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.

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.