04-3 DLMS/COSEM brings interoperability
DLMS/COSEM brings interoperability to meter data exchange
Since the publication of the DLMS/COSEM standards in 2002 (IEC 62056, EN 13757-1), growing global interest confirms that it is up to the expectations. See an overview of the market needs leading to its creation, and the technology developments enabling it. Understand the main principles and benefits for purchasers and vendors and learn about the latest developments.
Market liberalisation and global competition created a complex environment and drastically new needs for meter data collection. Developments in this area provide one of the core technologies enabling new ways of operating the energy market. Energy providers, focusing on customer satisfaction and striving to optimise the use of their resources, need metering systems that support all energy types, like electricity, gas, water etc. providing more data, more often, from geographically dispersed locations to more commercial partners. Accurate data on time, securely, with the lowest possible overall cost are needed.
Enabling technologies, like low-cost A/D converters, microcontrollers, DSP-s, high capacity memories and latest communication methods are driving a revolution started some 30 years ago in a conservative industry, dominated for more than a century by electromechanical meters and manual meter reading. Needs for more accuracy first and more data later are met by intelligent, communicating meters, spreading from large consumption applications to the lower, residential segments. Collection of the huge amount of data generated must be based on efficient solutions. Cost of system integration and maintenance becomes the key aspect.
The first data collection systems were inevitably based on proprietary solutions. Utilities, working in a stable environment, were ready to partner with a single or a few vendors and were able to dedicate resources to operate such systems.
Need for standardisation has been identified early and IEC 61107 has been published in 1992. It defined how to log on to the meter and retrieve data, but it did not say anything about how to find, identify, format and interpret them. Drivers based on manufacturer specific information had to be developed for each meter type and data collection system - an expensive and lengthy process. Maintenance is increasingly difficult in today’s environment of global expansion, mergers and takeovers leading to an inhomogeneous meter park and requiring consolidation of data collection systems.
A drastically new approach was taken by leading manufacturers and utilities, grouped in the DLMS UA. It builds on the principles and some elements of the latest standards for power system control, developed by IEC TC 57. The telecontrol industry was one of the first to realise that system integration of intelligent electronic devices requires a new architecture modelling the process by interface objects, and separating the model from the communication protocol. While the process model is relatively stable, communication technologies are changing fast. Splitting the models from protocols separates two problem domains, allowing them to develop at their own speeds and increasing flexibility.
DLMS/COSEM has been conceived to bring interoperability into meter data exchange. This has been achieved by developing an interface object model, common for all energy types, specifying the identification, formatting and interpretation of all data. The model is separated from the protocol, to allow using any media. Protocol suites for various communication profiles, based on widely used international standards have been developed. Standard extension rules, self-descriptive features, possibility to negotiate features and contexts render the specification future proof and allow the development of a generic data collection system, capable to connect to the meter, reveal its data structure, load parameters and collect data. DLMS/COSEM also provides advanced security features and selective access to data, not available before. To verify implementations, a conformance certification scheme is available.
The COSEM object model (Companion Specification for Energy Metering), described in the Blue Book defines interface classes close to the metering domain, like registers, profiles, clocks, schedules. An interface class definition describes the attributes with the data types usable, and methods allowing the modification of the attributes. Objects may interact with each other to perform functions like tarification, end of billing period etc. Handling of special events, like clock setting, power failures, is also defined.
The functionality of the meter – be it simple or complex - is modelled by instantiating the necessary number of the appropriate objects, identified and referenced by their logical name attributes as defined in the OBIS (Object Identification System) standard. Functionality can be freely organised to several logical devices within a physical device or can be even spread across several physical devices. COSEM does not standardise and by no way limits the functionality of the meter. The model supports a wide range of functions and new ones are continuously added. This year, for example, a range of objects modelling advanced power quality and loss compensation features have been defined.
The DLMS-based protocols (Device Language Message Specification) transport the data represented by attributes and methods of the objects. Access to these is the task of the DLMS services provided by the COSEM application layer, the top layer in any protocol stack. Data exchange is based on the Client/Server paradigm: the data collection system requests services and the meter provides them. In addition, the meter can initiate reporting events. The DLMS services are common to all interface classes. This allows defining new ones without affecting the protocol. A special Association object controls authentication and provides a view of the functionality of the meter, the objects, their attributes and methods available for a given client. Several associations may be defined, providing selective access to the various parties, according to their respective access rights. All data are clearly structured using ASN.1 and efficiently encoded using A-XDR. Protocol overhead can be significantly reduced by structuring data in profiles and transporting them in single requests/responses.
Lower layers of the protocol are selected according to the communication media. For PSTN and GSM networks, a 3-layer OSI-based protocol stack is defined. The HDLC based data link layer ensures the integrity of data transport and provides segmentation for long messages, like readout and load profiles. The physical layer supports the optical port, serial ports and PSTN or GSM modems.
This year a new profile allowing meter data exchange via the internet has been added, opening the way for DLMS/COSEM to co-exist with a range of internet applications, like file transfer, mail service etc. The COSEM application layer is supported by TCP/UDP encapsulated in a simple wrapper. Below the IP layer, any data link layer and physical layer can be used. The protocols are described in the new Green Book.
Interoperability is defined as the ability to exchange data between any meter and any data collection system having compatible capabilities. DLMS/COSEM comes complete with a conformance-testing scheme, specified in the Yellow Book. Although in itself it cannot guarantee interoperability, it is an important tool to verify the correctness of implementations. To date, 13 meter types have been certified.
Market acceptance is shown by growing number tenders specifying DLMS/COSEM, growing number of compliant meters – several hundred thousands are on the wall - and data collection systems supporting them, steadily growing membership of the DLMS UA, this year especially from the Far-East. Migration is an important issue, given the large number of existing meters and systems. For this purpose, most meters continue to offer IEC 62056-21 Mode C - in parallel with the DLMS/COSEM mode - although functionality may be reduced. In addition, protocol converters are available.
Purchasers and vendors benefit alike from using DLMS/COSEM, a truly open, global standard. Purchasers enjoy lower system integration and maintenance costs, a consolidated approach to meter functionality driven by the standard model, and enhanced control and security to access meter data. Vendors building on DLMS/COSEM can compete on the application and price/performance ratio instead of competing on technical details. The DLMS UA provides a platform to share and enrich experience.
The SELMA project in Germany (www.selma-project.de, Metering International 2004/2) is a good example. In this project, utilities and manufacturers work together to extend DLMS/COSEM with enhanced security features, like authentication, secure channels and certified device components. This extension requires only the definition of a few new interface classes, like daily load profiles with digital signature.