The picture below shows back to back cable. The cable decides which end to be DCE or DTE and it is usually marked on the cable. However, in some cases we might connect two routers back-to-back using the routers’ serial interfaces (Example: Inside the router labs). The router is the DTE (Data Terminal Equipment) and the external device is the DCE (Data Communications Equipment), where the DCE provides the clocking. A CSU/DSU (Channel Service Unit/Data Service Unit) is a digital-interface device used to connect a router to a digital circuit. When connecting a serial cable to the serial interface of the router, clocking is provided by an external device, such as a CSU/DSU device.
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