NMS, CLI, and SNMPv3
For the remainder of this article, we'll assume that each of our network devices hosts an embedded EMS. Implemented in this EMS are CLI and SNMPv3 entities that can both be accessed by an external NMS. This is schematically illustrated in Figure 2.
Figure 2 Devices hosting EMS and CLI entities accessible by an NMS.
Let's take a closer look at the elements of Figure 2.
The NMS box in the upper-left corner of Figure 2 contains an application suite employed to provide FCAPS-based facilities.  This software helps in enabling some of the network operator's important business processes, such as the creation, modification, or deletion of layer 2/3 VPNs, LSPs, and so on. As we saw in Figure 1, the NMS provides a software interface for these operations that can be used by the OSS.
We now have some idea of the typical OSS and NMS capabilities. The latter uses the EMS to access the management plane.  Looking at how the NMS interacts with the EMS completes the last piece of the jigsaw. The mechanism for this interaction is generally the CLI and/or SNMPthough others (such as XML) are increasingly being used.