Recommendation U.21
                        OPERATOR RECALL ON A TELEX CALL SET UP
                             ON A RADIOTELEGRAPH CIRCUIT
                             (New Delhi, 1960; amended at Geneva, 1964)
               The CCITT,
         considering
               (a) that experience  has  shown  that,  for  telex  calls  set  up  over  a
         radioteleg              circuit,               it               was
         useful to enable the telex  subscriber  to  cause  an  operator  to
         re-enter on a call in progress without interrupting it;
              (b)   that such re-entry may be of interest in  the  following
         cases as well as in the case of a defective connection:
               i)  When a subscriber decides, in the course of a call, to change  from  a
                  plain text to a cypher he can call the operators in the terminal  radio
                  exchanges and ask them to  interrupt  the  delay  signal,  which  might
                  otherwise disturb the synchronism between the cyphering apparatus  used
                  at the two ends.
               ii) When a subscriber has sent a message but waits a very long time for  a
                  reply from his correspondent, he  can  ask  the  operator  whether  his
                  message is still being stored  or  whether  it  is  expected  that  any
                  interruption to the radio circuit will continue. If  need  be,  he  can
                  then choose another means of communication (telegram or telephone call)
                  to send an urgent message to its destination;
               (c) that although it seems that re-entry by an  operator  will  be  limited
         mainly to national networks (for example by a subscriber calling the  controlling
         telex operator on the radiotelegraph circuit), international  standardization  of
         an operator recall signal would be useful if the controlling  telex  operator  on
         the radiotelegraph circuit  is  located  in  a  transit  country,  and  also  for
         intermediate manual switches; this would no doubt prove to be a  great  advantage
         when this possibility is generally utilized,
         unanimously declares the following view
               (1) If the Administrations concerned agree on the use of a  special  signal
         enabling a subscriber to recall an international telex operator's position making
         use of radiotelegraph circuits, such a recall must not cause release of a call in
         progress.
               (2) This operator recall signal will consist of the follow   
         sequence:  combinations  No.  28  (line-feed)  followed   by   four
         combinations No. 27 (carriage-return).
              (3)   The detection device causing re-entry  by  the  operator
         will be controlled by the receipt of four consecutive  combinations
         No.  27;  combinations  No.  28  will  only  be   used   to   avoid
         superposition of the text on the receiving teleprinter and will not
         have to be recognized by the detection device.
              (4)   The equipment for  discriminating  the  operator  recall
         signal will be switched off  by  a  sequence  of  four  consecutive
         combinations No. 19 (signal for transfer to data).























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