Roaming In The GSM Network

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Roaming In The GSM Network

• Ian Hughes,Elaine McGrath,Bryan Mc Laughlin, Orla
  Mahon
• Group H

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History of the GSM Networks
Global System for Mobile communication (GSM) was
established less than 50 years ago by the
European Telecommunications Standards Institute
as the access protocol for 900 MHz cellular
networks. However the history of communications
goes back a lot further. 1876 - Alexander Graham
Bell invented the telephone. This was the first
means of modern telecommunications. 1896 -
Guglielmo Marconi made the first radio
transmission. 1964 - The concept of sharing
resources was introduced into the radio telephone
networks. This means that networks could have
more subscribers on it than there were radio
channels. 1971 - Bell Telephone company proposed
a new cellular network concept, using a limited
frequency band. Bell introduced the Advanced
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What was involved and who did what Ian Hughes
put most of the Web Page together.He also
researched the topic of Guide To Roaming On
GSM with the topics of Structure of the cellular
network The GSM standard. Bryan Mc Laughlin
chose the topic of Key Words.He formed a table
with the Main Components/Procedures and their
meanings! Elaine Mc Grath put together a write
up on Technical Information.She gathered
information on Technical Data. Orla Mahon
chose the topic of Future Of GSM. Our other
topics included Inside Your GSM Phone(This was
about the phones components and their purpose),
History of GSM(Involves An introduction to the
GSM and the different Analogue Standards).
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Technical Information
Technical Data GSM uses the circa 900Mhz band
The frequency band used is 890-915MHz (mobile
transmit) and 935-960MHz (base transmit). To
allow maximum number of users access, each band
is subdivided into 124 carrier frequencies spaced
200 kHz apart, using FDMA techniques. Each of
these carrier frequencies is further subdivided
into time slots using TDMA. TDMA (Time Division
Multiple Access) has 8 time slots (i.e.
transmitting for one eighth of the time). Hence,
one radio channel can support 8 'full rate'
traffic. A more economical 'half rate' scheme
with 16 traffic channels is being introduced.
TDMA provides each user with the carrier
frequency for approximately 0.577ms. There is
also an extension band of 15 MHz in both
directions. The peak power of mobile stations
depends on the class of mobile. Precautions are
required to avoid Interference with other users.
Power control - 15 steps of 2dB - is provided.
The transmitter must be ramped up and down in
level in a controlled fashion at the beginning
and end of each time slot. Careful filtering is
also applied. Frequency hopping may he optionally
employed in order to avoid 'deadspots' and to
minimize interference from other signals. The
hopping rate is one hop per TDMA frame (4.6µs),
or 217 hops per second.
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Technical Information
The method of modulation used Is Gaussian Minimum
Shift Keying (GMSK), with a BT value of 0.3 at a
gross data rate of 270 kb/s. Phase and frequency
synchronization must allow for Doppler shift for
vehicle speeds up to 250km/h as well as for
frequency standard drift, and timing advance to
compensate for propagation delay due to round
trips for paths, in cells up to 35km radius.
The overall data rate for the radio channel is
270kb/s. This is split into B full rate or 16
half rate traffic channels, plus the signaling
channels. The coding is complex in order to have
the maximum chance to detect and correct the
errors encountered in a typical propagation path.
The output of the speech coder is encrypted,
coded and interleaved in a sophisticated way to
allow Forward Error Correction to be applied. The
data is then sent as bursts in time slots of
577µs, each containing 116 encrypted bits. There
are 8 or 16 of these time slots per TDMA frame,
and the receive and transmit time slots are
staggered so that the mobile station is not
receiving at the same instant as it is
transmitting, thus simplifying the filtering
requirements. With this scheme, there can be at
least one spare slot between transmit and
receive, leaving time for the synthesizer to
change frequency (whether or not hopping is
employed). The receiver also monitors adjacent
cell for one time slot each frame to determine
their signal strength to optimize a possible
handover.
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Guide To Roaming On GSM
Structure of the cellular network Cellular
network principle is based on the sub divisions
of the geographical area covered by the network
into a number of smaller areas. These areas are
called cells. Each of the cells has a fixed
station, which acts as a transmitter- receiver
serving all the mobile stations situated within
the boundaries of the cell. These days, when a
subscriber wants to make a call, the base station
(which is the control center for the group of
frequencies allocated by the network to that cell
along with the group of subscribers present in
the cell) allocates a sending frequency. When the
subscriber moves into another cell, he comes
under the control of another base station, which
allots him another frequency different from the
first.
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Technical Info.
When GSM was launched in the early 1990's, only
the half rate coder was available. This operated
at a basic rate of 13 kbps which became 22 kbps
gross when the coding for forward error
correction was added. The full rate codec
operates at 11.4 kbps gross, almost doubling the
capacity and spectral efficiency of' a network
vital to satisfy anticipated customer demand and
to cope with the huge number of new customers.
GSMs coding principle is known as RPE-LPE
(Regular Pulse Excitation - Long Term
Prediction), and sampled the analogue signal with
a 13 bit uniform PCM, at 8000 samples per second.
A sophisticated Discontinuous Transmission (DTX)
scheme is used to minimize radiated power for
battery efficiency and spectral efficiency. This
features voice activity detection, with insertion
of comfort noise during periods of 'silence', and
speech frame substitution, to minimize distortion
due to interference.
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Future of GSM
WAP What will mobile phones be like in the
future? The general consesus is that a new era
dawns in mobile communications with the advent of
WAP. WAP(Wireless Application Protocol) is a tag
based language similar to HTML. It will allow the
mobile phone to be used like an internet-ready
computer, allowing you to roam in a virtual world
of data, as well as our physical world. Goals
of the Wireless Application Protocol
Independent of wireless network standard.
Open to all. Will be proposed to the
appropriate standards bodies. Applications scale
across transport options. Applications scale
across device types. Extensible over time to new
networks and transports. Applicability The
Wireless Application Protocol will be applicable
to, but not limited to GSM-900, GSM-1800,
GSM-1900, CDMA IS-95, TDMA IS-136, 3G systems -
IMT-2000, UMTS, W-CDMA, Wideband IS-95.
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Future Of GSM
Mobile Multimedia The mobile multi-media kit
We've grown accustomed to the need for
Megabytes of disk space and ever-higher
transmission speeds for our new sophisticated
network applications. And soon we'll want to use
these high-powered applications on the move.GSM
has forseen this demand and work is well underway
to boost the speed of bearer servicesup to
115kbit/s, and higher-a significant leap from the
9.6kbit/s circuit switched data as it stands
today. For higher speeds, a new part of the GSM
standard is being developed. This is known as
High Speed Circuit Switched Data (HSCSD) and will
boost user capacity up to 64kbits/s, and higher.
Already in 1996, Ericsson, in conjunction with
Telia of Sweden, conducted the world's first live
demonstration of 64kbit/s data over GSM.
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Future Of GSM
HSCSD will facilitate any number of new
applications for mobile communications. For
example, it could be used for video conferencing.
Business travellers will be able to dial their
offices from their hotel rooms and, using
standard software on laptopcomputers, hold
videoconferences with their colleagues back at
the office. The same technology could be used
to assist emergency services. For example,
ambulances at the scene of a major incident will
be able to send real time video pictures and
medical data back to a specialist centre. Remote
clinics will similarly benefit by being able to
send scans and X-rays to medical specialists who
may be in another region or another country.
Expert medical advice will be quickly available
exactly where it is needed. Corporate users will
be able to use the World Wide Web and download
pages with a high graphics content in seconds.
They will also take advantage of the higher
speeds for accessing in-house LAN's via Intranets
(corporate Internets). Because HSCSD has an
in-built bandwidth on demand capability, it will
not matter which speed the user needs for any
particular activit, the service will provide
whatever speed they require, up to 6kbit/s.
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Future Of GSM
Interconnecting this 64kbit/s wireless capability
with public ISDN networks, for example, will give
mobile users complete end-to-end digital
connectivity with the attendant benefits of very
fast set up times and high link quality. The main
benefit of this approach is that it is designed
for the existing GSM infrastructure. There is no
need for operators to change their entire network
infrastructure in order to implement it.
Operators will therefore be able to implement
HSCSD in a quick and efficient way, once the
specification has been ratified.
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Guide To Roaming On GSM
The GSM standard. The GSM network provides
subscribers with 3 types of services. 1.Bearer
services. 2.Teleservices. 3.Supplementary
services. Subscribers gain access to the
network through their handset terminal known in
GSM as the mobile station. This consists of a
wireless telephone and a data terminal, which
receives messages from the network, and the Base
Transceiver Station (BTS) is the network terminal
equipment interfacing to the mobile.
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Guide to roaming the GSM
The BTS consists of a group of fixed transmitters
and receivers through which it transfers messages
to the mobile terminals present in the cell that
it controls. Next in the network is the BSCs
(Base Station Controllers) which controls the
traffic from the base station and act as channels
to the network sub-system. The BSCs are served by
Mobile Switching Centers (MSC's) which connect
the GSM network to the PSTN and ISDN public
switched networks and provide the interface
between the GSM network and data base and the
radio sub-system. The databases are 1) the VLR
(Visitor Location Register), 2) the HLR (Home
Location Register) of the switching center on
which the subscriber is based and 3) the
Authentication Center (AUC). These databases
allow the access rights of subscribers to be
checked and also to keep track of the location of
subscribers.
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History of the GSM Networks
Mobile Phone Service (AMPS) and so began the
cellular concept. 1982 - Standardization of the
(AMPS) system specifications. During the 80's,
several cellular radio networks entered service
around the world. These were however only
analogue systems. Today however, there are many
cellular radio networks and radio telephony is
almost commonplace. It is now possible to make a
call to anywhere, at any time