Fibre optics for beginners

Fibre optics is no longer something for telephone exchanges: it reaches homes through FTTH, links warehouses to their offices and joins the network cabinets of a school across different floors. People who see it for the first time get lost among codes and colours, but a few rules are enough to buy the right parts. Here they are, in the order you should ask yourself the questions.

1. Single-mode or multimode

This is the first choice, and there is no going back: the two types must not be mixed on the same link.

• Single-mode (OS2, 9/125 µm): for long distances, up to 10 km and beyond with the right modules. It is the fibre used for FTTH and for links between buildings. Patch cords usually have a yellow jacket.
• Multimode (50/125 µm): for short distances, inside a building or between nearby cabinets. There are several grades: OM2 in older installations, OM3 and OM4 in recent ones, with an aqua jacket. There is also OM1 (62.5/125), found in dated installations.

If the fibre is already installed, read the code printed on the jacket or ask whoever laid it: everything else depends on that.

2. The connector: LC, SC or ST

• LC: small, today the most common on switches, SFP modules and fibre patch panels.
• SC: square and larger, common in slightly older installations and in home FTTH outlets.
• ST: bayonet type, still found in some older installations.

When the two ends have different connectors you use a hybrid patch cord, for example SC on one side and LC on the other. Duplex means two paired fibres, one to transmit and one to receive, the most common case in business networks; simplex is a single fibre, as in FTTH.

3. UPC or APC: the colour matters

The connector end face can be polished flat (UPC, usually blue) or at an 8-degree angle (APC, green). They must never be mated with each other: the link works badly and you risk damaging the end faces. Home FTTH outlets almost always have green SC/APC connectors; to connect them to equipment with an LC/UPC port there is a dedicated hybrid patch cord.

4. SFP modules: bringing fibre into the switch

Many switches have SFP ports that take a small module, which turns the electrical signal into light. The module is chosen according to the fibre and the distance, and must be the same type at both ends of the link:

• 1000Base-SX: Gigabit over multimode, up to 550 m;
• 1000Base-LX: Gigabit over single-mode, up to 10 km;
• 10GBase-SR: 10 Gigabit over multimode, up to 300 m on OM3.

Before buying, check that the switch accepts third-party modules: some manufacturers lock them out.

5. No SFP ports on the switch? Media converters

A media converter is a small box with an RJ45 port on one side and fibre on the other. You need one when the switch or the computer only has copper ports. They are used in pairs, one at each end, or a media converter at one end and a switch with SFP at the other; either way the fibre type and wavelength must match. Those with an SFP slot are the most convenient: change the module and they work with any fibre.

6. Inside the cabinet

Fibres coming from outside are terminated in a 1U fibre patch panel, with the adapters on the front: from there patch cords run to the switch. Inside the panel the fibres are spliced to pigtails, a job that needs a fusion splicer and is usually done by the installer. Everything else, patch cords, adapters and modules, can be fitted by hand without special tools.

A safety note: the light travelling in the fibre is usually invisible, but it can harm your eyes. Never look into a connector or an adapter while the system is live. To find a broken fibre you use a visual fault locator, which sends a bright red light and makes the break glow.

Patch cords, adapters, patch panels and accessories are in Fibre optics, SFP modules and media converters in Switches, routers and network converters.

Need a hand?

If you have fibre already installed and don't know what to buy to connect it, send us a photo of the connectors and of the code on the jacket: we will tell you which patch cords and modules you need.