The problem of repeated requirements...

...in standards is well-known. As the BACNET-L list was reminded today by a little discussion about the Subscribe-COV service.

We received a question from Vamsi Krishna noting that Clause 13.4.1.5 reads, in part:

"If both the 'Issue Confirmed Notifications' and 'Lifetime' parameters are absent, then this shall indicate a cancellation request. If the 'Lifetime' parameter is present then the 'Issue Confirmed Notifications' parameter shall be present."

The question was what a device should do if only the 'Lifetime' parameter is absent. This caused a bit of discussion about whether this situation was even intended, or what should be done since it was not stated. Then Coleman noted that Clause 13.14.2 handled this:

If the 'Lifetime' parameter is not present but the 'Issue Confirmed Notifications' is present, then a value of zero (indefinite lifetime) shall be assumed for the lifetime.

But when I read 13.14.2 in its entirety I noted that it also repeated the requirements of 13.4.1.5. Clearly the three sentences belong together, but having two of them appear earlier led people, myself included, to stop at the first clause with these requirements.

The author of this material was even then an experienced standards writer, so I can only assume it was a cut&paste error. But it sure caused some confusion.