Practical intro to fiber-optic networks
I was looking into how to link a remote point in my mansion to the network and checked how it could work with a fiber-optic connection, since the router had a SFP+ socket.
TL/DR: I’ll go with a SFP+ bidi single-mode connection.
First of all, stay safe. Don’t look directly into a fiber with your eye. The light/laser is not powerful, but why do it? You won’t see anything anyway, as the light is in the infrared.
Ethernet vs fiber-optic
Ethernet networks are quite simple. Devices are linked directly to other devices and they “negociate” with the other device the highest speed they can use on the given cable. Everything can normally be connected to everything. See some numbers at the very end.
Fiber-optic networks need a bit more thought. The fiber cable ends in some type of connector that plugs into a transceiver which is plugged into equipment. There are multiple types of cable, connectors, transceivers, equipment and they need to be matched since they operate at a fixed, designed speed.
Speed
For practical purposes, 1 Gbps is not bad. It means you download ArchLinux (1.2 GB as of January 2025) in 10 seconds. I imagine you’re most probably limited by the server bandwidth. But, if it’s possible to have more for a small price increase, why not..
| Gbit/s | MB/s |
|---|---|
| 1 Gbps | 125 MB/s |
| 2.5 Gbps | 313 MB/s |
| 5 Gbps | 625 MB/s |
| 10 Gbps | 1.25 GB/s |
Fiber can be used at 1 Gbps, but it would be a shame to do that. Nowadays the SFP+ transceivers, having a nominal speed of 10 Gbps, are common.
Ethernet 1 Gbps is the norm but I see affordable 2.5 Gbps equipment. Ethernet 10 Gbps equipment is quite expensive.
Transceivers
The transceivers contain the LED or laser that emits light and the corresponding light sensor. There are many types of transceivers.
The SFP (small form-factor pluggable) form factor seems to be the most popular, due to its practicality. SFP is not standardized, but competing manufacturers adhere to a “multi-source agreement”, so the products should be compatible.
Various types of SFP transceivers are designed for specific speeds, just to name a few:
- SFP — 1 Gbps (named exactly the same as the form factor)
- SFP+ — 10 Gbps
- SFP28 — 25 Gbps
The sweet spot is SFP+, the “enhanced” SFP which has a nominal transfer speed of 10 Gbps. “Nominal” means “in name”, meaning in practice it could be lower, for example when there is a cable issue. Note the older variant, SFP (without +), has the same form factor but a much lower speed of 1 Gbps, so make sure it has a +.
Because the fiber optic cable is cheap, most transceivers are designed to work with duplex fiber cables, one to transmit and one to receive. In contrast, the “bidirectional” transceivers need a single fiber cable. These come in pairs, meaning one’s TX wavelength must match the other’s RX wavelength and the other way around. The price difference seems small, certainly does not matter for a handful of connections. My suggestion is to use bidirectional ones.
https://en.wikipedia.org/wiki/Small_Form-factor_Pluggable
Cables
The fiber is composed of a highly transparent core and the surrounding cladding. The core transmits the light signal. The cladding is an optical layer that traps the light and guides it back into the core.
Fun fact: the speed of the light in glass or plastic is ⅔ of the speed of light in vacuum—only 200,000 km/s.
There are two categories of fibre, depending on how many “modes of light” they support:
- single-mode (SMF), in which the light follows a single straight path, with minimal bouncing from the core-cladding interface.
- multimode (MMF), in which the light is sent through different “paths”, meaning at different angles. When the light hits the core-cladding interface it undergoes total internal reflection. The multimode is possible due to the wider core.
SMF is designed for long-distance transmissions, spanning hundreds of kilometers, whereas MMF is best suited for shorter distances, typically under 1 km. Note SMF can just as well be used on short distances.
All fiber types support 10 Gbps transmission, but the maximum distance varies:
| Cable type | Category | Core/cladding diameter | Max distance for 10 Gbps |
|---|---|---|---|
| OM1 | MMF | 62.5/125 μm | 33 m |
| OM2 | MMF | 50/125 μm | 82 m |
| OM3 | MMF | 50/125 μm | 220 m |
| OM4 | MMF | 50/125 μm | 400 m |
| OM5 | MMF | 50/125 μm | 400 m |
| OS1 | SMF | 9/125 μm | 10 km |
| OS2 | SMF | 9/125 μm | 40 km |
Note most MMF cables have a 50 μm core into a 125 μm cladding, while SMF cables have a 9 μm core into 125 μm cladding. It’s recommended to use higher-grade OM3, OM4 or OM5 fiber for 10 Gbps networks rather than the older OM1 and OM2.
MMF is cheaper, but these cables are cheap anyway, much cheaper than ethernet cables. For large-scale deployments, the differences probably add up, but for home networking it does not really matter.
Normally you buy cables of a specific length which have connectors at the end. These are called “jumper cables”. Profi equipment is required to cut/join fiber cable. When looking for cables, you might want to pick bend insensitive (BI) cables, which are specifically designed to reduce losses caused by bends.
Connectors
There are multiple types of connectors on the market, but the Lucent Connectors (LC) are very common because they are precise, compact and practical.
https://en.wikipedia.org/wiki/Optical_fiber_connector
Attenuators
When using SMF, pay attention to the optical-input power range of the transceiver, to avoid the signal overloading the transceiver. Since these modules are designed for distances of tens of kilometers, this can happen when using them on much shorter distances. I imagine this can also happen when using transceivers from different manufacturers.
An optical attenuator reduces the light intensity when the signal is too strong.
Multimode connections do not need attenuators.
https://en.wikipedia.org/wiki/Optical_attenuator
Duplex vs BiDi
Traditionally, the optical fiber was being used unidirectionally, meaning two cables were required to establish a connection between two points, one to transmit and one to receive. Nowadays, transceivers that send and receive data on the same optical fiber are a well-established technology.
Connections made with two cables require “duplex” connectors. The duplex LC connector is simply composed of two LC connectors, having twice the width: 12.7 mm (0.50 inches), about the size of an ethernet jack. In case the conduit in the wall is tight, you might prefer a BiDi connection, which requires a single cable with a single compact LC connector at each end.
Technically it should be possible to pull two separate cables and use them in duplex mode. Note since BiDi transceivers are only SMF, it seems unlikely that you can find simplex MMF cables. But these duplex LC connectors can normally be separated, so if you really want it, you can buy two duplex cables, separate all the duplex connectors, cut half of them and you have two separate simplex cables.
The “uniboot” cables have duplex LC connectors but the two fibers are inside the same round cable.
Fibre end polishing
You probably don’t need to know this, but it’s interesting.
The fibre end can be polished in multiple ways, from the worst to best connection quality:
- flat,
- PC (Physical Contact) is slightly convex,
- UPC (ultra Physical Contact) is almost round and
- APC (Angled Physical Contact) at an angle of 8 degrees.
When the signal quality is paramount, APC polishing is used. Us mere mortals will be using flat polish.
Ethernet cables
| Cable type | Speed | Distance |
|---|---|---|
| Cat5e | 1 Gbps | 100 m (328 feet) |
| Cat 6 | 10 Gbps | 55 m (180 feet) |
| Cat 6a | 10 Gbps | 100 m (328 feet) |
When looking for a cable, first pick the category, then look for cables having some “x/FTP” spec. The “FTP” part means the twisted pairs are each wrapped in foil to avoid crosstalk and electromagnetic interference (EMI).
The “x” specifies the shielding of the entire cable:
- U for unshielded
- S for braided shielding
- F for foil shielding
- SF for both S and F
