OER is used in Intelligent Transportation protocols and is highly suited for any protocol for which the speed of encoding/decoding is essential. Whereas in PER Aligned a fixed-size encoding occupying less than 8 bits may begin at any bit position within an octet and may end at any bit position within the same or another octet, in OER the encoding of every possible value of every ASN.1 type occupies a whole number of octets, and all the fields of the encoding are octet-aligned. Unlike PER Aligned, OER is completely octet-oriented. However, in contrast to PER, OER favors encoding/decoding speed and ease of implementation over compactness of the encodings. The Octet Encoding Rules (OER), like the Packed Encoding Rules (PER), produce compact encodings by taking advantage of information present in the ASN.1 schema to limit the amount of information included in each encoded message. The fastest ASN.1 encoding rules are the Octet Encoding Rules (OER). Note that DER is commonly used in security-related applications such as X.509 digital certificates. Two related encoding rules, the Distinguished Encoding Rules (DER) and the Canonical Encoding Rules (CER) are subsets of BER which eliminate some of the extra flexibility provided by BER. BER always sends a tag to indicate what kind of data follows, then a length indicating the length of the data that follows, then the value which is the actual data. The oldest encoding rules, the Basic Encoding Rules (BER), use a Tag-Length-Value (TLV) format for encoding all information. Note that all of the encoding rules are able to represent any messages you would like to exchange.
Each set of "encoding rules" has specific characteristics, such as compactness or decoding speed, which make it best suited for particular environments. ASN.1 has sets of rules precisely specifying how messages must be "encoded" for communication with other machines.
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