The channel service unit/data service unit (CSU/DSU) is a device that is positioned between the circuit connection provided by the WAN provider and the router on the other side. When the signal arrives from the digital WAN connection, it is converted into a signal the router can understand. The CSU/DSU device, which is a single box, contains two logical elements that provide different services:
- The CSU faces the service provider side
- The DSU provides data for the CPE (router)
The CSU/DSU can be a physical device placed between the demarcation point and the router or it can be integrated directly into a router (in this case, the demarcation point can be directly connected to the router). An example of a CSU/DSU device is shown in Figure 20.4 below:
Figure 20.4 – CSU/DSU Device
From the demarcation point, you can plug in the CSU/DSU device with a standard RJ45 cable but the wiring used inside it is called RJ-48c, which uses the same connector as Ethernet links but with a different set of pins. The connection from the CSU/DSU device to the router is usually a serial link (e.g., RS-232), which converts the digital signal coming from the WAN side into a “router-friendly” signal.
The CSU/DSU device can also have a number of diagnostic jacks that can be used with dedicated equipment to monitor the WAN signal’s health and status.