Keep legacy T1 and E1 services online when copper runs are exposed to interference, distance limits, or unreliable plant conditions. The Black Box MT660A-MM converts T1/E1 traffic for transport over multimode fiber, helping preserve link integrity for PBXs, CSUs, and routers that still anchor critical operations. Built for managed deployments, it supports AMI, B8ZS, and HDB3 line codes and includes diagnostics that help isolate faults before they become outages. Local and remote fiber loopback switches simplify verification during installation and maintenance, while the crossover switch on the RJ-45/RJ-48 port reduces setup friction when connecting equipment. This is the kind of converter chosen when uptime matters more than convenience. It fits rack-mounted environments, integrates cleanly into existing infrastructure, and gives operations teams the visibility needed to support legacy circuits without replacing working systems. For organizations protecting voice trunks, branch connectivity, or serial-era transport paths, it delivers a practical way to move mission-critical links onto fiber.