09-10-2010, 02:55 PM
Abstract
GSM (Global System for Mobile Communications) is the most popular standard for mobile phones in the world. GSM enables subscribers to use their phones in many parts of the world. GSM is a cellular network, which means that mobile phones connect to it by searching for cells. There are five different cell sizes in a GSM network, macro. micro, pico, femto and umbrella cells. The coverage area of each cell varies according to the implementation environment. Macro cells can be regarded as cells where the base station antenna is installed on a mast or a building above average roof top level. Micro cells are cells whose antenna height is under average roof top level; they are typically used in urban areas. Picocells are small cells whose coverage diameter is a few dozen metres; they are mainly used indoors. Femtocells are cells designed for use in residential or small business environments and connect to the service provider’s network via a broadband internet connection. Umbrella cells are used to cover shadowed regions of smaller cells and fill in gaps in coverage between those cells.
Mobile services based on GSM technology were first launched in Finland in 1991. Today, more than 690 mobile networks provide GSM services across 213 countries and GSM represents 82.4% of all global mobile connections. According to GSM World, there are now more than 2 billion GSM mobile phone users worldwide. GSM World references China as "the largest single GSM market, with more than 370 million users, followed by Russia with 145 million, India with 83 million and the USA with 78 million users."
GSM Versions : GSM versions are :
GSM 900
GSM 1800 - Also called DCS (Digital Cellular System)
GSM 1900 - Also called PCS (Personal Communication System)
GSM Rail
GSM Network: 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:
Switching system (SS),
The base station system (BSS),
The operation and support system (OSS).
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.
GSM Services :
GSM System provides integration of voice and data services.
Integration with PSTN and ISDN.
Services are what customer care about.
GSM services are categorized into three :
Bearer Services
Tele Services
Supplementary Services
Bearer Services : Bearer services enable transmission of data and voice.
Bearer services are divided into two parts :
Transparent services
Non-transparent services
Transparent Services :
Make only use of functions of physical layer (PHY).
Data transmission has constant delay and constant throughput
Use only FEC
No retransmissions
Data rates: 2.4, 4.8, 9.6 Kbps (depending on FEC used)
Non-transparent Services :
Make only use of functions of layers two and three
Can have retransmission
Can implement flow control
Very low error rate
Delay and throughput may vary depending on quality
Tele Services : Main focus of tele services is on voice and encrypted voice.
Other services include :
Message Services :
Short messaging service (message <= 160 chars)
SMS Applications : road conditions, stock quotes, news, e-mail header etc.
EMS : Enhanced messaging service (message <= 760 chars)
Small images, animated pictures can be used.
MMS : Multimedia messaging service
Large pictures : GIF, JPEG
Short video clips
Fax service
Supplementary Services : Supplementary services include :
Caller identification
Call forwarding
Multi-party communication
Features Of GSM :
Localisation & Calling Of GSM :
One of the main features of GSM system is the automatic, worldwide localisation of it’s users. The GSM system always knows where a user is currently located, and the same phone number is valid worldwide. To have this ability the GSM system performs periodic location updates.
The HLR (home location register) contains information about the current location, and the VLR that is currently responsible for the MS informs the HLR about the location of the MS when it changes. Changing VLRs (visitor location register) with uninterrupted availability of all services is also called roaming.
To locate an MS and to address the MS, several numbers are needed:
MSISDN (Mobile Station International ISDN Number) : The only important number for the user of GSM is the phone number, due to the fact that the phone number is only associated with the SIM, rather than a certain MS.
IMSI (International Mobile Subscriber Identity) : GSM uses the IMSI for internal unique identification of a subscriber.
TMSI (Temporary Mobile Subscriber Identity) : To disguise the IMSI that would give the exact identity of the user which is signaling over the radio air interface, GSM uses the 4 byte TMSI for local subscriber identification. The TMSI is selected by the VLR and only has temporary validity within the location area of the VLR. In addition to that the VLR will change the TMSI periodically.
MSRN (Mobile Station [Subscriber] Roaming Number) : This is another temporary address that disguises the identity and location of the subscriber. The VLR generates this address upon request from the MSC and the address is also stored in the HLR. The MSRN is comprised of the current VCC (Visitor Country Code), the VNDC (Visitor National Destination Code) and the identification of the current MSC together with the subscriber number, hence the MSRN is essential to help the HLR to find a subscriber for an incoming call.
All the numbers are needed to find a user within the GSM system, and to maintain the connection with a mobile station.