Unit 2 - Global System for Mobile (GSM) Communications

BSC Functions

Control of frequency hopping; performing traffic concentration to reduce the number of lines from the MSC; providing an interface to the Operations and Maintenance Center for the BSS; reallocation of frequencies among BTSs; time and frequency synchronization; power management; time-delay measurements of received signals from the MS.

Vital GSM Address and Identifier: International Mobile Subscriber Identity (IMSI)

Every registered user has this with a valid IMEI stored in their Subscriber Identity Module (SIM).

Why GSM?

Improved spectrum efficiency; international roaming; low-cost mobile sets and base stations (BSs); high-quality speech; comparability with Integrated Services Digital Network (ISDN) and other telephone company services; support for new services

Home Location Register (HLR)

The database used by the MSC for subscriber data and service information.

Equipment Identity Register (EIR)

This is the database that contains a list of all valid mobile equipment on the networks where the IMEI identifies each MS.

The Base Transceiver Station (BTS)

This station houses the radio transceivers that define a cell and handles the radio link protocols with the MS. It is usually placed in the center of a cell and has between 1 and 16 transceivers, depending on the density of users in the cell.

Subscriber Identity Confidentiality

To ensure subscriber identity confidentiality, the Temporary Mobile Subscriber Identity (TMSI) is used. Once the authentication and encryption procedures are done, the TMSI is sent to the MS, after the receipt the MS responds. For communications outside the location area, the LAI is necessary in addition to the TMSI.

GSM Network Area - Location Area (LA)

A group of cells form this area. This is the area that gets paged when a subscriber gets an incoming call. Each of these is assigned a LAI and is served by one or more BSCs.

Functions of OMC

Administration and commercial operations (subscription, end terminals, charging and statistics); security management; network configuration, Operation and Performance Management; maintenance tasks.

LAI Hierarchy

CC, which is 3 decimal places; MNC, which is 2 decimal places; Location Area Code (LAC), which is a maximum 5 decimal places or maximum of twice 8-bits coded in hexadecimal (LAC < FFFF)

Structure followed by MSISDN categories

Country Code (CC), which is up to 3 decimal places; National Destination Code (NDC), which is typically 2-3 decimal places; Subscriber Number (SN), which is a maximum of 10 decimal places.

Vital GSM Address and Identifier: Local Mobile Subscriber Identity (LMSI)

Each mobile station can be assigned with this, which is an original key, by the VLR. This key can be used as the auxiliary searching key for each mobile station within its region. It is assigned if the mobile station is register with the VLR and sent to the HLR. It is comprised of four octets (4x8 bits).

BTS Functions

Encoding, encrypting, multiplexing, modulating, and feeding the RF signals to the antenna; transcoding and rate adaptation; time and frequency synchronizing; voice through full or half rate services; decoding, decrypting, and equalizing received signals; random access detection; timing advances; uplink channel measurements.

Structure followed by MSRN

Follows the same structure of the MSISDN, the CC and NDC are based off of the visited network, while SN is based in the current mobile network.

GSM Security and Encryption

GSM is the most secured cellular communications system available today. It has its security methods standardized and maintains end-to-end security by retaining the confidentiality of calls and anonymity of the GSM subscriber.

Additional components of the GSM architecture

Home Location Register (HLR), Visitor Location Register (VLR), Equipment Identity Register (EIR), Authentication Center (AuC), SMS Servicing Center (SMS SC), Gateway MSC (GMSC), Chargeback Center (CBC), and Transcoder and Adaptation Unit (TRAU).

The Base Station Subsystem (BSS)

Is composed of the BTS and BSC, which communicate across the specified Abis interface, enabling operations between components that are made by different suppliers. The radio components of this subsystem may consist of four to seven or nine cells and may have one or more base stations. A separate high-speed line is connected from the BSS to the Mobile MSC.

Parts of IMSI

Mobile Country Code (MCC), which is 3 decimal places, internationally standardized; Mobile Network Code (MNC), which is 3 decimal places, for unique identification of mobile networks within the country; Mobile Subscriber Identification Number (MSIN), which is a maximum 10 decimal places and is the identification num er of the subscriber in the home network.

What can a GSM network be broadly divided into?

Mobile Station (MS), Base Station Subsystem (BSS), Network Switching Subsystem (NSS), and Operation Support Subsystem (OSS).

Mobile Station Authentication

The GSM network authenticates the identity of the subscriber through the use of a challenge-response mechanism. A 128-bit Random Number (RAND) is sent to the MS, which computes the 32-bit Signed Response (SRES) based on the encryptions of the RAND with the authentication algorithm (A3) using the individual subscriber authentication key (Ki). The Ki is never transmitted over the radio channel, as it is present in the subscriber's SI, as well as the AuC, HLR, and VLR databases.

Signaling and Data Confidentiality

The SIM contains the ciphering key generating algorithm (A8) that is used to produce the 64-bit ciphering key (Kc). The key is computed by applying the same RAND used in the authentication process to Kc A8 with the Ki. Encrypted voice and data communications between the MS and the network is accomplished by using the ciphering algorithm A5.

GSM Network Area - MSC/VLR Service Area

The area covered by one MSC.

GSM Network Area - Public Land Mobile Network (PLMN)

The area covered by one network operator and can contain one or more MSCs.

Mobile Service Switching Center (MSC)

The central component of the NSS. It performs the switching of calls between the mobile and other fixed or mobile network users, as well as the management of mobile services such as authentication.

Vital GSM Address and Identifier: Temporary Mobile Subscriber Identity (TMSI)

This can be assigned by the VLR, which is responsible for the current location of a subscriber. This needs to have only local significance in the area handled by the VLR and is stored on the network side only in the VLR and is not passed to the Home Location Register (HLR). With the current location area this identifies a subscriber uniquely. Can contain up to 4 x 8 bits.

The Mobile Station (MS)

This consists of the physical equipment, such as radio transceiver, display and digital signal processors, and the SIM card. It provides the air interface to the user in GSM networks. Other services are: voice tele-services, data bearer services, and the features' supplementary services.

The Base Station Controller (BSC)

This controller manages the radio resources for one or more BTSs. It handles radio channel setup, frequency hoping and handovers. This is the connection between the mobile and the MSC and it also translates the 13 Kbps voice channel used over the radio link to the standard 64 Kbps channel used by the PSTN or ISDN. It assigns and releases frequencies and time-slots for the MS.

Visitor Location Register (VLR)

This is a database that contains temporary information about subscribers that is needed by the MSC in order to service visiting subscribers.

What is GSM?

This is a digital cellular technology used for transmitting mobile voice and data services; it digitalizes and compresses data, then sends it down through a channel with two other streams of user data, each in its own time-slot.; the concept emerged from a cell-based mobile radio station in the early 1970s; this the most widely accepted standard in telecommunications and it is implemented globally; this is a circuit-switched system that divides each 200 kHz channel into eight 25 kHz time-slots; operates on 900MHz and 1800 MHz in most parts of the world, US it operates in the bands 850MHz and 1900 MHz; this owns a market share of more than 70% of the world's digital cellular subscribers; this makes use of a narrowband Time Division Multiple Access (TDMA) technique for transmitting signals.

Authentication Center (AuC)

This is a protected database that stores a copy of the secret key stored in each subscriber's SIM card, which is used for authentication and ciphering of the radio channel.

Vital GSM Address and Identifier: Mobile Station Roaming Number (MSRN)

This is an interim location dependent ISDN number, assigned to a mobile station by a regionally responsible Visitor Location Register (VLR). Using this, the incoming calls are channelled to the MS.

Vital GSM Address and Identifier: Mobile Subscriber ISDN Number (MSISDN)

This is the authentic telephone number of a mobile station and based on the SIM, a mobile station can have many of these numbers. Each subscriber is assigned with a separate number to their respective SIM.

GSM Network Area - Cell

This is the basic service area; one BTS covers one of these. Each of these is given a Cell Global Identity (CGI), a number that uniquely identifies this area.

Vital GSM Address and Identifier: Location Area Identity (LAI)

This is within a Public Land Mobile Network (PLMN), and a Location Area identifies this. It is a hierarchy that is based on international standard and structured in a unique format.

Vital GSM Address and Identifier: International Mobile Station Equipment Identity (IMEI)

This looks more like a serial number which distinctively identifies a mobile station internationally. This is allocated by the equipment manufacturer and registered by the network operator, who stores it in the Equipment Identity Register (EIR).

What is a Subscriber Identity Module (SIM)?

This provides personal mobility so that the user can have access to all subscribed services irrespective of both the location of the terminal and the use of a specific terminal. This card needs o be in a GSM cellular phone to receive calls, make calls, or receive other subscribed services on that phone.

MS Functions

This provides the receptor for SMS messages, enabling the user to to toggle between the voice and data use. The mobile facilitates access to voice messaging systems and also provides access to the various data services available in a GSM network.

The Network Switching Subsystem (NSS)

This subsystem is made up of functional elements, the main part of which is the Mobile Service Switching Center (MSC).

The Operation Support Subsystem (OSS)

This subsystem is the implementation of the Operations and Maintenance Center (OMC), which is connected to all equipment in the switching system and the BSC.

Part of IMEI

Type Approval Code (TAC), which is 6 decimal places, centrally assigned; Final Assembly Code (FAC), which is 6 decimal places, assigned by the manufacture same as the Serial Number (SNR); Spare (SP) is 1 decimal place. IMEI = TAC + FAC + SNR + SP

Vital GSM Address and Identifier: Cell Identifier (CI)

Using this (maximum 2 x 8 bits), the individual cells that are within a location area can be recognized. When the Global Cell Identity (LAI + CI) cells are combined, then it is uniquely defined.