Chapter 8

Data Transmission and Modems

Agenda

• Circuit• Mode of transmission• Digital transmission• Analog transmission for digital signals - modems

– Methods

– Classification

– Interfaces

– Functions

– Selection Criteria

Circuit

• Signal rate– No. of signal changes (amplitude, frequency, or

phase) on a circuit per second– Baud– Types: dibits, tribits, and quadbits

• Speed– No. of bits that a circuit can carry in 1 second– Bits per second (bps) used for measurement

Mode of Transmission

• Data flow

• Physical connection

• Timing

Data Flow - US

• Simplex transmission– Television and radio

• Half-duplex (HDX) transmission– CB radio, terminal

• Full-duplex (FDX) transmission– Telephone, computer to computer

Physical Connection

• Parallel transmission– Fast– Simple– Line cost

• Serial transmission– Complicated transmitter and receive– Decomposing and reconstructing

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

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

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)

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 -)

Digital Transmission of Digital Signals

• Digital transmitter/receiver (data service unit/channel service unit (DSU/CSU))– Simpler & cheaper

• Transmitter for shaping & timing the signal, interface between DTE and line

• Receiver for protection of excessive voltage, diagnostic and testing

Analog Transmission of Digital signals (Modems – I)

• Modulation and Demodulation– Receiver (modulation), control unit (auto dial & auto

answer), transmitter (demodulation), & power supply– Equalizer in transmitter & adaptive equalizer in

receiver (adjust to fit the characteristics of the telephone line)

• 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

Modems - II

• Methods– Trellis code modulation: signal processor, error

correction for error free transmission, high speed

• 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 33.6

kbps – V.90 for 56 kbps

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.

Modem Classification

• Simplex, half-duplex, full-duplex transmission modems

• Asynchronous or synchronous transmission modems

• Acoustically coupled modems (portable)

• Limited distance modems/short haul modems (less than 20 miles)

• Modem eliminators/null modems (cable less than several thousand feet)

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 Interface

Specification (DOCSIS) by CableLabs– For large files and not logon process

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

Modem Interfaces

• Between modem and line

• Between DTE and modem

Between Modem and Line

• Two- to four-wire cable

• RJ-11 plug

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

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

RS-232-D Interface

• Specifications for the 25-pin connector

• Addition of a shield on the cable

• Redefinition of protective ground and some pins

RS-449 Interface

• 37 pins

• 4,000 feet cable

• Not well accepted

RS-336 Interface

• Automatic dialing from computer to remote terminals for data collection

• Not for high speed transmission

X. Interfaces• X.21 interface

– Digital connection to a digital public telephone network

• X.21bis interface– Terminal to packet switch network via analog

line

Current Loop Interface

• Represent 1 and 0 by presence or absence of an electrical current

• Simple and inexpensive

• Nonstandard

• Teletywriter system

Other Modem Functions

• Reverse channel for signal one another to determine the speed

• Auto dial/auto answer

• Modem diagnostics– Loop back for accuracy checking

Modem Selection Criteria

• Digital or analog signals

• Asynchronous or synchronous

• Speed

• Distance

• Type of line

• Cost

• Functions

Spread Spectrum

• Analog signal for analog or digital data

• Frequency hopping ad direct sequence

• Generated by Pseudo random number generator

Points to Remember

• Circuit• Mode of transmission• Digital transmission• Analog transmission for digital signals - modems

– Methods

– Classification

– Interfaces

– Functions

– Selection Criteria

Discussion

• Design the data transmission and modem specification for a bank