A CSU/DSU acts as a translator between the LAN data format and the WAN data format. Such a conversion is necessary because the technologies used on WAN links are different from those used on LANs. In reality, you can think of a CSU/DSU as a digital modem; but unlike a normal modem, which changes the signal from digital to analog, a CSU/DSU changes the signal from one digital format to another. Figure 3.16 shows how a CSU/DSU might fit into a network.
Figure 3.16 How a CSU/DSU is used in a network.
A CSU/DSU has physical connections for the LAN equipment, normally via a serial interface, and another connection for a WAN. Traditionally, the CSU/DSU has been in a separate box from other networking equipment; however, the increasing use of WAN links means that some router manufacturers are now including the CSU/DSU functionality in routers or are providing the expansion capability to do so.
Is a CSU/DSU a Gateway?
A CSU/DSU is in a sense a gateway device because it translates data from one format (that used on the LAN) to another (that used on the WAN). The same can be said of a normal modem.