ty, extensibility, and scalability. These characteristics are achieved in TMN using object-oriented design principles (see Figure 2.5). Operations are directed to manage objects and notifications are emitted from managed objects. Consistency constraints of the managed object are specified as a part of the behav- ior definition of the attribute. The value of an attribute can determine the behavior of the managed object. 2.6 Logical Architecture TMN logical architecture splits telecom management functionality into a set of hier- archal layers. This logical architecture is one of the most important contributions of TMN which helps to focus on specific aspects of telecom management application development. Though new detailed models were built based on TMN as a reference, the TMN logical architecture still forms the basic model which reduces manage- ment complexity by splitting management functionality SO that specializations can be defined on each of the layers (see Figure 2.6). TMN Model Managed object Operations Attributes Notifications behavior Figure 2.5 Managed object. Business management layer Service management layer Network management layer Element management layer Network element layer Figure 2.6 TMN logical architecture (TMN pyramid). The layers in the TMN logical architecture are: 1. Network element layer (NEL): This is the lowest layer in the model and it cor- responds to the network element that needs to be managed and from which management information is collected. This layer can also include adapters to adapt between TMN and non-TMN information. So both the Q-adapter and the NE in the physical TMN model are located in the NEL and NEF and QAF functions are performed in network element layer. 2. Element management layer (EML): This is the layer above the network element layer and deals with management of elements in the NEL. The development of an element management system (EMS) falls in this layer and the functionalities discussed in the first chapter performed by element managers are grouped in this lo