Data Transmission and Modems

1
Chapter 8

• Data Transmission and Modems

2
Agenda

• Circuit
• Mode of transmission
• Digital transmission
• Analog transmission for digital signals - modems
• Methods
• Classification
• Interfaces
• Functions
• Selection Criteria

3
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

4
Mode of Transmission

• Data flow
• Physical connection
• Timing

5
Data Flow - US

• Simplex transmission
• Television and radio
• Half-duplex (HDX) transmission
• CB radio, terminal
• Full-duplex (FDX) transmission
• Telephone, computer to computer

6
Physical Connection

• Parallel transmission
• Fast
• Simple
• Line cost
• Serial transmission
• Complicated transmitter and receive
• Decomposing and reconstructing

7
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

8
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

9
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)

10
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 -)

11
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

12
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

13
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

14
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.

15
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)

16
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

17
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

18
Modem Interfaces

• Between modem and line
• Between DTE and modem

19
Between Modem and Line

• Two- to four-wire cable
• RJ-11 plug

20
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

21
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

22
RS-232-D Interface

• Specifications for the 25-pin connector
• Addition of a shield on the cable
• Redefinition of protective ground and some pins

23
RS-449 Interface

• 37 pins
• 4,000 feet cable
• Not well accepted

24
RS-336 Interface

• Automatic dialing from computer to remote
  terminals for data collection
• Not for high speed transmission

25
X. Interfaces

• X.21 interface
• Digital connection to a digital public telephone
  network
• X.21bis interface
• Terminal to packet switch network via analog line

26
Current Loop Interface

• Represent 1 and 0 by presence or absence of an
  electrical current
• Simple and inexpensive
• Nonstandard
• Teletywriter system

27
Other Modem Functions

• Reverse channel for signal one another to
  determine the speed
• Auto dial/auto answer
• Modem diagnostics
• Loop back for accuracy checking

28
Modem Selection Criteria

• Digital or analog signals
• Asynchronous or synchronous
• Speed
• Distance
• Type of line
• Cost
• Functions

29
Spread Spectrum

• Analog signal for analog or digital data
• Frequency hopping ad direct sequence
• Generated by Pseudo random number generator

30
Points to Remember

• Circuit
• Mode of transmission
• Digital transmission
• Analog transmission for digital signals - modems
• Methods
• Classification
• Interfaces
• Functions
• Selection Criteria

31
Discussion

• Design the data transmission and modem
  specification for a bank