The tray is usually made of plastic or metal and can hold a varying number of fibers, depending on the size of the box. In real fiber optic networks, cables are rarely installed as one continuous, uninterrupted length. Along transmission routes—whether in access networks, metro networks, or backbone infrastructure—fiber cables must be joined, branched, repaired, or reserved for future expansion. But every one of. An optical cable split fiber box, also known as a fiber distribution box or fiber optic splice closure, is a device used to terminate, splice, and distribute optical fibers.