At the client’s location ATU-Rs perform modulation and demodulation of signals for optimum transmission over copper wires. The ATU-Rs work in conjunction with hardware equipped with NICs (Network Interface Cards) to transmit voice and data signals over PSTN to provider locations. ATU-Rs use error correction schemes like forward error correction where redundant data is added to the messages to enable the receiver to perform error checking without asking the sender for more data and echo cancellation using echo suppressers. This reduces any disruptions that might occur due to noise at the PSTN level (Littman, 2002).
The previously mentioned DSLAMs and ADSL transceivers situated at telephone exchanges and central offices are classified as ATU-Cs (ADSL Terminal Unit-Central Office). Hubs, bridges and routers located at central offices charged with redirecting voice calls to PSTN networks and data, voice and video to IP networks are also classified as ATU-Cs. The DSL capable technologies used to implement backbone infrastructure connecting various central offices include ATM, SONET/SDH, IP, Fiber Channel, Frame Relay and Gigabit Ethernet (Littman, 2002).