, and other supported systems.
Billing: Collecting and processing subscriber billing data.

Home Location Register (HLR)

HLR is a permanent storage database that includes subscriber information like service profile, activity status, and location information. When a user by a new SIM, their data is stored in HLR. It contains IMSI, MSISDN, Service subscription information (prepaid/postpaid), Billing information, roaming restrictions, supplementary services, etc.

Visitor Location Registrar (VLR)

The primary purpose of the VLR database is to reduce signaling between MSC and HLR, thus putting a less burden on HLR. VLR temporary store information of the visiting subscriber. VLR includes the following information
  • Temporary mobile subscriber identity (TMSI)
  • Location area identity (LAI)
  • Mobile Station Roaming Number (MSRN)
  • Cell Phone status

Authentication Center (AUC)

AUC performs an authentication function to protects against intruders in the air interface.
Generally associated with HLR required to access and update subscriber data continuously.
The authentication process will habitually process begins whenever a user initializes any process in the system. AUC uses authentication keys and algorithms like (SRES, RAND, KC, ETC) to provide extra security.

Equipment identity register (EIR)

The Equipment Identity Register consists of IMEIs data distributed in three list
White List:  List of IMEIs that are allowed to use the network without any trouble.
Gray List: This list consists of IMEIs that have received a warning message for using faulty software or equipment. Such IMEIs can be possibly get banned at any time.
Black List: IMEIs that are banned for illegal activity or for some other reason. Such IMEIs are not allowed to use network resources. The primary purpose of creating these lists is to deal with fraudulent, stolen, and banned devices. 

4. Operations and maintenance center (OMC)

Nowadays, networks are getting more complex; that is why they require more attention because no one wants a system that has up and downs. Operation and Maintenance Centre (OMC) is used to make sure that the network runs smoothly. OMC performs four primary operations which is
  • Network Monitoring
  • Network Measurement
  • Network Development
  • Fault management
OMC will generate alarms in case of error or fault is detected. Some errors (software related) are fixed automatically, and others would require a visit. OMC collects data for analysis, which plays a vital role in network optimization.

Reference Book: 
1. Mishra, Ajay R.-Fundamentals of network planning and optimization 2G_3G_4G _ evolution to 5G-Wiley (2018)
2. Sauter, Martin "From GSM to LTE‐Advanced Pro and 5G"
3. Gottapu Sasibhushana Rao "Mobile CellularCommunication"