CSC 335 Data Communications and Networking

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CSC 335 Data Communications and Networking

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

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Motivation

• Next topics
• transmission mode
• transmission standard
• multiplexing- many signals of different carrier
  frequencies sharing a single medium to
  communication with many receivers
• data compression

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Transmission Mode

• A transmission mode defines the way in which a
  bit group goes from one device to another. It
  also defines whether bits may travel to both
  directions simultaneously or whether devices must
  take turns sending and receiving. The former
  relates to the choice of grouping and the latter
  relates to the timing of transmission.

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Serial vs. Parallel Communication

• The first choice regarding transmission mode is
  how sender and receiver can choose to group data
  bits.
• Parallel transmission means that a group of bits
  is sent simultaneously by using a separate line
  (wire) for each bit. Usually this transmission
  mode is used between short-distance connections.
• Serial transmission means that a single wire is
  used to send a group of data bits one bit at a
  time. Serial communication is used mostly between
  distant connections.

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Serial vs. Parallel Communication
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Asynchronous Communication

• There are two ways to provide serial
  communication Synchronous and asynchronous
  transmission.
• In a broader sense, asynchronous transmission
  means that the sender and the receiver dont have
  to synchronize before transmission. In an
  asynchronous system, the receiver must be ready
  to accept data whenever it arrives.

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Asynchronous Communication

• In a more technical sense, communication hardware
  is classified as asynchronous if the electrical
  signal doesnt contain information that the
  receiver can use to determine where individual
  bits begin and end.

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Asynchronous

• Data transmitted on character at a time
• 5 to 8 bits
• Timing only needs maintaining within each
  character
• Resync with each character

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Asynchronous (diagram)
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Asynchronous - Behavior

• In a steady stream, interval between characters
  is uniform (length of stop element)
• In idle state, receiver looks for transition 1 to
  0
• Then samples next seven intervals (char length)

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Asynchronous - Behavior

• Then looks for next 1 to 0 for next char
• Simple
• Cheap
• Overhead of 2 or 3 bits per char (20)
• Good for data with large gaps (keyboard)

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Synchronous - Bit Level

• Block of data transmitted without start or stop
  bits
• Clocks must be synchronized
• Can use separate clock line
• Good over short distances
• Subject to impairments
• Embed clock signal in data
• Manchester encoding
• Carrier frequency (analog)

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Synchronous - Block Level

• Need to indicate start and end of block
• Use preamble and postamble
• e.g. series of SYN (hex 16) characters
• e.g. block of 11111111 patterns ending in
  11111110
• More efficient (lower overhead) than async

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Synchronous (diagram)
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Traditional Configurations