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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 Residual direct-current

(dc) components or zero frequencies are undesirable 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 Includes timing

information in the data being transmitted to prevent misinterpretation

Lack of synchronization

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Line Coding Unipolar Polar Bipolar

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Unipolar 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 Potential problems:

DC component Lack of synchronization

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Polar Uses two voltage levels, one positive and one

negative Alleviates DC component Variations

Nonreturn to zero (NRZ) Return to zero (RZ) Manchester Differential Manchester

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Nonreturn to Zero (NRZ) Value of signal is always positive or negative NRZ-L

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) 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) 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?

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 Disadvantage

Requires two signal changes to encode each bit; more bandwidth necessary

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Manchester 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 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 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 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. Modulator : creates a analog signal from binary data Demodulator : recovers the binary data from the modulated

signal

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V.32 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 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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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.

Traditional Modems

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Traditional Modems

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RS232 Interface

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Introduction Specifies the interface between DTE and DCE:

V.28 : mechanical and electrical characteristics V.24 : functional and procedural characteristics

Even used in applications where there is no DCE 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 DTE

data terminal equipment e.g. computer, terminal

DCE data communication equipment connects DTE to communication lines e.g. modem

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Mechanical Characteristics 9-pin connector

9-pin connector is more commonly found in IBM-PC 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 Single-ended

one wire per signal, voltage levels are with respect to system common (i.e. signal ground)

Mark: –3V to –15V represent Logic 1, Idle State (OFF)

Space: +3 to +15V 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 InterfaceRS-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

RxRx

GndGnd

ComputerDevice

Transmit connects to Receive

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Function of Signals TD: transmitted data RD: received data DSR: data set ready

indicate whether DCE is powered on DTR: data terminal ready

indicate whether DTR is powered on turning off DTR causes modem to hang up the line

RI: ring indicator ON when modem detects phone call

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Function of Signals DCD: data carrier detect

ON when two modems have negotiated successfully and the carrier signal is established on the phone line

RTS: request to send ON when DTE wants to send data Used to turn on and off modem’s carrier signal in multi-

point (i.e. multi-drop) lines Normally constantly ON in point-to-point lines

CTS: clear to send ON when DCE is ready to receive data

SG: signal ground

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Flow Control Means to ask the transmitter to stop/resume

sending in data Required when:

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