Try to Learn about this device, This is not commonly shown in CCNA Labs.
The CSU/DSU provides signal timing for communication and is used for interfacing with the digital transmission facility.
•Essentially, the CSU/DSU is used by a router to connect to a digital
line in much the same way that a PC uses a modem to connect to an analog line.
•A CSU/DSU for T1 will typically provide:
–Framing: ESF, D4(SF)
–Line Coding: AMI, B8ZS
–Cabling to Router: V.35, RS-232
–(more on these later)
The CSU/DSU may also be built into the router. Like the card shown below.