A channel service unit/data service unit (CSU/DSU) is a piece of data comunications equipment that performs the following functions:
- Acts as a tranciever
- Connects data terminating equipment to dedicated circuits such as T1 and T3.
- A CSU/DSU performs multiplexing and demultiplexing on T1 and T3 circuits.
- May have the ability to add and drop channels from a T1 or T3.
- Modern CSU/DSU's split the arriving data stream into multiple voice channels and/or multiple data channels.
Here is a picture of the back of an external, stand-alone CSU/DSU for a T1 (Courtesy Kentrox Corporation's website).
Downloaded from http://www.kentrox.com/products/datasmart%5F656/back.html
Image cropped to show a single device
The port labled "DATA PORT 1" is a V.35 connector that can be wired to a Cisco router to provide data. This is the most commonly used connector for most T1 to router connections where an external CSU/DSU is used. Many Cisco routers now incorporate a CSU/DSU directly in the router.
The port labled "Ethernet" is used to connect to the CSU/DSU from the local computer network to monitor the device. Kentrox CSU/DSU's provide an SNMP v2c agent to manage the device and a DHCP client to provide auto-configuration of the device on the local computer network.