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GSM Protocol Architecture

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GSM Family Snapshot

GSM Family

GSM Protocol Architecture

As I explained in my previous article you have following interfaces in the Pure GSM netwoek
Um Interface: Between MS and BSS
A Interface: Between BSS and NSS
Abis Interface: Between BTS and BSC (both are components of BSS)

Now lets see how protocols connect them with each other.

GSM Control Plane
GSM Control Plane

GSM Data Plane
GSM Data Plane Protocol in TM Mode
GSM Data Plane Protocol in Non TM Mode

posted Jul 17, 2014 by Salil Agrawal

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GSM i.e. Global System for Mobile Communications is a standard developed by ETSI and popularly called 2G-GSM and has more then 80% market share in 2G market.

GSM network can be divided into following four sections

  1. Mobile Station (MS)
  2. Base Station Subsystem (BSS)
  3. Network Switching Subsystem (NSS)
  4. Operation Support Subsystem (OSS)

GSM Architecture

Interfaces in GSM network

  1. Um Interface: Between MS and BSS
  2. A Interface: Between BSS and NSS
  3. Abis Interface: Between BTS and BSC (both are components of BSS)


Mobile Station (MS):

A MS consist of Physical equipment called Mobile Equipment and SIM (Subscriber Identification module)
ME is uniquely identified by an IMEI (International Mobile Equipment Identity) whereas SIM identifies as user.

Base Station Subsystem (BSS)

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

Base Transceiver Station (BTS):
- Encodes,encrypts,multiplexes,modulates and feeds the RF signals to the antenna.
- Frequency hopping
- Communicates with Mobile station and BSC
- Consists of Transceivers (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
- Transcoding and rate adaptation functionality
- Handover for each MS
- Radio Power control
- It communicates with MSC and BTS

Network Switching Subsystem (NSS)

Consist of four parts MSC, HLR, VLR, and EiR

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

Home Location Registers (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.

Visitor Location Registers (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

Equipment Identity Register (EIR)
- Database that is used to track handsets using the IMEI (International Mobile Equipment Identity)
- Made up of three sub-classes: The White List, The Black List and the Gray List
- Only one EIR per PLMN

Operation Support Subsystem (OSS)

The OSS consists of the Operation and Maintenance Center (OMC) and responsible for
- regular network operation
- subscriber administration
- call billing

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