Frame relay interface types

As shown in Figure 23, frame relay enables communication between user devices such as routers and hosts. The user devices are also called "data terminal equipment (DTE)." They are connected to a frame relay network through the DTE interface. The devices that provide access to the frame relay network for DTEs are called "data communications equipment (DCE)." A DCE is connected to a DTE with a DCE interface on the user network interface (UNI) side and to a frame relay switch in the frame relay network with a network-to-network interface (NNI) on the NNI side. The switches in the frame relay cloud are interconnected with the NNI.

In actual applications, a DTE interface can connect to only a DCE interface, and an NNI interface can connect to only an NNI interface. On a frame relay switch, the frame relay interface should be an NNI or DCE interface.

As shown in Figure 23, Router B and Router C form a simple frame relay network, to which DTE devices Router A and Router D are attached. The DTE and DCE are identified on only the UNI interface; a virtual circuit between two DTE devices can be assigned different DLCIs on different segments.