TMN's Physical Architecture defines the following building blocks:
Building blocks always implement the function blocks of the same name (e.g. Network Elements perform Network Element Functions, Mediation Devices perform Mediation Functions etc.).
It is possible to implement multiple function blocks (of the same or of a different type) into a single building block. The Operations System, for example, may be used to implement multiple OSFs, but may also be used to implement an OSF, MF and a WSF. In the case a building block implements multiple function blocks of different types, "the choice on the building block's name is determined by the predominate usage of the block".
Figure 16 shows which function blocks may be implemented into which building blocks.
A special kind of building block is the Data Communication Network (DCN). As opposed to the others, this building block does not implement any TMN function block. In fact, the DCN is used by other building blocks for the exchange of management information; the DCN's task is to act as a transport network.
At first sight it seems strange that TMN defines a building block that does not implement any function block. The existence of the DCN can be understood however when we remember that previous TMN drafts (e.g. [8]) modelled the DCF as a function block. According to these drafts, the DCF had to be implemented by a DCN and, in that case, each building block implemented at least one function block. In 1990 it was decided however to model the DCF no longer as a function block [9]. After this decision was made, the standard was not rewritten in a consistent way and the DCN is therefore still modelled as a building block.