Digital Cellular System
Digital
cellular systems are those which incorporate digital modulation techniques.
Digital systems provide significant improvements in capacity and system
performance. The United States Digital Cellular System (USDC) was created in
the late 1980s to handle more users in a given spectrum allotment. USDC is a
TDMA (time division multiple access) technologies. USDC can provide up to six
times the capacity of AMPS.
The USDC
standard has the same 45MHZ FDD scheme as AMPS. The dual-mode USDC/AMPS system,
which had been implemented in Canada and Mexico, was standardised as interim
standard 54 ( IS-54) by the electronic industry north American Digital Cellular
(NADC). The USDC system is intended to share the same frequency reuse scheme
and base stations as the AMPS system, enabling base stations and consumer units
to be supplied with both AMPS and USDC channels from within the same type of
hardware. USDC forward and reverse control channels utilise the same signalling
method as AMPS phones to ensure compatibility.
Global
System for Mobile Communications (GSM)
The secondary
standard is the Global System for Mobile (GSM). It was created to address the
fragmentation issues that plagued Europe's first cellular infrastructure. GSM
was the world's first cellular system to define digital modulation as well as
network-level architecture and services. It is the most widely used second-generation
(second generation) technology in the world.
GSM was
originally designed to be a pan-European cellular service that offered several
communication services via ISDN, but it is now the world's most advanced
standard for new cellular radio and personal communication devices globally. As
of 2001, GSM has over 350 million consumers worldwide.
In 1991, GSM
was initially launched on to European market. For marketing purposes, GSM
changed its name to the global system for mobile communications in 1992. By the
end of 1993, numerous non-European nations in South America, Asia, and
Australia had embraced GSM, which supports personal communication service (PCS)
in the newly established 1.8GHZ to 2.0GHZ radio bands throughout the world.
GSM Services
GSM services
adhere to ISDN standards and are classed as teleservices or data services.
Teleservices include both conventional mobile phone traffic and mobile
generated traffic. Data service includes both computer-to-computer
communication and packet-switched traffic. User services are classified into
three kinds.
Telephone
Services — This service includes emergency dialling and facsimiles. Videotex
and Teletex are also supported by GSM, although they are not part of the GSM
standard.
Bearer or data services - These services include packet-switched
methods and transfer rates ranging from 300 bps to 9.6 kbps. Data can be
transmitted in either transparent or non-transparent mode (where GSM provides
standard channel coding for user data) (where GSM offers special coding
efficiency based on specific data services).
Supplementary ISDN services — These are digital services that
include all diversions, closed user groups, and caller identifications. These
services are not available in analogue mobile networks. Short message services
(SMS) are another supplementary service that allows GSM customers and base
stations to send alphanumeric pages of a certain length.
SMS can be used for safety and advisory purposes, such as
broadcasting highway or weather information to all GSM users within the reception
range.
GSM's most notable feature is the Subscribers Identity
Module ( SIM ). SIM cards are memory devices that contain information such as
the subscriber's identification number, the networks and countries where the
subscriber is eligible for services, private keys, and other user-specific
information. A SIM card with a four-digit personal ID number is used by a
subscriber to activate services from any GSM phone.
SMS services are available as smart cards (credit card-sized
cards that can be put into any GSM phone) or plug-in modules. All GSM phones
are similar and inoperable unless a SIM card is inserted. Subscribers may
insert their SIM card into any appropriate terminal, such as a hotel phone,
public phone, or any portable or mobile phone, and have all incoming GSM calls
routed to that terminal and all outbound calls billed to their home phones, regardless
of where they are in the globe.
The system's provision of air privacy is the second prominent
characteristic of GSM. In contrast to analogue FM cellular phone systems, which
may be easily observed, eavesdropping on a GSM radio transmission is nearly
difficult. Encrypting the digital bitstream sent by a GSM transmitter with a
secret cryptographic key known only to the cellular carrier allows for privacy.
For each user, this key evolves. Before designing GSM equipment or developing a
GSM system, each carrier and GSM equipment manufacturer must sign a memorandum
of understanding (MOU). The Memorandum of Understanding (MOU) is an
international agreement that permits nations and carriers to share encryption
algorithms and other private data.
GSM System Architecture
The GSM framework is made up of three primary linked
subsystems. They are as follows: 1) Base station subsystem (BSS), 2) Network
and switching subsystems, and 3) Operation support subsystem (OSS). Through
specific network interfaces, the subsystems communicated with one another and
with the users.
The mobile station (MS) is also a subsystem, although it is
typically regarded to be part of the BSS for architectural purposes. The BSS,
also known as the radio subsystem, offers and controls radio transmission
routes between the mobile switching centre and the base station (MSC). The BSS
also controls the radio interface between mobile stations and all GSM
subsystems. Each BSS is composed of numerous base station controllers (BSCs),
which connect the MS to the NSS through MSCs.
NSS manages system switching and connects MSCs to other networks such as PSTN and ISDN.
Figure: GSM Architecture
The OSS permits engineers to monitor, diagnose, and
troubleshoot all elements of the GSM system and hence supports its operation
and maintenance. This subsystem communicates with the others in the GSM
network.
The picture is a block schematic of the GSM architecture. The
radio air interface connects the mobile stations (MSs) to the base station
subsystem (BSS). The BSS is made up of several BSCs that are linked together by
a signal MSC. Each BSC may manage hundreds of Base Transceiver Stations (BTSs).
Some BTSs may be co-located at the BSC, while others may be dispersed widely
and physically linked to the BSC through a microwave connection or special
leased lines.
The picture represents the various interfaces used in GSM.
The interface that links a BTS to a BSC is known as the Abis interface. GSM has
standardised the Abis interface for all manufacturers, which transfers traffic
and maintenance data. BTS and BSC equipment from the same vendor might be used
to minimise small differences.
Physically, the BSCs are interconnected to the MSC through
dedicated/leased lines or a microwave link. The A interface, which is specified
within GSM, is the interface between a BSC and an MSC. The A-interface employs
the signalling correction control portion (SCCP) of the SS7 protocol. It allows
for communication between the MSC and the BSS, as well as network
communications between individual subscribers and the MSC. An interface enables
a service provider to employ multiple manufacturers' base stations and
switching equipment.
The NSS is in charge of routing GSM calls between external
networks and the radio subsystem's BSCs. The MSC is the NSS's core unit, and it
manages traffic between all of the BSCs.
NSS has three databases. They are as follows: 1) Home
location registers ( HLR ), 2) Visitor location registers ( VLR ), and 3)
Authentication centres ( AUC ). The HLR is a database containing subscriber and
location information for each user who lives in the same city as the MSC. Each
GSM user in a certain GSM market is given a unique international mobile
subscriber identifier (IMSI). Each home user is identified by this number.
The VLR is a database that temporarily saves the IMSI and
customer information for each roaming subscriber who visits a certain MSC's
coverage area. The VLR is linked to multiple neighbouring MSCs in a certain
region and stores subscription information for every visiting user in the area.
When a travelling mobile is registered in the VLR, the MSC
provides the required information to the visiting subscriber's HLR so that
calls to the roaming mobile can be routed correctly over the PSTN by the
roaming user's HLR.
The Authentication Center is a highly secure data centre that
maintains each HLR and VLR subscriber's authentication and encryption
credentials. The Authentication Center maintains a database known as the
Equipment Identify Register (EIR), which identifies stolen or fraudulently
altered phones that communicate identify data that does not match the information
in either the HLR or VLR.
The OSS provides support for one or more operation
maintenance centres (OMC), which monitor and maintain the operation of each MS,
BS, BSC, and MSC in a GSM system.
The OSS performs three functions: 1) maintain all telecommunications hardware and network operations with a specific market, 2) manage all pricing and invoicing procedures, and 3) manage all mobile equipment in the system.
Figure: Various interfaces used in GSM