6? Transmission of Digital Data: Interfaces and MODEMs

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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)
• 6.5 ??

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6.1 Digital Data Transmission

• of 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

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Digital Data Transmission

• ????(synchronous transmission)
• send bits one after another without start/stop
  bits or gaps. It is the responsibility of the
  receiver to group the bits
• ?? ??(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
• microcomputer
• computer
• printer
• fax machine and so on

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DTE-DCE Interface(contd)

• DCE(Data Circuit-Terminating Equipment)
• is any device 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)
• to define the connection between a DTE and DCE

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DTE-DCE Interface(contd)

• Standards
• EIA
• EIA-232
• EIA-442
• EIA-449
• ITU-T
• V series (???? ?? ??? ??)
• X series (??? ???? ?? ?????)

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DTE-DCE Interface(contd)

• EIA-232(RS-232)
• defines the mechanical, electrical and
  functional characteristics
• ???? ?(mechanical)
• define the interface as a 25-wire cable with a
  male and a female 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
• 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)

• ? synchronous full-duplex transmission

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DTE-DCE Interface(contd)
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DTE-DCE Interface(contd)

• Null Modem
• not needed to connect two compatible digital
  devices directly
• 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

Pin
Functions
Category
Pin
Functions
Category
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36
37
Shield Signal rate indicator Unassigned Send
data Send timing Receive data Request to
send Receive timing Clear to send Local
loopback Data mode Terminal ready Receive
ready Remote loopback Incoming call Select
frequency Terminal timing Test mode Signal ground
Receive Common Unassigned Send data Send
timing Receive data Request to send Receive
timing Clear to send Terminal in service Data
mode Terminal ready Receive ready Select
ready Signal quality New signal Terminal
timing Standby indicator Send common
I I I I I I II I I I II II I II
II I I I I I I I II I I I II II I II II
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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 using 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
• DB-15 connector

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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
• Demodulator converts an analog signal to a
  digital signal

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MODEMs(contd)

• Transmission
• 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
• ASK(Amplitude Shift Keying)
• Baud rate in half-duplex ASK

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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

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MODEMs(contd)

• PSK and QAM(??)

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

• Modem Standards
• Bell modem
• ITU-T modem

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MODEMs(contd)

• Bell Modems(contd)

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MODEMs(contd)

• Bell Modems

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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