Optical Line Terminal (OLT)

Overview

An optical line terminal is the operator end of a fiber-to-the-home network. A single passive optical network splits one fiber out of the OLT into a tree that reaches up to 128 homes, with nothing powered between the exchange and the subscriber. The OLT sits at the root of many such trees, aggregating thousands of subscribers into a chassis that runs continuously, which is why it is built around redundancy and hot-swap servicing rather than a single sealed board.

Cards slide into the Chassis and seat onto the Switch-Fabric Backplane, a passive board whose high-speed lanes connect every slot. The Control Card holds the switch fabric and management processor that tie the system together. Subscriber fibers land on PON Line Card modules, while the Uplink Card aggregates all that traffic onto the core network. Two Power Supply modules run in parallel so either can fail without an outage, the Fan Tray pulls cooling air through the cards, and the Management Port gives operators out-of-band control.

How it works

A PON is a shared medium, and the OLT is its scheduler. Downstream, a PON line card broadcasts every frame to all ONUs on the tree; each home filters out only the frames addressed to it. Upstream is harder, because many homes share one fiber and must never transmit at once. The PON MAC on the line card hands each ONU precise time slots and, through a ranging process, accounts for each subscriber's distance so their bursts arrive back-to-back without colliding. The optics that receive those bursts work in burst mode, re-locking to a new signal level every few microseconds.

Traffic that survives the PON scheduling is just packets, and the control card's switch fabric forwards them between the subscriber-facing line cards and the network-facing uplink at full rate. Everything rides a common timing reference distributed across the backplane so the PON slots stay aligned. Because the design assumes years of unbroken operation, line cards, power modules, and the fan tray all pull from the front of a live chassis, letting an operator grow capacity or replace a fault without darkening the homes behind it.