THE DRAWINGS CONTAINED IN THIS RECOMMENDATION HAVE BEEN DONE IN AUTOCAD.
          Recommendation U.75
                          AUTOMATIC CALLED TELEX ANSWERBACK CHECK
                       (Malaga-Torremolinos, 1984; amended at Melbourne, 1988)
                The CCITT,
          considering
                (a) that there is a need to check the answerback of the called telex number
          [e.g. delivery from a  telex-Teletex  conversion  facility  (CF)/  or  store  and
          forward unit (SFU)];
                (b) that Recommendation F.60 defines a preferred structure  for  the  telex
          answerback;
                (c) that different forms of answerback exist,
                (d) that Recommendation F.60 bis defines a structure for a telex answerback
          associated with an intermediate document storage device;
          unanimously declares
                that the following requirements are recommended  for  automatic  answerback
          check of a called telex terminal by an administration's equipment:
          1      Case 1: reference information for the check is provided by the calling 
                 subscriber
                This information  can  be  in  total  or  part  of  the  called  subscriber
          answerback (contiguous printable characters and space). There is  no  restriction
          on the number of characters supplied.
                In this case, the called party answerback check consists of  verifying  the
          presence of the provided character string. Considering the  information  provided
          in the directories and terminal identifications, allowance is to be made for  the
          following differences:
                 -   one character mismatch in the letter part;
                 -   one hyphen or one space is ignored in the national call number.
          2      Case 2: no information on the answerback of the called terminal is 
                 provided by the calling subscriber
                The reference  information  for  the  answerback  check  is  the  selection
          information provided by the calling subscriber.
                In this case, the called party answerback check consists of:
                 -   extracting the national call number and F.69 code from the answerback;
                 -   comparing the obtained national call number and  F.69  code  with  the
                     supplied  selection  information  code.  Allowance  is  made  for   the
                     following cases of mismatch:
                     a)  a positive  national  call  number  match  without  a  valid  telex
                        national identification code (TNIC) match,
                     b)  a  match  between  the  least  significant  part  of  the  supplied
                        selection information and the national call  number  obtained  from
                        the called party answerback,  considered  to  be  positive  if  the
                        difference in field length is limited to two characters.
          3      An algorithm which meets the preceding criteria for cases 1 and 2 is shown
          in Figure 1/U.75
                In some circumstances, it may be necessary to  compare  the  answerback  of
          the called  subscriber  with  the  answerback  received  and  recognized  at  the
          beginning of the call.
                In such cases, if the received string consists of more characters than  the
          previously recognized answerback, then a check should be made as to  whether  the
          recognized answerback is part of the received string.
                                          FIGURE 1/U.75 - CCITT 76220

          Note 1 - Check the automatic emitting speed and wait for the end of  the  answerback.  The
          answerback is considered to have ended after detection of a 300 ms period of idle.
          Note 2 - "Retry" refers to another attempt to trigger the answerback.
          Note 3 - If unsuccessful, perform one retry if allowed in the protocol.
          Note 4 - The answerback provided could be a contiguous part of the expected answerback  or
          all of it.
          In case of  a  return  call  to  the  calling  subscriber  (e.g.  PDN  (positive  delivery
          notification) or NDN (negative delivery notification) delivery) the stored  calling  telex
          answerback is considered as a "provided" one.
          Note 5 - This comparison is to verify the presence of the provided character string in the
          received answerback, allowing one character mismatch in the letter part.
          Note 6 - A zero in the selection, but not in the  answerback  in  front  of  the  national
          number is to be ignored. If the received figure group is shorter than  the  selected  one,
          consider it as match, but make a note in the call record "received  figure  group  is  not
          complete". It is possible that the received figure group includes the F.69 code.





                                                         Fascicle VII.2 - Rec. U.75   PAGE1

         Note 7 - Forward message, but make a note "area code-check was not possible" into the call
         record.
         Note 8 - If called A/B is not available from previous procedures.
         Note 9 - If a digit "0" appears at this stage between the  F.69  code  and  the  "NCN"  it
         should be ignored.

































































         PAGE2   Fascicle VII.2 - Rec. U.75