The Evolution of CAT5 Performance and Jack Design
What constitutes Cat5 performance is a continually revised set of specifications. These revisions do not necessarily obsolete existing equipment. It depends strictly on the quality of the initial design. Some of the revised specifications came about because problems were found, while others filled a need not originally contemplated.
For instance, the original specification for connector crosstalk called for it to be measured with a special calibrated plug having a permissible crosstalk of 40 to 42dB. When it was discovered that various laboratories could not verify each other's results, TSB40 was created, using 5 calibrated plugs, one between 40 and 40.5dB, another greater than 41.5 dB and the other three somewhere in between. Thus, anyone who had diligently designed his compensation to cover the range between 40 and 42 dB had a design that was robust under this elaboration.
Another major problem that surfaced was short links failing TSB67. This problem was created by jack compensation schemes that were unbalanced; they were not symmetrically applied to the two wires that make up a pair. Again, a designer familiar with radiation and signal balance would have applied the necessary compensation in a balanced mode, resulting in a robust design under the specifications in 568A.
Today the EIA/TIA TR41.8.1 Committee is developing a new specification for far end cross talk (FEXT) because the IEEE 1000BASET Committee needs to be assured that cable will meet what are now unspecified limits. While NEXT was developed for applications that listen to one pair whole transmitting on another; FEXT is now being developed for applications where signals are transmitted in the same direction on two or more pairs and the cross contamination needs to be constrained. This specification is being developed is such that most of the existing cable plants will be able to meet it. Note that jacks that exceed CAT5 limits for NEXT also have improved FEXT (most of the problem is in the jacks rather than the wire) and will easily pass this yet to be ratified specification.
There has been some concern about applications whose bit rate exceeds 100Mb/s. Both FDDI over copper and ATM are designed to work with CAT5 cable while FDDI further restricts the jack use to the outer pairs. Design engineers face creating higher speed applications needing new cabling vs. fitting the design to existing cabling. One option is to use complex and costly coding schemes that allow more bits within a given bandwidth. One company has demonstrated packing 155 Mb/s in a bandwidth of 30 MHz by using an 8 by 8 amplitude-phase matrix with adaptive compensation. The 1000BASET Committee is even discussing resorting to crosstalk cancellation with the committed intent of running Gigabit Ethernet on existing cable plant.
There are other issues, such as return loss and balance, which will be added to future cabling standards in order to meet the needs of ever-greater bandwidth. The pressure is to utilize the very large but still youthful installed base of CAT5. The trend is towards incorporating the historically costly complex coding and compensation schemes onto single inexpensive chips (Moore's Law in action).
As the jacks are the weakest link in the channel, how well the modular jacks are compensated is crucial to the future success of CAT5. ETS modules and patch panels use compensated jacks that far exceed EIA/TIA 568B Commercial Building Telecommunications Cabling Standard for crosstalk, return loss and insertion loss. The basic design is well balanced, with more compensation elements symmetrically applied than required by the standard. ETS jacks have passed UL requirement for safety and performance. ETS jacks will out-perform any and all jacks while other brands merely meet CAT5 performance.
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