Packed encoding rules (PER) are ASN.1 encoding rules for producing a compact transfer syntax for data structures described in ASN.1, defined in 1994.
This Recommendation or International Standard describes a set of encoding rules that can be applied to values of all ASN.1 types to achieve a much more compact representation than that achieved by the BER and its derivatives (described in ITU-T Rec. X.690 | ISO/IEC 8825-1).
It uses additional information, such as the lower and upper limits for numeric values, from the ASN.1 specification to represent the data units using the minimum number of bits. The compactness requires that the decoder knows the complete abstract syntax of the data structure to be decoded, however.
There are two variations of packed encoding rules: unaligned and aligned. With the unaligned encoding, the bits are packed with no regard for octet (byte) boundaries. With aligned encoding, certain types of data structures are aligned on octet boundaries, meaning there may be some number of wasted padding bits. Unaligned encoding uses the least number of bits, but presumably at some cost in processing time.
The packed encoding rules also define a restricted set of encoding rules, called CANONICAL-PER, which is intended to produce only a single possible encoding for any given data structure. CANONICAL-PER's role is therefore similar to the role of DER or CER.
Documents: ITU-T_X.691, ISO 8825-2.