This chapter is from the book
A Channel Service Unit/Digital Service Unit (CSU/DSU), sometimes called Data Service Unit, is a device that converts the digital signal format used on LANs into one used on WANs. Such translation is necessary because the networking technologies used on WANs are different from those used on LANs.
The CSU/DSU sits between the LAN and the access point provided by the telecommunications company. Many router manufacturers are now incorporating CSU/DSU functionality into their products.