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E1E1--E2 CME2 CM
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OverviewOverview ofof GSMGSM
22/3/2011
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AGENDA
Cellular Concept
Reuse of FrequencyTypes of cells
GSM Architecture
GSM NW elements
GSM Areas
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Cellular Systems
A cell corresponds to the covering area of one
transmitter or a small collection of transmitters.
The size of a cell is determined by the transmitter's
power.
transmitters in order to enable the efficient reuse of the
frequencies.
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Reuse of Frequency
Frequencies used in a cell will be reused several cells
away.
The distance between the cells using the same
frequency must be sufficient to avoid interference.
capacity in number of users.
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Cluster
Group of cells.
The number of cells in a cluster must be determined so
that the cluster can be repeated continuously within the
covering area of an operator.
, , .
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A cluster is a group of cells
No channels are reused within a cluster
Clusters
A seven Cell Cluster
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Deciding the cluster size
The smaller the number of cells per cluster is, the bigger
the number of channels per cell will be. The capacity of
each cell will be therefore increased
However a balance must be found in order to avoid the
clusters when small size clusters are opted.
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Types of Cells
Macro cells :-large cells for remote and sparsely
populated areas
Micro cells:- for densely populated areas
Selective cells:-cells with a particular shape and
covera e for e . Cells located at the entrances of
tunnels where coverage of 360 degrees is not
needed
Umbrella cells:-covers several micro cells; reduce the
number of handovers
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Different Types of Cells
MACRO-CELL:antenna radiating above roofs
---> Wide Coverage ( 35 km)
EXTENDED-CELL:
macro cell with system coverage
extension ( 120 km) for coasts...
MICRO-CELL:
Antenna below the roofs
---> small coverage
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Architecture of GSM NW
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Mobile Station
Mobile Station = Terminal + SIM
Terminal can be fixed (20W), portable (8W), Handheld (
2W, 0.8W)
The only element that personalizes a terminal is the SIM
-
Identification Number (PIN)
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Functions of SIM
Authentication of the validity of the MS when
accessing the networkUser authentication
Storage of subscriber-related information, which can
be: data fixed during administrative phase (e.g.,subscriber identification), and temporary network
data (e.g., cell location identity).
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Base station Subsystem-BTS
BTS (Base Transceiver Station) is usually placed in thecenter of a cell
Corresponds to the transceivers and antennas used in
each cellHas between 1 and 16 transceivers depending on the
density of users in the cell.
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Base station Subsystem-BSC
BSC (Base Station Controller) controls a group of BTS
and manages their radio resources
Is in charge of handovers, frequency hopping, exchange
functions and control of the radio frequency power levels
.
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Network and Switching Subsystem
The Mobile services Switching Center (MSC)
The Gateway Mobile services Switching Center
(GMSC)Home Location Register (HLR)
Visitor Location Register (VLR)
The Authentication Center (AuC)The Equipment Identity Register (EIR)
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Functions of MSC
Call handling (w.r.t.mobile nature e.g., paging)
Management of logical radio-link channel during calls
Management of MSC-BSS signalling protocol
Handling location registration and ensuring interworking
between Mobile Station and VLR
- -
Acting as a gateway MSC to interrogate the HLR
Exchange of signalling with other system entities
Standard functions of a local exchange switch in the fixed
network (example: charging)
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HLR contains
Identity of mobile subscriber
ISDN directory number of mobile station
Subscription information on teleservices and bearer
services
Supplementary services
Location information for call routing
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VLR contains
Identity of mobile subscriber
Any temporary mobile subscriber identity
ISDN directory number of mobile
A directory number to route calls to a roaming station
Copy of (part of) the subscriber data from the HLR
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EIR Lists
White or Valid list - List of valid MS equipment identities
Grey or Monitored list - List of suspected mobiles under
observation
Black or prohibited list - List of mobiles for which service
.
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Authentication Centre (AuC)
Contains subscriber authentication data called
Authentication Keys (Ki)
Generates security related parameters needed toauthorize service using Ki
Generates unique data pattern called a Cipher Key
(Kc) needed for encrypting user speech and data
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Other NW elements
The Operations and Maintenance Center (OMC)-R is
the centralized maintenance and diagnostic heart of the
BSS. It allows the network provider to operate, administer,
and monitor the functioning of the BSS.
Billing Centre: periodically polls the disk records of each
.
Service Center: The Service Center interfaces with the
MSCs to provide special services, such as the SMS
Note:The Billing Center and Service Center are not a
basic part of the GSM system.
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GSM AREAS
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Frequency Bands
The band 890-915 MHz has been allocated for the uplink
direction (transmitting from the mobile station to the
base station).
The band 935-960 MHz has been allocated for the
the mobile station).
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Discontinuous Transmission
Works on VAD (Voice Activity Detection)
DTX helps to reduce interference between different cells
and to increase the capacity of the system.
It also extends the life of a mobile's battery.
connection is dead.
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From receiverEarpiece
Comfortnoise
speech
decoder
Digital toAnalogConverter
Discontinuous Transmission
Features
To transmitterMicrophone speech
encoder
Voice Activity
Detection
Analog toDigitalConverter
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Timing Advance
The aim of the TA is that the signals coming from
different mobiles arrive to the base station at the right
time.The base station measures the timing delay of the
mobile stations.
If the bursts corresponding to a mobile station arrivetoo late and overlap with other bursts, the base
station tells, this mobile, to advance the transmission
of its bursts.
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Power Control
Power levels are adjusted so that the power is nearly the
same for each burst.
A base station also controls its power level.
The mobile station measures the strength and the
.
If the mobile station does not receive correctly the signal,
the base station changes its power level.
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Power Control
BTS commands MS at different
distances to use different power levels
so that the power arriving at the BTSs Rx is
approximately the same for each TS
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Discontinuous Reception
It is a method used to conserve the mobile station's
power.
The paging channel is divided into sub channels
corresponding to single mobile stations.
' '
in the sleep mode during the other sub channels of the
paging channel.
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Equalization
An equalizer estimates the channel impulse response
of the GSM system and then constructs an inversefilter.
The receiver knows which training sequence it must
wait for. The equalizer will then, comparing the
received training sequence with the training
sequence it was expecting, compute the coefficients
of the channel impulse response.
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