Why fibre?
Fibre carries light through a thin glass core instead of electricity through copper. This gives it three big advantages: much longer distances (kilometres instead of the 100 m limit of copper Ethernet), immunity to electrical interference, and no electrical connection between buildings, which avoids problems with ground loops and lightning.
Single-mode vs. multimode
| Multimode | Single-mode | |
|---|---|---|
| Core | 50 or 62.5 µm | About 9 µm |
| Distance at 10 Gbps | Hundreds of metres | 10 km and beyond |
| Cable grades | OM1–OM5 (OM3/OM4 common, often aqua) | OS1/OS2 (often yellow) |
| Optics cost | Lower | Higher |
| Typical use | Inside a building or data centre | Between buildings, across campuses, provider links |
In multimode fibre, light takes many paths (modes) through the wide core, and they arrive at slightly different times. This spreading of each pulse is called modal dispersion, and it limits the distance. The tiny core of single-mode fibre allows only one path, so pulses stay sharp over long distances.
One strand each way
Most fibre Ethernet links use a pair of strands: one to transmit (TX) and one to receive (RX). The TX of one end must connect to the RX of the other end, so the pair is crossed.
- 1. A transmits on one strand… …which arrives at B's receiver.
- 2. B transmits on the other. If one strand breaks, traffic may still flow in one direction only. This is a unidirectional link, which UDLD (covered in the Spanning Tree course) can detect.
Connectors and transceivers
| Item | What it is |
|---|---|
| LC | Small push-and-latch connector, usually in a duplex pair; the most common type on switches |
| SC | Larger, square push-pull connector; common on older and service provider equipment |
| SFP | Hot-pluggable transceiver for 1 Gbps (SFP+ for 10 Gbps, SFP28 for 25 Gbps) |
| QSFP+ / QSFP28 | Four-lane transceivers for 40 Gbps / 100 Gbps |
⚠️ Both ends must use matching optics (for example, SR with SR) and the right fibre type. An SR optic on single-mode fibre, or an LR optic on multimode fibre, may not work at all, or may work unreliably.
SFP and SFP+ modules in practice
Most switches do not have fibre sockets built in. Instead, they have empty SFP (Small Form-factor Pluggable) slots. You slide in a small module, called a transceiver, that matches the cable you want to use. The module converts electrical signals into light, and light back into electrical signals.
SFP: 1 Gbps
Modules such as 1000BASE-SX (multimode) or 1000BASE-LX (single-mode). Copper 1000BASE-T SFP modules with an RJ45 socket also exist.
SFP+: 10 Gbps
The same size and shape as SFP, but faster: 10GBASE-SR, 10GBASE-LR and others. Most SFP+ slots also accept a 1 Gbps SFP module.
DAC cables
A DAC (Direct Attach Copper) cable has an SFP+ plug fixed to each end. It is a cheap choice for very short links (a few metres) inside a rack.
- Hot-pluggable: you can insert and remove modules while the switch is running.
- Both ends must match: the same speed, the same standard (SR with SR, LR with LR) and the right fibre type.
- An SFP+ module in a plain SFP slot usually does not work, because the slot cannot run at 10 Gbps.
- Vendor coding: some switches only accept their own brand of module, or show a warning for other brands.
The names on the modules (SX, LX, SR, LR) tell you the standard and the fibre type. The Ethernet standards lesson explains how to read them.
Learn more: Ethernet Physical Layer Standards
- Fibre carries light, so it reaches long distances and is not affected by electrical interference.
- Multimode (wider core, cheaper optics) is used inside buildings; single-mode (9 µm core) is used for kilometres.
- Most links use two strands, with TX crossed to RX. LC is the usual connector.
- SFP slots take 1 Gbps modules and SFP+ slots take 10 Gbps modules. Both ends must match.
Check yourself
You need a 10 Gbps link between two buildings 8 km apart. Which fibre and optics should you use?
You connect a new fibre link between two switches, but it won't come up. Both optics have been tested and work. What is a common, simple cause?