Module 5

The Outside Plant: CO to Couch

Every fiber connection is a single unbroken thread of glass that begins in a building full of equipment and ends beside your sofa. This module walks that thread, hop by hop, naming every box, cabinet, and cable it passes through along the way.

OSP vs ISP

The fiber network splits into two worlds, divided by one line — the demarcation point (the "demarc"), marked by a small enclosure called a NID (Network Interface Device). For fiber-to-the-home (FTTH) that line sits at the home; in FTTB/curb architectures the demarc sits wherever the operator's responsibility ends.

Walk the cable, CO to home

This is the spine of the module. A single fiber signal leaves the Central Office and arrives at your home after passing through a fixed, ordered sequence of elements. Here is the whole path at a glance:

  CENTRAL OFFICE                        STREET / NEIGHBORHOOD                      HOME
  ┌──────────┐   F1 feeder   ┌─────┐   F2 distribution   ┌────────┐   F3 drop   ┌──────────┐
  │ OLT      │══════════════►│ FDH │═══════════╗════════►│ FAT /  │────────────►│ Terminal │
  │ (CO)     │  144–1728 fib │ 1:4 │  12–144   ║         │ NAP    │  1–2 fiber  │ box →    │
  └──────────┘               └─────┘  fibers   ▼         │ 1:8    │             │ NID/ONT  │
                            primary        ┌─────────┐   └────────┘             └──────────┘
                            splitter       │ splice /│   secondary               demarc =
                                           │ junction│   splitter                boundary
                                           └─────────┘
                            1:4 (primary) × 1:8 (secondary) = 1:32 total
                            splice/junction may hold a first-stage splitter
  feeder area  ───────────►  distribution area  ──────────────►  drop / user area
   (= CORE)                    (= DISTRIBUTION)                       (= ACCESS)

The journey divides into four zones — core/CO → feeder → distribution → drop/user — mapping cleanly onto the networking layers core → distribution → access. The hierarchy below shows how fiber count shrinks and splitting increases as you move outward.

Now walk each element in turn — click any stop in the widget below to inspect what it holds.

1. Central Office (CO) — the source

The Central Office (CO) is the building where the fiber network begins (on cable-TV networks, the head end). Inside sits the OLT (Optical Line Terminal), the powered "brain" that lights the fiber and sources every downstream signal. Everything from here out is the OSP.

2. Feeder cable (F1) — the high-count trunk

The feeder cable (F1) is the high-count trunk leaving the CO — typically 144, 288, 432, 864, or 1728 fibers typical, since one route serves many neighborhoods. The "fat pipe": the core layer.

3. FDH — the street cabinet and primary splitter

The FDH (Fiber Distribution Hub) is the feeder→distribution transition, where the trunk fans toward neighborhoods. It houses the primary splitter — a single centralized 1:32 in some designs, or a low-ratio 1:4 first stage in a cascaded design where a 1:8 second stage waits at the FAT (so the two stages multiply: 1:4 × 1:8 = 1:32 total) typical. It also provides cross-connect flexibility: any incoming (feeder) fiber can be patched to any outgoing (distribution) fiber, so the cabinet re-routes without re-splicing.

4. Distribution cable (F2) — through the neighborhood

The distribution cable (F2) is the medium-count cable along the street — typically 12–144 fibers typical — branching toward groups of homes inside splice closures.

5. Splice closures — sealed junctions

A splice closure (FOSC, Fiber Optic Splice Closure) is a sealed, weatherproof enclosure rated IP67/IP68 where fibers are joined for branching, repair, or cable transitions. Two shapes: dome/butt (cables enter one end) and inline/horizontal (both ends). May hold a first-stage splitter in distributed designs.

6. FAT / NAP / FDT — the access terminal

The access terminal sits at the curb, at the edge of distribution, where individual drop cables plug in via pre-connectorized ports. It frequently holds the secondary 1:8 splitter typical — the stage that fans the signal out to houses on the block. The most name-inconsistent box in the network (see below).

7. Drop cable (F3) — the last leg to the house

The drop cable (F3) is the small cable — often 1–2 fibers, sometimes a flat "figure-8" — from the access terminal to one home. It uses bend-insensitive fiber (ITU-T G.657) to tolerate sharp bends. The access layer's final hop.

8. Terminal box — outside the house

The terminal box is the exterior box on the home where the outdoor drop transitions to indoor fiber — the handoff from hardened OSP to the indoor world.

9. NID / ONT — the demarc, in the home

Finally the signal reaches the demarcation point. The NID (Network Interface Device) is the enclosure that marks it; the ONT (Optical Network Terminal) is the active device that terminates the PON (Passive Optical Network), converting optical into Ethernet, phone, and Wi-Fi. The ONT is the downstream counterpart to the OLT.

A warning about names

How cable gets installed

Placing fiber across miles of terrain comes down to three methods — a tradeoff between cost, speed, protection, and ease of future repair.

Supporting infrastructure

None of that cable floats in mid-air. A catalog of poles, pipes, and boxes holds and protects it — the structures you will meet most often: