er-domain or cross-domain network links. This hierarchical organization of network inventory optimizes the data needs of service provisioning, fault management and performance management functions because the intra-domain and the inter-domain databases can all be viewed together as a single "logical and integrated NMS database." While this is a simple explanation of the concept of multi-domain inventory management, in reality, the organization of domains can be pretty complex - especially in a layered network such as IP over DWDM. In such cases, the IP domain has no physical connectivity unless a "circuit" is provisioned through the DWDM layer. And hence, the activation process and the inventory process need to be closely aligned. Logical Integrated NMS Database Inter-Domain Database Intra-Domain Database Domain 1 Domain 2 Domain 3 Figure 40. Hierarchical Multi-Domain Inventory Management The Ultimate Guide to Building and Delivering OSS/BSS Furthermore, this is a logical view of various databases. In reality, the intra-domain and inter-domain databases can be housed within the same physical database or be in separate databases. Some network domains will be legacy, and hence the intra-domain inventory for this network will be stored within a legacy system. The legacy system can then be just treated as another domain database and resources between the legacy and next-generation domains can be inventoried in an inter-domain database. Next-Generation OSS Logical Integrated NMS Database Manage Inventory Inter-Domain Manager Intra-Domain Domain Manager Domain Manager Legacy Inventory OSS(s) of Legacy Upload Network Inventory/ Topology Inventory Physically Housed in Legacy OSS Discovery Discovery Network Network Domain 1 Domain 2 Legacy Domain Figure 41. Relationship of Next-Generation Inventory Database with Legacy OSS Figure 41 shows how the next-generation OSS inventory databases maintained at any or all of the EML/NML/SML layers are related to the data in a legacy OSS. Each of these contributes towards the integrat