chapter 8 data transmission and modems. agenda circuit mode of transmission digital transmission...
TRANSCRIPT
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