Prof. Choong Seon HONG

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Transmission of Digital DATA Interfaces and
Modems

• Prof. Choong Seon HONG

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6? Transmission of Digital Data Interfaces
and MODEMs

• 6.1 Digital Data Transmission
• 6.2 DTE-DCE Interface
• 6.3 Other Interface Standards
• 6.4 ??(MODEM)

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6.1 Digital Data Transmission

• Primary concern when considering the
  transmission of data from one device to another
  is the wiring

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Digital Data Transmission(contd)

• Parallel Transmission
• By grouping, we can send data n bits at a time
  instead of one
• ?? ??(speed)
• ?? ??(expansive)
• ????(short distance) ?? 25 feets

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Digital Data Transmission(contd)

• Parallel transmission

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Digital Data Transmission(contd)

• Serial Transmission
• one bit follows another
• ?? ?? channel, ??
• ?? require interface converter

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Digital Data Transmission(contd)

• Serial Transmission

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Digital Data Transmission(contd)

• ??? ??(Asynchronous Transmission)
• send one start bit(0) at the beginning and one or
  more stop bits(1) at the end of each byte
• ex) the connection of a terminal to a computer
• ?? cheap and effective

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Digital Data Transmission

• ????(synchronous transmission)
• Sending bits one after another without start/stop
  bits or gaps. It is the responsibility of the
  receiver to group the bits
• The receiver counts the bits as they arrive and
  groups them in eight-bit units
• ?? ??(speed)

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6.2 DTE-DCE Interface

• DTE(Data Terminal equipment) ??? ????
• DCE(Data Circuit-terminating Equipment) ??? ??
  ?? ??

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DTE-DCE Interface(contd)

• DTE(Data Terminal Equipment)
• is any device that is a source of destination
  for binary digital data
• terminal
• Personal computer
• printer
• fax machine and so on

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DTE-DCE Interface(contd)

• DCE (Data Circuit-Terminating Equipment)
• is any device (functional unit) that transmits
  or receives data in the form of an analog or
  digital signal through a network
• Modulator/demodulator

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DTE-DCE Interface(contd)

• ???(Standards)
• defining the connection between a DTE and DCE

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DTE-DCE Interface(contd)

• Standards
• EIA (Electronic Industries Association)
• EIA-232
• EIA-442
• EIA-449
• ITU-T (International Telecommunications
  Union-Telecommunications Sector)
• V series (???? ?? ??? ??)
• X series (??? ???? ?? ?????)

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DTE-DCE Interface(contd)

• EIA-232(RS-232)
• defining the mechanical, electrical and
  functional characteristics
• ???? ?? (mechanical specification)
• defining the interface as a 25-wire cable with a
  male (plug) and a female (receptacle) DB-25 pin
  connector
• length may not exceed 15m(50 feet)
• ???? ?? (electrical specification)
• define voltage levels and the type of signal(?
  NRZ-L)

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DTE-DCE Interface(contd)

• Sending the data
• To be recognized as data, the amplitude of a
  signal must fall between 3 and 15 volts or
  between 3 and 15 volts

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DTE-DCE Interface(contd)

• Control and Timing
• Only 4 wires out of 25 available in an EIA-232
  interface are used for data functions
• Signals must be sent using OFF less than 3 volts
  and ON greater than 3 volts

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DTE-DCE Interface(contd)

• ?? ?? (Functional Specification)

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DTE-DCE Interface(contd)

• Functions of Pins in EIA-232, DB-9

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DTE-DCE Interface(contd)

• ? synchronous full-duplex transmission

Step1 preparation Step2 Readiness Step3
Set up
Step1 preparation Step2 Readiness Step3
Set up
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DTE-DCE Interface(contd)
Step4 Data Transfer Step 5 Clearing
Step4Data Transfer Step 5 Clearing
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DTE-DCE Interface(contd)

• Null Modem
• not needed to connect two compatible digital
  devices directly
• Ex) connecting two DTEs in the same building
• Using regular data pin connections with and
  without DCEs

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DTE-DCE Interface(contd)

• Crossing Connections
• Null modem pin connection

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6.3 Other Interface Standards

• needs more speed and/or distance
• EIA-449
• EIA-530
• X.21

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Other Interface Standards(contd)

• EIA-449
• DB-37 pin
• DB-9 pin
• DB-37 and DB-9 connections

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Other Interface Standards(contd)

• Pin Functions
• category I pins
• are compatible with those of EIA-232
• category II pins
• have no equivalent in EIA-232

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Other Interface Standards (contd)

• DB-37 pins

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Other Interface Standards(contd)

• DB-9 pins

Pin
Function
EIA-232 Equivalent
1 2 3 4 5 6 7 8 9
Shield Secondary receive ready Secondary send
data Secondary receive data Signal ground Receive
common Secondary request to send Secondary clear
to send Send common
1 14 16 7 12 19 13
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Other Interface Standards(contd)

• Electrical specification RS-423 and RS-422
• EIA-449 uses two other standards to define its
  electrical specification
• RS-423 (unbalanced circuits)
• RS-422 (balanced circuits)

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Other Interface Standards(contd)

• RS-423 Unbalanced Mode

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Other Interface Standards(contd)

• RS-422 Balanced Mode

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Other Interface Standards(contd)

• Canceling of noise in balanced mode

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Other Interface Standards(contd)

• EIA-530
• is a new version of EIA-449 that uses DB-25
  pins

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Other Interface Standards(contd)

• X.21
• is an interface designed by the ITU-T to
  address many of the problems existing in the EIA
• eliminating most of the control circuits of the
  EIA standards
• DB-15 connector
• Byte timing (pins 7 and 14) byte
  synchronization
• Control and indication
• Pin 3 equivalent of request to send (control)
• Pin 5 equivalent of clear to send (indication)

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Other Interface Standards

• DB-15 pins

Pin
Function
Pin
Function
1 2 3 4 5 6 7 8
Transmit data or control Control Receive data or
control Indication Signal element timing Byte
timing Reserved
Shield Transmit data or control Control Receive
data or control Indication Signal element
timing Bye timing Signal ground
9 10 11 12 13 14 15
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6.4 MODEMs

• The most familiar type of DCE

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MODEMs(contd)

• MODEM
• stands for modulator/demodulator
• Modulator converts a digital signal to an
  analog signal
• using the digital-to-analog mechanism of ASK,
  FSK, PSK, and QAM
• Demodulator converts an analog signal to a
  digital signal

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MODEMs(contd)

• Transmission Rate
• Bandwidth
• Each line has an upper limit and a lower limit on
  the frequencies of the signals it can carry. This
  limited range is called the bandwidth

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MODEMs(contd)

• Telephone line bandwidth
• A telephone line has bandwidth of almost 3000 Hz

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MODEMs(contd)

• Modem speed
• The bandwidth required for ASK transmission is
  equal to baud rate of signal
• in half-duplex ASK, because the effective
  bandwidth of a telephone line is 2400Hz, the
  maximum bit rate is also 2400.

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MODEMs(contd)

• Baud rate in full-duplex ASK
• Although ASK has a good bit rate, it is not used
  today because of noise

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MODEMs(contd)

• FSK(Frequency Shift Keying)
• Baud rate in half-duplex FSK
• Baud rate in full-duplex FSK
• Max baud rate Max bit rate 2400 (fc1
  fc0)/2

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MODEMs(contd)

• Baud Rate in Full-Duplex FSK

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MODEMs(contd)

• PSK and QAM(??)

Theoretical bit rates for modems
Half-Duplex
Full-Duplex
Encoding
ASK, FSK, 2-PSK 4-PSK, 4-QAM 8-PSK,
8-QAM 16-QAM 32-QAM 64-QAM 128-QAM
2400 4800 7200 9600 12,000 14,400 16,800
1200 2400 3600 4800 6000 7200 8400
256-QAM
19,200
9600
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MODEMs(contd)

• Modem Standards
• Bell modem
• The first commercial modems in the early 1970s
• ITU-T modem

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MODEMs(contd)

• Bell Modems(contd)

For flow control or error control
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MODEMs(contd)

• Bell Modems
• 212 --gt 201 Same technology, data rate double

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MODEMs(contd)

• ITU-T Modem Standards(contd)

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MODEMs(contd)

• ITU-T Modem Standards

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MODEMs(contd)

• ITU-T/Bell Compatibility

ITU-T
Bell
Baud Rate
Bit Rate
Modulation
V.21 V.22 V.23 V.26 V.27 V.29
103 212 202 201 208 209
300 600 1200 1200 1600 2400
300 1200 1200 2400 4800 9600
FSK 4-PSK FSK 4-PSK 8-PSK 16-QAM
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MODEMs(contd)

• Intelligent Modems
• contain software to support a number of
  additional function, such as automatic answering
  and dialing
• AT Commandparameter commandparameter
• Ex) ATDT4088648902

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MODEMs(contd)

• AT commands

Command
Meaning
Parameters
The number to dial 0 or 1 0 or 1 n
A B D E H L P T
Put modem in answer mode Use V.22bis at 1200
bps Dial the number Enable/disable echo
printing Put modem on/off hook Adjust speaker
volume Use pulse dialing Use tone dialing
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MODEMs(contd)

• Traditional Modems

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MODEMs(contd)

• 56K Modem

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MODEMs(contd)

• Cable Modem