Mobile Computing Architecture – GSM Network Architecture

Mobile Computing Architecture – GSM Network Architecture

Mobile Computing Architecture – GSM Network Architecture – The full form of GSM is group special mobile (GSM) and later it was named as a global system for mobile communications (GSM).

It was founded in 1965. GSM is the most successful and popular mobile telecommunication system. GSM is used by over 800 million people and in over 190 countries.

The main aim of GSM was to provide a mobile phone system that allows users to move throughout Europe and allows voice services compatible with the ISDN and PSTN systems.

 

 

Mobile Computing Architecture  – Versions of GSM

GSM is a particularly second-generation system, replacement of the first-generation analog system. This system was not capable to give high worldwide data rates as promised by the third generation systems.

GSM has initially been deployed in Europe with 890-915 MHz for uplinks and 935-960 MHz downlinks. This GSM system is called GSM 900.

The next version is called GSM 1800 MHz, 1710-1785 MHz uplink and 1805-1880 MHz downlink. This system is called DCS or the digital cellular system-1900.

The next versions of GSM are GSM 400. It is deployed in sparsely populated areas as a replacement of the analog system.

The next more advanced version of GSM is GSM-RAIL. It has been introduced in several European countries for railroad systems.

 

GSM-RAIL

GSM-RAIL uses separate frequencies for their channels. GSM-RAIL (GSM-R, 2002), also includes so many additional services that are not available in previous versions.

 

Mobile Computing Architecture  – GSM Features

GSM-R offers 19 special channels for railroad operators for voice and data traffic.

GSM Meaning

Special features of GSM-R are as follows-

 

• Emergency calls with acknowledgments
• Voice group call service (VGCS),
• Voice broadcast service (VBS).
• Calls are prioritized in this system for instance high priority calls pre-empts the low priority calls.
• Calls have short set-up times:
• Emergency calls less than 2 s, group calls less than 5 s.
• Calls can be directed to all users at a certain location
• The most important use of GSM-R is the control of trains, switches, gates, and signals.

 

However, the main purpose of GSM is voice and data services. But we can predict various future applications for mobile communications particularly data-driven applications.

 

Mobile Computer Services

GSM Mobile Services

GSM system has defined three kinds of services:

• Bearer Services
• Tele-Services
• Supplementary services

 

Bearer Services in GSM

Bearer Services allow transparent and nontransparent, synchronous and nonsynchronous data transmission services.

 

• Transparent bearer services in GSM

Transparent bearer services use the functions of the physical layer to transmit data.

Data transmission has a constant delay and throughput if no transmission errors occur.

 

Forward error correction (FEC) is used in GSM to improve the quality of data transmission. FEC codes redundancy into the data stream helps to reconstruct the original data if any transmission error occurs during communicating the data.

 

Transparent services do not recover lost data in the situation of shadowing or interruption because of handovers.

Using transparent and nontransparent services, GSM provides various bearer services for internetworking PSTN, ISDN, and packet-switched public data networks such as X.25. X.25 is available worldwide.

 

• Nontransparent Bearer Services

Nontransparent bearer services use protocols of layers 2 and 3 for error correction and flow control.

These services use transparent bearer services using radio link protocol.

This radio link protocol (RLP) includes the high-level data link control (HDLC) mechanism and special selective reject mechanism to trigger retransmission erroneous data.

 

TeleServices of GSM

GSM provides voice-oriented teleservices. Teleservices include voice transmission, message services, and basic data communication services or PSTN and ISDN services.

The most important service is the telephony services.

 

Telephony Services

GSM provides high-quality digital voice transmission. It offers a bandwidth of 3.1 kHz of analog phone systems.

A special codec or coder-decoder is used for voice transmission.

 

Other codecs are used for the transmission of analog signals for communication with traditional modems such as the fax service.

 

Emergency Number Service of GSM

This service is free of cost and essential for all the service providers. Emergency number service is of the highest priority service and pre-empting other connections.

The service is automatically set up with the closest emergency center.

 

Short Message Service

SMS is a simple short message service. It allows 160 characters of messages.

SMS services do not use the standard data channel of GSM. It uses unused capacity in the channels.

 

Supplementary services

In addition to teleservices and bearer services, GSM providers provide various supplementary services.

Examples o supplementary services are as follows-

1. Identification
2. Call redirection
3. Forward incoming calls
4. Close user group service – Through a company-specific GSM sub-network facility only members of a group can communicate.
5. Multi-party communication service

 

 

GSM System Architecture

 

 

GSM has a complex hierarchical system architecture. It includes various entities, interfaces, and acronyms.

GSM consists of three subsystems as given below –

1. Radio Subsystem
2. Network Switching subsystem
3. Operation subsystem

GSM – Radio Sub System

Radio subsystem comprises the entire radio specific entities such as

• A mobile station (ms) and
• Base station subsystem (BSS)

 

RSS and NSS connected with “a” interface and further connected with OSS via the “o” interface.  All the subsystems of GSM are as follows-

 

1. Base station subsystem – BSS
2. Base transceiver subsystem – BTS
3. Base station controller -BSC
4. Mobile station – ms

 

Base station subsystem – BSS

• A BSS comprises many base station subsystem. Each BSS is controlled by BSC or base station controller.
• A BSS does all functions that are necessary to manage and maintain radio connections to a mobile station, for instance, coding and decoding of voice, and rate adaptation to and from the wireless network.
• A BSS contains several BSC and BTS.

 

Base transceiver subsystem – BTS

The base transceiver comprises all radio entities and equipment such as

• Antennas, Signal processing, Amplifiers, etc. necessary for radio transmission
• A BTS form a radio cell or using sectorized antennas several cells and that are connected to the mobile station via the Um interface (ISDN U Interface for mobile ) and to the BSC via the Abis interface.
• The Um-interface contains all the entities required for wireless transmission such as TDMA, FDMA, etc.

 

 

Base station controller –BSC

• The base station controller manages all the BTS.
• It reserves radio frequencies, handles handovers, from one BTS to another BTS within the BSS.
• BSC multiplexes radio frequency channels onto the fixed network connections at the ‘A’ interface.

 

Mobile station – ms

• The mobile station comprises user equipment and entities as well as the software required for communication with the GSM network system.
• The mobile station consists of user-independent hardware and software and subscriber entity module or ‘Sim’ that stores all the user-related data that is relevant for the GSM system.

 

Network and Switching Subsystem

Network and Switching subsystem connects the wireless with the standard public network.

NSS performs handovers between different BSS.

NSS performs following functions-

• Worldwide localization of mobile users
• Support charging, accounting, and roaming of users between various providers in different countries

NSS consists of following switches and databases

• Mobile services switching center –MSC
• MSCs are high-performance digital ISDN switches.
• MSC manages several BSCs in geographical regions
• MSC setups connections to other MSC via ‘A’ interface
• MSC has some more connections to other fixed networks such as connection with PSTN and ISDN.
• Using additional interworking functions MSC is connected to Public Data Network (PDN) also, such x.25.
• MSC handles all the signaling required for connection setup, connection release and hangovers of connections to other MSCs.

 

Home location register – HLR

The home location register is the most important database in the GSM system. It stores all the user-specific data such as-

• Mobile Subscriber ISDN number – MSISDN
• Information on subscriber services such as forwarding, roaming restrictions, and the international mobile subscriber identity – IMSI
• Dynamic information is also needed such as the current location area (LA) of the mobile station.
• As soon as the mobile station leaves its current location area, the information of the HLR is immediately updated. This information is needed to localize the mobile station in the worldwide GSM network.
• HLR also stores present MSC and VLR.
• All these user-specific information elements only exist once for each user in a single HLR, that also facilitates charging and accounting.

 

Visitor Location Register – VLR

• The visitor location register is associated with each MSC.
• VLR is dynamic database stores all important information of the mobile user such as its current location area, and current MSC.
• Whenever a new user changes its location area and MSC, VLR copies its data from its HLR.
• VlRs capable to store the data of millions of customers.

 

Operation Subsystem – OSS

OSS is the third subsystem of GSM. It contains all the necessary functions for network operations and maintenance.

It contains the following entities-

• Operation and Maintenance Center
• Authentication Center
• Equipment Entity Register

 

Operation and Maintenance Center

Operation and Maintenance (OMC) monitors and controls network entities via O interface. OMC management functions are as below-

• Traffic monitoring
• Status reporting of network entities
• Subscriber management
• Security management
• Accounting and billing

 

Authentication Center (AUC)

AUC is situated in the protected part of the home location register.

AUC protect user identity and data transmission. It contains algorithms for user authentication.

 

Equipment Entity Register

EIR is a data repository that keeps records of all the equipment or devices related to GSM. It also keeps records of valid devices including mobile phones and also invalid devices or stolen devices.

 

GSM Radio Interface

In GSM 900, 124 channels are used. Each channel is 200 kHz wide. GSM 1800 uses 374 channels.

GSM 900

In GSM data is transmitted in small portions called bursts that are used for data transmission inside a time slot for both user data as well as signaling data.

The burst is only 546.5 us long and contains 148 bits. The 30.5 us are used as guard space to avoid overlapping with other bursts.

The details of 148 bits are as below-

• The first and last 3 bits of a normal burst is set to 0 and can be used to improve the receiver performance
• The tanning sequence in the middle of a slot is used to adapt the parameters of the receiver to the current path propagation characteristics and to select the strongest signal in the case of multipath propagation.
• A flag S indicates whether the data field is user data or network data.
• 57 bits on both the side of the tanning sequence are used for user data.

The following figure shows the TDM used in GSM. Each of the 248 channels is separated
in time through a GSM TDMA frame

How GSM works

GSM Logical Channels and Frame Hierarchy

GSM uses two kinds of channels

• Logical Channels
• Traffic Channels

 

GSM Traffic Channels – GSM TCH

GSM uses traffic channels to transmit voice and fax data.

Two types of TCHs have been defined in the GSM.

Full-rate TCH – TCH/F

Half-rate TCH – TCH/h

 

Full-rate TCH – TCH/F

A TCH/F has a data rate of 22.8 kbits/s.

 

Half-rate TCH – TCH/h

TCH/h has data rate of 11.4 kbits/s.

Voice codecs used in the beginning of GSM standardization consumed 13 kbit/s. Remaining capacity of the TCH/F (22.8 kbit) was used for error correction.

Improved codecs give better voice quality in terms of speed and used with TCH/h, however voices quality decreased after the use of improved codecs.

The standard codecs system for voice are called full rate or FR,13 kbits/s and half rate or HR, 5.6 kbit/s.

A new codec system enhanced full rate or EFR, provides better voice quality than full rate codec if transmission error is low.

 

GSM Control channels – CCH

Different control channels are used in the GSM network architecture to control the following –

Control channels are communication channels used in a system (such as a radio control channel), which are dedicated to the sending and/or receiving of command messages between devices (such as a base station and a mobile radio).

On the GSM system, the control channel sends messages that include paging (alerting), access control (channel assignment) and system broadcast information (access parameters and system identification).

 

Three groups of Control channels – CCH are defined and these groups of channels have further sub channels

 

Broadcast control channel – BCCH

Common control channel – CCCH

Dedicated control channel – DCCH

 

Broadcast control channel – BCCH

A base transceiver system or BTS uses a broadcast control channel to provide information to all mobile stations within a cell. Information transmitted in this channel is as follows-

• Cell identifier
• Options present within the cell about frequency hopping
• Frequency available inside the cell and in the neighboring cells
• BTS gives frequency correction information via frequency correction channel (FCCH) and also information about time synchronization via synchronization channel (SCH).

FCCH and SCH are sub-channels of BCCH.

 

Common control channel – CCCH

The common control channel exchanged all the information about the connection setup between a mobile station and BTS.

For calls toward the direction of the Mobile station, BTS uses a paging channel (PCH) for paging the correct mobile station.

If a mobile station wants to set up the connection it uses a random access channel or RACH, to send data to the BTS.

RACH implements multiple accesses using slotted Aloha. All the mobile stations within a cell may access RACH channel.

If any collision occurs with other mobile stations in a GSM system, the BTS uses the access grant channel or AGCH to signal a mobile station that it can use a TCH or SDCCH for further connection setup.

 

 

Dedicated control channel – DCCH

 

Dedicated control channels are bidirectional channels while all other channels mentioned above are unidirectional channels.

As long as a mobile station has not set up a TCH with the BTS, the mobile station uses the Standalone dedicated control channel (SDCCH) at a low data rate of 782bit/s for signaling. This includes signaling for authentication, registration or other data required for setup TCH.

TCH and SDCCH have a slow associate dedicated control channel (SACCH) also which is used for exchanging system information like system quality and signal power level.

If more signaling data required to be shared a GSM uses fast associated dedicated control channel (FACCH).

FACCH uses the time slots that are otherwise used by TCH. This is required in the case of handovers where BTS and Mobile station needs to share a large amounts of data in minimum time.