CMTS interoperability testing often incorporates an in-house cable network with a data generator to simulate Internet downstream traffic and subscribers' upstream traffic. The network consists of tens of kilometers of fiber cable in spools. Typically, engineers at a manufacturer's CMTS test lab will use several rolls of fiber and coax to create several cable lengths. It's important that the engineers not just test at the longest possible cable lengths. Because the cable modems send data upstream in time slices, the CMTS must compensate for transmission delays, and it must communicate with modems at many different distances.
CMTS manufacturers have racks full of cable modems lining the walls of their test labs.
The coax cable that simulates a trunk connects to the racks of modems on their RF sides. As many as four amplifiers may reside along one cable trunk. A single trunk can support about 500 modems. Additional rolls of coax attach to the amplifiers but don't connect anywhere. These rolls simulate unterminated cable lines reserved for new subscribers.