Transcript Chapter 8 Data Transmission and Modems

Chapter 8
Data Transmission and Modems
Agenda
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Circuit
Mode of transmission
Digital transmission
Analog transmission for digital signals - modems
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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
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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
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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
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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
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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
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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
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Circuit
Mode of transmission
Digital transmission
Analog transmission for digital signals - modems
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Methods
Classification
Interfaces
Functions
Selection Criteria
Discussion
• Design the data transmission and modem
specification for a bank