10GBase-T taps twisted-pair wiring

The IEEE in June approved the publication of the 802.3an-2006 standard, otherwise known as 10GBase-T. This document describes a physical layer (PHY) transmission device for 10Gbps Ethernet over twisted-pair copper. While running 10G Ethernet over this type of wiring was once thought to be impossible, standards makers relied on four technical building blocks to make 10GBase-T a reality: cancellation, analog-to-digital conversion, cabling enhancements and coding improvements.

Work on 10GBase-T began in 2002, and the IEEE task force that wrote the specification agreed that 10GBase-T would require roughly 1,000 times better cancellation of internal cable impairments, at more than six times the speed of 1000Base-T. Similar to 1000Base-T, 10GBase-T transmits on the wire in both directions simultaneously, so the weak far-end signals are received in the presence of echo and near-end crosstalk from the transmitters sending to the opposite end of the wire.

In consequence, echoes from the entire length of the wire needed to be canceled. In addition, 10GBase-T needed to expand the degree of noise reduction to the extent that significant cancellation of far-end crosstalk was required. This was solved through new parallel transform-based processing techniques, similar to those used in radar processing, and novel combined cancellation and equalization methods perform the required processing within a reasonable level of complexity.

The analog-to-digital converters posed another challenge. 10GBase-T would need to digitize the signal to approximately 10 bits of accuracy at least 800 million times each second. Digital correction and parallelization were brought to bear, and techniques were demonstrated that provided the required conversion speed and accuracy while dissipating low single-digit watts of power.

With this reduction to the noise of the transceiver and the crosstalk within the cable, the major source of noise in the system became crosstalk from other 10GBase-T links in adjacent cables. Cross-industry collaboration between cabling vendors and 10GBase-T PHY experts worked to minimize the alien crosstalk characteristics of copper cabling. Cabling vendors are putting the finishing touches on a cabling specification (TIA/EIA-568 Addendum 10), which defines new Category 6A cabling for 100-meter 10GBase-T, as well as a technical bulletin to address installed cabling (TSB-155), which describes testing and mitigation techniques. Several vendors have shipped pre-standard Category 6A cabling systems.

The final building block, coding improvements, shows that the ever increasing levels of silicon integration render commercially feasible what was once thought impractical. 10GBase-T uses a state-of-the-art low-density parity check (LDPC) code that approaches fundamental coding limits. LDPC codes were discovered in the 1960s but have been rarely implemented, because until recently decoders were considered too complex for commercial application.

In the late 1980s technologists realized that this computational cutoff rate for coding was not a fundamental barrier, and in the early 1990s this barrier was broken with highly integrated decoders for science applications, including deep space communications. 10GBase-T technology is pioneering the commercial use of these high-performance codes to achieve a low bit error rate.