Ethernet cables
Inside the outer jacket are for twisted pairs, eight copper wires total, and those twists are not there for decoration. They help fight two major enemies, electromagnetic interference (or EMI) and crosstalk, which is basically electrical signals interfering with each other.
Because ethernet cables use electric impulses to send the data, these cables cables have to be twisted like this so the communication won’t get messed up. We have to differ UTP and STP which stands for unshielded twisted pair or shielded twisted pair. Unshielded means here is no extra metallic shield around the wires, but they are twisted to reduce the interference. In bigger environments (or with PoE) you will more often see STP, which is the same idea but with more armor around it.
Straight-Through cs crossover
Devices like PCs used to send traffic on specific pins and receive on others. Switches were designed to complement that, which is why straight-through cable worked when connecting a PC to a switch. The pins lined up in a way that made sense. But if you connect similar devices together like PC to PC or switch to switch, you had a problem. That’s where crossover cable came into play.
Nowadays there exists a technology which is called Auto-MDIX, which is basically smart enough to figure all that out automatically and adjust the pins for you.
The pinout you need to know
The wire colours go like this:
- White orange
- Orange
- White green
- Blue
- White blue
- Green
- White brown
- Brown
That order matters! If you get one wire wrong the cable may not work at all.
PoE
PoE which stands for “Power over Ethernet” solves a problem which is, on the first glance, not to bad. Every device which is connected to network cable, needed to be plugged in to receive some power. And yes, the whole thing really started with IP phones. Back in the days, Cisco pushed hard into IP telephony and realized that running both, a power cable and Ethernet cable to every desk phone was not ideal. So, they came up with their own early version, called Cisco Inline Power. That idea became the standardized PoE we use today.
PoE is amazing because it makes deployment easier, cheaper and faster.
How PoE evolved
| Standard | Type | Watts |
| 802.3af | Type 1 PoE | 15.4 watts per port |
| 802.3at | Type 2 PoE (PoE+) | 30 watts per port |
| 802.3bt | Type 3 PoE (PoE++) | 60 watts per port |
| 802.3bt | Type 4 PoE (PoE++) | 90 watts per port |
These standards are backwards compatible, so if your switch supports a newer type of PoE, means the old types work as well.
Active PoE vs passive PoE
This part matters a lot!
If you don’t know the difference you could literally fry a device!
Active PoE is the smart version. The switch and the device, talk to each other and negotiate how much power is needed. The communication often happens via CDP (Cisco Discovery protocol) or LLDP (Link Layer Discovery Protocol).
Passive PoE just sends the power. Always, no handshake, no negotiation just power. That’s why passive PoE can be dangerous.
Fiber optic cables
Fiber optic cables are fast, go far and ignore EMI. Three reasons to use Fiber in the first place.
A Fiber optic cables sends light impulses through the cable. In the center of each Fiber optic cable there is a core which is the tiny center where the light travels and the cladding, which prevents the the light from escaping. The light moves throught the core and reflects in a controlled way so it stays inside the cable.
Single Mode vs. Multimode
There are two types of Fiber optic cables.
Multimode has a larger core, which means the light has more room to bounce around on it’s journey. That’s great for shorter distances and common inside-building use cases. Single mode has a much smaller core, so the light goes more directly through the cable, which means it can go much longer distances.
Single mode wins on distance and bandwidth, but that does not mean it’s the answer for everything. Single mode is more expensive, and for a lot of real-world jobs like connection switches in data centers, Mulitmode is perfect.
Connectors, SFPs, and Fiber
Common Fiber connectors
| Connector name | Acronym | Form factor | Common use |
| Lucent connector | LC | Small factor, snap-in clip (resembles an RJ45 but smaller). Usually duplex (two joined together). | The absolute standard for SFP/SFP+ transceivers and high-density patch panels. |
| Subscriber Connector | SC | Square, push-pull latching mechanism. Larger than LC. | Common in older enterprise networks, GBIC modules, and carrier FTTH (Fiber to the Home) boxes. |
| Straight Tip | ST | Round, bayonet-style twist-lock (similar to a BNC coax connector). | Legacy networks, campus backbones, and industrial environments. |
SFP Transceiver Form Factors
These are the hot-swappable hardware modules inserted into switch bays to convert electrical signals to optical signals.
| Module type | Common speeds | Cable type | Typical Deployment |
| SFP (Mini-GBIC) | 1 Gbps | MMF / SMF | Legacy uplinks, standard edge switch connections. |
| SFP+ | 10 Gbps | MMF / SMF / DAC | Modern core/distribution switching, NAS storage, and server uplinks. |
| SFP28 | 25 Gbps | MMF / SMF | Data center top-of-rack switches and high-performance server NICs. |
| QSFP+ / QSFP28 | 40 Gbps / 100 Gbps | MMF / SMF / MPO | High-bandwidth switch-to-switch aggregation and data center backbones (uses 4 parallel channels). |
Fiber optic cable types
Fiber is broadly split into Multimode (short range, multiple light paths) and Single mode (long range, single light path).
| Fiber class | Designation | Max distance (Typical | Color | Common Application |
| Multimode | OM1 | Up to 300m (at 1 Gbps) | Orange | Legacy installations; obsolete for new 10Gbps+ builds. |
| Multimode | OM3 | Up to 300m (at 10 Gbps) | Aqua | Laser-optimized; standard for short-range building/rack links. |
| Multimode | OM4 | Up to 400m (at 10 Gbps) | Aqua / Erika violet | High-speed data centers and short-range backbone links. |
| Single mode | OS2 | Up to 10km – 40km+ | Yellow | Long-haul campus backbones, metropolitan networks. |