Much is happening in the 10GBASE-T space, particularly in the area of channel performance.
July 1, 2004
For this month’s article I wanted to provide an update on the work of the TIA TR 42.7 subcommittee in support of 10 Gig Ethernet, which last met in June.
It was recognized that the operation of 10 Gig Ethernet (10GBASE-T) will require channel performance to be characterized at least to 500 MHz and possibly as high as 625 MHz.
This frequency range is beyond the maximum currently specified for Category 6 cabling, which is 250 MHz.
As a starting point for discussion, it was originally proposed in the IEEE 10GBASE-T task force, to take the current specification limits for channel performance and extrapolate the values up to 625 MHz.
Extensive data submitted in the TIA subcommittee and task groups indicated that for certain parameters, namely the Near End Crosstalk (NEXT) and Power Sum Near End Crosstalk (PSNEXT), the channel performance can be worse than the extrapolated limits.
This is primarily due to the roll-off in connector NEXT above 250 MHz.
This roll-off follows a slope of between 40 dB to 60 dB compared to the extrapolated limits, which follow a 20 dB slope.
What does this mean and is it significant? Traditionally, NEXT between pairs is the dominant form of interference that restricts the available bandwidth of a channel.
However, 10 Gig Ethernet will implement sophisticated Digital Signal Processing (DSP) techniques to cancel out as much as 40 dB of NEXT.
Therefore, the residual NEXT that is left over after cancellation is no longer a critical parameter for 10 Gig Ethernet. The critical parameter is the alien crosstalk from adjacent cables, which cannot be cancelled out effectively.
A lot of effort within the TIA TR 42.7 Subcommittee and task groups is focused on defining methods for measuring alien NEXT and for mitigating alien NEXT performance within cable bundles and between patch cord assemblies.
Some examples for mitigating alien NEXT performance include ensuring that cables are loosely installed by avoiding, as much as possible, dressing or combing cables over their entire length.
If tie-wraps or Velcro wraps are used, ensure that they are loosely fitted (easily moveable) and spaced further apart (1 to 1.5 m).
If cables are installed in conduits, ensure that the recommended fill ratio is not exceeded. Overfilled conduits (greater than 40% fill) will have the same effect as a tightly bundled cable.
To illustrate the effect of tightly bundled cables, the TIA TR 42.7.2 Cable Working group conducted an experiment where they bundled six cables around a victim cable in the center with tie wraps spaced approximately 0.1 m (3-inches) apart for a distance of 90 meters (much worse than a typical installation).
First draft submitted
The alien crosstalk from all the surrounding cables was measured on the victim cable. The level of alien crosstalk was about 41 dB at 100 MHz. As a second step, the tie wraps were removed and the cables were allowed to resume a natural orientation due to residual stresses in the cable and gravity.
The alien crosstalk was re-measured and improved by about 9 dB or three times less alien crosstalk coupling at 100 MHz. Thus it is seen that tight bundling can have a significant effect on alien crosstalk performance.
It should be noted that alien crosstalk is not a concern for existing applications such as 1000BASE-T and 100BASE-TX. For the 10GBASE-T application, the level of alien NEXT needs to be about 15 dB or 5 times lower than the alien NEXT for 1000BASE-T to support a maximum distance of 100 meters.
Alternatively, the distance needs to be reduced from 100 meters to 55 meters.
The characteristics of “installed base” Category 6 cabling for 10GBASE-T applications are contained in a draft Telecommunications Systems Bulletin TSB-155.
The first draft of TSB-155 has been submitted to IEEE 802.3an task force for consideration in the development of their standard.
In addition to characterizing “installed base” cabling, TIA TR 42.7 has a project to develop augmented Category 6 cabling with improved transmission performance.
The objective is to support a maximum 100 meter distance for 10GBASE-T. The principal enhancements include a lower Insertion Loss (equivalent to Class F cabling in the ISO standards) and significantly improved alien NEXT performance.
It was agreed at the meeting that a target date for a first draft specification for augmented Category 6 will be October.
Paul Kish is Director, IBDN Systems & Standards at NORDX/CDT. He is also vice chair of the TR-42 engineering committee.
Disclaimer: The information presented is the author’s view and is not official TIA correspondence.