The IEC61850 standard is by far the most complete communication standard in the substation automation field. The ASN.1 coding specification is the most basic coding method of the MMS protocol used by IEC61850. Here, we will introduce the "stone" of the IEC61850 protocol communication - ASN.1 coding.
The network protocol stack used in early computer communication does not adopt uniform coding rules when transmitting data. If the information of different coding rules is to be communicated between two computers, the developer must implement codec decoding of different information.
The application of the ASN.1 (abstract syntax tag) specification is a landmark change for computer network communication, which makes communication parties pay more attention to the content of information exchange, rather than the specific codec process.
The ACSI (Abstract Communication Service Interface) defined by the IEC 61850 protocol is specifically mapped to the MMS (Manufacturing Message Specification) protocol, and the MMS protocol uses the coding specification of ASN.1. The IEC61850 standard no longer cares about the specific communication process, but focuses on the data exchange model and interoperability between the IEDs in the substation. Therefore, the application of ASN.1 is the IEC61850 standard in practicality, normativeness, flexibility and ease. One of the reasons why scalability is stronger than traditional statutes.
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The ASN.1 specification is located in the sixth layer of the ISO/OSI seven-layer open interconnection model. It is mainly divided into two parts: grammar rules and coding rules: grammar rules are used to describe the specific composition of information objects, including data types, content order or Structure; the encoding rules define the specific codec syntax of the information.
The data types defined by ASN.1 have both simple basic data types and complex structure types, as shown in the following table.
Coding rules :
ASN.1 provides a variety of coding rules, such as BER (Basic encoding rules), DER, CER, PER, etc. This paper mainly introduces the basic coding rules BER used by IEC61850 in MMS codec. Other coding rules can refer to ASN.1 official website. Documentation.
The coding structure adopted by the ASN.1 basic coding rule BER consists of three parts: tag Tag, length Length, and content Value, which is referred to as TLV structure. The basic coding rule uses 8-bit groups as the basic transmission unit.
There are 4 types of tagged tags, general classes, application classes, context-dependent classes, and special classes. The classification information is as follows:
The ASN.1 BER encoding rule defines a batch of simple types of tag values, but these tag values ​​cannot meet the needs of MMS applications. For example, integer and unsigned integers cannot be distinguished. Therefore, the MMS extension defines a set of dedicated tag values. Available in the ISO/IEC 9506-2 specification.
The Length field indicates the number of octets contained in the Value section.
Content Value Part For simple types, the Value field is the actual value of the data. For the structure type, the Value field is a TLV structure of one or more data, forming a hierarchical nested structure.
ASN.1 provides a rich data organization and flexible extension mechanism to describe very complex data.
MMS syntax specification implementation :
ISO/IEC 9506 – 1 and ISO/IEC 9506 – 2 define the service specification and protocol specification of MMS. These two parts are the core part of the MMS protocol. The specification is mainly composed of ASN.1 syntax, such as:
VariableSpecificaTIon ::= CHOICE {
Name ObjectName,
invokeID Unsigned32,
variableDescripTIon IMPLICIT SEQUENCE {
Address Address,
typeSpecificaTIon TypeSpecificaTIon
},
}
Defines a selection of complex structure type VariableSpecification, the first member is a BITSTRING bit string, the second member is an unsigned 32-bit integer, and the third member is a sequence of complex types that emphasize the order of members.
It can be seen from the above that the ASN.1 specification plays a very important role in the implementation of the MMS protocol. Correct data coding is an important prerequisite for ensuring the application of IEC61850.
Zhiyuan Electronics E8000 and E8300 power quality analysis and monitoring terminals are the first to pass the IEC61850 conformance test certification of Cape Lab. Each group of MMS messages is carefully designed and highly conforms to the ASN.1 coding specification, making the communication between the device and the main station more robust. stable.
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