CSC 335 Data Communications and Networking

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CSC 335 Data Communications and Networking

• Lecture 4b Communication and Multiplexing
• Dr. Cheer-Sun Yang

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Standards for Communication
A standard for communication defines among others
the timing of signals and the electrical details
of voltage and current. If two venders follow a
given standard, their equipment will
interoperate.
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Standards for Communication

• There are several organizations that are
  contributing to ensure that communication
  hardware built be different venders will
  interoperate. The defines the specifications for
  various communication hardware.
• International Telecommunications Union (ITU)
• the Electronic Industries Association (EIA)
• Institute for Electrical and Electronic
  Engineers (IEEE)

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Interfacing

• Data processing devices (or data terminal
  equipment, DTE) do not (usually) include data
  transmission facilities
• Need an interface called data circuit terminating
  equipment (DCE)
• e.g. modem, NIC
• DCE transmits bits on medium
• DCE communicates data and control info with DTE
• Done over interchange circuits
• Clear interface standards required

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Interfaces between DTE and DCE
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Characteristics of Interface

• Mechanical
• Connection plugs
• Electrical
• Voltage, timing, encoding
• Functional
• Data, control, timing, grounding
• Procedural
• Sequence of events

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V.24/EIA-232-F

• ITU-T v.24
• Only specifies functional and procedural
• References other standards for electrical and
  mechanical
• EIA-232-F (USA)
• RS-232
• Mechanical ISO 2110
• Electrical v.28
• Functional v.24
• Procedural v.24

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Electrical Specification

• Digital signals
• Values interpreted as data or control, depending
  on circuit
• More than -3v is binary 1, more than 3v is
  binary 0 (NRZ-L)
• Signal rate lt 20kbps
• Distance lt15m
• For control, more than-3v is off, 3v is on

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Functional Specification

• Pins nine most frequently used pins
• Data Terminal Ready(pin 20) as terminal or
  computer is powered up
• Data Set Ready (pin 6) as modem is powered up
• Carrier Detect (pin 8) as modem detects a
  carrier on the telephone line

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Functional Specification(contd)

• Request to Send (pin 4) terminal wants to send
  data
• Clear to Send (pin 5) modem ready to accept
  data
• Transmit (pin 2) data transmitted
• Receive (pin 3) data received

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Mechanical Specification
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Procedural Specification

• E.g. Asynchronous private line modem
• When turned on and ready, modem (DCE) asserts DCE
  ready
• When DTE ready to send data, it asserts Request
  to Send
• Also inhibits receive mode in half duplex
• Modem responds when ready by asserting Clear to
  send
• DTE sends data
• When data arrives, local modem asserts Receive
  Line Signal Detector and delivers data

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Dial Up Operation (1)
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Dial Up Operation (2)
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Dial Up Operation (3)
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DTE and DCE Interfacing
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Data Transmission Using RS-232
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Transmission Using RS-232
This figure shows that RS-232 uses negative 15
volts to represent a 1 bit and positive 15 volts
to represent a 0 bit.
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Null Modem

• Sometimes we may need to allow two devices such
  as PC to communicate directly, that is, with no
  network or DCEs between them.
• Your first reaction may be connecting two RS232
  interface together. However, they both try to
  send a signal to request to send, or receive data
  from the same pin.

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Null Modem
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Summary of RS-232 Main Features
RS-232 is a popular standard used for
asynchronous serial communication over short
distance between a computer (DTE) and a modem or
ASCII terminal(DCE). RS-232 precedes each
character with a start bit, follows each
character with an idle period at least one bit
long (stop bit), and send each bit in exactly the
same length of time.