Fibre loss budget
Add up the worst case before you buy optics, then compare it with what the light meter says after installation. The defaults are the TIA maximums. If your connectors are rated lower loss, use the figure from their datasheet.
A typical patch-panel-to-patch-panel link has two mated pairs, one at each panel. Each extra patch point adds a pair.
Copper and speed
A channel is at most 100 m: 90 m of fixed cable plus 10 m of cords in total. Long patch cords and coiled slack in the ceiling count towards it.
| Speed | Minimum cable | Distance | Notes |
|---|---|---|---|
| 1000BASE-T | Cat 5e | 100 m | All four pairs. |
| 2.5GBASE-T | Cat 5e | 100 m | Watch alien crosstalk in large bundles. |
| 5GBASE-T | Cat 6 | 100 m | Often works on Cat 5e. Test it rather than assume. |
| 10GBASE-T | Cat 6A | 100 m | On Cat 6 UTP, up to 55 m and less in large bundles. |
Cheap cable that is copper-clad aluminium (CCA) rather than solid copper is not compliant with any category standard, whatever the jacket says. It fails insertion loss, breaks when flexed, and runs hot with PoE. If a reel is unusually light or cheap, scrape a conductor.
Test failures explained
What a failed certification line means, and the usual cause.
| Result | What it means | Usual cause |
|---|---|---|
| Open | A conductor is not connected end to end. | Cable cut or nicked, or a conductor not pushed fully into the IDC slot. |
| Short | Two conductors touch. | Crushed cable, a staple through it, or stray copper at a termination. |
| Reversed pair | The two wires of one pair are swapped, for example pin 1 to pin 2. | Wires of a pair laid the wrong way round at one end. |
| Crossed pair | A whole pair lands on the wrong pins, for example 1-2 on 3-6. | One end terminated T568A and the other T568B. |
| Split pair | Every pin connects to the right pin, but the signal pairs are made from wires of two different pairs. | Wrong colour order at both ends. It passes a simple continuity tester and fails NEXT, so only a certifier finds it. |
| Length | Permanent link over 90 m or channel over 100 m. | Long run, or a lot of slack coiled in the ceiling. Patch cords count towards the channel. |
| Insertion loss | Too much signal lost along the link. | Too long, a hot cable (PoE in big bundles), or cable that is not solid copper. |
| NEXT | Too much crosstalk near the tester end. | Too much untwist at a termination (keep it under 13 mm), mismatched connector category, or a split pair. The tester's fault location tells you which end. |
| Return loss | Signal reflected back by impedance changes. | Kinked or crushed cable, over-tight ties, mixed categories, poor patch cords. |
| Resistance unbalance | The two wires of a pair have different DC resistance. | A poor termination on one wire. It matters for PoE, which runs current down the pair. |
Fibre
| Symptom | Usual cause |
|---|---|
| High loss at a connector | Dirty end face, by a distance the most common fibre fault. Inspect, clean, inspect again, then connect. |
| High loss along the run | A tight bend or crushed section (macrobend), or a bad splice. An OTDR shows where. |
| High reflectance | A dirty or damaged end face, an air gap, or an APC connector mated to a UPC one. |
| No light at all | Transmit and receive swapped. On MPO trunks, check that the polarity method matches at both ends. |
| Fails only at 40G or 100G | The link is inside the 10G budget but not the SR4 one. Standard-grade connectors use most of a 1.5 to 1.9 dB budget. |
Colour codes
T568A and T568B
| Pin | T568A | T568B |
|---|---|---|
| 1 | White/green | White/orange |
| 2 | Green | Orange |
| 3 | White/orange | White/green |
| 4 | Blue | Blue |
| 5 | White/blue | White/blue |
| 6 | Orange | Green |
| 7 | White/brown | White/brown |
| 8 | Brown | Brown |
Either works. Pick one for the site and use it at both ends. Mixing them is a crossed pair.
Fibre jackets and connectors
| Fibre | Jacket | Connector |
|---|---|---|
| OM1, OM2 | Orange | Beige |
| OM3 | Aqua | Aqua |
| OM4 | Aqua, or heather violet in Europe | Aqua or heather violet |
| OM5 | Lime green | Lime green |
| OS1a, OS2 | Yellow | Blue for UPC, green for APC |
Never mate a green (APC, angled) connector to a blue (UPC, flat) one. The loss is high and the end faces can be damaged.
Power over Ethernet
| Standard | At the switch port | At the device | Pairs used |
|---|---|---|---|
| 802.3af (Type 1) | 15.4 W | 12.95 W | 2 pairs |
| 802.3at (Type 2) | 30 W | 25.5 W | 2 pairs |
| 802.3bt (Type 3) | 60 W | 51 W | 4 pairs |
| 802.3bt (Type 4) | 90 W | 71.3 W | 4 pairs |
Higher PoE classes heat cable bundles, and warm cable loses more signal. For Type 3 and Type 4 in large bundles, follow the cable maker's bundle-size guidance, and prefer Cat 6A, whose larger conductors run cooler. Budget the switch's total PoE power as well as the per-port class.
Before you accept a cabling installation
The checklist to agree with the installer before work starts, and to hold them to before you take over the network.
- Every horizontal run is under 90 m, in proper containment, and terminated at both ends on outlets and patch panels.
- Every link is certified with a field tester, and you have the full results, not a summary.
- Outlets follow a sequential numbering scheme that survives a second floor, such as 1-01 to 1-99 and 2-01 to 2-99, and the labels match the panels.
- Floor-standing cabinets where you can: 800 mm deep by 600 mm wide is a sensible minimum, with about a metre clear at the sides and front so the panels come off for work.
- Dedicated, filtered power to each cabinet, with a PDU inside it.
- The cabinet is laid out to an agreed drawing: patch panels, cable management and switch positions.
- Written warranty terms for the installation, who is responsible for faults afterwards, and how to contact them.
- As-built drawings and the test results handed over together.
UK rules worth knowing
- BS EN 50173 covers the design of generic cabling, and BS EN 50174 covers installation, including separation from power cables.
- BS 6701 requires telecoms cable permanently installed in buildings to meet at least CPR Euroclass Cca-s1b,d2,a2. Cheaper Eca cable does not comply.
- Construction Products Regulation: permanently installed cable must carry a Euroclass and a declaration of performance. Ask for the declaration.
- BS 7671, regulation 521.10.202: wiring systems must be supported so they will not collapse prematurely in a fire. Most cabling specifications hold data cable on the same route to the same rule, so plastic ties and trunking alone at high level are the classic fail. Use metal fixings at intervals.
- Labelling and records: label both ends of every link, the panel and the outlet, and hand over the test results with the as-built drawing. The certification report is the warranty evidence.