Chapter 8:Data CommunicationFundamentals

Business Data Communications, 4e

Three Components of Data Communication

Data Analog: Continuous value data (sound, light,

temperature) Digital: Discrete value (text, integers, symbols)

Signal Analog: Continuously varying electromagnetic wave Digital: Series of voltage pulses (square wave)

Transmission Analog: Works the same for analog or digital signals Digital: Used only with digital signals

Analog Data-->Signal Options

Analog data to analog signal Inexpensive, easy conversion (eg

telephone) Data may be shifted to a different part of

the available spectrum (multiplexing) Used in traditional analog telephony

Analog data to digital signal Requires a codec (encoder/decoder) Allows use of digital telephony, voice mail

Digital Data-->Signal Options

Digital data to analog signal Requires modem (modulator/demodulator) Allows use of PSTN to send data Necessary when analog transmission is used

Digital data to digital signal Requires CSU/DSU (channel service unit/data

service unit) Less expensive when large amounts of data are

involved More reliable because no conversion is involved

Transmission Choices

Analog transmission only transmits analog signals, without regard

for data content attenuation overcome with amplifiers signal is not evaluated or regenerated

Digital transmission transmits analog or digital signals uses repeaters rather than amplifiers switching equipment evaluates and

regenerates signal

Data

Signal

TransmissionSystem

A

DD

DA

A

Data, Signal, and Transmission Matrix

Advantages of Digital Transmission

The signal is exactSignals can be checked for errorsNoise/interference are easily filtered outA variety of services can be offered over one lineHigher bandwidth is possible with data compression

Why Use Analog Transmission?

Already in placeSignificantly less expensiveLower attentuation ratesFully sufficient for transmission of voice signals

Analog Encoding of Digital Data

Data encoding and decoding technique to represent data using the properties of analog wavesModulation: the conversion of digital signals to analog formDemodulation: the conversion of analog data signals back to digital form

Modem

An acronym for modulator-demodulatorUses a constant-frequency signal known as a carrier signalConverts a series of binary voltage pulses into an analog signal by modulating the carrier signalThe receiving modem translates the analog signal back into digital data

Methods of Modulation

Amplitude modulation (AM) or amplitude shift keying (ASK)Frequency modulation (FM) or frequency shift keying (FSK)Phase modulation or phase shift keying (PSK)

Amplitude Shift Keying (ASK)

In radio transmission, known as amplitude modulation (AM)The amplitude (or height) of the sine wave varies to transmit the ones and zerosMajor disadvantage is that telephone lines are very susceptible to variations in transmission quality that can affect amplitude

1 0 0 1

ASK Illustration

Frequency Shift Keying (FSK)

In radio transmission, known as frequency modulation (FM)Frequency of the carrier wave varies in accordance with the signal to be sentSignal transmitted at constant amplitudeMore resistant to noise than ASKLess attractive because it requires more analog bandwidth than ASK

1 1 0 1

FSK Illustration

Phase Shift Keying (PSK)

Also known as phase modulation (PM)Frequency and amplitude of the carrier signal are kept constantThe carrier signal is shifted in phase according to the input data streamEach phase can have a constant value, or value can be based on whether or not phase changes (differential keying)

0 0 1 1

PSK Illustration

0 1 1

Differential Phase Shift Keying (DPSK)

0

Analog Channel Capacity: BPS vs. Baud

Baud=# of signal changes per secondBPS=bits per secondIn early modems only, baud=BPSEach signal change can represent more than one bit, through complex modulation of amplitude, frequency, and/or phaseIncreases information-carrying capacity of a channel without increasing bandwidthIncreased combinations also leads to increased likelihood of errors

Voice Grade Modems

Cable Modems

DSL Modems

Digital Encoding of Analog Data

Primarily used in retransmission devicesThe sampling theorem: If a signal is sampled at regular intervals of time and at a rate higher than twice the significant signal frequency, the samples contain all the information of the original signal.8000 samples/sec sufficient for 4000hz

Converting Samples to Bits

QuantizingSimilar concept to pixelizationBreaks wave into pieces, assigns a value in a particular range8-bit range allows for 256 possible sample levelsMore bits means greater detail, fewer bits means less detail

Codec

Coder/DecoderConverts analog signals into a digital form and converts it back to analog signalsWhere do we find codecs? Sound cards Scanners Voice mail Video capture/conferencing

Digital Encoding of Digital Data

Most common, easiest method is different voltage levels for the two binary digitsTypically, negative=1 and positive=0Known as NRZ-L, or nonreturn-to-zero level, because signal never returns to zero, and the voltage during a bit transmission is level

NRZ-L

Differential NRZ

Differential version is NRZI (NRZ, invert on ones)Change=1, no change=0Advantage of differential encoding is that it is more reliable to detect a change in polarity than it is to accurately detect a specific level

NRZI

Problems With NRZ

Difficult to determine where one bit ends and the next beginsIn NRZ-L, long strings of ones and zeroes would appear as constant voltage pulsesTiming is critical, because any drift results in lack of synchronization and incorrect bit values being transmitted

Biphase Alternatives to NRZ

Require at least one transition per bit time, and may even have twoModulation rate is greater, so bandwidth requirements are higherAdvantages Synchronization due to predictable

transitions Error detection based on absence of a

transition

Manchester Code

Transition in the middle of each bit periodTransition provides clocking and dataLow-to-high=1 , high-to-low=0Used in Ethernet

Manchester Code

Differential Manchester

Midbit transition is only for clockingTransition at beginning of bit period=0Transition absent at beginning=1Has added advantage of differential encodingUsed in token-ring

Differential Manchester

Digital Encoding Illustration

Asynchronous and Synchronous Transmission

Asynchronous Transmission Data are transmitted one character at a time. Timing (synchronization) is maintained within

each character, by the use of start elements and stop elements.

Synchronous Transmission A block of bits is transmitted in a steady

stream without start and stop codes. Synchronization

A separate clock line To embed the clocking information in the data signal

Asynchronous Transmission

Synchronous Transmission

To determine the beginning and end of a block of bits Begins with a preamblepreamble bit pattern Ends with a postamblepostamble bit pattern

Frame =

preamble + control + data + postamble

Digital Interfaces

The point at which one device connects to anotherStandards define what signals are sent, and howSome standards also define physical connector to be used

Generic CommunicationsInterface Illustration

DTE and DCE

DTE DTE

host com puter term inal

in terface in terface

m odem m odem

DCE

Four Characteristics of Interfaces

Mechanical: The actual physical connection of the DTE to the DCE.Electrical: Voltage levels and timing of voltage changes.Functional: The functions that are performed.Procedural: The sequence of events for transmitting data.

RS-232C (EIA 232C)

EIA’s “Recommended Standard” (RS)Specifies mechanical, electrical, functional, and procedural aspects of the interfaceUsed for connections between DTEs and voice-grade modems, and many other applications

EIA-232-D

new version of RS-232-C adopted in 1987improvements in grounding shield, test and loop-back signalsthe prevalence of RS-232-C in use made it difficult for EIA-232-D to enter into the marketplace

RS-449

EIA standard improving on capabilities of RS-232-Cprovides for 37-pin connection, cable lengths up to 200 feet, and data rates up to 2 million bpscovers functional/procedural portions of R-232-C electrical/mechanical specs covered by

RS-422 & RS-423

Functional Specifications

Specifies the role of the individual circuitsData circuits in both directions allow full-duplex communicationTiming signals allow for synchronous transmission (although asynchronous transmission is more common)

Table 8.6(Page 200)

Functionalspecification

Procedural Specifications

Multiple procedures are specifiedSimple example: exchange of asynchronous data on private line Provides means of attachment

between computer and modem Specifies method of transmitting

asynchronous data between devices Specifies method of cooperation for

exchange of data between devices

Mechanical Specifications

25-pin connector with a specific arrangement of leadsDTE devices usually have male DB25 connectors while DCE devices have female In practice, fewer than 25 wires are generally used in applications

DB-25 Female

DB-25 Male

RS-232 DB-25 Connectors

RS-232 DB-25 Pinouts

RS-232 DB-9 Connectors

Limited RS-232

RS-422 DIN-8

Found on Macs

DIN-8 Male DIN-8 Female

Electrical Specifications

Specifies signaling between DTE and DCEUses NRZ-L encoding Voltage < -3V = binary 1 Voltage > +3V = binary 0

Rated for <20Kbps and <15M greater distances and rates are

theoretically possible, but not necessarily wise

RS-232 Signals (Asynch)

Even Parity

Odd Parity

No Parity