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 – feet cable length –2, ,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