Basic Cellular Repair Module.pdf

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Basic Cellular Repairs
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Basic Cellular repairs
GSM Theory
During the early 1980s, analogue cellular telephone systems were experiencing
rapid growth in Europe, particularly in Scandinavia and the United Kingdom, but
also in France and Germany. Each country developed its own system, which was
incompatible with everyone else's in equipment and operation. This was an
undesirable situation, because not only was the mobile equipment limited to
operation within national boundaries, which in a unified Europe were increasingly
unimportant, but there was a very limited market for each type of equipment, so
economies of scale, and the subsequent savings, could not be realised.
The Europeans realised this early on, and in 1982 the Conference of European
Posts and Telegraphs (CEPT) formed a study group called the Groupe Spécial
Mobile (GSM) to study and develop a pan-European public land mobile system.
The proposed system had to meet certain criteria:
• good subjective speech quality,
• low terminal and service cost,
• support for international roaming,
• ability to support handheld terminals,
• support for range of new services and facilities,
• spectral efficiency, and ISDN compatibility.
In 1989, GSM responsibility was transferred to the European Telecommunication
Standards Institute (ETSI), and phase I of the GSM specifications were published
in 1990. Commercial service was started in mid-1991, and by 1993 there were 36
GSM networks in 22 countries, with 25 additional countries having already
selected or considering GSM . This is not only a European standard - South Africa,
Australia, and many Middle and Far East countries have chosen GSM. By the
beginning of 1994, there were 1.3 million subscribers world-wide . The acronym
GSM now (aptly) stands for Global System for Mobile telecommunications.
The developers of GSM chose an unproven (at the time) digital system, as
opposed to the then-standard analogue cellular systems like AMPS in the United
States and TACS in the United Kingdom. They had faith that advancements in
compression algorithms and digital signal processors would allow the fulfilment of
the original criteria and the continual improvement of the system in terms of
quality and cost. The 8000 pages of the GSM recommendations try to allow
flexibility and competitive innovation among suppliers, but provide enough
guidelines to guarantee the proper interworking between the components of the
system. This is done in part by providing descriptions of the interfaces and
functions of each of the functional entities defined in the system.
SIM Subscriber Identity Module
HLR Home Location Register
MS Mobile Station
VLR Vistor Location Register
BTS Base Transceiver Station
EIR Equipment Identity Register
BSC Base Station Controller
AC Authentication Center
MSC Mobile services Switching Center
PSTN Public Switched Telecomm
Network
VLR Visitor Location Register
ISDN Integrated Services Digital
Network
A GSM network is composed of several functional entities, whose functions and
interfaces are defined. Figure 1 shows the layout of a generic GSM network. The
GSM network can be divided into three broad parts. The Mobile Station is carried
by the subscriber, the Base Station Subsystem controls the radio link with the
Mobile Station. The Network Subsystem, the main part of which is the Mobile
services Switching Center, performs the switching of calls between the mobile
and other fixed or mobile network users, as well as management of mobile
services, such as authentication. Not shown is the Operations and Maintenance
center, which oversees the proper operation and setup of the network. The Mobile
Station and the Base Station Subsystem communicate across the Um interface,
also known as the air interface or radio link. The Base Station Subsystem
communicates with the Mobile service Switching Center across the A interface.
Mobile Station
The mobile station (MS) consists of the physical equipment, such as the radio
transceiver, display and digital signal processors, and a smart card called the
Subscriber Identity Module (SIM). The SIM 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. By inserting the SIM card into
another GSM cellular phone, the user is able to receive calls at that phone, make
calls from that phone, or receive other subscribed services.
The mobile equipment is uniquely identified by the International Mobile
Equipment Identity (IMEI). The SIM card contains the International Mobile
Subscriber Identity (IMSI), identifying the subscriber, a secret key for
authentication, and other user information. The IMEI and the IMSI are
independent, thereby providing personal mobility. The SIM card may be protected
against unauthorized use by a password or personal identity number.
Base Station Subsystem
The Base Station Subsystem is composed of two parts, the Base Transceiver
Station (BTS) and the Base Station Controller (BSC). These communicate across
the specified A-bis interface, allowing (as in the rest of the system) operation
between components made by different suppliers.
The Base Transceiver Station houses the radio tranceivers that define a cell and
handles the radio-link protocols with the Mobile Station. In a large urban area,
there will potentially be a large number of BTSs deployed. The requirements for a
BTS are ruggedness, reliability, portability, and minimum cost.
The Base Station Controller manages the radio resources for one or more BTSs. It
handles radio-channel setup, frequency hopping, and handovers, as described
below. The BSC is the connection between the mobile and the Mobile service
Switching Center (MSC). The BSC also translates the 13 kbps voice channel used
over the radio link to the standard 64 kbps channel used by the Public Switched
Telephone Network or ISDN.
Network Subsystem
The central component of the Network Subsystem is the Mobile services
Switching Center (MSC). It acts like a normal switching node of the PSTN or
ISDN, and in addition provides all the functionality needed to handle a mobile
subscriber, such as registration, authentication, location updating, handovers,
and call routing to a roaming subscriber. These services are provided in
conjuction with several functional entities, which together form the Network
Subsystem. The MSC provides the connection to the public fixed network (PSTN
or ISDN), and signalling between functional entities uses the ITU-T Signalling
System Number 7 (SS7), used in ISDN and widely used in current public
networks.
The Home Location Register (HLR) and Visitor Location Register (VLR), together
with the MSC, provide the call-routing and (possibly international) roaming
capabilities of GSM. The HLR contains all the administrative information of each
subscriber registered in the corresponding GSM network, along with the current
location of the mobile. The current location of the mobile is in the form of a
Mobile Station Roaming Number (MSRN) which is a regular ISDN number used to
route a call to the MSC where the mobile is currently located. There is logically
one HLR per GSM network, although it may be implemented as a distributed
database.
The Visitor Location Register contains selected administrative information from
the HLR, necessary for call control and provision of the subscribed services, for
each mobile currently located in the geographical area controlled by the VLR.
Although each functional entity can be implemented as an independent unit, most
manufacturers of switching equipment implement one VLR together with one
MSC, so that the geographical area controlled by the MSC corresponds to that
controlled by the VLR, simplifying the signalling required. Note that the MSC
contains no information about particular mobile stations - this information is
stored in the location registers.
The other two registers are used for authentication and security purposes. The
Equipment Identity Register (EIR) is a database that contains a list of all valid
mobile equipment on the network, where each mobile station is identified by its
International Mobile Equipment Identity (IMEI). An IMEI is marked as invalid if it
has been reported stolen or is not type approved. The Authentication Center 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.
SMS
The Short Message Service (SMS) is the ability to send and receive text messages
to and from mobile telephones. The text can comprise of words or numbers or an
alphanumeric combination. SMS was created as part of the GSM Phase 1
standard. The first short message is believed to have been sent in December
1992 from a Personal Computer (PC) to a mobile phone on the Vodafone GSM
network in the UK. Each short message is up to 160 characters is length when
Latin alphabets are used, and 70 characters in length when non-Latin alphabets
such as Arabic and Chinese are used.
There is no doubting the success of the Short Message Service- the market in
Europe alone has reached over three billion short messages per month despite
little proactive marketing by network operators and phone manufacturers. Key
market drivers over the next two years such as the Wireless Application Protocol
(WAP) will continue this growth path.
For a relatively simple messaging service, there certainly are a lot of elements
that need to be taken into account when developing and deploying SMS! However
operators who take the time and trouble to invest in SMS will find appreciative
customers and appreciating revenues.
WAP
There are three major parts of any WAP-enabled system, namely the WAP
Gateway , the HTTP Web Server , and the WAP Device itself, which is interacting
with the WAP Gateway, as Figure 1 illustrates below. The WAP Gateway sends
WML- formatted content to the WAP device, whilst the WAP gateway must
communicate with the Web server using the Web's primary protocol, HTTP.
Figure 1
All Web servers can communicate with a variety of information sources, using a
number of different integration tools and protocols, for example a Web server can
serve pages of information that are generated by tools such as Active Server
Pages (ASP), ColdFusion, or PHP . Database integration is achieved using protocols
such as CGI (the Common Gateway Interface) or, more likely,ODBC, as Figure 2
illustrates.
Figure 2
The Wireless Application Protocol (WAP) is a hot topic that has been widely hyped
in the mobile industry and outside of it. WAP is simply a protocol- a standardized
way that a mobile phone talks to a server installed in the mobile phone network.
It is amazing how in just six months, it has become imperative for all Information
Technology companies in Nordic countries and beyond to have a WAP division.
Many many advertising agencies and "dot.coms" have announced WAP services.
WAP is hot for several reasons:
1. It provides a standardized way of linking the Internet to mobile phones,
thereby linking two of the hottest industries anywhere.
2. Its founder members include the major wireless vendors of Nokia, Ericsson
and Motorola, plus a newcomer Phone.com.
3. By April 2000, the WAP Forum had over 350 member companies.
4. Mobile information services, a key application for WAP, have not been as
successful as many network operators expected. WAP is seen as a way to
rectify this situation.
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