DLMS User Association, COSEM Architecture and Protocols, Sevent Edition
DLMS User Association 2009-11-30 V 7.05
DLMS UA 1000-2 ed.7
16/84
© Copyright 1997-2009 DLMS User Association
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.