Specifications

RM2151 - RADIO TELEX COMMUNICATION Page 2-1
2 RADIO TELEX COMMUNICATION.
2.1 Introduction
This chapter describes the basic properties of radiotelex systems employing error correction in the form of ARQ
(Automatic Retransmissions Request) and FEC ( Forward Error Correction). The technical details of the error
correction is defined by the CCIR ( The International Consultative Committee ) in recommendation Rec. 476-3 and
the new Rec. 625. Theese specifications are generally accepted and permit equipment from different manufacturers
to communicate via common radio circuits.
2.2 Basic system description
The advantage of telex communication as compared to voice communications is obvious to anyone accustomed
to communication by an ordinary telex machine. For mobile communication, especially if HF-radio circuits are the
primary transmission media, the quality of the communication is subject to interference effects from a variety of
sources. The result of this noise and interference is an annoyance and inconvenience but it is not particulary
damaging for the voice communications since the meaning of the message can be understood without recognizing
every word. Telex communication, however, should not be a subject to character mutilation because it might be
impossible to distinguish the intended information from the characters actually received. As an example, this could
be position information, tonnage reports and payroll information. Therefore means must be used to overcome the
adverse results of noise and interference which are inherent on HF-radio circuits.
The answer to reliable and efficient telex communication via radio is error detection and correction. In 1970 the CCIR
drafted the recommendation for error detection and correction, defining a constant-ratio code for automatic error
detection and giving detailed requirements for the error correction.
As traffic increased, CCIR have found it necessary to recommend a revised procedure for radiotelex communication.
Together with the introduction of 9-digit selective call numbers, the new recommendation Rec. 625 has been
approved by CCIR in May 1986.
Data coding
The recommended code is a 7-unit synchronous code which uses only those bit combinations having a ratio of 3
Mark bits to 4 Space bits to represent valid characters. There is 35 of the 3/4 ratio combinations, allowing all 32
CCITT-2 characters to be represented.
The CCIR recommended code needs no start or stop bits because it is synchronous. No error-detecting parity is
required because the Mark/Space ratio is constant. This allows efficient transmission of messages, since all
character bits are used to determine the character.
Error detection
In addition to allowing more efficient use of transmission time, the CCIR code is more effective in detecting errors
than the parity-equipped CCITT-2 code. The CCIR code detects all combinations of bit inversions odd or even,
except the rare inversion of an equal number of Space and Mark bits in one character, preserving the 3/4 ratio. Thus
the undetectable combinations of the CCIR code will occur much less often than those of the CCITT code.
Error correction
The elementary error detection system does what it is designed to do - detect errors. The usefulness of the system
lies in its ability to correct errors. Depending on the application several methods exist for error correction. Two basic
methods are recommended. ARQ and FEC.
2.3 ARQ operation
The first method is named ARQ (Automatic Retransmissions Request).
ARQ is workable between two stations only, and involves sending data in small blocks, then listening between blocks
for the receiving station to request either the next block, or retransmissions of the last block because of an error.
In ARQ mode, two stations communicate directly with one another. The station which initiates the connection is
designated the Master and the other is the Slave. Initially the Master is the sending station, but at any later instance
the Slave may take the role of sending station and sends information to the Master. This is like a telephone
conversation; The Master is the one who makes the initial call, at any later time the Slave or the Master may be
talking. Data is transmitted in blocks of three characters. Between the block of characters, the sending station waits
for a reply of a single character from the receiving station, indicating the validity of the received data. If an error occurs
in a data block, the receiving station sends a request for repetition. The sending station, upon receiving a request
for repetition. retransmits the block. The request may be repeated up to 32 times, until the complete block have been
received error free. After 32 repetitions the Master station automatically initiates a new call. If this new call is
unsuccessful and two-way communication cannot be re-established, both Master and Slave reverts to the Standby
condition. If the new call is successful the two units automatically continue the interrupted communication.With the
introduction of CCIR Rec. 625, the two stations will, as part of the calling procedure, exchange identities. If, at any
later stage, the connection is broken because of errors (called rephasing or restart), the stations will only reestablish
the connection with the same station as before, thus avoiding the annoying problem with a third station grabbing an
existing connection.