UTP cable: why eight wires determine the network speed
Understand why four conductors often limit Ethernet to 100 Mbit/s, how the four pairs enable Gigabit, and how to diagnose a faulty cable.
A NAS, a computer and a router all have Gigabit ports, yet copying a file tops out around 10 to 12.5 MB/s. Storage is not always the culprit. This figure is often the tell-tale sign of an Ethernet link negotiated at 100 Mbit/s instead of 1 Gbit/s: 100 Mbit/s divided by eight yields 12.5 MB/s before you even subtract headers and other losses.
The culprit may be a four-conductor cable, a broken pair, an incomplete crimp, or equipment limited to 100 Mbit/s. To understand why, you need to look at what is actually inside the jacket of an Ethernet cable.
A normal UTP cable contains eight conductors
UTP stands for Unshielded Twisted Pair, or unshielded twisted pair. Under the jacket there are four pairs, that is eight conductors. Each pair pairs a solid colour with its white-striped version:
- white-orange and orange ;
- white-green and green ;
- white-blue and blue ;
- white-brown and brown.
The twist is not decorative. The two conductors of a pair carry complementary signals. Interference picked up along the way affects the two wires of a pair in a similar manner and the receiver can cancel much of it. Each pair has a different twist pitch to limit crosstalk between pairs.
You must preserve this geometry right up to the connector. Untwisting several centimetres to ease the crimp degrades performance, even if a simple continuity test shows that all eight wires are connected.
Why four wires suffice at 100 Mbit/s, but not at 1 Gbit/s
Older 100BASE-TX links use two pairs: one for transmission and one for reception. That is why some economy or older cords contain only four conductors. They can work correctly at 100 Mbit/s, but cannot establish a compliant 1000BASE-T link.
Copper Gigabit Ethernet uses the four pairs simultaneously in both directions. The 2.5GBASE-T, 5GBASE-T and 10GBASE-T rates also use the four pairs.
| Standard | Rated speed | Pairs used | Conductors required |
|---|---|---|---|
| 10BASE-T | 10 Mbit/s | 2 | 4 |
| 100BASE-TX | 100 Mbit/s | 2 | 4 |
| 1000BASE-T | 1 Gbit/s | 4 | 8 |
| 2.5GBASE-T and 5GBASE-T | 2.5 and 5 Gbit/s | 4 | 8 |
| 10GBASE-T | 10 Gbit/s | 4 | 8 |
A single poorly crimped conductor can therefore leave a 100BASE-TX link functional while blocking Gigabit. Depending on the network controllers and the nature of the fault, negotiation may fall back to 100 Mbit/s, fail completely, or produce an unstable link with errors and renegotiations.
Eight visible wires do not prove the cable is good
A basic tester mainly checks continuity and pin order. It detects a broken wire, an inversion or a short circuit, but does not necessarily measure the high-frequency quality of the link. A cable can show eight lights in the correct order and yet remain unable to hold its rated speed because of an over-untwisted pair, a bad connector, excessive length, too tight a bend radius, or strong interference.
For a structured installation, a certifier measures in particular insertion loss, near-end and far-end crosstalk, return loss and electrical length. For a home diagnosis, the quickest test usually remains temporarily swapping the cord for a short, certified cable whose Gigabit operation is already known.
T568A, T568B and crossover cables
The cabling standards T568A and T568B use the same eight conductors but place the orange and green pairs differently. A straight-through cable uses the same scheme at both ends. An old crossover cable combines T568A at one end and T568B at the other to cross transmission and reception.
In a modern network, Auto-MDI/MDIX detects the pair direction automatically. It is therefore generally no longer necessary to make a crossover cable to connect two computers directly, a computer to a NAS, or two switches. The essential point remains the consistency of the crimp and the presence of the four pairs.
Which category to choose?
Category describes measurable electrical performance, not just a number printed on the jacket. The quality of the connectors, wall jacks, patch panel and installation counts as much as the cable.
| Category | Reasonable use | Reference distance | Advice |
|---|---|---|---|
| Cat 5 | Old 100BASE-TX | up to 100 m | Replace during a renovation |
| Cat 5e | 1GBASE-T, 2.5GBASE-T | up to 100 m | 5GBASE-T may work on a qualified link |
| Cat 6 | 1, 2.5 and 5 Gbit/s | up to 100 m | 10GBASE-T depends on length and external crosstalk |
| Cat 6A | 10GBASE-T | up to 100 m | Predictable choice for a new 10 Gbit/s installation |
For 10GBASE-T on Cat 6, the often-cited rule of 55 m is not a universal guarantee. Recommendations distinguish links that must work up to about 37 m from those that may work between 37 and 55 m depending on external crosstalk. Only a measurement of the installed link gives a reliable answer. For 100 m at 10 Gbit/s, choose Cat 6A.
Cat 7 and Cat 8 address other constraints and do not automatically constitute a better purchase for a home RJ45 jack. A consistent Cat 6A installation, with certified components end to end, is generally more useful than a spectacularly marked cord plugged into ordinary jacks.
2.5 and 5 Gbit/s on an existing cabling
The IEEE 802.3bz standard introduced 2.5GBASE-T and 5GBASE-T to exploit much of the already-installed Cat 5e and Cat 6 cabling. 2.5 Gbit/s on Cat 5e up to 100 m is a normal target of the standard. 5 Gbit/s is specified on Cat 6 up to 100 m and may work on Cat 5e under defined conditions, but an old long and heavily bundled link may require qualification because of external crosstalk.
Auto-negotiation selects the best common speed announced by the two interfaces. It does not, however, turn a faulty cable into a compliant one. Some controllers have a downshift mechanism that attempts a lower speed after several link-establishment failures: useful for maintaining connectivity, but revealing a problem if the hardware should work faster.
Round cable, flat cable, copper and CCA
A flat cord is not automatically limited to 100 Mbit/s. A properly designed and certified product can reach its advertised speed. Conversely, very thin cables without documentation leave less margin, bend more easily and sometimes hide low-gauge conductors. For fixed cabling, a round four-pair cable, certified and installed without crushing, remains the most robust choice.
Also check the conductor material. A CCA cable uses copper-clad aluminium. Its electrical resistance is higher, it heats up more with PoE and it is more fragile when crimped. Copper-pair communication cables in CCA do not meet the TIA cabling requirements, which demand solid or stranded copper conductors.
- Solid copper: suited to fixed links in walls and cable runs.
- Stranded copper: more flexible, suited to short patch-panel cords.
- CCA: to avoid for structured cabling, especially with PoE.
The conductor gauge, often expressed in AWG, also counts. The higher the AWG number, the thinner the conductor. For a long PoE link or a heavily powered device, the cumulative resistance can become as significant as the data rate.
Why the number of conductors also matters for PoE
Power over Ethernet carries power over the cable pairs. Depending on the PoE generation, two or four pairs may be used. A cable missing two pairs may seem sufficient for an old 100 Mbit/s link, but it reduces the power-supply options, increases the risk of overheating if the conductors are too thin, and prevents the modern modes that distribute current over the four pairs.
For a camera, a Wi-Fi access point, a phone or another PoE device, choose a compliant copper cable, respect the bend radius and avoid tightly bundled bundles exposed to high temperatures.
Checking the negotiated speed on a Synology NAS
In DSM, open Control Panel → Network → Network Interface, then expand the relevant interface. The « Network status » line shows the negotiated speed, the duplex and the MTU.
On a manageable switch, check the port speed and duplex. On a non-manageable model, the colour or blink rate of the lights may distinguish 100 Mbit/s, 1 Gbit/s and sometimes 2.5 Gbit/s; the exact meaning depends on the manufacturer.
Note: link speed is not the speed of a file. A 1 Gbit/s link carries at most 125 MB/s in theory. In a real transfer, about 110 to 118 MB/s is common with good disks and an efficient protocol. Encryption, SMB, storage, the processor, small files and other users further reduce the result.
Diagnostics procedure when Gigabit falls to 100 Mbit/s
- Read the link speed at both ends before launching a copy test.
- Unplug and replug the cord to restart auto-negotiation.
- Try a known short cable, Cat 5e or higher, with eight copper conductors.
- Change port on the switch or router to rule out a damaged connector.
- Inspect the connectors: consistent T568A or T568B order, contacts seated, jacket held and pairs barely untwisted.
- Test the eight conductors with a continuity tester, then qualify the link if the fault persists.
- Check the port error counters: CRC, link losses and renegotiations may reveal a marginal cable.
- Measure with iperf3 to separate network performance from that of the disks and the file share.
Avoid forcing 1 Gbit/s manually as a first solution. Copper Gigabit depends on auto-negotiation for several parameters and a forced setting fixes neither a broken pair nor a bad crimp.
Conclusion
The number of conductors is not a manufacturing detail. Four wires can suffice for an old 100BASE-TX link, while 1000BASE-T and higher rates need the four pairs. If a transfer tops out near 12.5 MB/s, start by checking the negotiated speed and the cable before blaming the NAS or the disks.
For a new installation, keep a simple rule: eight copper conductors, a suitable category, consistent connectors, pairs preserved up to the contacts and end-to-end validation. The marking on the jacket promises a category; only the complete link proves the performance.
Technical references
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About the author
Jeremy Kraft runs the infrastructure and the self-hosted services of the IBCSC. These guides describe configurations that were actually deployed and re-checked before publication.
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