ABSTRACT
Since the evolution of man he has
always tried learn and gain as much as possible for himself and his community,
to relate him self to the various cultures castes and creeds of mankind. This
Quest for realizing and exploring has lead to inventive thinking and a constant
probation and to relate to each other throughout the world they have fore bared
the gap of communication to expunge one such a minuscule but major hurdle a
common Global System for communications was founded communication on the move
with a bit to bit elaboration was analyzed and the forth presentation bears the
facts of a Global System For Mobile communication and on which basis it is
employed along with the assets & environment .
Ø 1st Generation cellular mobiles
phones used analog channels.
Ø 2nd Generation cellular mobiles
use digital channels.
1st generation
analog cellular mobile communication contained of analog channels, each cell
has bunch of different freq channels & each costumer is given a particular
frequency channel.
2nd generation
digital cellular mobile communications constituted of digital channels, a
number of costumers share a particular frequency channel dynamically through
TDMA.
ü Basic
Cell Structure
v Be it digital or cellular, the regions are
always divided into hexagons and the MU is connected to BS as usual.
ü GLOBAL
SYSTEM FOR MOBILE COMMUNICATION (GSM);
Ø 2nd Generation constitutes of
GSM.
Ø GSM is not a software but a standardization
group.
It was established in 1982 to create common
European mobile standard.
Cellular industry is the most rapidly
increasing industry currently there are millions of cellular subscribers.
ü What does G
S M work on ?
Ø GSM works on TDMA (Time Division Multiple Access).
Ø All stations transmit data on same
frequency but their transmissions are separated in time.
The
TDMA system is designed for use in a range of environments and situations, from
hand portable use in a downtown office to a mobile user traveling at high speed
on the freeway
Ø Time Division Multiple Access
Ø TDMA
has 2M channels of bandwidth say x
hertz.
Ø Reverse channel->f, f+x, f+2x,……f
+(M-1)x
Ø Forward channel->f, f+x, f+2x,……f +(M-1)x
+ x
Ø x = Bandwidth
Ø f = center frequency of lowest frequency
channel.
Ø When connection is setup it is full duplex
and the user has two frequencies allotted to him.
Ø f , f +x.
Half of the frequency is for forward
channel and half of it is for reverse channel.
Forward channel: The channel traversing
from the base station to mobile unit.
Reverse channel: The channel traversing
from the mobile unit to base station.
Hence we can say that the physical channel
is subdivided into number of sub channels.
ü Basic operations
Ø To send voice traffic on GSM (TDMA) mobile
we need to,
Ø Perform analog to digital conversion,
Ø Error detection and correction,
Ø Channel access has to be done properly,
Eg :- PCM , delta modulation.
Since in TDMA, data is sent in the form of
frames.
For proper access a memory location holds
the information about the time slots allotted to each user.
ü TDMA
design considerations.
Ø Number of logical channels.
Ø Maximum cell radius.
Ø Frequency.
Ø Maximum vehicle speed ( Vm ).
Ø Maximum coding delay.
Ø Maximum delay spread ( Xm ).
Ø Bandwidth.
Usually 835km, 900Mhz,Vm==250km/hr, 20 ms to avoid
adding unduly delays Xm==10;this is the difference in propagation delay among
different multi path signals arriving at the same antenna
Bandwidth:
Not to exceed 200khz,corresponding to 25khz per
channel.
ü Advantages
of T D M A.
Ø Easily adapted to voice and data traffic.
Ø Has a data rate
of expandable multiples of 64kbps (up to120Mbps).
Ø Enables various
services including multimedia and video conferencing.
Ø No interference.
Ø Extended battery
life and talk time.
Ø TDMA is the only
technology that offers an efficient utilization of hierarchical cell structures
( HCS’s ) offering Pico, micro, and
Macro cells .
Þ Various services like fax, voice band data,
short message service (SMS).
Þ Extended battery life since the mobile is
only transmitting a portion of the time (from 1/3 to 1/10) of the time during
conversations.
Þ HCSs allow coverage for the system to be
tailored to support specific traffic and service needs. By using this approach,
system capacities of more than 40-times AMPS can be achieved in a
cost-efficient way.
Ø TDMA allows service compatibility with the
use of dual mode handsets.
Ø TDMA has increased resistance to multi path
distortion.
Ø TDMA
allows handoff to/from an analog AMPS channel.
Ø The most widely deployed digital technology
at the present time is TDMA.
ü TDMA frame format
ü GSM
network
Ø GSM provides recommendations, not
requirements.
Ø The GSM specifications define the functions
and interface requirements in detail but do not address the hardware.
Þ TDMA functions (that is; to provide
connection between two remote MU) is the basic function of GSM and to achieve
that we need physical and mechanical support proper routing, control and
processing of data.
Þ Not specifying hardware makes it easier on
the designer so that he can buy his components according to his choice.
ü GSM
network elements.
Ø The GSM network is divided into three major
systems:
1) The switching system (SS).
2) The base station system (BSS).
3) The operation and support system (OSS).
·
GSM
inter working unit (GIWU)
1) Gateway mobile services switching center
(GMSC)—
2) Authentication center (AUC)—
3) Home location register (HLR)—
4) Visitor location register (VLR); mobile services switching center (MSC)—
5) Equipment identity register (EIR)—
6) Base
Station Controllers (BSCs) and the base transceiver stations (BTSs).Message center (MXE)
Ø The switching system (SS) :
Ø It is responsible for performing call
processing and subscriber-related functions.
Ø It constitutes of
Ø Home location Register (HLR):
Þ The HLR is a database used for storage and
management of subscriptions. The HLR is considered the most important database,
as it stores permanent data about subscribers, including a subscriber's service
profile, location information, and activity status. When an individual buys a
subscription from one of the PCS operators, he or she is registered in the HLR
of that operator.
Ø Mobile
Services Switching Center (MSC):
Þ The MSC performs the telephony switching
functions of the system. It controls calls to and from other telephone and data
systems. It also performs such functions as toll ticketing, network
interfacing, common channel signaling, and others.
Ø VLR
Visitor Location Register:
Þ The VLR is a database that contains
temporary information about subscribers that is needed by the MSC in order to
service visiting subscribers. The VLR is always integrated with the MSC. When a
mobile station roams into a new MSC area, the VLR connected to that MSC will request
data about the mobile station from the HLR. Later, if the mobile station makes
a call, the VLR will have the information needed for call setup without having
to interrogate the HLR each time.
Ø All radio-related functions are performed
in the BSS (Base Station Service) it consists of
Ø Authentication
center (AUC):
A unit called the AUC provides
authentication and encryption parameters that verify the user's identity and
ensure the confidentiality of each call. The AUC protects network operators
from different types of fraud found in today's cellular world.
Ø Equipment Identity Register (EIR):
The EIR is a database that contains
information about the identity of mobile equipment that prevents calls from
stolen, unauthorized, or defective mobile stations. The AUC and EIR are
implemented as stand-alone nodes or as a combined AUC/EIR node.
Ø Base Station Controllers-BSC
The BSC provides all the control functions
and physical links between the MSC and BTS. It is a high-capacity switch that
provides functions such as handover, cell configuration data, and control of
radio frequency (RF) power levels in base transceiver stations. A number of
BSCs are served by an MSC.
Ø Base Transceiver Stations BTS:
The BTS handles the radio interface to the
mobile station. The BTS is the radio equipment (transceivers and antennas)
needed to service each cell in the network. A group of BTSs are controlled by a
BSC.
Ø The Operation and Support System-OSS
Ø The implementation of OMC is called the
operation and support system (OSS).
Ø ADDITIONAL FUNCTION ELEMENTS:
Ø MXE: Message center
Ø MSN:
Mobile service node
Ø GMSC: Gateway mobile services switching center
Ø GIWU: GSM inter working unit
The operations and maintenance center (OMC)
is connected to all equipment in the switching system and to the BSC. The
implementation of OMC is called the operation and support system (OSS). The OSS
is the functional entity from which the network operator monitors and controls
the system. The purpose of OSS is to offer the customer cost-effective support
for centralized, regional, and local operational and maintenance activities
that are required for a GSM network. An important function of OSS is to provide
a network overview and support the maintenance activities of different
operation and maintenance organizations.
ü Advantages
and disadvantages.
Ø GSM carries all the advantages and
disadvantages of TDMA.
Ø The most prominent advantage can be stated
as the application of TDMA through proper protocols and standards thus making
it feasible.
Ø Only disadvantage is that vacant time slots
are encountered whenever a particular MU is idle but this we can overcome using
synchronous TDMA.
ü Personal communication services (PCS) for
GSM
Frequency band: The frequency range specified for GSM is 1,850 to
1,990 MHz (mobile station to base station).
Þ Duplex distance—The duplex distance is 80 MHz. Duplex distance is the
distance between the uplink and downlink frequencies channel separation .The
separation between adjacent carrier frequencies. In GSM, this is 200 KHz .
A channel has
two frequencies, 80 MHz apart.
Þ Modulation: Gaussian minimum shift keying (GMSK).
Þ Transmission rate— 270 kbps.
Þ Access method— TDMA
Speech coder—GSM uses linear predictive coding (LPC). The purpose
of LPC is to reduce the bit rate. The LPC
Modulation—Modulation is the process of sending a signal by
changing the characteristics of a carrier frequency. This is done in GSM via
Gaussian minimum shift keying (GMSK).
Transmission rate—GSM is a digital system with an over-the-air bit rate
of 270 kbps.
Access method—GSM utilizes the time division multiple access (TDMA)
concept. TDMA is a technique in which several different calls may share the
same carrier. Each call is assigned a particular time slot.
Speech coder—GSM uses linear predictive coding (LPC). The purpose
of LPC is to reduce the bit rate. The LPC provides parameters for a filter that
mimics the vocal tract. The signal passes through this filter, leaving behind a
residual signal. Speech is encoded at 13 kbps.
ü Facts
about GSM
Ø Right now, GSM is the accepted cellular
standard in Europe; South America; North America; Southeast Asia etc.
Ø Scandinavia first deployed cellular services for commercial use.
Ø Call waiting is not supported by GSM.
conclusion:
This paper resolute the
evolution of inventive thinking along with human requirements to meet the
challenges of urban global communications.
The major disadvantage of
TDMA is the non-availability of the call-waiting feature. Which can be overcome
by Asynchronous TDMA.
The organization
of asynchronous TDMA is done through artificial intelligent Single input Memory
Modules commonly known as SIMM.
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