Vintage ADTRAN DSU III AR (Data Service Unit) is an all-rate DSU/CSU. In the pre-broadband era, this was the "bridge" between a business's internal network (like a router or a mainframe) and the digital telephone lines (DDS or Switched 56) provided by the phone company.
​Front Panel: The LCD and keypad allowed technicians to configure the unit and run diagnostics like "loopback tests" to see where a connection was failing.
​Rear Ports:
​V.35 & EIA-232: These are heavy-duty serial ports used to connect to routers or computers.
​TELCO: This is where the high-speed digital line from the utility would plug in.