CSU/DSU (Channel Service Unit/Data Service Unit)CSU/DSU refers to two separate hardware communication devices that are often used together and can be combined into one box. The devices are needed to connect networking equipment such as routers to digital communications lines such as T1 lines. The CSU serves as the terminator for the signal, while the DSU is similar to a modem in that it changes the signals coming from network equipment to bipolar digital signals necessary for transmission over digital lines.