Slide 17 of 41
Notes:
PER got it almost right! It does not use TLV, and in the very initial design (long before it became a Standard), tags were totally ignored.
However, PER does need to assign an integer value to each alternative of a CHOICE and to each element of a SET in order to produce unambiguous encodings. This integer could have been assigned based on the textual order of the alternatives and elements, but then an “editorial” change which re-ordered these alternatives and elements would have affected the bits on the line.
So the decision was taken that PER would effectively re-order alternatives of CHOICEs and elements of SETsinto an order determined by their tags before assigning an integer value to them to avoid ambiguity in the encoding.
So PER still needs the tags, and all the rules on tags that were initially specified to ensure that BER encodings were unambiguous still apply.
Of course, when writing an ASN.1 specification, you don’t really want to make it only work with PER and not BER, so you will need to obey the rules of tagging anyway.
Hope you are not getting too impatient - we are close now to describing the actual rules (at least, some of them!)
