Download - Data communications
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Data two types
Analog Form of sinusoidal wave pattern…
changing states Digital
Form of ON/OFF pattern… pulses
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Communication
Analog data transmission States changes (follows SIN wave) Maintenance cost is low But.. Cost of transmission is high Effect of environment is very high Amplifier circuit loses data
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Digital data transmission Follow ON/OFF pattern Mainainance cost is high Transmission cost is low Environment factor… low No loss while using amplifier/reapeter
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Channel Characteristic
Ideal channel Should convey the maximum from
sender to receiver Should not ALTER…(additional noise) No distance restriction Convey cost should be maintain
Type Analog digital
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Close look to the digital channel
Channel carries BITS Measurement… how many BIT/ second
called bit rate (bps) The bps is the rate at which the channel
can carry BITS (digital data) Distribution of bits determine bit rate
shorter the duration greater bps
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TRANSMISSION MODES
Simplex
Only one way communication … unidirectional flow
Interactive part is absent… so no ackw Examples… keyboard/printer.. Radio, TV Cheapest… but low efficient
Sender receiverDATA
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Half duplex
Both communication thru same medium
Only one is active at a time … no one at the same time
Either send or receive at a time Examples HDD, RAM
Sender/Receiver
ReceiverSender
DATA
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Full Duplex
Simultaneous transmission in both direction
Full interactive communication Examples… telephone/mobile costly
SenderReceiver
ReceiverSender
Data
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Asynchronous Mode Transmission
Referred as ON/OFF (Start/stop) transmission
Transmission takes place character by character
Character sequence
Irregular time interval
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Each channel is started by ‘start’ bit and ended by ‘stop’ bit
Channel remain unused between the two character… hence at each character start and stop bit is required to notify the receiver
Summery :- Data is transmitted character by character at irregular time interval
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Synchronous Mode Transmission
Character are grouped as a block Series of such blocks are
transmitted Each block is started by HEADER
and ended with TRAILER information and each block may contain hundreds of characters
Indefinite time interval between blocks
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Summary Synch
Entire blocks of characters are framed and transmitted
Expensive Efficient Need for BUFFER and accurate synch and
is required.
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Asynchronous Data is transmitted character by
character. Less costly… but not efficient No buffer is required… but channel will
remain unused
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Type’s Of Media
Guided media Signals are guided thru solid media
(like copper wire) Thru cables
Unguided media Signals are not guided… not thru solid
medium (use of air) Usage of radio waves
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Guided Media
Twisted pair cable Two insulated wires (1mm thickness)
With each other Less expensive FD transmission Can be used for analog and digital Flow efficient is carries the signal
depends on thickness and distance Very efficient for short distance (less
then 100meters)
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More noise friendly Normally Used in LAN If more twisted per centimeter… results
less noise effect and better will be the quality
Easy to maintain If used less then 100 meters can give
up to 9600bps
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CO-AXIAL CABLE Better shielding … higher data bps
@longer distances …several tens of bps at distances up to thousands feet
Used for analog (75 ohm cable) and digital (50 ohm cable) communication
Costlier then twisted pair
Copper wire
Insulating material
Wire mesh conductor
Protective plasticcovering
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OPTICAL FIBER Inner core… glass/plastic… conducts
the light…size is in microns(1/25000 inch)
Cladding … reflects the light Most expensive Data rate up to 100 mbps to 2Gbps No effect of EM noise Mainly for digital Half duplex (FD cause interference)
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Electrical signalTo
Light converter
Light signal ToElectrical signal
Converter Fiber optic
Fiber corer
cladding
jacket
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UNGUIDED MEDIA Radio wave can travel ideally with
the speed of light (in vacuum) –cover long distance
RF are omni directional RF is subjected to interference at
any frequency
Hz
RadioMicrowave infrared
Vis
ibl
e lig
ht
UV X Ray4 5 6 7 8 9 10 11 16
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TERRESTRIAL MICROWAVE TRAN. 4 TO 6 GHz and 21 to 23 GHz Cheaper then fiber optic 1 to 4 mbps travel in straight line
(hence line of sight is required) Cost depend on distance Long distance telephone, cellular, TV,
link to cities etc..
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SATELLITE MICROWAVE TRANS. One antenna is on a satellite 4 -6 GHz and 11-14GHz Use of satellite- cost Normally uplink is 6 KHz and downlink
is 4 KHz Earth based station required careful
adjustment Can reach most remotes places on
earth
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INFRAERED Used for short distance communication Do not pass thru solid object Generally cheaper Used for wireless LAN, remote controls
etc.. licensing is not required.
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MODEMS
Types A) Landline :- connected to PSTN…
having jacks RJ11 Internal – inside computer External –separate device… outside the
computer… connected to serial port PCMCIA – small size normally used for
laptops. Personal computer memory card
(designed by) international associatioan.
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B) Wireless Radio transmitters/receiver generally used
for mobile device. RJ11 is not there instead they can access
thru radio waves. If it is out of range – no use.
C) LAN Allow shared remote access to LAN.
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Standards Bell Modem :- designed by bell lab. There are
103/113 series,202 series, 212 series, 201 series, and 208/9 series.
ITU- T modem- V.22,V.26,V.29 V.26 from 1200-2400 bps… user phase shift
keying. V.22 bit- 600 baud line… during each signal
period (baud) the modem conveys 4 data bit 600*4= 2400 bps.
V.29 operating at 2400 baud *4 =9600 bps.
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Encoding Techniques
Analog data to Analog signal Digital data to Digital signal Digital data to Analog signal Analog data to Digital signal
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Analog data to Analog signal
Types of modulation 1) Amplitude
Carrier
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Modulated
Amplitude Modulation
Double side band Transmittedcarrier
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Modulation To modulate to mix the signal with the carrier. Process of encoding signals (information) for the
transmission Translate the source signal, base band to a
band pass signal (high frequency compared to the source frequency).
Source signal – MODULATING signal. Band pass signal – MODULATED signal. MODULATION is done by varying the amplitude
or frequency of high frequency carrier according to the modulating signal.
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Amplitude Modulation Amplitude of high frequency carrier signal
is varied accordance to the instantaneous amplitude of the modulating signal.
Easy Environment friendly. Strength decrease with distance.
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Frequency Modulation FM signal constant MODULATED but
frequency VARIES IN ACCORDING TO THE SIGNAL to be transmitted.
Mixing of two frequency high frequency (carrier) with the signal (low frequency) compound frequency varying according to signal.
Least affected by noise. Requires high bandwidth than AM.
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Frequency Modulation carrier
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Phase Modulation The shape of the carrier signal (phase) is
made to change at given pint of time. The difference between two sine signals is a
phase angle… normally 180 out of phase. USES:- medium speed modems use phase
modulation to convert digital signals into phase modulated signals. This process of phase shifting keying (PSK) allows modem to modulate and demodulate
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Phase Modulation carrier
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Digital data to Digital signal
Digital data to digital signal conversion.
Equipment less complex and expensive then digital data to analog modulation equipment.
One logic state represented by positive the other by negative voltage
Data rate Rate of data transmission in bps.
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Schemes of D to D 1)Non return to zero –level (NRZ-L)
To different voltage for 0 & 1 bits Voltage constant during bit interval No transmission i.e. return to 0 voltage E.g. absence of voltage for 0, constant
positive for 1 More often negative voltage for 1 value
and positive for the other
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0 1 0 0 1 1 0 0 0 1
NRZ-L
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2) non return to 0 interval Constant voltage pulse for duration of
bit Data encoded else presence or absence
of signal transmission at beginning of bit time
Transition (low to high or high to low) denotes binary 1
No transition denotes binary 0
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0 1 0 0 1 1 0 0 0 1
NRZ-I
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3)Manchester Transition in middle of each bit period Transition serves as clock and time Low to high represent 1 High to low represent 0
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0 1 0 0 1 1 0 0 0 1
Manchester
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4)Defrential Manchester Mid bit transition is clocking only Transition at start of a bit period
represent 0 No transition at start of a bit period
represent 1
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0 1 0 0 1 1 0 0 0 1
Differential Manchester
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Digital Data to Analog Signal
Public telephone system 300 Hz to 3400 Hz.
Amplitude Shift Keying (ASK). Frequency Shift Keying (FSK). Phase Shift Keying (PSK).
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ASK (Amplitude Shift Keying) Values represented different amplitude
of carries. Usually one amplitude as 0. i.e. presence or absence of carrier is
used. Susceptible to sudden gain changes. Inefficient. Up to 1200 bps on voice grade line. Used over optical fiber.
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0 0 1 1 0 1 0 0 0 1 0
ASK
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Binary Frequency Shift Keying Most common form is Binary FSK. Two binary values represented by 2
different frequencies (near carrier). Less susceptible to error than ASK. Up to 1200 bps on voice grade lines. High frequency radio . Even higher frequency on LANs using
co-ax.
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0 0 1 1 0 1 0 0 0 1 0
BFSK
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Binary phase shift keying (BPSK) Phase of carrier signal is shifted to
represent data. Binary PSK
Two phases represent two binary digits. Differential PSK
Phase shifted relative to previous transmission rather than some reference signal.
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0 0 1 1 0 1 0 0 0 1 0
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Analog Data To Digital Signal
Digitization Conversion of analog data in to digital
data. Digital data can then be transmitted
using NRZ-L. Digital data can then be converted to
analog signal. Analog to digital conversion done using a
codec. Pulse code modulation conversion of
analog data in to digital data.
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Digitizer Modulator
Analog data (voice) Digital Data
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Pulse code modulation It’s a digitizing process in which analog
is represented in digital form. The sound are transformed in to pulse
by codec…sampling of the amplitude of the analog signals at very short interval of time… the sampled valued converted in to digital number of 0’s and 1’s… and finally it is transmitted.
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At the receiving, the original A/D is reversed… voltage values are converted read and production of the exact signal will be achieved.
If a signal is sampled at regular interval at a rate higher than twice the highest signal frequency, the samples contain all the information of the original signal.
Voice data limited to below 4000Hz. Required 8000 samples per second. Each sample assigned digital value.
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CODEC (Compressor/DECompressor) Its an electronic circuit that convert analog
to digital. Converts human voice in to digital code
using pulse code modulation. The resulting digital signal can travel
through all digital communication equipment… provides more reliable and less costly compared to analog.
Its also converting back to voice. CODEC electronics used in digital phone.
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Multiplexing
Multiplexing is a set of techniques that allows the simultaneous transmission of multiple signal across a single data link.
Whenever the transmission capacity of a medium linking two devices is greater then the transmission needs of the devices, the link can be shared in order to maintained the utilization of the link, much at one cable can carry a hundreds of TV channel.
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MUX
FDM TDM
Synch Asynch
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Frequency division mux (FDM) In FDM signal generated by each sending
device modulated different carrier frequencies. These modulated signals are then combined in to a single composite signal that can be transported by the link. The carrier frequencies have to be different enough to accommodate the modulation and demodulation signals.
(refer fig.) The first PC terminal is sending “1010” where as second terminal is sending “0110”. The multiplexing process starts by applying amplitude modulation in to each signal by using different carrier frequencies as f1 and f2
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FDM mux process
1 0 1 0
Amplitude Modulation
WithCarrier f1
Amplitude Modulation
WithCarrier f2
0 1 1 0
Modulated signal +
Signal connected
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In demux process, we use filters to decompose. The multiple signal in to its constitute signals. Then each signal is passed to a amplitude demodulation process to separate the carrier signal from the message signal. Then the message signal is sent to the waiting receiver.
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Filter
Filter
Bandwidth f1
Bandwidth f2
Amplitude
Amplitude
1 0 1 0
0 1 1 0
Signal withCarrier f1
Signal withCarrier f2
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Time Division mux (TDM) In the TDM multiple transmission can
occupy a single link by subdividing them and interleaving the portion. We say that TDM is a round robin use of a frequency.
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Synch TDM The mux allocate exactly the same time
slot each device at all times, whether or not a device has any thing to transmit. Time slot 1 ,for example is assigned to device 1 alone and can not be used by any other device.
FRAME: In synch TDM, a frame consist of one complete cycle of time slots. Thus the number of slots in frame is equal to the number of inputs.
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MUX
4 3 2 1 4 3 2 1 4 3 2 1 4 3 2 1
1
2
3
4
Synch TDM: mux process
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AAAA
BB
C
DDD
MUX
A D A D B A D C B A
1
2
3
4
Synch TDM: mux process
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AAAA
BB
C
DDD
DEMUX
A D A D B A D C B A
Synch TDM: demux process
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Asynch TDM In asynch TDM each slot in a frame is not
dedicated to the fix device. Each slot contain an index of the device to be sent to and a message. Thus the number of slots in a frame is not necessary to be equal to the number of inputs devices. More than one slots in a frame can be allocated for an input device. Asynch TDM allows maximization the link. It allows a number of lower speed input lines to be multiplexed to a single higher speed line.
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AAAA
BB
C
DDD
1
MUX
A D A D B A D C B A
Synch TDM: mux process
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FRAME: In asynch TDM, a frame contain a fix number of time slots. Each slot has an index of which device to receive.
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MULTIPLE ACCESS TECHNOLOGIES FOR WIRELESS
COMMUNICATION
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COMMUNICATION : Fixed BAND of Frequency Spectrum.
Multiple Access Methods - WHY ? SHARE THE FREQUENCY SPECTRUM. Differentiates the signals from different
sources , without degrading the Quality. Different techniques of SHARING …
called Multiple Access Methods / Techniques / Schemes / Technologies.
MULTIPLE ACCESS TECHNOLOGIES FOR
WIRELESS COMMUNICATION
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MULTIPLE ACCESS TECHNOLOGIES FOR
WIRELESS COMMUNICATION
THREE Basic Multiple Access Methods currently in use :- FDMA
FREQUENCY DIVISION MULTIPLE ACCESS
TDMA
TIME DIVISION MULTIPLE ACCESS
CDMA
CODE DIVISION MULTIPLE ACCESS
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FDMA (Frequency Division Multiple Access)
Users SHARE the available spectrum in the FREQUENCY domain.
Assigns the individual CHANNEL ( Unique Frequency) to users - Allocated band is called TRAFFIC CHANNEL. Hence .. Different Users …..Different Traffic Channels.
FrequencyF1 F2 F3 F5F4
Cha
nnel
1
Use
r A
Cha
nnel
2
U
ser
B
Cha
nnel
3
U
ser
C
Cha
nnel
4
Use
r D
Cha
nnel
5
Use
r E
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FDMA (Frequency Division Multiple Access)
If User A is in USE .. Channel 1 will not be allotted to others. Disadvantage -> When Channel is not in Use … can not be used by others .. Wastage of Resource.
Each Channel has Very LOW Bandwidth …. Hence Implemented normally in Narrow band Systems.
Requires TIGHT filtering to reduce the Channel Interference .
Channel ID = Frequency Slot ID.
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TDMA (Time Division Multiple Access)
Spectrum is divided in narrow frequency bands (Like FDMA) and further divided into a number of time slots.
Each User is allotted a Time Slot that permit access to the frequency channel for that duration of the time slot.
Frequency
Time
User A
User B
User C
User D
User E
User F
User G
User H
User I
T1
T2
T3
F1 F2 F3
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TDMA (Time Division Multiple Access)
Traffic Channel ID = Frequency Slot ID + Time Slot ID
Periodic train of time slots … make a FRAME. Each User shares a frequency with several users. Transmission for any user is non continuous. Allocation of different numbers of Time Slots per
frame to users … Better Utilization of Spectrum… Analog Systems used FDMA .. Digital Systems used
TDMA.
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Spread Spectrum Multiple Access
PN Code - pseudo-noise code … random binary Sequence / Code.
SSMA - a) Frequency Hopped Multiple Access (FHMA) & b) Direct Sequence Multiple Access(DSMA). FHMA :- Carrier Frequencies of individual user are
VARIED in a pseudo random way. Based on the PN code of the user .. Each user occupy
the narrow band channel at one particular time. Because of the PN … Signals changes channels
rapidly. Difference between FHMA & FDMA is that the FHMA
signal changes channels at rapid interval.
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CDMA (Code Division Multiple Access)
DSMA is also called CDMA. Unique PN code is assigned to unique user. Users share the Block of frequency spectrum on the
basis of PN code.
Frequency
PN codes
User A
User B
User C
Code 1
Code 2
Code 3
F1
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CDMA (Code Division Multiple Access)
Channel ID = PN Code ID Utilizes the entire spectrum of allotted spectrum -
All the PN code modulated signals from the users are transmitted over the entire spectrum. And at the receiving end the signals classified as per the copy of PN sequence .
Unlike FDMA - TDMA … There is no LIMIT of number of users … but increase in users degrades the quality.
Each user operate independently with NO knowledge of other users.
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CORDLESS TELEPHONE SYSTEMS
Cordless Telephone System - provide the user limited range and mobility. Coverage rang is few Tens of Meters to Few hundred Meters.
Public Telephone Network .. (DoT)
Fixed Station -
Base StationHandset
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CELLULAR TELEPHONE SYSTEMS
The concept was developed in early 70’s by Bell Laboratories
Extension of your wireless connection to the public telephone network for any user location within the range of the system.
The principle of cellular system…To divide a large geographic area into cells.
Each adjacent Cell Transmitters operate on different frequencies to avoid interference.
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CELLULAR TELEPHONE SYSTEMS
Transmitted power and height of antenna of each CELL is low so that the same set of frequency can be used for different cells far apart.
Hence theoretical coverage range and capacity of a cellular system are therefore UNLIMITED.
Each Cell is represented by HEXAGONE.
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A cellular System –An Overview.
Basic Cellular System - Mobile Stations+Base Stations+Mobile Switching Center.
Public Telephone Network
Mobile Switching
Center MSCTo Other MSC
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A cellular System - An Overview.
Mobile Station - Contains a transceiver+ antenna+Control Circuitry
Base Station - Bridge between MS and MSC MSC - Coordinated the activities of all BS and
connect them to PTN. Plus Billing & System Maintenance.
The Channel used for VOICE transmission from BS to MS …called Forward Voice Channel (FVC).
The Channel used for VOICE transmission from MS to BS …called Reverse Voice Channel (RVC).
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How it WORKS ?
When MS is turned ON … Searches for the strongest FVC.
When a Call is made for MS.. MSC dispatches the request to all BS.
The Mobile Number is broadcasted as a paging message.
MS acknowledges the Paging message. BS relays this ACK to MSC . MSC instructs the BS to select particular
frequency Channel for communication .
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How it WORKS ?
BS TO RING sends DATA message in FVC TO RING the MS.
During CALL .. MSC handles the transmitted power and controls the channel between BS and MS in order to maintain the Quality ( as MS is likely in MOBILE mode)
When MS goes out of range of BS … called HANDOFF …
Two other Channels are also Used besides FVC & RVC … a) FCC & b) RCC
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How it WORKS ?
When MS Originates Call… sends all information to BS.
BS passes information to MSC MSC Validates .. And If required help from Public
Telephone Network requested. And the two way PATH will be maintained till
the Call Lasts.
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Handoff
During the ongoing call if BS senses the LOW power Quality from MS , it requests neighboring BS to check the signal level . ( This happens when MS moves to different cell while in USE)
If the signal is BETTER , current BS signals the MS to switch over to new BS and inform the new BS to take over.
This change of SPEECH channel is called “Handoff”
This changeover will not be noticed / experienced by the user.
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Frequency REUSE
BS in adjacent cells are assigned channel groups … totally different from the neighboring cells.
BS antennas are designed to cover the particular cell.
CF
GD
CE
F
GDAA
BB
E
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Wireless Systems Standards
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1G Cellular Systems
Based on Analog Cellular Systems Concept
Depends on Frequency Band , Channel Spacing and channel coding
Individual calls use different channels and the Spectrum is shared on the basis of FDMA
Uses Analog FM for speech transmission Normally uses 7 Cell reuse pattern –
provision for Cell splitting.
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2G Cellular System
Completely DIGITAL Cellular System Increased in Capacity ( 3 to 10 times) MS Terminal Size Reduction Reduces the Power requirements … Increases
the battery life Improved Reception Highly Secured … Interference prone
environment. Cell Splitting … Better Wide Area Roaming More Popular
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2G Cellular System
Spectrum Sharing in the digital environment can be based on ; TDMA : Each Radio Channel is partitioned in to
number of time slots - each user is assigned a frequency/time slot COMBINATION
CDMA : A radio Channel is used SIMULTANEOUSLY by multiple mobile users , and the signals from different users are distinguished by SPREADING them on the basis of PN code.
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Global System for Mobile(GSM)
Introduced in Europe in 1990. World’s most popular standard now. A memory device that stores the
subscriber Id , Networks, Countries where he’s entitled to get services , personal information is inserted into GSM phones .(Subscriber Identity Module - SIM). Example TOI dated 18th March.
Without SIM – non operational. Encryption is possible … More secured
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GSM Architecture
BTSBSC
PSTN
Other MSCMSCHLR
VLR
AUC
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GSM Architecture MS (Mobile Station)
• Low Power Requirement…0.8-8.0w• SIM is Required
BSS (Base Station System)• BSC+BTS (Base Transceiver Station)• Responsible For Radio Channel Allocation/
Monitoring (BSC)• Power Control (BSC)• Handoff Management (BSC) – Reduce The Burden
of MSC• Digital Signal Processing (BTS)
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GSM – System Architecture
MSC Doesn’t contain Info regarding MS . Call Setup, Supervision & End / Routing BILLING MOBILITY Management Management with Other MSCs , PSTN .
Home Location Register – HLR Centralized Database of MS falling under MSC Refer for every Incoming Call
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GSM – System Architecture
VLR – Visitor Location Register Temporarily stores the MS … Each roaming MS
visiting MSC. AUC – Authentication Center
Strongly protected database which handles the authentication and encryption keys of every MS
Interfaces :Between BSC & MSC ::: A InterfaceBetween BSC & BTS ::: Abis InterfaceBetween BTS & MS ::: GSM Radio Air InterfaceSS7 Protocol ::: Signal Correction control part
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GSM Specifications
RC :: 890 – 915 MHz FC :: 935 – 960 MHz Separation ::: 45 MHz Channel Spacing ::: 200 kHz Each Channel is TIME SHARED
between 8 subscribers using TDMA Total number of channel :::
125 (25MHz bandwidth) * 08 = 1000 approximately
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GSM Specifications
Channel Frame :
TS0 TS1 TS2 TS4 TS5 TS6 TS7TS3
4.615ms
576.92μs156.25bits
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GSM Traffic Channels
Traffic Channels (TCHs) Carry digitally encoded user SPEECH or DATA
Control Channels (CCHs) Carry signaling and synchronizing commands
between BS & MS Full Rate :
User Speech / Data … one TS per Frame Half Rate :
Same time slot but sent in alternate frames Two half rate channel users would share the same
time slot but would alternately transmit during every other frame
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TCHs - Types
Full Rate TCH TCH/FS … Full rate speech channel carries
@13kbps TCH/F9.6 … Full rate DATA channel @9600bps TCH/F4.8 … Full rate DATA channel @4800bps TCH/F2.4 … Full rate DATA channel @2400bps
Half Rate TCH TCH/HS … half rate of the full rate channel …
6.5kbps TCH/H4.8 …half rate DATA @4800bps TCH/H2.4 …half rate DATA @2400bps
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CCH - Types
Three Main Control Channels – Broadcast Channel (BCH), Common Control Channel (CCCH) & DEDICATED Control Channel (DCCH)
BCH – Operates only on Forward link … Synchronization for all MS
Broadcast Control Channel – BCCH … used to broadcast info. Such as cell & network identity . Plus … Channel structure , channel availability and congestion parameters.
Frequency Correction Channel – FCCH … allows each MS to synchronize its internal frequency as of BS
Synchronization Channel – SCH … used to identify the serving BTS
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CCH - Types
Common Control Channel – CCCH – used to page specific MS , assign signaling signals to specific MS and receive requests for service from MS
Paging Channel – PCH … provides paging signals from BSC to all MS in the cell … used to provide cell broadcast ASCII text messages to all MS – SMS feature.
Random Access Channel – RACH … reverse link used by MS .. Used by MS to originate calls
Access Grant Channel – AGCH … used by the BSC to provide forward link communication to the MS and carries signals which instructs MS to operate on particular physical channel .
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CCH - Types
Dedicated Control Channels (DCCH) – bi-directional in nature like traffic channels …
Stand-alone Dedicated Control Channel –SDCCH …ensures that MS will remain connected with BSC while MSC verify the MS …
Slow Associated Control Channel – SACCH … carries general info. Between the BTS and MS… on the forward , regular signals to MS like transmitted power … in reverse , it carries received signal strength , quality of TCH info.
Fast Associated Control Channel – FACCH … carries urgent messages same as of SDCCH … urgent message like handoff request .