The test system simulates the physical devices that are necessary to complete the network with logical objects called emulators. An emulator processes incoming control and/or bearer traffic in a manner consistent with the device that it simulates, and sends the appropriate control messages or responses to another emulator or physical device.
Emulators communicate by means of an address that is associated with a physical port on the test server. Each of the test ports on the test server is configured with a pool of virtual addresses.
You define the communication settings for the configurable emulators that are used in a test case and, to some extent, you can also define their behavior. Part of the emulator's definition includes assigning a address to the emulator, and specifying the routing that the emulator will use. In some cases, a number of emulators of the same type — multiple instances of an emulator — can be defined. An instance of an emulator is often referred to as a node or an emulated node. The type of emulators available in a test case is dependent on the test case and the options that you choose in the test definition.