Not all Cats prefer IT

Category 6 performance represents a significant step forward in the overall high-speed performance of copper utp/stp links, increasing rated performance to 200 MHz, and the pushing of copper links with RJ-45 style connectors to their maximum limit. However, there has been a growing industry concern that while Cat 6 performance can be achieved with certain "tuned combinations" of connectors and plugs, it may no longer be possible for installers to assume full mix-and-match compatibility between generic components.

To provide compatibility with existing work-area equipment, the draft ISO/IEC specifications for Cat 6 and Class E cabling support the use of an RJ-45 eight-position modular interface at the telecoms outlet. The draft specifications stipulate that Cat 6 configurations will sustain a positive attenuation to crosstalk ratio (ACR) up through at least 200 MHz installed performance (using a 250 MHz test). Overall, the RF field testing parameters required to certify Cat 6 cabling to 250 MHz test levels require improvements in measurement precision by as much as 10 dB over Cat 5e. In response to these stringent Cat 6 requirements, a whole new generation of field testers has evolved, providing unprecedented Level III test accuracy.

In essence, we currently have Cat 6 and Class E draft specifications that are being refined toward worldwide industry agreement along with field test capabilities that are closely tracking the draft requirements. In addition, we have cable that clearly meets the Cat 6 objectives along with a number of connector manufacturers that claim to have a total Cat 6 solution. However, the weak point in overall Cat 6 viability remains the RJ-45 connection.

Obviously, the goal of Cat 6 is to specify a 200 MHz solution that is fully backward compatible with existing Cat 5 and 5e. However, squeezing Cat 6 performance out of the RJ-45 form and function constraints is requiring optimization techniques that run counter to maintaining such universal compatibility. Although an ideal connector design would likely rely upon new overall spatial relationships, symmetry and shape to avoid inter-pair coupling at 200 MHz performance, the constraint of the existing RJ-45 form factor has left electrical tuning techniques as the sole avenue of improvement open to connector designers.

The performance of the RJ-45 modular plug is specified as a terminated open circuit (TOC) measurement with NEXT performance in the range of 40-41·5 dB. For Cat 5 implementations, special compensation circuitry has already been incorporated into telecom outlet designs which are tuned to achieve required performance with RJ-45 connectors. However, achievement of Cat 6 requirements will likely require NEXT performance at the connector of approximately 48 dB at 200 MHz.

Although the obvious solution would appear to be improve the TOC performance of new RJ-45 designs, studies conducted by a major cable/connector manufacturer have shown that the Cat 5 electrical tuning is optimized for a finite range. As a result, higher performing "super plugs" may present significant backward-compatibility problems. The evidence indicates that tuned combinations of improved-TOC plug-and-jack connectors can effectively support Cat 6 speeds. However, if an RJ-45 plug with greater than 44 dB of TOC is connected to an existing Cat 5 jack it can produce mismatched compensation and degraded performance that fail to even meet Category 5 requirements.

Of course, NEXT compatibility is only one key parameter of the connector interface in a structured cabling environment that can be impacted by electrical tuning mismatches between the modular plug and jack. It is very likely that other key parameters such as balance and FEXT could also be negatively affected by mixing Cat 6 components within some existing Cat 5 environments.

In addition to presenting a problem of backward compatibility, the lack of an industry wide standard for electrically tuning new Cat 6 components, means there is a definite risk of failure when attempting to mix RJ-45 plugs and jacks from different vendors.

From the cable installers' standpoint, it is becoming increasingly important to consciously select and test for cross compatibility between connector components throughout their structured wiring implementations. These incompatibility issues can also pose a problem when it comes to the accepted practice of both installers and network administrators to create patch cords in the field. In addition, because the customers' equipment interfaces can cause incompatibility problems, the advent of Cat 6 may require a higher level of customer assistance, well beyond the point of final cable certification.

It is already well accepted that the RJ-45 connector will have to be discarded for higher speed implementations, such as the Cat 7 specifications. In the meantime, it is clear that we are pushing the existing modular connector form factor to the limits of its capabilities, creating an environment of significant concern for cable installers. The bottom line is that all industry participants, including installation contractors, cable/connector manufacturers and test equipment suppliers, need to increase their focus on the issue of generic compatibility.

If Cat 6 is going to succeed in the long run, working partnerships and technology-sharing arrangements between key component manufacturers will be a vital factor in maintaining a reliable foundation of mix-and-match capability for installers. In addition, the development and delivery of joint training curricula between cable connector manufacturers and test equipment suppliers will play an important role in assisting installers to test and certify optimally-matched combinations of Cat 6 components.