Line Coding, Modem, RS232 interfacing sequences.

Line Coding Process of converting binary data to a digital signal

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

Self-Synchronization Includes timing information in the data being transmitted to prevent misinterpretation Lack of synchronization

Line Coding Unipolar Polar Bipolar

Unipolar Simplest method; inexpensive Uses only one voltage level Polarity is usually assigned to binary 1; a 0 is represented by zero voltage

Unipolar Potential problems: DC component Lack of synchronization

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

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 Transition between voltage, not voltage itself, represents a 1 bit

NRZ-L and NRZ-I Encoding

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

RZ Encoding

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

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

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

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)

Bipolar AMI Neutral, zero voltage represents binary 0 Binary 1s represented by alternating positive and negative voltages

MODEM

Telephone Modems A telephone line has a bandwidth of almost 2400 Hz for data transmission

Modem stands for modulator/demodulator. Modulator : creates a analog signal from binary data Demodulator : recovers the binary data from the modulated signal

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.

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.

Traditional Modems 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

RS232 Interface

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

DTE, DCE and RS232

Vocabulary DTE DCE data terminal equipment e.g. computer, terminal data communication equipment connects DTE to communication lines e.g. modem

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

9-Pin RS232 Connector

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

RS232 Logic Waveform

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

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

Function of Signals DCD: data carrier detect RTS: request to send 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

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