In modern metering devices (meters of electrical energy, heat, water, gas), as a rule, proprietary protocols are used to transmit data. As a result, metering devices from different manufacturers become incompatible, and this not only complicates the modernization of systems, the introduction of innovations, but also deprives the market of energy accounting systems of “free” competition.

The solution to this problem is to use open protocols, for example, protocols that comply with the IEC 62056 standard (DLMS / COSEM).

Links to published parts of a publication series

1. DLMS / COSEM is an open protocol for data exchange with metering devices. Part 1: Overview
2. DLMS / COSEM is an open protocol for data exchange with metering devices. Part 2: interface classes, metering device model

General information about IEC 62056 (DLMS / COSEM)

DLMS / COSEM is a stack-based protocol based on the concepts of the OSI model, which regulates the exchange of data between metering devices and data collection systems, which is based on the client-server architecture. The underlying specifications in this standard are DLMS and COSEM. The following is a brief description of these specifications.

DLMS stands for Distribution Line Message Specification and is an application-level specification that does not depend on lower levels and, as a result, on communication channels. This specification was developed to standardize messages sent over distribution lines. In this notation, this standard is published under IEC 61334-4-41. Subsequently, the concept of DLMS has undergone changes, and the abbreviation DLMS has become deciphered as the Device Language Message Specification. The goal of the changes was to provide a unified environment for structural modeling and data exchange with metering devices. In its current form, the standard regulates: remote reading of readings from metering devices, remote control, as well as additional services for measuring any type of energy resource (electricity, water, gas, heat).
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COSEM stands for COmpanion Specification for Energy Metering and is a specification that reflects an interface model of metering devices that provides a representation of their functionality. The interface model uses an object-oriented approach.

According to COSEM, a metering device is a physical device consisting of logic devices . Each logical device has a unique identifier (globally), called the logical device name . The information contained in each logical device is available through interface objects . In turn, access to interface objects within a logical device is carried out through association objects . The association object provides information about the resources available in the logical device, depending on the access rights.

Each interface object consists of attributes and methods. The attributes contain information about the object and the functionality that it represents. For example, for an object representing the function “Measuring the frequency of the electrical network”, the attributes will display information about the frequency value (for example, 50) and the unit of measurement (for example, Hz). Methods, in turn, allow you to change or view values. For example, you can reset the values ​​of certain attributes using the Reset () method, if there is one in the corresponding interface object. Methods are not necessarily present in interface objects.

Interface objects that have common characteristics (the same attributes and methods) make up the interface class . The reverse is also true; interface objects are instances of the interface class. The interface class is identified by two parameters: the class identifier ( class_id ) and version ( version ). It should be noted that each interface object, within a logical device, is unique and uniquely identifies information independent of the manufacturer of the metering device, represented by this object using a logical name (the first attribute of any interface object), class identifier and its version.

The DLMS / COSEM standard is fully described in the so-called four color books:

• The blue book describes the object model COSEM of the metering device and the object identification system;
• The green paper describes the architecture and protocols;
• The yellow book covers all issues related to testing for compliance with the standard;
• The white paper contains a glossary of terms.

These books are available free of charge to all members of the DLMS UA (User Association). Excerpts from these books are available to users who are not members of DLMS UA, which can be obtained here .

The difference of the standard DLMS / COSEM from other standards for data exchange with metering devices

The most commonly used standard for data exchange with metering devices is the FLAG protocol, standardized for electricity metering purposes as IEC 61107. There are also other widely used standards:

• The Euridis protocol is used mainly in France and is oriented to transmit information over twisted pair. This protocol is standardized for use in the field of electric power industry, IEC 62056-31: 1999;
• MBUS protocol for heat accounting, standardized by CEN TC 294 as EN1434-3: 1997;
• The IEC 60870-5-102: 1996 protocol for transmitting total integrated values, standardized by IEC TC 57;
• The ANSI C12.18 (optical port), C12.19 (utility tables), C12.21 (communication trough telephone modems) protocols used in North America.

Here we will not make a detailed comparison of the standards, but dwell only on the main points that allow the DLMS / COSEM standard to fully meet the needs of the liberal energy market.

First, the DLMS / COSEM determines the interface model that is valid for any type of energy source (electricity, gas, water, heat, etc.). Each interface object has a standardized unique identifier by which data is identified. This model is completely independent of those protocol levels that transport data. As a result, the system built on the basis of the DLMS / COSEM protocol is open for expansion by adding new interface classes and versions without changing the services providing access to the interface objects, thereby preserving interoperability.

Secondly, the definitions of interface classes standardize a number of widely used functional metering devices: registration of consumption (electricity, heat, water, gas), tariff planning (implementation of multi-tariff metering devices), measurement of power quality, and others. Unambiguous interpretation of data is guaranteed by the fact that The attributes of the interface class use well-defined types (array, structure, boolean, integer, long, etc.) information about which, if necessary, is passed along with the data.

Thirdly, DLMS / COSEM provides controlled and secure access to information inside the metering device for various participants in the energy market. DLMS / COSEM defines three levels of access to the metering device, open access (none), password access (low level) and access with authentication (high level). In addition, information transmitted over communication lines can be encrypted, it is also governed by the standard.

Fourth, since the interface model is completely independent of the communication environment, a wide range of data interfaces can be used without changing the interface model and data management mechanism in data collection systems. Today, serial interfaces and data transmission over the Internet are supported.

Fifth, unlike older protocols, where (for example, in the case of using IEC 61107) special drivers for data acquisition systems were needed for each new measuring device, DLMS / COSEM allows you to create unified drivers through which it becomes possible to communicate with metering devices of different types from different manufacturers.

These unique combinations of features that are not available in any other protocols currently known allow to fully implement the idea of ​​a liberal energy market, make it competitive and open to innovation, and simplify system upgrades.