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Global system for mobile communication (GSM) - Simplified

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Global system for mobile communication (GSM) is a globally accepted standard for digital cellular communication. GSM is the name of a standardization group established in 1982 to create a common European mobile telephone standard that would formulate specifications for a pan-European mobile cellular radio system operating at 900 MHz. It is estimated that many countries outside of Europe will join the GSM partnership.

Cellular is one of the fastest growing and most demanding telecommunications applications. Throughout the evolution of cellular telecommunications, various systems have been developed without the benefit of standardized specifications. This presented many problems directly related to compatibility, especially with the development of digital radio technology. The GSM standard is intended to address these problems.

From 1982 to 1985 discussions were held to decide between building an analog or digital system. After multiple field tests, a digital system was adopted for GSM. The next task was to decide between a narrow or broadband solution. In May 1987, the narrowband time division multiple access (TDMA) solution was chosen.

GSM provides recommendations, not requirements. The GSM specifications define the functions and interface requirements in detail but do not address the hardware. The reason for this is to limit the designers as little as possible but still to make it possible for the operators to buy equipment from different suppliers. The GSM network is divided into three major systems: the switching system (SS), the base station system (BSS), and the operation and support system (OSS).

GSM Architecture:

GSM Architecture

The Switching System:

The switching system (SS) is responsible for performing call processing and subscriber-related functions. The switching system includes the following functional units.

home location register (HLR) —The HLR is a database used for storage and management of subscriptions. The HLR is considered the most important database, as it stores permanent data about subscribers, including a subscriber's service profile, location information, and activity status. When an individual buys a subscription from one of the PCS operators, he or she is registered in the HLR of that operator.

mobile services switching center (MSC) —The MSC performs the telephony switching functions of the system. It controls calls to and from other telephone and data systems. It also performs such functions as toll ticketing, network interfacing, common channel signaling, and others.

visitor location register (VLR) —The VLR is a database that contains temporary information about subscribers that is needed by the MSC in order to service visiting subscribers. The VLR is always integrated with the MSC. When a mobile station roams into a new MSC area, the VLR connected to that MSC will request data about the mobile station from the HLR. Later, if the mobile station makes a call, the VLR will have the information needed for call setup without having to interrogate the HLR each time.

authentication center (AUC) —A unit called the AUC provides authentication and encryption parameters that verify the user's identity and ensure the confidentiality of each call. The AUC protects network operators from different types of fraud found in today's cellular world.

equipment identity register (EIR) —The EIR is a database that contains information about the identity of mobile equipment that prevents calls from stolen, unauthorized, or defective mobile stations. The AUC and EIR are implemented as stand-alone nodes or as a combined AUC/EIR node.

The Base Station System (BSS):

All radio-related functions are performed in the BSS, which consists of base station controllers (BSCs) and the base transceiver stations (BTSs).

BSC —The BSC provides all the control functions and physical links between the MSC and BTS. It is a high-capacity switch that provides functions such as handover, cell configuration data, and control of radio frequency (RF) power levels in base transceiver stations. A number of BSCs are served by an MSC.

BTS —The BTS handles the radio interface to the mobile station. The BTS is the radio equipment (transceivers and antennas) needed to service each cell in the network. A group of BTSs are controlled by a BSC.

The Operation and Support System:

The operations and maintenance center (OMC) is connected to all equipment in the switching system and to the BSC. The implementation of OMC is called the operation and support system (OSS). The OSS is the functional entity from which the network operator monitors and controls the system. The purpose of OSS is to offer the customer cost-effective support for centralized, regional, and local operational and maintenance activities that are required for a GSM network. An important function of OSS is to provide a network overview and support the maintenance activities of different operation and maintenance organizations.

Additional Functional Elements:

message center (MXE) —The MXE is a node that provides integrated voice, fax, and data messaging. Specifically, the MXE handles short message service, cell broadcast, voice mail, fax mail, e-mail, and notification.

mobile service node (MSN) —The MSN is the node that handles the mobile intelligent network (IN) services.

gateway mobile services switching center (GMSC) —A gateway is a node used to interconnect two networks. The gateway is often implemented in an MSC. The MSC is then referred to as the GMSC.

GSM interworking unit (GIWU) —The GIWU consists of both hardware and software that provides an interface to various networks for data communications. Through the GIWU, users can alternate between speech and data during the same call. The GIWU hardware equipment is physically located at the MSC/VLR.

posted Feb 20 by Sanketi Garg

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Following is the list of various identifiers used in GSM.
1. IMEI - International Mobile Equipment Identity
2. IMSI - International Mobile Subscriber Number
3. TMSI - Temporary Mobile Subscriber Identity
4. LMSI - Local Mobile Subscriber Identity
5. MSISDN - Mobile Subscriber International ISDN number
6. MSRN - Mobile Subscriber Roaming Number
7. LAI - Location Area Identity
8. CI - Cell Identifier
9. BSIC - Base Station Identity Code

IMEI - International Mobile Equipment Identity

Its an internationally-unique serial number allocated to Mobile Station (MS) hardware at the time of manufacture. It is registered by the network operator & stored in Authentication Center (AuC) for Validation purpose. This number consists of type approval code, final assembly code and serial number of the mobile station. The network stores the IMEI numbers in the Equipment Identity Register (EIR).

IMSI - International Mobile Subscriber Number

Each registered user is uniquely identified by its international mobile subscriber identity (IMSI). It is stored in the subscriber identity module (SIM). A mobile station can only be operated if a SIM with a valid IMSI is inserted into equipment with a valid IMEI.

There are following parts of an IMSI:
Mobile Country Code (MCC): 3 decimal places, internationally standardized.
Mobile Network Code (MNC): 2 decimal places, for unique identification of mobile network within the country.
Mobile Subscriber Identification Number (MSIN): Maximum 10 decimal places, identification number of the subscriber in the home mobile network.

TMSI - Temporary Mobile Subscriber Identity

Its used to protect the true identity (IMSI) of a subscriber. It is issued by & stored within a VLR (not in the HLR) when an IMSI attach takes place or a Location Area (LA) updates takes place. The issues TMSI only has validity within a specific LA. The TMSI is used for security purposes, so that the IMSI of a subscriber does not have to be transmitted over the air interface. Its a temporary identity, which regularly gets changed.

LMSI - Local Mobile Subscriber Identity

The VLR can assign an additional searching key to each mobile station within its area to accelerate database access. This unique key is called the Local Mobile Subscriber Identity (LMSI). The LMSI is assigned when the mobile station registers with the VLR and is also sent to the HLR.

MSISDN - Mobile Subscriber ISDN Number

The real telephone number of a mobile station is the mobile subscriber ISDN number (MSISDN). It is assigned to the subscriber (his or her SIM, respectively), such that a mobile station set can have several MSISDNs depending on the SIM.

The MSISDN categories follow the international ISDN number plan and therefore have the following structure:
Country Code (CC) : Up to 3 decimal places.
National Destination Code (NDC): Typically 2-3 decimal places.
Subscriber Number (SN): Maximum 10 decimal places.

MSRN - Mobile Subscriber Roaming Number

It is temporary, location-dependent ISDN number issued by the parent VLR to all MSs within its area of responsibility. It is stored in VLR & associated HLR but not in the MS. The MSRN is used by VLR associated MSC for call routing within MSC/VLR service area.

MRSN = CC + NDC + SN

LAI - Location Area Identity

Each location area within PLMN (Public Landline Mobile Network) has an associated Intenationally unique identifier (LAI). The LAI is broadcasted regularly by the BTSs on the Broadcast Control Channel (BCCH), thus uniquely identify each cell within an associated location area (LA). It structure is as follows,

LAI = MCC + MNC + LAC

MCC= Mobile Country Code (of the visited country)
MNC= Mobile Network Code (of the serving PLMN)
LAC= Location Area Code

CI - Cell Identifier

Within an LA, the individual cells are uniquely identified with a cell identifier (CI), maximum 2 x 8 bits. Together with the global cell identity (LAI + CI) calls are thus also internationally defined in a unique way.

BSIC - Base Station Identity Code

Each BTS is issued with a unique identity, the BSIC & is used to distinguish neighbouring BTSs. It is needed to identify that the frequency strength being measured by the mobile station is coming from a particular base station.

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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

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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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