Transcript Slide 1

TELEX OVER
RADIO
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.
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.
PC
STATION 1
Modem
RX
TX
TX
RX
STATION 2
Modem
PC
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
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.
• 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)
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.
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.