Copper networking cables consisting of paired, twisted conductors used for Ethernet, telephone, and other data communications, standardized by TIA/EIA-568.
Table of Contents#
- Overview
- Cable Types (Shielding)
- Wiring Standards
- Categories and Performance
- Installation Guidelines
- Cable Testing
- Troubleshooting
- See Also
- Sources
1. Overview#
Twisted pair cables are the most common type of network cabling in local area networks. The wires are twisted together in pairs to reduce electromagnetic interference (EMI) and crosstalk between adjacent pairs. The twist rate (twists per meter) varies between pairs within the same cable to further reduce interference.
Twisted pair cables use RJ-45 connectors for Ethernet and are available in various categories (CAT 5e through CAT 8) with different performance characteristics. The choice of cable depends on the required bandwidth, distance, and environment.
2. Cable Types (Shielding)#
2.1. Unshielded Twisted Pair (UTP)#
The most common and basic type. The cable contains pairs of wires twisted together with no additional shielding. UTP relies entirely on the twist rate to reduce EMI and crosstalk. It is lighter, more flexible, and less expensive than shielded alternatives.
Best for: Standard office and residential installations with low EMI.
2.2. Shielded Twisted Pair (STP)#
Similar to UTP, but the twisted pairs are wrapped with a braided copper shield or foil screening around the entire cable bundle. STP requires proper grounding at both ends to be effective; without grounding, the shield can act as an antenna and increase interference.
Best for: Environments with moderate EMI (near motors, fluorescent lighting).
2.3. Foil Twisted Pair (FTP)#
Each individual twisted pair is wrapped in a foil shield, protecting against crosstalk between pairs and external EMI. Also called ScTP (Screened Twisted Pair).
Best for: High-density installations where crosstalk between pairs is a concern.
2.4. Shielded Foil Twisted Pair (S/FTP)#
Combines both FTP and STP shielding: each pair is individually foil-wrapped, and the entire cable bundle is enclosed in a braided shield. This provides the highest level of protection against both EMI and crosstalk.
Best for: Industrial environments, data centers, and areas with high EMI (near heavy machinery, power lines).
2.5. Shielding Designation Summary#
| Designation | Individual Pair Shielding | Overall Cable Shielding |
|---|---|---|
| U/UTP | None | None |
| F/UTP | None | Foil |
| U/FTP | Foil | None |
| S/FTP | Foil | Braid |
| SF/FTP | Foil | Foil + Braid |
The designation format is
Overall/Pair: the first letter(s) describe the overall cable shield, and the letters after the slash describe the individual pair shield.
3. Wiring Standards#
Two wiring standards exist for terminating RJ-45 connectors. Both use the same pin assignments for data transmission; the difference is which color pairs are on pins 1-2 and 3-6.
3.1. TIA/EIA-568A#
| Pin | Color Description |
|---|---|
| 1 | White/Green (green stripe on white) |
| 2 | Green (solid green) |
| 3 | White/Orange (orange stripe on white) |
| 4 | Blue (solid blue) |
| 5 | White/Blue (blue stripe on white) |
| 6 | Orange (solid orange) |
| 7 | White/Brown (brown stripe on white) |
| 8 | Brown (solid brown) |
3.2. TIA/EIA-568B#
| Pin | Color Description |
|---|---|
| 1 | White/Orange (orange stripe on white) |
| 2 | Orange (solid orange) |
| 3 | White/Green (green stripe on white) |
| 4 | Blue (solid blue) |
| 5 | White/Blue (blue stripe on white) |
| 6 | Green (solid green) |
| 7 | White/Brown (brown stripe on white) |
| 8 | Brown (solid brown) |
3.3. Usage Notes#
- Straight-through cable: Both ends use the same standard (both 568A or both 568B). Used for connecting devices to switches, routers, or patch panels.
- Crossover cable: One end is 568A, the other is 568B. Used for connecting two similar devices directly (e.g., PC-to-PC). Modern devices with Auto-MDI/MDIX make crossover cables unnecessary.
- 568B is the most commonly used standard in commercial installations. 568A is common in residential and government installations.
- Consistency matters more than which standard you choose. Use the same standard throughout an installation.
4. Categories and Performance#
| Category | Bandwidth | Maximum Speed | Certified Distance | Notes |
|---|---|---|---|---|
| CAT 3 | 16 MHz | 10 Mbps | 100 m | Legacy, telephone and 10BASE-T |
| CAT 4 | 20 MHz | 16 Mbps | 100 m | Legacy, Token Ring |
| CAT 5 | 100 MHz | 100 Mbps | 100 m | Obsolete, replaced by 5e |
| CAT 5e | 100 MHz | 1 Gbps | 100 m | Minimum for modern networks |
| CAT 6 | 250 MHz | 10 Gbps (certified 55 m) / 1 Gbps (100 m) | 55 m for 10GBASE-T, 100 m for 1000BASE-T | See distance notes below |
| CAT 6a | 500 MHz | 10 Gbps | 100 m | Alien crosstalk tested; recommended for 10G |
| CAT 7 | 600 MHz | 10 Gbps | 100 m | S/FTP shielded; uses GG45 or TERA connectors |
| CAT 7a | 1000 MHz | 10 Gbps | 100 m | Supports future 25G/40G applications |
| CAT 8 | 2000 MHz | 25/40 Gbps | 30 m | Data center switch-to-server connections |
4.1. CAT 6 Distance Clarification#
CAT 6 is certified for 10GBASE-T at distances up to 55 meters under the TIA-568-C.2 standard. At distances beyond 55 meters, alien crosstalk (interference from adjacent cables) may cause errors at 10 Gbps. In practice:
- 55 m is the certified maximum for 10 Gbps operation
- Some installations achieve 10 Gbps at distances up to 70-80 m in low-noise environments, but this is not guaranteed or standards-compliant
- For reliable 10 Gbps at 100 m, use CAT 6a or higher
- CAT 6 supports 1 Gbps at the full 100 m without issues
4.2. Solid vs Stranded Conductors#
| Type | Use Case | Properties |
|---|---|---|
| Solid core | Permanent runs through walls, patch panels | Better electrical performance, less flexible, use punch-down termination |
| Stranded | Patch cables between devices and wall jacks | More flexible, withstands repeated bending, use crimped connectors |
5. Installation Guidelines#
5.1. Bend Radius#
The minimum bend radius is critical to prevent damage to the cable's internal structure, which degrades performance:
| Cable Type | Minimum Bend Radius |
|---|---|
| UTP (4-pair) | 4x cable outer diameter (approximately 25 mm / 1 inch) |
| STP/FTP | 8x cable outer diameter (approximately 50 mm / 2 inches) |
| During installation (pulling) | 8x cable outer diameter for UTP |
Exceeding the bend radius can change the twist rate, increase crosstalk, and reduce signal quality. This is especially critical for CAT 6 and above.
5.2. Pull Force#
- Maximum pull force: 110 N (25 lbf) for 4-pair UTP
- Never pull cables by the connector
- Use cable lubricant for long conduit runs
- Do not exceed the cable's rated tensile strength
5.3. Separation from Power#
| Cable Type | Minimum Separation from Unshielded Power |
|---|---|
| UTP | 200 mm (8 inches) for parallel runs; crossing at 90 degrees is acceptable |
| STP/FTP | 50 mm (2 inches) for parallel runs |
5.4. General Best Practices#
- Maximum channel length: 100 m total (90 m permanent link + 10 m patch cables)
- Avoid running cables parallel to fluorescent lights, motors, or power lines
- Use cable management (trays, j-hooks) to prevent excessive weight on lower cables in bundles
- Label both ends of every cable run
- Leave service loops at patch panels for future retermination
- Maintain the pair twist as close to the connector as possible (untwist no more than 13 mm for CAT 6)
- Ground all shielded cables at one end to avoid ground loops
6. Cable Testing#
6.1. Basic Continuity Testing#
A basic cable tester verifies:
- Wire map - correct pin-to-pin connections (detects opens, shorts, crossed pairs, reversed pairs)
- Continuity - each conductor is connected end-to-end
6.2. Certification Testing#
A cable certifier (e.g., Fluke DSX series) performs comprehensive tests to verify compliance with the cable category standard:
| Test | What It Measures |
|---|---|
| Insertion loss (attenuation) | Signal strength loss over the cable length |
| NEXT (Near-End Crosstalk) | Interference between pairs at the near end |
| FEXT (Far-End Crosstalk) | Interference between pairs at the far end |
| Return loss | Signal reflected back due to impedance mismatches |
| Propagation delay | Time for signal to travel end-to-end |
| Delay skew | Difference in propagation delay between pairs |
| ACR-F (Attenuation to Crosstalk Ratio, Far-end) | Signal quality metric |
6.3. Quick Diagnostics with Linux#
# Check link speed and duplex (should show expected values)
ethtool <interface>
# Check for errors on the interface
ip -s link show <interface>
# Look for CRC errors, frame errors, or carrier errors
# These often indicate cable problems
ethtool -S <interface> | grep -i error6.4. Common Test Failures and Causes#
| Test Failure | Likely Cause |
|---|---|
| Wire map failure | Incorrect termination, damaged connector |
| High insertion loss | Cable too long, poor connections, damaged cable |
| NEXT failure | Pairs untwisted too far at termination, poor quality cable |
| Return loss failure | Impedance mismatch from kinks, tight bends, or connector issues |
Troubleshooting#
| Issue | Cause | Solution |
|---|---|---|
| No link light on switch/NIC | Cable not connected, damaged cable, or bad connector | Test continuity; re-terminate connectors; try a known-good cable |
| Intermittent connectivity | Loose connector, cable damage, or EMI | Re-crimp connectors; check for cable damage; move away from EMI sources |
| Link at 100 Mbps instead of 1 Gbps | Only 2 of 4 pairs working (pins 1,2,3,6) | Test all 8 conductors; re-terminate if pairs 4,5,7,8 are open or shorted |
| High packet loss or CRC errors | Cable exceeds maximum length, poor termination, or crosstalk | Verify cable length under 100 m; re-terminate with proper twist maintenance; upgrade cable category |
| 10G link won't establish | CAT 6 run exceeds 55 m or alien crosstalk from adjacent cables | Shorten run to 55 m or upgrade to CAT 6a; separate from adjacent cables |
| Performance degrades near power lines | EMI from unshielded power cables | Maintain minimum separation; use STP/FTP cable; cross power at 90 degrees |
| Grounding issues with shielded cable | Shield grounded at both ends causing ground loop | Ground the shield at one end only (typically the patch panel end) |