Download Line Coding

Line Coding, Modem, RS232
interfacing sequences.
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Line Coding

Process of converting binary data to a digital signal
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DC Components
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Residual direct-current
(dc) components or
zero frequencies are
undesirable
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Some systems do not
allow passage of a dc
component; may distort
the signal and create
output errors
DC component is extra
energy and is useless
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Self-Synchronization
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Includes timing
information in the data
being transmitted to
prevent
misinterpretation
Lack of synchronization
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Line Coding
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Unipolar
Polar
Bipolar
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Unipolar
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Simplest method; inexpensive
Uses only one voltage level
Polarity is usually assigned to binary 1; a 0 is
represented by zero voltage
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Unipolar
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Potential problems:
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DC component
Lack of synchronization
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Polar
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Uses two voltage levels, one positive and one
negative
Alleviates DC component
Variations
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Nonreturn to zero (NRZ)
Return to zero (RZ)
Manchester
Differential Manchester
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Nonreturn to Zero (NRZ)
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Value of signal is always positive or negative
NRZ-L
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Signal level depends on bit represented; positive
usually means 0, negative usually means 1
Problem: synchronization of long streams of 0s or
1s
NRZ-I (NRZ-Invert)
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Inversion of voltage represents a 1 bit
0 bit represented by no change
Allows for synchronization
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NRZ-L and NRZ-I Encoding
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Return to Zero (RZ)
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In NRZ-I, long strings of 0s may still be a
problem
May include synchronization as part of the
signal for both 1s and 0s
How?
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Must include a signal change during each bit
Uses three values: positive, negative, and zero
1 bit represented by positive-to-zero
0 bit represented by negative-to-zero
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RZ Encoding
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RZ Encoding
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Disadvantage

Requires two signal changes to encode each bit;
more bandwidth necessary
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Manchester
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Uses an inversion at the middle of each bit interval
for both synchronization and bit representation
Negative-to-positive represents binary 1
Positive-to-negative represents binary 0
Achieves same level of synchronization with only 2
levels of amplitude
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Differential Manchester
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Inversion at middle of bit interval is used for
synchronization
Presence or absence of additional transition at
beginning of interval identifies the bit
Transition means binary 0; no transition means 1
Requires two signal changes to represent binary 0;
only one to represent 1
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Bipolar Encoding

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Uses three voltage levels: positive, negative,
and zero
Zero level represents binary 0; 1s are
represented with alternating positive and
negative voltages, even when not
consecutive

Alternate mark inversion (AMI)
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Bipolar AMI
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Neutral, zero voltage represents binary 0
Binary 1s represented by alternating positive and
negative voltages
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Telephone Modems

A telephone line has a bandwidth of almost
2400 Hz for data transmission
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Modem stands for modulator/demodulator.
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Modulator
: creates a analog signal from binary data
Demodulator : recovers the binary data from the modulated
signal
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V.32
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ITU-T's V.32 standard was issued in 1989 for
asynchronous, full-duplex operation at 9600 bps.
Although designed for asynchronous DTEs, two
V.32 modems actually communicate synchronously.
A circuit converts the asynchronous data stream into
synchronous blocks, invisible to the application.
V.32 supports modulation rates of 2400, 4800, and
9600 bps.
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V.32bis
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ITU-T's V.32 standard was issued in 1991 for
asynchronous, full-duplex operation at 14.4
Kbps.
V.32bis is an extension of the V.32
technology. V.32bis supports modulation
rates of 2400, 4800, 9600 bps and 14.4
Kbps.
Data compression and error correction can
increase the throughput rates.
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Traditional Modems
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After modulation by the modem, an analog
signal reaches the telephone company
switching station where it sampled and digitized
to be passed through the digital network.
Bit rate is 56,000bps.
Uploading :33.6kbps.
Downloading 56kbps.
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Traditional Modems
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RS232 Interface
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Introduction
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Specifies the interface between DTE and DCE:
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Even used in applications where there is no DCE
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V.28 : mechanical and electrical characteristics
V.24 : functional and procedural characteristics
e.g. connecting computer to printer, magnetic card reader,
robot, … etc.
Introduced in 1962 but is still widely used
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DTE, DCE and RS232
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Vocabulary
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DTE
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data terminal equipment
e.g. computer, terminal
DCE
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data communication equipment
connects DTE to communication lines
e.g. modem
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Mechanical Characteristics
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9-pin connector
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9-pin connector is more commonly found in IBMPC but it covers signals for asynchronous serial
communication only
Use male connector on DTE and female
connector on DCE
N.B.: all signal names are viewed from DTE
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9-Pin RS232 Connector
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Electrical Characteristics
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Single-ended
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Mark: –3V to –15V
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represent Logic 1, Idle State (OFF)
Space: +3 to +15V
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one wire per signal, voltage levels are with respect to
system common (i.e. signal ground)
represent Logic 0, Active State (ON)
Usually swing between –12V to +12V
Recommended maximum cable length is 15m, at
20kbps
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RS232 Logic Waveform
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RS-232 Interface
RS-232 is the Serial interface on the PC
Three major wires for the Serial interface:
• Transmit - Pin 2
• Receive - Pin 3
• Ground - Pin 7 (25 pin connector)
- Pin 5 (9 pin connector)
Tx
Tx
Computer
Device
Rx
Rx
Gnd
Gnd
Transmit connects to Receive
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Function of Signals
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TD: transmitted data
RD: received data
DSR: data set ready
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DTR: data terminal ready
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indicate whether DCE is powered on
indicate whether DTR is powered on
turning off DTR causes modem to hang up the line
RI: ring indicator
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ON when modem detects phone call
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Function of Signals
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DCD: data carrier detect
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RTS: request to send
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ON when DTE wants to send data
Used to turn on and off modem’s carrier signal in multipoint (i.e. multi-drop) lines
Normally constantly ON in point-to-point lines
CTS: clear to send
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ON when two modems have negotiated successfully
and the carrier signal is established on the phone line
ON when DCE is ready to receive data
SG: signal ground
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Flow Control
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Means to ask the transmitter to stop/resume sending
in data
Required when:
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DTE to DCE speed > DCE to DCE speed
(e.g. terminal speed = 115.2kbps and line speed =
33.6kbps, in order to benefit from modem’s data
compression protocol)
 without flow control, the buffer within modem will overflow –
sooner or later
the receiving end takes time to process the data and thus
cannot be always ready to receive
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