IEC 61334-6:2000 pdf download
IEC 61334-6:2000 pdf download.Distribution automation using distributionline carrier systems -Part 6:A-XDR encoding rule.
5 Rules for encoding
A-XDR encoding of any ASN,1 type results in an integral number of bytes, each containin9 eight bits. This series of bytes begins with the first byte of the encoding of the identifier field 01 the outermost ASN.1 type — in these terms, this byte can be considered as the most significant byle For the purpose Of this standard, the following identification schema applies:
• the bytes of the A-XDR encoding are not systematically numbered, but sometimes, when it helps for clear understanding, comments apply (for example, I byte of the value. etc.);
• bits of any byte are numbered from 1 to 8, where bit 8 is the most significant.
5.1 The Identifier field
The purpose of the Identifier field is to Indicate the type of the encoded value. Provided that the sender and the receiver are operating exactly the same specification of the abstract syntax, the Identifier field conveys information only in cases when
a) one type of data should be selected between different alternatives — CHOICE:
b) the presence of an OPTIONAL component in a SEQUENCE type should be indicated;
C) the presence of a DEFAULT component in a SEQUENCE type should be indicated.
A-XDR encoding contains an Identifier field only In these cases. In addition. A.XDR encodes an Identifier when encoding the Identifier is required by the ASN.1 specification (ASN.1 explicit tagging. see 6.7).
In case a), A-XDR requires that all the alternatives of the CHOICE will be specified at ASN.l level as explicitly tagged types (see 6.7). In these cases. the encoded tag forms the identifier field.
In cases b) and C). the presence or the absence of the OPTIONAL or DEFAULT component is indicated by a so-called BOOLEAN presence flag The Identifier field for these component values is the A-XDR encoding of the value of the presence flag (see 6.9).
On the other hand, A-XDR may be forced to encode the Identifier field, when the ASN.1 definition contains ASN.1 explicit tags (see 6.7). A-XDR encoding of such types is defined to be the same as their BER encoding. The aim of this support is to force the length to be encoded, for example, in order to allow for the easy omission of some structures. The Identifier field for those types is the encoded value of he ASN.1 tag and occupies an integral number of bytes, at least one, as specified in ITU-T Recommendation X.209.
5.2 The Length field
In A-XOR the Length field (when this field is present) immediately precedes the Contents field. represents explicitly the length of the Contents field and occupies an integral number of bytes. Provided that the sender and the receiver are operating exactly the same specitication of the abstract syntax, the Length field conveys information only in cases when a variable length ASN,1 type is to be encoded. The possible cases are
a) variable length INTEGER:
b) variable length BIT STRING:
C) variable length BYTE STRING:
d) variable length SEQUENCE OF type.
A-XDR encodes the Length field only in the above cases. and, in addition, in one more case. when it is required by the ASN.1 specification (ASN.1 explicit lagging, see 6.7).
In a), b), C) and d), the Length field is encoded as a variable length integer. For the additional case, the same encoding applies as defined In BER (see ITU-T Recommendation X.209) except for the restriction that only the definite form can be used. Using the indefinite form (footnote 7) for the Length field Is not allowed within A-XDR.
5.3 The Contents field
The Contents field is the substance of the encoding. conveying the actual value, It consists of zero or more bytes, and shall encode the data value as specified in the following clauses.
