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Chapter 8

Data Transmission and Modems

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Agenda

Circuit

Mode of transmission

Digital transmission Analog transmission for digital signals - modems

Methods

Classification

Interfaces

Functions

Selection Criteria

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Circuit

Signal rate

No. of signal changes (amplitude, frequency, or

phase) on a circuit per second Baud

Types: dibits, tribits, and quadbits

SpeedNo. of bits that a circuit can carry in 1 second

Bits per second (bps) used for measurement

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Mode of Transmission

Data flow

Physical connection

Timing

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Data Flow - US

Simplex transmission

Television and radio

Half-duplex (HDX) transmission

CB radio, terminal

Full-duplex (FDX) transmission

Telephone, computer to computer

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Physical Connection

Parallel transmission

Fast

Simple

Line cost

Serial transmission

Complicated transmitter and receive

Decomposing and reconstructing

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Timing

Asynchronous transmission Start/stop bits for character synchronization

Mark (1or stop) /space (0 or start ) bits for bit

synchronization

Simple, inexpensive, slow speed transmission

For personal computer

Synchronous transmission

Clock circuitry One to four synchronization characters for each block

of data

Large amount of data on dedicated line

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Digital Signals

Unipolar (positive voltage for 1, no voltage for 0)

Bipolar, nonreturn-to zero (NRZ)

Manchester coding (low-to-high is 1, high-to- low is 0) Differential Manchester (no transition at the beginning

of the bit period is 1, second transition at the beginning

of the bit period is 0)

Benefits: self-clocking, and error detection

Bipolar, return-to-zero

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Benefits of Digital Transmission

Better data integrity (detect & correct error)

Higher capacity cables (fiber-optic)

Easier integration (voice, data, video, etc.)

Better security and privacy (encrypt data)

Lower cost (large-scale integrated circuitry)

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Digital Transmission of Analog

Signals Quantization

Quantizing noise or digitizing distortion

Codec (coder/decoder): analog-to-digital (A/D)converter & digital-to-analog (D/A) converter

Methods

Pulse code modulation (256 integers, 8000 per second)

Adaptive differential pulse code modulation(difference)

Delta modulation (1 for +, 0 for -)

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Analog Transmission of Digital

signals (Modems I)

Modulation and Demodulation

Receiver (modulation), control unit (auto dial & autoanswer), transmitter (demodulation), & power supply

Equalizer in transmitter & adaptive equalizer in

receiver (adjust to fit the characteristics of the telephoneline)

Methods

Frequency shift keying (FSK)

Phase shift keying (PSK): 180 degree (1 bit), 90 degree(2 bits), 45 degree (3 bits)

Differential phase shift keying (DPSK)

Quadrature amplitude modulation (QAM): combination

of 8 phases & 4 relative amplitudes

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Modems - II

Methods

Trellis code modulation: signal processor, error

correction for error free transmission, highspeed

Standards by ITU-T

V.32 for 9.6 kbps & V.32bis for 14.4 kbps

V.34 for 28.8 kbps & V.34+ or V.34bis for 33.6kbps

V.90 for 56 kbps

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Modems - III

Handshaking

Exchange signals between modems

Test the characteristics of circuit and quality determine baud rate, modulation technique and

error control

Data compression

V.42bis by ITU-T

Microcom Networking Protocol (MNP 7 or 10)by Microcom, Inc.

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Modem Classification

Simplex, half-duplex, full-duplex

transmission modems

Asynchronous or synchronous transmissionmodems

Acoustically coupled modems (portable)

Limited distance modems/short haulmodems (less than 20 miles)

Modem eliminators/null modems (cable less

than several thousand feet)

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Modem Classification - II

Facsimile modems

Error correction and data compression

Modem for fiber-optics circuits

Digital-electrical to digital-optical

Cable modems

DTE to cable television system cable

Data Over Cable Service InterfaceSpecification (DOCSIS) by CableLabs

For large files and not logon process

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Modem Classification - III

Reverse channel

Slow speed reverse channel for signaling one another

Auto dial/auto answer

Hayes Microcomputer Product, Inc.

Command AT for attention

Internet service provider usage

Dial-backup public telephone network connection for

leased telephone line

Internal modem

Single circuit card

No switch to change the parameters and operations

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Modem Interfaces

Between modem and line

Between DTE and modem

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Between Modem and Line

Two- to four-wire cable

RJ-11 plug

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Between Modem and DTE

RS-232-C interface

V.24 interface

RS-232-D interface

RS-449 interface

RS-336 interface

X.21 interface X.21bis interface

Current loop interface

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RS-232-C Interface

By EIA and ITU-T V.24

Characteristics

25-pin connector at both end

digital and serial

Female for DCE and male for DTE

Pins 2 and 3 to pass data, other for signaling

Limitations

50 -100 feet cable length

2,400 - 9,600 bps

Hard to difference 0 and 1 when speed is high

and cable is long

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RS-232-D Interface

Specifications for the 25-pin connector

Addition of a shield on the cable

Redefinition of protective ground and some

pins

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RS-449 Interface

37 pins

4,000 feet cable

Not well accepted

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RS-336 Interface

Automatic dialing from computer to remote

terminals for data collection

Not for high speed transmission

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X. Interfaces

X.21 interface

Digital connection to a digital public telephone

network

X.21bis interface

Terminal to packet switch network via analog

line

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Current Loop Interface

Represent 1 and 0 by presence or absence of

an electrical current

Simple and inexpensive

Nonstandard

Teletywriter system

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Other Modem Functions

Reverse channel for signal one another to

determine the speed

Auto dial/auto answer

Modem diagnostics

Loop back for accuracy checking

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Modem Selection Criteria

Digital or analog signals

Asynchronous or synchronous

Speed

Distance

Type of line

Cost

Functions

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Spread Spectrum

Analog signal for analog or digital data

Frequency hopping ad direct sequence

Generated by Pseudo random number

generator

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Points to Remember

Circuit

Mode of transmission

Digital transmission Analog transmission for digital signals - modems

Methods

Classification

Interfaces

Functions

Selection Criteria

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Discussion

Design the data transmission and modem

specification for a bank