TELEX OVER RADIO

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TELEX OVER RADIO
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The radio telex technique is built upon
experience gathered from the design of the land
telex network. By means of radio telex, a ship
station is given the possibility of sending
messages from the ship to the international telex
network. Narrow Band Direct Printing (NBDP) is
often used in radio terminology.
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The system

• When a ship has traffic on hand, it will
  establish a radio telex connection on the radio
  frequencies assigned for telex purpose (List of
  Coast Stations). The ship station consists of a
  transmitter, a receiver, a special adapter called
  MODEM (MOdulator DEModulator), and a
  teleprinter.

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PC
STATION 1
Modem
RX TX
TX RX
STATION 2
Modem
PC
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The Modem

• The modem has a very important function in the
  radio telex system. Without a modem, the system
  would not be able to function. The modem takes
  care of the adjustment of signals from the text
  editing equipment to the radio transmitter, and
  vice versa.

• Automatic Error Correction (ARQ)
• Changing the transmitter direction
• Programming selective call numbers
• Calling

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Modulation

• In maritime communications the Upper Side Band
  (USB) is used for all kinds of communication.
• On telex channel, the centre frequency is placed
  in the middle of the USB at 1500Hz.

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• Method of modulation is called Frequency Shift
  Keying (FSK).
• FSK is achieved when transmitted signals
  alternates between two frequencies
• Two frequencies SPACE and MARK
• F1B emission class
• F frequency modulation
• 1 One channel containing digital
  information, no subcarrier
• B Electronic telegraphy, intended to be
  decoded by machine (Radioteletype and digital
  modes)

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ARQ (Automatic Error
Correction)

• Ordinary land telex alphabet of 5
• ARQ systems alphabet of 7
• Uses two way communication
• Ensures that every character has the correct
  proportion of 4 to 3
• If the proportion is wrong, will request for a
  repetition.

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FEC (Forward Error
Correction)

• Use for distress communication, NAVTEX, traffic
  lists, news bulletin, etc.
• Error correction is achieved by using FEC
• Every character is transmitted twice with an
  interval of 280 milliseconds
• If the character is received as garbled, it will
  be printed as a star ( ), or only a space.

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