Fundamentals: What is Computer Twisted Pair?

Twisted pair cable is the most popular type of copper cabling used in local area networks (LAN) and traditional telephone systems. Its fundamental element is a pair of insulated copper conductors twisted together along their entire length. A typical network cable contains four such pairs placed within a common outer protective sheath.

The Magic of Twisting

"Twisting" is not an aesthetic procedure; it's a key engineering element designed to combat the ubiquitous problem in electronic communications: Electromagnetic Interference (EMI). Twisting conductors provides two key benefits:

• External Noise Protection: When external magnetic field passes through a twisted pair, it induces current in both conductors. Because the conductors are twisted, each is alternately closer and farther from the noise source. This causes the induced noise current in one conductor to be opposite (180 degrees out of phase) to the noise current in the other. When a differential receiver at the end of the line reads the difference between the two conductors, the opposite noise signals effectively cancel each other out, protecting the original data signal.
• Crosstalk Reduction: Twisting also closes the magnetic field of the cable itself, preventing signal interference from one pair to adjacent pairs in the same cable. This type of internal interference is called crosstalk. To further enhance this effect, each pair in a multi-pair cable is twisted with a slightly different density (twist rate).

Shielding Types (XX/YTP Standard)

For additional protection against strong EMI interference, twisted pair can include metallic shielding. The modern XX/YTP nomenclature describes the type of overall and individual pair shielding.

• U/UTP (Unshielded / Unshielded Twisted Pair):

  This is the most popular and basic type, often simply called UTP. It has no metallic shield. The cable consists only of four twisted pairs in a non-metallic outer sheath. It is suitable for most office and home environments with low EMI levels.

• F/UTP (Foiled / Unshielded Twisted Pair):

  This cable has an overall foil shield made of aluminum (F) that wraps all four pairs together. Individual pairs remain unshielded (UTP). This construction, formerly known as FTP, provides good protection against external EMI interference.

• U/FTP (Unshielded / Foiled Twisted Pair):

  This cable has no overall shield (U), but each pair individually is wrapped with its own foil shield (FTP). This design excellently minimizes crosstalk between pairs, making it suitable for high-speed applications. Formerly known as STP.

• SF/UTP (Shielded and Foiled / Unshielded Twisted Pair):

  Has a robust double overall shield consisting of both braid (S) and foil layer (F). Individual pairs inside are unshielded (UTP). Provides very high protection against external interference.

• S/FTP (Shielded / Foiled Twisted Pair):

  This high-performance cable combines an overall braid shield (S) with individual foil shields on each pair (FTP). Provides excellent protection against both external EMI interference and internal crosstalk. Often used in Category 7/7A installations.

Performance Categories and Classes

Twisted pair cabling is standardized in performance categories (TIA) and classes (ISO/IEC/EN), which determine its ability to carry signals, primarily through maximum specified frequency (bandwidth).

• Category defines requirements for individual components (e.g., the cable itself, connector).
• Class defines requirements for the entire transmission path (so-called "channel").

Popular Categories:

• Category 5e / Class D: The most popular standard currently in use. Supports frequencies up to 100 MHz and is designed for Gigabit Ethernet (1 Gb/s).
• Category 6 / Class E: Supports frequencies up to 250 MHz, offering better performance and less crosstalk than Cat 5e. Solidly supports Gigabit Ethernet and can handle 10 Gigabit Ethernet on shorter distances (up to 55 meters).
• Category 6A / Class EA: The "A" stands for "Augmented" (enhanced). Supports frequencies up to 500 MHz and is designed for reliable 10 Gigabit Ethernet operation at full 100-meter distance.
• Category 7 / Class F and Category 7A / Class FA: These standards support even higher frequencies (600 MHz and 1000 MHz) and require fully shielded cables (S/FTP) and specialized connectors other than RJ-45 to achieve their full potential. They are designed for future high-speed applications exceeding 10 Gb/s.

Connection Standards: T568A and T568B

To ensure interoperability, there are two defined standards specifying how the eight wires in a twisted pair cable should be terminated in an 8P8C (RJ-45) connector: T568A and T568B.

The only difference between these two standards is the swapping of the green and orange pairs (Pairs 2 and 3). Consistency is key - a single installation should use one standard throughout the system. While T568B is more common in the United States and newer installations, T568A is specified in some government contracts and is compatible with older wiring schemes.

Cable Types: Straight vs. Crossover

Depending on how T568A/B standards are applied at each end, two types of cables are created:

• Straight Cable: Both ends of the cable are terminated using the same standard (e.g., T568B at both ends). This is the most popular cable type, used to connect a device (such as a computer) to a network device (such as a switch or router).
• Crossover Cable: One end is terminated per T568A and the other per T568B. This causes the transmit and receive pairs to be crossed, enabling direct connection of two similar devices (e.g., two computers or two older switches) without a central network device.

Auto MDI/MDI-X: Modern Significance

Currently, the need for crossover cables has significantly decreased. Most modern network devices have Auto MDI/MDI-X functionality, which automatically detects the type of cable used and configures the port accordingly. This technology enables the device to work correctly regardless of the cable type used, largely eliminating the need for crossover cables.

Installation Rules and Best Practices

Proper installation is crucial for achieving the specified performance of a cabling system. Errors can degrade performance even when using high-quality components.

• Minimum Bend Radius: Never bend the cable tighter than its specified minimum bend radius (typically 4 times the cable diameter for UTP). Excessive bending changes the cable's geometry and its impedance, causing reflections and signal degradation.
• Separation from Power Cables: Maintain separation of data transmission cables from power cables to avoid EMI interference. If crossing is unavoidable, it should be done at a 90-degree angle to minimize inductive coupling.
• Termination: At the connector, untwist the pairs for the shortest possible length. Standards specify maximum untwisted pair length (e.g., 13 mm or 0.5 inches) to maintain crosstalk performance.
• Grounding: For shielded cables (F/UTP, S/FTP, etc.), the shield must be properly grounded, typically at both ends, to effectively drain induced interference. An ungrounded shield can act like an antenna, worsening interference.
• Cable Management: Avoid overtightening cable ties, as this can deform the cable and change its geometry. Use appropriate organizers to avoid tangling and maintain performance.