Chapter 8:Data Communication

FundamentalsBusiness 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 exact Signals can be checked for errors Noise/interference are easily filtered out A variety of services can be offered over one line Higher bandwidth is possible with data

compression

Why Use Analog Transmission?

Already in place Significantly less expensive Lower attentuation rates Fully sufficient for transmission of voice

signals

Analog Encoding of Digital Data Data encoding and decoding technique to

represent data using the properties of analog waves

Modulation: the conversion of digital signals to analog form

Demodulation: the conversion of analog data signals back to digital form

Modem

An acronym for modulator-demodulator Uses a constant-frequency signal known as a

carrier signal Converts a series of binary voltage pulses into an

analog signal by modulating the carrier signal The 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 zeros

Major 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 sent Signal transmitted at constant amplitude More resistant to noise than ASK Less 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 constant The carrier signal is shifted in phase according

to the input data stream Each 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 second BPS=bits per second In early modems only, baud=BPS Each signal change can represent more than one bit,

through complex modulation of amplitude, frequency, and/or phase

Increases information-carrying capacity of a channel without increasing bandwidth

Increased combinations also leads to increased likelihood of errors

Voice Grade Modems

Cable Modems

DSL Modems

Digital Encoding of Analog Data

Primarily used in retransmission devices The 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

Quantizing Similar concept to pixelization Breaks wave into pieces, assigns a value in a

particular range 8-bit range allows for 256 possible sample levels More bits means greater detail, fewer bits means

less detail

Codec Coder/Decoder Converts analog signals into a digital form and converts

it back to analog signals Where do we find codecs?

Sound cards Scanners Voice mail Video capture/conferencing

Digital Encodingof Digital Data

Most common, easiest method is different voltage levels for the two binary digits

Typically, negative=1 and positive=0 Known as NRZ-L, or nonreturn-to-zero level,

because signal never returns to zero, and the voltage during a bit transmission is level

Differential NRZ

Differential version is NRZI (NRZ, invert on ones)

Change=1, no change=0 Advantage of differential encoding is that it is

more reliable to detect a change in polarity than it is to accurately detect a specific level

Problems With NRZ

Difficult to determine where one bit ends and the next begins

In NRZ-L, long strings of ones and zeroes would appear as constant voltage pulses

Timing 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 two

Modulation rate is greater, so bandwidth requirements are higher

Advantages Synchronization due to predictable transitions Error detection based on absence of a transition

Manchester Code

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

Differential Manchester

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

Digital Encoding Illustration

Digital Interfaces

The point at which one device connects to another

Standards define what signals are sent, and how

Some 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

RS-232C (EIA 232C)

EIA’s “Recommended Standard” (RS) Specifies mechanical, electrical, functional,

and procedural aspects of the interface Used for connections between DTEs and

voice-grade modems, and many other applications

EIA-232-D

new version of RS-232-C adopted in 1987 improvements in grounding shield, test and

loop-back signals the 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-C provides for 37-pin connection, cable lengths up to

200 feet, and data rates up to 2 million bps covers functional/procedural portions of R-232-C

electrical/mechanical specs covered by RS-422 & RS-423

Functional Specifications

Specifies the role of the individual circuits Data circuits in both directions allow full-

duplex communication Timing signals allow for synchronous

transmission (although asynchronous transmission is more common)

Procedural Specifications Multiple procedures are specified Simple 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 leads

DTE 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 DCE Uses 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)

Odd Parity

Even Parity

No Parity