Global System of Mobile Communication (2G)
In 1982, the Conference of European Posts and
Telegraphs (CEPT) nominated a group called the Groupe
Spécial Mobile (GSM) to develop a public land mobile
system that could operate across Europe with the
objectives of:
•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
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:
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.
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).
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.
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).
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
management.
functions,
The
it
information
also
on
handles
the
mobility
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.
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.
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.
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.
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.
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.
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.
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.
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.
•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.
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.
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.
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.
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)
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.
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.
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.
•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.
•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.
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.
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.
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.
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 operator’s 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.
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
The Wireless Access protocol operates over a variety of
wireless bearer mechanisms, such as GSM’s 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:
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).
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.
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.
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.
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:
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.
On receiving the result of the request the
acknowledgement is sent. Reliable service at is
accomplished
retransmission
this
and
layer
by
selective
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,
user-to-user reliability in addition to protocol
acknowledgement, asynchronous transactions,
optimal
out-of-band
acknowledgements,
concatenation
to
information,
delayed
and
message
improve
over-the-air
efficiency. WTP is massage oriented protocol,
which
makes
it
suitable
browsing applications.
for
interactive
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.
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.
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