DIfference between NMS and EMS

The telecommunications management network (TMN) provides a framework for achieving interconnectivity and communication across heterogeneous operating systems and telecommunications networks.

TMN describes telecom network management from several viewpoints: a logical or business model, a functional model, and a set of standard interfaces. Each of these is critically important and interdependent.

TMN is the overall management network but a subset of it i.e. NMS deals with a group of devices that can be logically grouped such as the kind of technology it is supporting (WLAN, CDMA, GSM, WIMAX, etc) or it can be vendor specific also. EMS is a subset of NMS where in the focus is upon each device or element of that particular network which is managed by one NMS... which in turn gets managed via TMN.

Another way to look at it is....

TMN gives a holistic overview a birds eye view of the overall managed network... a TMN can be broken down into logical entities as NMS... and each of these network elements or devices which fall under a NMS which can be managed independently needs an EMS to be present.