Telecommunications

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Telecommunications

• Communications Model
• Trasmission Modes
• Transmission Direction
• Transmission Media
• Communication Networks

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Communications Model
Message
Medium
Sender
Receiver
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Transmission of Digital Data over Analog Links
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Transmission Modes
start bit
start bit
end bit
end bit
start bit
end bit
byte
byte
byte
Asynchronous transmission Low-speed transmission
of one character at a time, using control bits
(i.e., start stop bits)
byte
byte
byte
byte
Synchronous transmission High-speed simultaneous
transmission of large blocks of data
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Transmission Direction

• Simplex transmission
• Transmission occurs in one direction only
• e.g., radio or TV waves
• Half-duplex transmission
• Transmission occurs in both directions, but only
  one at a time
• e.g., Walkie-Talkie, CB radio, pagers
• Full-duplex transmission
• Transmission occurs in both directions
  simultaneously
• e.g., telephones

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Transmission Medium wire-based

• Twisted wire pair
• A pair of copper wires are twisted to avoid
  signal interference
• The cost is low, and widely used in local loops
  of telephone net.
• Subject to communication noises
• Coaxial cable
• Layers of copper conductors covered with
  shieldings
• can transmit data, video, voice signals over one
  line
• Fiber optics cable
• Fastest wire-based transmission medium
• The cost is too high, and ideal as a corporate
  back-bone cable to interconnect many LANs

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Transmission Medium wireless

• Terrestrial microwave
• microwave dishes needed at certain intervals
  (appr. 50km)
• allows for very high-speed transmissions, and
  used by phone companies or common carriers
• Satellite microwave
• involves uplinks downlinks travelling to and
  from the satellite
• widely used for GPS applications
• Cellular communication
• An area is divided up into many small cells, and
  an antenna installed in each cell handles signal
  transmissions

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Microwave and Satellite Transmissions - Contd
The satellite and the antenna
Microwave antenna
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Mobile telecommunications
IMT-2000 video phone
Cellular phone
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Communications Networks

• LAN(Local Area Network)
• ideal for small area networking needs (e.g.,
  room, floor, building)
• widely used to support computing needs in the
  office environment
• MAN(Metropolitan Area Network)
• a backbone cable linking small LANs throughout
  an organization
• e.g., A network interconnecting agencies of an
  insurance firm
• WAN(Wide Area Network)
• networking support covers a large geographical
  area
• used by airlines, banks, etc.