Hindrances to High-Speed Data Transfer | Speed Bumps

Electricity flowing through a cable is nothing more than electrons moving inside the cable and bumping into each other—sort of like dominoes falling. For a signal to be received properly by the receiver, enough electrons must make contact all the way through the cable from the sender to the receiver. As the frequency on a cable (and consequently the potential data rate) increases, a number of phenomena hinder the signal's travel through the cable (and consequently the transfer of data). These phenomena are important not only to the person who has to authorize cable purchase but also to the person who tests and certifies the cable.

The current specifications for Category 5e, 6, and 6A cabling outline a number of these phenomena and the maximum (or minimum) acceptable values that a cable can meet and still be certified as compliant.

Due to the complex modulation technology used by 1000Base-T Ethernet, and even more so with 10GBase-T, the TIA has specified cabling performance specifications beyond what was included in the original testing specification. These performance characteristics include power-sum and pair-to-pair crosstalk measurements, delay skew, return loss, and ELFEXT. Some of these newer performance characteristics are important as they relate to crosstalk—for example, AXT (alien crosstalk) to express the interaction between cables in a cable bundle. Although crosstalk is important in all technologies, faster technologies such as 1000Base-T and 10GBase-T are more sensitive to it because they use all four pairs in parallel for transmission.

All these requirements are built into the current version of the standard, ANSI/TIA-568-C.

Many transmission requirements are expressed as mathematical formulas. For the convenience of humans who can't do complex log functions in their heads (virtually everyone!), values are pre-computed and listed in the specification according to selected frequencies. But the actual requirement is that the characteristic must pass the "sweep test" across the full bandwidth specified for the cable category. So performance must be consistent and in accordance with the formula, at any given frequency level, from the lowest to the highest frequency specified.

The major test parameters for communication cables, and the general groupings they fall into, are as follows:

• Attenuation (signal-loss) related
  • Conductor resistance
  • Mutual capacitance
  • Return loss
  • Impedance
• Noise-related
  • Resistance unbalance
  • Capacitance unbalance
  • Near-end crosstalk (NEXT)
  • Far-end crosstalk (FEXT)
  • Power-sum NEXT
  • Power-sum FEXT
  • Alien crosstalk (AXT)
• Other
  • Attenuation-to-crosstalk ratio (ACR)
  • Power-sum ACR
  • Propagation delay
  • Delay skew