How to connect a logical device?
There are two different situations; the first being, that the data collection system (client) knows the structure of the metering device (server) and the second that the device is unknown to the client.
The client must know in advance the physical device address, which is the lower MAC address of the device if it is connected to a LAN, e.g. when multi-drop is used. In case of modem connection, the telephone number and the modem connection parameters need to be known. If security mechanisms are used, the password or the ‘secret’ needs to be known as well.
When the structure of the metering device is known, the process is the following.
1. Connect the physical layer. Optionally, negotiate the protocol stack.
2. Connect the data link layer using the known lower HDLC address and the known upper HDLC address of the selected logical device within the physical device. Optionally, negotiate data link layer parameters.
3. Establish the application association, negotiate the application context (SN or LN referencing), the xDLMS context and perform authentication, using the known password or HLS secret.
4. Access the selected attributes and methods, using the appropriate service set. This is GET/SET/ACTION when using LN referencing and READ/WRITE when using SN referencing.
5. Terminate the data link layer connection – this will release the application association as well – and establish another association or terminate the physical connection.
The actions to perform when the structure of the metering device is not known are more complicated. They will be as follows:
1. Connect the physical layer. Optionally, negotiate the protocol stack.
2. Connect the data link layer using the reserved address of the public client (0x10), the lower HDLC address of the physical device and the upper HDLC address of the management logical device (0x01). Optionally, negotiate data link layer parameters.
3. Establish the application association, negotiate the application context (SN or LN referencing) and the xDLMS context. No authentication is required.
4. Access the SAP assignment object (in case of LN referencing, logical name 0.0.41.0.0.255), in case of SN referencing 0xFC00) to retrieve the list of logical devices and their SAP-s. The SAP_assignment_list attribute delivers the SAP of each logical device with its logical_device_name. When short name referencing is used, the base_name of the COSEM logical device name object is 0xFD00.
5. Disconnect the data link layer and connect again using the address of the appropriate client address and the upper HDLC address of the logical device selected.
6. Establish the application association, negotiate the application context (SN or LN referencing), the xDLMS context and perform authentication (password must or HLS secret must be known).
7. Retrieve the object-list from the association object with access rights to attributes and methods.
8. Browse through the list and find the objects of interest in the logical device.
9. Access the selected attributes and methods using the appropriate service set. This is GET/SET/ACTION when using LN referencing and READ/WRITE when using SN referencing.
10. Terminate the data link layer connection – this will release the application association as well – and establish another association or terminate the physical connection.
The physical device may support installation of the metering device by calling in to the data collection system when it is installed.