Ratified in 2006, the IEEE 802.3an 10GBASE-T specification for 10-Gigabit Ethernet (10GigE) on unshielded twisted pair copper cabling offers two important attributes to network managers and IT professionals planning data centers and enterprise networks. First, it supports legacy copper cabling, and for new installations maintains the structured copper cabling paradigm, as well as support for RJ-45 connections and patch panels. Secondly, 10GBASE-T will enable over time lower-cost 10GigE interconnects by enabling high-density 10-gigabit switches.

Because of its lower cost and plug-and-play simplicity, unshielded twisted pair (UTP) copper cabling remains the media of choice for in-building horizontal runs and data center cabling. As much as 80 percent of the cabling inside buildings today is Category 5e or better, and Category 5e channels can be qualified to support 10GBASE-T operation. On Category 6 links, the standard supports distances from 55 meters to 100 meters. In addition, augmented Category 6 or Category 6a supports 10GBASE-T links up to 100 meters.

The challenge of echo and near-end crosstalk cancellation in 10GBASE-T is one of the major complexity challenges to implementation.

10GBASE-T enables network managers to scale their networks to 10-gigabit speeds, while taking advantage of their investment in installed copper cabling infrastructure. For new installations, it leverages the cost-effectiveness of copper structured cabling.

To increase Ethernet data rates to 1 Gbps, 1000BASE-T Ethernet uses four pairs in a Category 5e cable, with bidirectional signaling. The transceivers are required to cancel both echo and near-end crosstalk (NEXT) on each pair of wires, and cancellation of far-end crosstalk, while recommended, was not required.

To get another order of magnitude in the bit rate, 10GBASE-T takes this several steps further, both expanding the signaling rate and increasing the number of levels in the transmitted signal.

In order to achieve this, a low-parity density check code is employed, as well as substantial improvements to receiver sensitivity, echo and crosstalk cancellations. Challenges consist of impairments related to the propagation of the signals themselves, such as insertion loss and intersymbol interference, that are caused by the limited bandwidth and real impedance of the cable itself, plus degradations due to interfering effects, such as echo, near-end crosstalk and far-end crosstalk. In addition, background noise and other radiated signals, such as alien crosstalk, can reduce the received signal-to-noise ratio.

Vulnerable to noise

With wired Ethernet systems, as frequency increases, the received signal will become weaker, making it more vulnerable to noise, culminating in high power loss at 400 MHz. In addition to the need to encode the 10GigE signal to fit within this limited bandwidth, the slope of the attenuation, from less than 3 decibals at low frequencies to a 40-decibal loss at 400 MHz, drives the need for efficient equalization. Because hardware trades will affect robustness, physical layer evaluators should test not only the longest links a physical layer can operate on, but also numerous intermediate length and patch cord configurations.

Beginning with 1000BASE-T, and continuing to 10GBASE-T, the effects of echo and NEXT have been cancelled substantially in the receiver. The practical implication is that in addition to cabling connectors and interfaces, echo and NEXT formed by the transmitted signal reflecting off minor imperfections in the twists should be cancelled. As a result, echo cancellers can be required to have taps covering the entire length of the channel. These taps should continue to adapt to changes in the channel in order to respond to mechanical changes in the wire.

The echo and NEXT cancellation in 10GBASE-T is one of the major complexity challenges to implementation. The techniques used in 1000BASE-T solutions, if implemented in a straight-forward way, result in a complexity increase. Because of the inherent variability and randomness of the impulse responses, simple techniques to extend the length of the impulse response cancelled, such as continuous-time analog filters or digital filters, do not easily lend themselves to flexible solutions that will work on a variety of wiring.

To gain both the advantage of highly precise digital signal processing and efficient analog processing, one approach involves both analog and digital signal processing. This approach allows for robust cancellation of NEXT and echo by implementing cancellers hundreds of taps long, and sharing computation between all 16 echo and NEXT cancellation paths.

Because of the trades made in the design of echo and near-end crosstalk cancellers, testing the system's ability to respond to perturbations in the cable is important. Bit errors may occur from the transients involved in bending a cable, but the system should recover after adjusting to the new echo and NEXT environment.

The final within-cable impairment is far-end crosstalk (FEXT). FEXT is interference produced by signals emanating from adjacent transmitters at the far end of the 10GigE link. For 10GBASE-T, however, FEXT interference produces a significant loss in the signal-to-noise ratio that would prevent 10GBASE-T transmission at intermediate lengths, as well as on long lines.

Performance testing recommended

Because FEXT is dispersed similar to the desired received signal, canceling it can require a significant amount of signal processing hardware. One tradeoff is to combine equalization and FEXT cancellation, resulting in a four to six times reduction in FEXT canceller complexity. Because levels of FEXT can vary significantly depending on cabling and connectors, testing the performance of these transceivers at all lengths is important. FEXT is a dominant issue for intermediate-length cabling, between 20 meters and 50 meters, with two to four connector channels.

Since the advanced signal processing in 10GBASE-T manages to mitigate the sources of noise and distortion within an individual four-pair cable sheath, this ultimately leaves the system limited in performance by external crosstalk from other cables in close proximity, called alien crosstalk. Alien crosstalk can come either from sources at the same end of a transmitting link (alien NEXT), or from sources transmitting at the other end, or somewhere along the length of the link (alien FEXT).

Alien NEXT sources are effectively mitigated through cabling design, qualification and installation practices. Alien FEXT, however, has proven to be a tougher problem for cabling. To avoid the issue of a short-distance link far-end cross talking into a link on a long line, the 10GBASE-T standard requires transmitters to reduce their transmit power to only the level needed on their link. Thus, short-link transmitters will transmit with a "power backoff" from the nominal power used for longest, 100-meter links.

Three cabling systems exceed the 10GBASE-T alien crosstalk requirements: Category 6 cable constructed with a foil screen (FTP), augmented Category 6 cabling and Category 7 cabling, supporting links up to 100 meters. For new cabling installations, customers should be evaluating Category 6a cabling, and for those customers who prefer screened cabling, Category 7 or Category 6 FTP.

George Zimmerman and Bruce Tolley are with Solarflare Communications, Irvine, Calif.

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