The Association objects, what are they?
In DLMS/COSEM, the association objects play a role of a ‘gatekeeper’, controlling access to the information in the meter and providing information on what is available.
As there are two referencing methods, there are also two types of association objects, one for Short Name and one for Logical Name referencing.
Although they perform the same task, the Association SN and the Association LN objects are slightly different.
The logical name of the current association object is 0.0.40.0.0.255. In case of SN referencing, the base_name of the current association object is 0xFA00.
Several instances of association objects are possible. For their identification, the value group E is used. At a time, there is only one association in a logical device, which is current. This is to avoid the complexity of safe ‘multi-user’ data handling in a meter. On the other hand, nothing forbids having access at the same time to multiple logical devices, if this is supported by the lower layers.
Both the Association LN and SN objects have an object_list attribute, providing the list of interface objects ‘visible’ in the given association. In case of LN referencing, the object_list provides the class_id, the version, the logical_name and the access rights to the attributes and methods for each object. In case of SN referencing, the object_list provides the base_name, the class-id, the version and the logical_name of each object. To retrieve the access rights, the get_attributes&methods method can be used. Default access right may be used (e.g. read only to attributes, no access to methods).
The Association LN object also holds the information about the associated partners (client-logical device), the application context, the xDLMS context, the authentication mechanism, the LLS secret (when this mechanism is used) and the association status.
The methods of the Association LN object support High Level Security Authentication and allow adding objects to the object_list or to remove them.
The methods of the Association SN object support reading objects by their logical name, to retrieve access rights and they also support the High Level Security Authentication process.