Gmdss Radiotelex Terminal; Introduction - Sailor HC4500 Operating Instructions Manual

GMDSS Radiotelex Terminal

Introduction

The GMDSS Radiotelex Terminal is an option used for handling
transmission/reception of telex messages over radio. The terminal
consists of a printer and a keyboard, connected to the transceiver
control unit which provides the interface to the DSC/telex modem
located in the transceiver unit. The keyboard is equipped with an
affixed template for function keys and indicator lamps.
The GMDSS Radiotelex Terminal was designed in accordance with
relevant IMO, ITU and ETSI recommendation/specifications and
has been approved for shipboard installations to be operating
within the Global Maritime Distress and Safety System.
It supports world-wide ship-to-ship, shore-to-ship and ship-to-shore
communication by utilizing the radiotelex protocols described in
ITU- Rec. 625 to overcome the deficiencies of the HF medium. In
case of two-way communication an ARQ (Automatic Repetition
reQuest) algorithm for error correction is thus used, and when
sending to more than one station an FEC (Forward Error Correc-
tion) algorithm is used.
To facilitate error detection the source text consisting of 5-bit telex
characters is coded to a constant weight (3/4 ratio of mark and
space bits) 7-bit code. In FEC mode the message is sent in time
diversity i.e. each character is sent twice with a time interval by
interleaving the original character stream with a delayed version of
itself. The receiving station thus has two chances to receive the
character correctly. If both are in error a '*' is printed. FEC
broadcast calls are used for sending collective messages to
several stations simultaneously. A special class of FEC allows
selective calling by means of call codes. The message is transmit-
ted in inverted format and only receiving stations with the correct
call codes will receive the message.
ARQ operation involves two stations. The information sending
station (ISS) sends the information in blocks of 3 characters and
listens in the interval between the blocks for an acknowledgement
character to be received from the information receiving station
(IRS) indicating whether or not the latter has detected any
erroneous character(s) in which case the block will be repeated by
the ISS. Both the stations involved in a communication session
may initiate an OVER sequence to change the direction of
information flow or a BREAK sequence to terminate the connec-
tion. The station which initiates the connection becomes the
'master' station by transmitting the call signal of another station
after going from 'standby' to 'phasing' state. The called station
becomes the 'slave'. When it recognizes its own call signal it will
also leave 'standby' and enter 'phasing' state by transmitting an
appropriate control character. After having verified the other station's
identity both stations will proceed to 'traffic' state and start exchang-
ing messages. If the quality of the radio link deteriorates resulting in
a large number of block repetitions, both stations will automatically
advance to the 'rephasing' state, in which the 'master' station tries to
call the 'slave' again, as it did in the 'phasing' state, without any of
them terminating the connection now under re-establishment. Both 9
digit and 5/4 digit call signals are supported and the corresponding
switching between the new protocol (ITU-R M. 625) and the old ITU-
R M. 476 is automatically performed.
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