Mental model
The physical layer determines how fast and how far. Get this wrong and the rest of the stack works perfectly while users complain about slow networks. Two big categories of media:
- Copper twisted pair — cheap, terminates with RJ45, runs up to 100 meters. The default for desk drops, APs, IP phones, cameras.
- Fiber optic — more expensive, multiple connector types, runs from 100m to many kilometers. The default for switch-to-switch uplinks, datacenter spine-leaf, building-to-building, ISP last mile.
Within each category, multiple grades exist for different bandwidth and distance targets.
Copper twisted pair — Cat ratings
All Ethernet over copper uses 8 wires in 4 twisted pairs. The category number determines bandwidth and distance.
| Cat | Max bandwidth | Max distance | Used for | Year |
|---|---|---|---|---|
| Cat5 | 100 Mbps | 100 m | Legacy 10/100 Ethernet | 1995 |
| Cat5e | 1 Gbps | 100 m | Most modern access ports | 2001 |
| Cat6 | 1 Gbps full / 10 Gbps to 55 m | 100 m / 55 m | Better margins, multi-gig PoE | 2002 |
| Cat6A | 10 Gbps | 100 m | Modern Wi-Fi 6/6E AP uplinks, datacenter copper | 2008 |
| Cat7 / Cat7A | 10 / 40 Gbps | 100 / 50 m | Niche; not widely deployed in N. America | 2010 |
| Cat8 | 25-40 Gbps | 30 m | Datacenter top-of-rack short-haul copper | 2016 |
Common confusions:
- Cat6 caps at 1 Gbps over 100m, but supports 10 Gbps if the run is ≤55m. Many people quote “Cat6 = 10 Gbps” without the distance caveat.
- Cat6A is the practical 10 Gbps copper standard for full 100m runs. If you’re building today and want headroom, use Cat6A everywhere.
- Cat7 is rarely seen in US enterprise. Most installers skip from Cat6A to fiber for longer or faster runs.
- Cat8 is short-haul only. Designed for ToR copper jumpers in datacenters; useless for general office cabling.
Shielding
- UTP (Unshielded Twisted Pair) — the standard. Cheaper, easier to terminate.
- STP / FTP / S/FTP — Shielded variants. Reduce alien crosstalk, more expensive, harder to ground properly, required for some industrial environments.
For 99% of enterprise installs, UTP is fine. STP only matters in high-EMI environments (factories, hospitals near MRI).
Termination
RJ45 connectors on both ends. Two wiring standards exist:
- T568A — green pair on pins 1/2
- T568B — orange pair on pins 1/2 (more common in N. America)
Both work. The rule: be consistent across both ends of a cable. Mixing A and B on the same cable creates a crossover (used to be needed for switch-to-switch; auto-MDIX handles it now).
Fiber optic — types and modes
Fiber transmits light, not electricity. Two fundamental types:
| Type | Core diameter | Color | Used for | Distance |
|---|---|---|---|---|
| Multi-mode (MMF) | 50 µm or 62.5 µm | Orange / Aqua / Lime | Short-range, intra-building | up to ~550 m |
| Single-mode (SMF) | 9 µm | Yellow | Long-range, inter-building, ISP | 10 km – 80+ km |
Why the difference? Single-mode’s tiny core forces light into a single path → less dispersion → much longer distance. Multi-mode’s wider core allows multiple paths → cheaper transceivers but signal disperses faster.
Multi-mode grades (OM)
Multi-mode fiber comes in OM grades. Higher number = better bandwidth × distance product.
| Grade | Color | 10 Gbps reach | 40/100 Gbps reach |
|---|---|---|---|
| OM1 | Orange | 33 m | not supported |
| OM2 | Orange | 82 m | not supported |
| OM3 | Aqua | 300 m | 100 m |
| OM4 | Aqua / Magenta | 400 m | 150 m |
| OM5 | Lime green | 400 m | 150 m (also supports SWDM) |
OM3 and OM4 are the modern defaults for datacenter MMF. OM1/OM2 are legacy and should be replaced.
Single-mode (OS1 / OS2)
OS1 and OS2 are similar; OS2 is slightly better for outdoor/long-haul. Single-mode supports speeds from 1G to 800G+ over distances from a few hundred meters (with cheap transceivers) to 80+ km (with long-haul DWDM).
Connectors
Multiple fiber connector types exist. Common ones:
| Connector | Where |
|---|---|
| LC | Most common in datacenters and modern switches. Small form factor, two fibers per duplex pair. |
| SC | Older, larger; still common in carrier deployments. |
| MPO / MTP | 12 or 24 fibers in one connector. Used for high-density spine-leaf and breakout cables. |
| ST | Legacy bayonet, mostly retired. |
Polish type matters for connector loss — UPC (blue, lower loss) vs APC (green, even lower loss, used in PON / carrier networks). Don’t mix them at a single connection.
Transceivers — what plugs into the switch port
A switch port doesn’t natively know what cable it’s talking to. You insert a transceiver (a small pluggable module) into the switch’s SFP/QSFP cage, which converts between the switch’s electrical interface and the optical or copper media on the cable.
| Form factor | Speed | Cable | Use |
|---|---|---|---|
| SFP | 1 Gbps | MMF / SMF / copper (RJ45) | Access switch uplinks |
| SFP+ | 10 Gbps | MMF / SMF / DAC | 10 Gbps server / switch ports |
| SFP28 | 25 Gbps | MMF / SMF / DAC | Modern server NICs |
| QSFP+ | 40 Gbps (4×10) | MMF / SMF / DAC / AOC | Spine-leaf, datacenter aggregation |
| QSFP28 | 100 Gbps (4×25) | MMF / SMF / DAC / AOC | Modern spine, datacenter backbone |
| QSFP-DD | 400/800 Gbps | MMF / SMF | Hyperscale datacenters |
Cable choices for transceiver speeds:
- DAC (Direct Attach Copper) — short (≤7 m) passive copper cable with transceivers built into both ends. Cheap. Use for top-of-rack server connections.
- AOC (Active Optical Cable) — same idea but with fiber. Longer (up to ~30 m) than DAC. Used for mid-rack to top-of-rack runs that don’t justify pluggable optics.
- Pluggable optics + structured fiber — most flexible. Use for inter-switch links and anything over ~10 m.
Vendor lock — the dirty secret
Many switch vendors “validate” only their own-branded transceivers. A generic 10G SFP+ from a third-party (FS, ProLabs) physically works and often costs 1/10 the price of a Cisco-branded one — but on stricter platforms (Catalyst 9000, Nexus), the port may refuse the optic without service unsupported-transceiver configured.
SW1(config)# service unsupported-transceiver
SW1(config)# no errdisable detect cause gbic-invalid
This is technically supported on most Cisco gear. Use third-party optics with eyes open.
Distance + bandwidth chart — the reference card
| Goal | Use |
|---|---|
| 1 Gbps to a desk, ≤100m | Cat5e UTP + RJ45 |
| 10 Gbps to a desk or AP, ≤100m | Cat6A UTP + RJ45 (and a 10G port) |
| 1 Gbps switch-to-switch in the same building, ≤300m | OM3 + 1G SFP optics |
| 10 Gbps switch-to-switch in the same building, ≤300m | OM3 + 10G SFP+ optics |
| 25-100 Gbps spine-to-leaf in datacenter, ≤100m | OM4 + 25/100G optics |
| 10 Gbps between two buildings, ≤10 km | Single-mode + 10GBASE-LR SFP+ |
| 10-40 Gbps to an ISP, ≤40 km | Single-mode + 10GBASE-ER / 40GBASE-ER4 |
| Server NIC to ToR, ≤7 m | DAC cable (passive) |
| Server NIC to ToR, ≤30 m | AOC cable |
Common mistakes
-
Quoting “Cat6 = 10 Gbps” without distance. It’s 10 Gbps only to 55m. For full 100m runs at 10G, use Cat6A.
-
Mismatched transceivers. Both ends must use the same standard. A 10G-SR (multi-mode short range) on one end and 10G-LR (single-mode long range) on the other won’t link.
-
Mixing MMF and SMF. Single-mode optics aimed into multi-mode fiber lose power; multi-mode optics into single-mode get refracted away from the core. Either way, no link.
-
OM3 over 40G QSFP+ at 400m. OM3 caps at 100m for 40/100G. Use OM4 or single-mode for longer.
-
Forgetting MPO polarity. 12-fiber MPO trunks have a polarity (A, B, or C) — wrong polarity means the TX of one device hits the TX of the other. Buy patch cords that match your trunk type.
-
PoE+ on Cat5e. Works for short runs but heat buildup is a real concern. Cat6 minimum for sustained PoE+, Cat6A for PoE++ (802.3bt).
-
Buying expensive Cat8 for office cabling. Cat8 is short-haul datacenter copper. For a typical desk drop, Cat6A is the modern standard.
-
Connecting APC to UPC. Polish mismatch = ~1 dB+ loss. Stick with one polish type per fiber path.
Lab to try tonight
This is hard to lab without real cables and switches. What you can do:
- Look at your existing patch panels. Identify the cable Cat rating (printed on the jacket).
- Open your switch and look at its uplink ports. SFP cage? SFP+? QSFP? Note the form factor.
- If you have an SFP-capable switch, pull out a transceiver and read the label. It’ll say something like
1000BASE-LX(1G single-mode) or10GBASE-SR(10G multi-mode short range). - From the CLI,
show interface Gi1/1 transceiver(Cisco) shows the optical power — useful for “is this fiber okay” diagnostics. - In CML or EVE-NG, fiber/copper choice is abstracted away — but you can simulate distance/loss issues by changing link delay parameters.
Cheat strip
| Concept | Plain English |
|---|---|
| Cat5e | 1 Gbps to 100m — the access-port standard |
| Cat6A | 10 Gbps to 100m — the modern AP / 10G access standard |
| Cat8 | 25-40 Gbps to 30m — datacenter ToR copper only |
| Multi-mode (MMF) | Short-range fiber, ~550m max, OM3/OM4 = aqua jacket |
| Single-mode (SMF) | Long-range fiber, 10-80 km, OS2 = yellow jacket |
| OM3 / OM4 | Multi-mode grades; OM4 better at 40/100G |
| OS1 / OS2 | Single-mode grades; OS2 for outdoor / long-haul |
| LC | Most common modern fiber connector |
| MPO / MTP | 12 or 24 fibers in one connector — high-density spine-leaf |
| SFP / SFP+ / SFP28 | 1 / 10 / 25 Gbps pluggable transceivers |
| QSFP+ / QSFP28 | 40 / 100 Gbps pluggable transceivers |
| DAC | Direct-attach copper, ≤7m, cheapest 10/25G option |
| AOC | Active optical cable, ≤30m, pre-terminated fiber |
service unsupported-transceiver | Allows 3rd-party optics on strict Cisco platforms |
| Distance + speed first | Pick the cable, then pick the transceiver. Not the other way around. |