Business data communications 4e chapter 8

Three Components of Data Communication ✘ Data ✘ Analog: Continuous value data sound, light, temperature ✘ Digital: Discrete value text, integers, symbols ✘ Signal ✘ Analog: Continuously

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 (en co der/ dec oder)

✘ Allows use of digital telephony, voice mail

Digital Data >Signal Options

✘ Digital data to analog signal

✘ Requires modem ( mo dulator/ dem odulator)

✘ 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

Signal

Transmission System

A

D A

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

✘ An acronym for modulator-demodulator

✘ Uses a constant-frequency signal known as a

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

✘ Co der/ Dec oder

✘ 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 Encoding

of 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

✘ 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

Generic Communications

Interface Illustration

DTE and 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

✘ 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

✘ 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 duplex communication

full-✘ 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 MaleRS-232 DB-25 Connectors

RS-232 DB-25 Pinouts

RS-232 DB-9 Connectors

✘ Limited RS-232

RS-422 DIN-8

✘ Found on Macs

✘ 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