The Full Form of CSU is‍ Channel Service Unit.

Channel Service Unit or CSU is a digital communication device that is used to connect a digital line to a digital device. Channel Service Units (CSUs) can be used to link local area networks (LANs) into a wide area network (WAN) using telecommunications carrier services such as: Digital data service (DDS) T-carrier services such as a T1 line

A CSU/DSU (Channel Service Unit/Data Service Unit) is a hardware device about the size of an external modem that converts a digital data frame from the communications technology used on a local area network (LAN) into a frame appropriate to a wide-area network (WAN) and vice versa. For example, if you have a Web business from your own home and have leased a digital line (perhaps a T-1 or fractional T-1 line) to a phone company or a gateway at an Internet service provider