Global System of Mobile Communication (2G) In 1982, the
Conference of European Posts and Telegraphs (CEPT) nominated a
group called the Groupe Spcial Mobile (GSM) to develop a public
land mobile system that could operate across Europe with the
objectives of:
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Low mobile device and service cost Good speech quality
International roaming capability Ability to support handheld mobile
devices Extensibility for adding new services and facilities
Compatibility with the ISDN
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The GSM system supports a variety of data services at rates
upto 9600 bps. The GSM network is also capable of supporting call
forward (such as call forwarding when the mobile subscriber is
unreachable by the network), call barring of outgoing or incoming
calls, caller identification, call waiting, and multi-party
conversations. Fig.15.4 shows the layout of a generic GSM network.
A GSM network consists of three major subsystems:
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The mobile Station: In the GSM network the Mobile Station (MS)
consists of the equipment, also often referred to as the terminal,
and a removable Subscriber Identity Module (SIM) in the form of a
smart card. The SIM card offers personal or identity mobility. In
the GSM network the mobile equipment is uniquely identified by the
International Mobile Equipment Identity (IMEI) assigned at the time
of manufacturing.
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The SIM card identity is independent of the IMEI. It uses the
International Mobile Subscriber Identity (IMSI) for identifying the
subscriber to the system, a secret key for authentication and other
information. The independence of IMEI and the IMSI and the use of
IMSI alone to identify the subscriber on the GSM network provide
personal mobility with regards to the mobile equipment. The SIM
card also has a provision for protection against unauthorized use
by use of a password or Personal Identity Number (PIN).
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The Base Station Subsystem The base station subsystem is made
up of two important components, the Base Transceiver Station (BTS)
and the Base Station Controller (BSC). Base transceiver station is
typically a radio transceiver that operates within a cell defined
by the power and footprint of the antenna used. It deploys and
communicates with the mobile station through radio link
protocols.
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Large and dense cellular networks may deploy a large number of
BTSs, thus the requirements for BTS are ruggedness, reliability,
portability, and minimum cost. One or more of base transceiver
stations operating in a cell, are controlled by a base station
controller. It manages the radio resources for the BTS,
radio-channel setup, frequency hopping, and handovers. On the other
hand, the BSC is connected to the Mobile service Switching Center
(MSC).
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The Network Subsystem: The MSC forms the core of the network
subsystem. It works like any ISDN or PSTN switching centre and
performs the switching of calls between the mobile users, and
between mobile and fixed network users. In addition to the normal
call switching functions, it also handles mobility management. The
information on the registration; authentication; location; call
handovers; routing, in case of roaming users, are all handled by
the MSC uses four databases, viz., home location register, visitor
location register, authorization, and equipment identity
register.
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The Home Location Register (HLR) maintains registration and the
required administrative information for all subscribers registered
in the GSM network along with the current location of the mobile.
The location of the mobile device is typically stored as the
signaling address used by the Visiting Location Register (VLR)
associated with the mobile station. The home location register,
along with the current location and other information of the VLR,
is used for managing roaming and call routing.
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Spectral Allocation The GSM uses 25 MHz for the mobile device
to base station transmission (uplink) and an additional 25 MHz for
the base station to the mobile device (downlink) transmission. The
International Union (ITU), the managing body for the international
allocation of radio spectrum, allocated the bands 890 -915 MHz for
uplink and 350 -960 MHz for downlink transmission for mobile
network in Europe.
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The allocation of 25 MHz for the analog system had reserved 10
MHz for future use. GSM networks were initially built using this 10
MHz and later expanded to full spectrum.
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Multiple Access GSM networks use a combination of Frequency
Division Multiple Access (FDMA) and Time Division Multiple Access
(TDMA). The 25 MHz of limited radio spectrum allocated for the use
in GSM networks is shared by all users by dividing the bandwidth
among as many users as possible. GSM networks divide up the 25 MHz
radio spectrum in 124 carrier frequency channels that are allotted
200 KHz each. Each base station is allotted at least one or more
carrier frequencies. Each base station uses TDMA by dividing the
carrier channel in to time slots.
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The fundamental unit of time in this TDMA scheme is called a
burst period and it lasts 15/26 micro second (or approximately
0.577 micro seconds). Eight burst periods are grouped into a TDMA
frame (120/26 micro second, or approximately 40615 micro second),
which forms the basic unit for the definition of logical
channels.
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One physical channel is one burst period per TDMA frame.
Channels are defined by the number and position of their
corresponding burst periods. All these definitions are cyclic, and
the entire pattern repeats approximately every 3 hours. Channels
can be divided into dedicated channels, which are allocated to a
mobile station, and common channels, which are used by mobile
stations in idle mode.
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3G Networks The enhanced data rates offered by EDGE through the
evolution of second generation (2G) GSM and TDMA networks were
still not fast enough for many multimedia mobile applications. The
wireless network technology offered the next generation (3G) of
solutions that provides high speed bandwidth to handheld
devices.
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Third generation (3G) networks are derived from the Universal
Mobile Telecommunications Services (UMTS) for high speed networks
that enable a variety of data intensive applications. CDMA200 A
third generation solution for mobile networking that evolved from
existing wireless standard is CDMA it is also known as IMT IS-95.
It supports 3G services as defined by the International
Telecommunications Union (ITU) for IMT-2000.
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W-CDMA Wideband Code Division Multiple Access is a standard
defined by the ITU standard and is derived from Code Division
Multiple Access (CDMA) standard. The standard is officially called
IMT-2000 direct spread. It is a 3G mobile wireless technology that
supports high speed transfers to mobile and portable wireless
devices.
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In the local area access mode it supports data rates of 2 Mbps
for transferring multimedia information. In WCDMA the signal is
coded and transmitted in spread spectrum mode over a 5 MHz carrier
band compared to 200 KHz carrier band used for CDMA.
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In addition to these important widely adopted standards there
are several variants that are also in use. The high data transfer
rates offered by 3G networks is capable of running multimedia
services that combines voice and data. The following data rates are
supported by 3G wireless networks: 2.05 Mb per second to stationary
devices. 384 Kb per second for slowly moving devices, such as a
handset carried by a walking user. 128 Kb per second for fast
moving devices, such as handsets in moving vehicles.
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These data rates are highest achievable under exclusive use
conditions. This means that in case of delivery to a stationary
device, the 2.05Mb per second rate is achieved when one user
occupies the entire capacity of the base station. Thus the normal
work load environment data rates attained are lower if there is any
other traffic. The actual data rates achieved by a user in practice
depend upon the number of calls and other traffic in progress.
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3G Standard The International telecommunications Union (ITU)
has worked out certain standards for 3G networks. CDMA has emerged
as the leading mechanism for 3G. The five ITU approved 3G standards
are as follows: CDMA 2000 WCDMA TD-SCDMA FDMA/TDMA TDMA-SC
(EDGE)
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CDMA uses a spread spectrum mechanism. In the spread spectrum,
a message consisting of Y bits per second is converted into a
longer message of kY bits and then transmitted at a higher rate.
The k is called the spreading factor. The spreading of messages
seems counter intuitive for attaining higher rates. The spread
spectrum has been used in military communication as it provided
immunity from jamming signals.
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In CDMA each transmitting entity uses a unique code assigned to
it. The coding scheme uses the user code for transmitting 1 and its
complement for transmitting a 0. The data bit stream is converted
into a coded bit stream and transmitted using the full frequency
spectrum rather than a limited frequency slot, as in FDMA, or time
slot, as in TDMA.
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Some of the important features of 3G networks are: The new
ratio spectrum relieves the overcrowding in existing systems. It
provides more bandwidth because the same frequencies can be used by
more than one pair of users. The adoption of 3G network based on IP
packets offers better interoperability between service providers.
The standard supports fixed and variable data rates.
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The 3G networks have devices that are backward compatible with
those of existing networks. It offers support to always-on devices
as it provides packet-based services using internet protocol
packets. The high data transfer rates support the smooth
functioning of multimedia services. Although some degree of
backward compatibility is supported, the cost of upgrading base
stations and cellular infrastructure to 3G is very high.
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Handsets that can use 3G services are complex products. The
higher power talks time and larger batteries. Thus, through
miniaturization of technology will alleviate the problem, handsets
and exited higher cost. Base stations need to be closer to each
other, which implies that service providers will incur more
cost.
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Wireless Access Protocol (WAP) The WAP protocol is the leading
standard for information services on wireless terminals like
digital mobile phones. WML is the language used to create the pages
displayed in a WAP browser. The wireless application protocol (WAP)
is the bridge that assists in developing technology independent
access to the Internet and telephony services from wireless
devices. It provides a mobile device user with the ability to
access the same set of information available on the Internet,
Intranets, or through the World Wide Web. That they could access
through their desktops.
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Since earlier attempts to provide internet access from wireless
devices used proprietary protocols and technology, they were
limited by the capability of wireless networks and handheld
devices. WAP addresses these issues by developing a standard
architecture for wireless access to net by utilizing the Internet
standard protocols with suitable modifications. The wireless
environment faces distinct constraints of lower connection,
stability, higher latency and lower available bandwidth.
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The architecture for building systems with wireless application
protocol utilizes Wireless Markup Language (WML) and WML Script to
produce content suitable for WAP enabled devices that makes optimal
use of small displays and makes one hand navigation possible. WAP
is a lightweight protocol requiring only the minimal resources
available on the devices to produce scalable content offering
deftly adaptively from one two line text displays available on
basic devices to graphic screens available on palmtops and newer
phone devices.
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The client, i.e., mobile devices, uses the lightweight WAP
stack to communicate with the WAP gateway for sending the URL
through the wireless system operators network to the WAP gateway
and a WAP browser that can interpret the binary codes of compact
WML and the WML script content delivered to it. The WAP gateway is
the interface that interconnects the wireless service operators
network with the internet. The requests received from mobile
devices are transformed to Hyper Text Transfer Protocol (HTTP) and
submitted to the Internet hosts.
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WAP is a layered protocol consists of following layers:
Wireless Application Environment (WAE) Wireless datagram Protocol
(WDP) Wireless transaction Protocol WTP) Wireless transport Layer
security (WTLS) Wireless Session Protocol (WSP), and Bearer
networks
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The Wireless Access protocol operates over a variety of
wireless bearer mechanisms, such as GSMs GPRS and EDGE, CDMA, CDPD,
IS-136, and iDEN. The WAP works on a variety of bearer networks
which may support the packet, or connection oriented services.
Users of WAP are shielded from the details of the bearer network.
The various protocol layers and the application environment of WAP
that offer bearer network transparency to applications are
described as follows:
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Wireless Datagram Protocol (WDP) The WDP has to directly deal
with the heterogeneous bearer network environment. One of the
important functions WDP has to perform is to offer the higher
layers of the protocol a consistent interface irrespective of the
underlying bearer. the bearer may or may not support the Internet
Protocol (IP) services. In case of bearers with IP support it uses
the User Datagram Protocol (UDP).
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In case IP less bearers such as GSM, it follows the WAP
specification to carry out the function. Thus WDP provides
operational transparency over one of the available bearer services,
thereby making the upper layers of the WAP stack independent of the
bearer. WDP accomplishes operational transparency over the widely
varying services offered by the bearer through the adaptation sub
layer. The adaptation layers map WDP functions to services offered
by different bearers. In case where the bearer is IP capable, WDP
functions in exactly in the same manner as the standard User
Datagram Protocol (UDP) of the Internet.
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Wireless Transaction Layer Security (WTLS) This is operational
layer implemented over WDP, offers a secure transport service
interface to higher layers in order to preserve the transport
service interface of WDP. the WTLS layer provides end-to-end
security features, which includes: Confidentiality using data
encryption algorithms. Data integrity using message authentication
codes. Authentication through digital certificates Non-repudiation
also through digital certificates and message authentication
codes.
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WTLS is derived from the Internet standard TLS protocol. It
offers standard connection security and also optimizations through
on-the-fly payload compression to increase the effectiveness of
datagram service running on a low-bandwidth network.
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Wireless Transaction Layer (WTP) In the WTP layer context a
transaction is defined as a request/response. The responsibility of
the layer is to offer an efficient transaction service over the
secure as well as insecure datagram service. It is a lightweight
transaction service that supports a request/response service. The
transaction services offered by the WTP can be put in the following
three class of services:
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Class 0: Unreliable push service Class 1: reliable push service
Class 2: reliable transaction service Unreachable push service is a
one-way communication service that does not bother to resend the
request in case it is lost in transaction. Reliable push service,
on the other hand, waits for acknowledgement from the receiver and
in case of lost request/ timeout way service in which a data
request is sent and the sending stack waits for the result.
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On receiving the result of the request the acknowledgement is
sent. Reliable service at is accomplished this layer by selective
retransmission and duplicate removal. Additionally, like the TCP in
the internet protocol stack, WTP is also responsible for taking
care of segmentation / reassembly of larger packets port number
addressing,
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user-to-user reliability in addition to protocol
acknowledgement, asynchronous transactions, optimal out-of-band
information, delayed acknowledgements, and message concatenation to
improve over-the-air efficiency. WTP is massage oriented protocol,
which makes it suitable for interactive browsing applications.
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Wireless Session Protocol (WSP) The WSP layer is a stripped
down version of the Internet standard, Hyper Text transfer Protocol
(HTTP / 1.1). One of the important features of this protocol is to
support the suspension and resumption of a session. In an unstable
connection situation that is prevalent in the mobile environment,
users who may be disconnected can continue the operation from
exactly the same point where the device had been disconnected.
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Context encoding, for efficiently transferring the contents in
a low bandwidth environment, is also addressed by the layer. the
following functionalities are offered and addressed by this layer:
protocol feature negotiation (capability negotiation) compact
encoding of data session suspend / resume long lived session states
asynchronous requests common facility for confirmed data push.