What is the purpose of the upper HDLC address?
DLMS/COSEM is based on the Client/Server paradigm, where the data collection system acts as the Client and the metering equipment acts the Server, providing services to the Client.
The Server is modelled as a physical device containing one or more logical devices, each logical device supporting one or more application associations with the Client(s) and two or more COSEM objects.
In the three-layer, connection oriented, HDLC based communication protocol, addressing is performed by the MAC layer. The lower HDLC address is used for addressing the physical device and the upper HDLC address is used for addressing the various logical devices within the same physical device. Application associations are established between a Client and a Logical Device in the Server. The associations are identified by the pair of HDLC addresses (Client HDLC address – Server lower HDLC/upper HDLC address). The HDLC based data link layer supports the lower HDLC/upper HDLC address structure by using the extended addressing mechanism provided by the HDLC standard ISO/IEC 13239. It is to be noted, that for the purposes of DLMS/COSEM a new frame format type 3, containing both the source and the destination addresses have been added to ISO/IEC 13239.
In a full OSI protocol stack the “Transport Services Access Point” has the same role as the upper HDLC address in the three-layer, connection oriented, HDLC based protocol stack used with COSEM. TSAP is a point in the transport layer of an OSI based product that can be accessed for addressing purposes. This address can be used in addition to the main network addressing.
Using a non-OSI example, the role of the upper HDLC address is similar to the “port number”. A computer, a single physical device is running various applications like web, mail, ftp. The computer can be accessed using its IP address, and the various applications – which are similar to DLMS/COSEM logical devices - can be accessed using their port numbers. Many people know that the assigned port number for World Wide Web HTTP is 80 and the standard port for Simple Mail Transfer Protocol is 25.