DLMS/COSEM通信框架概述

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"DLMS Green Book, DLMS/COSEM通信框架概述" 《Excerpt_GB7》是DLMS用户协会发布的一份文档，该文档摘自《能源计量DLMS/COSEM架构和协议的伴侣规范》的第二版第七次修订版。DLMS（Distributed Local Measurement and Supervision）和COSEM（Companion Specification for Energy Metering）是智能电网通信中的关键标准，主要用于电能表和其他计量设备的数据交换和管理。 1. **DLMS用户协会**：DLMS User Association是一个国际组织，致力于推广和维护DLMS通信协议，确保其在电能计量领域的应用。 2. **DLMS/COSEM通信框架**：这个框架定义了设备间的通信方式，包括客户端/服务器操作模式、连接导向的操作以及协议识别服务，以确保不同设备之间的互操作性和互联性。它提供了一种结构化的方法来处理能源数据的采集、传输和处理。 3. **客户端/服务器操作**：在DLMS/COSEM系统中，设备可以作为客户端发起请求，而其他设备作为服务器响应请求。这种操作模式支持分布式系统中的数据交换。 4. **连接导向的操作**：通信过程建立在连接的基础上，确保了数据传输的可靠性和安全性。 5. **互操作性和互联性**：COSEM通过定义标准接口和协议，使得不同供应商的设备能够在同一网络中无缝工作。 6. **协议识别服务**：这一服务确保不同设备能够识别和理解彼此的通信协议，从而实现正确的数据交换。 7. **应用模型**：文档中描述了多种应用模型，如DLMS/COSEM服务器模型、基于DLMS/COSEM的客户端模型和数据收集系统模型，这些模型指导了系统设计和实现。 8. **服务器模型**：DLMS/COSEM服务器模型定义了提供服务的设备，例如电能表，如何处理来自客户端的请求。 9. **客户端模型**：基于DLMS/COSEM的客户端通常是指需要从服务器获取数据或向服务器发送命令的设备，如数据采集器或远程读表系统。 10. **数据收集系统模型**：这部分描述了如何构建一个能够从多个DLMS/COSEM设备收集数据的系统。 11. **访问要求**：规定了对系统和数据的访问控制，确保只有授权的实体可以访问和操作数据。 12. **系统集成与电表安装**：讨论了DLMS/COSEM设备在实际电网环境中的部署和集成策略。 13. **迁移**：文档还涵盖了从旧系统迁移到DLMS/COSEM平台的过程和技术考虑。 14. **物理层服务**：虽然未详细展开，但物理层服务是通信的基础，包括信号传输、错误检测和纠正等，对于确保数据的准确无误传输至关重要。《Excerpt_GB7》提供了关于DLMS/COSEM通信协议的全面概述，是理解和实施智能电表通信解决方案的重要参考资料。这些规范和标准对于电力公司、设备制造商以及从事智能电网技术开发的工程师来说具有很高的价值。

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Interoperability is an application level notion: a client AP is interoperable with a server AP, if it is

able to establish AAs using the A-Associate service of the standard connection-oriented ACSE, as it

is specified in Clause 9.

AAs may be established between a client AP and server APs using various application contexts,

authentication mechanisms and xDLMS contexts as well as other parameters. For example, a client

AP may establish an AA with a server AP with an application context using short name (SN)

referencing and with another server AP with an application context using logical name (LN)

referencing. Although the messages exchanged depend on the application context of the AA

established, both server APs are interoperable with the client AP if it is able to establish the AA

using the right context with both server APs. With this, using the services of the standard ACSE for

AA establishment ensures interoperability in COSEM. In the case of pre-established AAs – see 4.2

– the choice of all parameters has in effect been previously made and fixed, for each AA.

On the other hand, interconnectivity is a protocol level notion: in order to be able to exchange

messages, the client and the server APs should be interconnectable and interconnected.

Before the two APs can establish an AA, they must be interconnected. The two APs are

interconnected, if each peer protocol layer of both sides, which needs to be connected, is

connected. In order to be interconnected, the client and server APs should be interconnectable and

shall establish the required connections. Two APs are interconnectable if they use the same

communication profile.

With this, interconnectivity in DLMS/COSEM is ensured by the ability of the COSEM AP to establish

a connection between all peer layers, which need to be connected.

4.4 Ensuring interconnectivity: the protocol identification

service

In DLMS/COSEM, AA establishment is always initiated by the client AP. However, in some cases, it

may not have knowledge about the protocol stack used by an unknown server device (for example

when the server has initiated the physical connection establishment). In such cases, the client AP

must obtain information about the protocol stack implemented in the server.

The COSEM framework provides a specific, application level service to for this purpose: the

protocol identification service. It is an optional application level service, allowing the client AP to

obtain information – after establishing a physical connection – about the protocol stack

implemented in the server. The protocol identification service, uses directly the data transfer

services (PH-DATA.request /.indication) of the PhL; it bypasses the other protocol layers. It is

recommended to support it in all communication profiles that have acces to the PhL.

4.5 Application models

COSEM models metering equipment as a set of logical devices, hosted in a single physical device.

Each logical device models a subset of the functionality of the metering equipment as these are

seen through its communication interfaces. The various functions are modelled using COSEM

interface objects.

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The metering equipment on the left hand side comprises “n” logical devices and supports the 3-

layer, CO, HDLC-based communication profile.

The COSEM AL is supported by the HDLC-based data link layer. Its main role is to provide a

reliable data transfer between the peer layers. It also provides addressing of the logical devices in

such a way, that each logical device is bound to a single HDLC address. The Management logical

device is always bound to the address 0x01. To allow creating a LAN so that several metering

devices at a given metering site can be reached through a single access point, another address, the

physical address is also provided by the data link layer. The logical device addresses are referred

to as upper HDLC addresses, while the physical device address is referred to as a lower HDLC

address.

The PhL supporting the data link layer provides serial bit transmission between physical devices

hosting the client and server applications. This allows using various interfaces, like RS 232, RS

485, 20 mA current loop, etc. to transfer data locally through PSTN and GSM networks etc.

The metering equipment on the right hand side comprises “m” logical devices.

The COSEM AL is supported by the COSEM TL, comprising the internet TCP or UDP layer and a

wrapper. The main role of the wrapper is to adapt the OSI-style service set, provided by the

COSEM TL to and from TCP and UDP function calls. It also provides addressing for the logical

devices, binding them to a SAP called wrapper port. The Management logical device is always

bound to wrapper port 0x01. Finally, the wrapper provides information about the length of the

APDUs transmitted, to help the peer to recognise the end of the APDU. This is necessary due the

streaming nature of TCP.

Through the wrapper, the COSEM AL is bound to a TCP or UDP port number, which is used for the

DLMS/COSEM application. The presence of the TCP and UDP layers allows incorporating other

internet applications, like FTP or HTTP, bound to their standard ports respectively.

The TCP layer is supported by the IP layer, which is in turn may be supported by any set of lower

layers depending on the communication media to be used (for example Ethernet, PPP, IEEE 802

etc.).

Obviously, in a single server it is possible to implement several protocol stacks, with the common

COSEM AL being supported by distinct sets of lower layers. This allows the server to exchange

data via various communication media with clients in different AAs. Such a structure would be

similar to the structure of a DLMS/COSEM client show below.

4.7 Model of a DLMS/COSEM based client

Figure 7 shows the model of a DLMS/COSEM based client as an example.

The model of the client – obviously – is very similar to the model of the servers:

 in this particular model, the COSEM AL is supported either by the HDLC-based data link layer or the

COSEM TL, meaning, that the AL uses the services of one or the other, as determined by the APs. In

other words, the APDUs are received from or sent through the appropriate supporting layer, which in turn

use the services of its supporting layer respectively;

 unlike on the server side, the addressing provided by the HDLC layer has a single level only, that of the

Service Access Points (SAP) of each Application Process (AP).

As we have seen, client APs and server logical devices are identified by their SAPs. Therefore, an

AA between a client and a server side AP can be identified by a pair of client and server SAPs.

The COSEM AL may be capable to support one or more AAs simultaneously. Likewise, lower layers may be

capable of supporting more than one connection with their peer layers. This allows data exchange between

clients and servers simultaneously via different ports and communication media.

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DLMS/COSEM client

using both 3-layer, CO, HDLC based and TCP-UDP/IP based communication profile

COSEM Transport layer

COSEM Application layer

HDLC based data link layer

# 0w # 0z# 01

Physical layer

e.g. RS 232, RS 485, optical port, current loop

Port # 2Port # 1 Port # n

Appl_

Process_1

Public client

Appl_

Process_2

Appl_

Process_m

ACSE

xDLMS

ASE

Port # 2Port # 1 Port # n

Web

pages

Files

FTP HTTP

DLMS/COSEM wrapper

# 01

# 0w

# 0z

Network layer

Data link layer

e.g. PPP

Data link layer

e.g. Ethernet

Transport layer

TCP or UDP

Data link layer

e.g. ATM

Physical layer

e.g. RS 232

Physical layer

e.g. Ethernet

Physical layer

e.g. PPP

FTP

port

HTTP

port

DLMS/

COSEM

port

Figure 7 – Model of a DLMS/COSEM based client using multiple protocol stacks

4.8 Model of a DLMS/COSEM based data collection system

Figure 8 shows the model of a DLMS/COSEM based data collection system, consisting of two

metering sites and two remote data collection systems is shown.

On metering site #1, metering equipment use the TCP-UDP/IP based communication profile and

they are connected to an Ethernet LAN. In addition, a local data collection system (DCS) is

installed. Each physical device has its own IP address. The entry point to the metering site is the

same as that of the LAN. Meters can be reached remotely through the internet, locally through the

local DCS, or directly by a Hand Held Unit (HHU). On the optical port, meters may communicate

either using the 3-layer, CO, HDLC-based communication profile or the TCP-UDP/IP based

communication profile using PPP.

On metering site #2, metering equipment use the 3-layer, CO, HDLC-based communication profile.

To be able to reach them through a single WAN access point, they are connected to a bus, for

example RS 485. The address of a physical device on the LAN is provided by its lower HDLC

address. As RS 485 does not provide a protocol for handling collisions on the bus, the client may

exchange data with the servers on the LAN one by one. In other words, the task of the bus master

is performed by the client. The access point of the LAN is a modem with an RS 485 interface. Its

address is provided by the WAN, which can be either the PSTN or the GSM telephone network. For

local data exchange, a portable DCS, connected directly to the RS 485 bus may be used. In this

case, remote access may not be available during the local communication. Like at metering site #1,

direct local data exchange is available using an HHU.

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