Standards definition is an oft-told story - Technology Information

Communications News, April, 2001

Like the tale of Pandora's box, the fable of CAT 6 standards will end with a promise of hope.

Category 6 copper cabling may be twice as good as Category 5, but it has been more than twice as difficult to establish as a standard. The Category 6 development cycle promises to follow much the same pattern as the development of CAT 5. The cycle always begins with the introduction of a new, higher-performance cable.

Standards bodies--such as the Telecommunications Industry Assn. (TIA) and the International Organization for Standardization (ISO)--begin to define standards for the cable and appropriate connecting hardware. During this process, "prestandard" products begin to make their way into the market. Only after a cabling system has been standardized, and has been installed by enough customers to represent a viable market, do local area network (LAN) equipment manufacturers develop products grounded on that level of performance.

OPENING PANDORA'S BOX

End-users, however, want assurance that the system they are installing will support the next high data rate application looming on the horizon. When the next-generation applications arrive, the system will not have been designed around the performance of one particular manufacturer's cabling system; it will have been based on compliance with the Category 6 cabling standard.

In late 1997, the International Organization for Standardization (IOS or ISO) proposed the Class E/Category 6 system, which would provide the power sum ACR (PSACR) performance at 200 MHz that Category 5 provides at 100 MHz. By mid-1998, performance requirements were proposed for all aspects of the Category 6 cabling system: cable, connectors, patch cords, links and channels. Unfortunately, getting from CAT 5 to CAT 6 has been much more challenging than getting from CAT 3 to CAT 5. Not only does the behavior of the components be come more complex as frequencies get higher, notions of what needs to be tested have changed as network protocols become more sophisticated.

Category 5 specifications were based on two-pair applications, such as 10/100Base-T and token ring. Gigabit Ethernet, however, has completely changed the way cabling system performance is viewed. 1000Base-T is unique in that it uses all four pairs of the cable. In addition, each pair transmits and receives simultaneously.

LAN applications also have evolved from having to distinguish between only two signal levels (10Base-T), to three signal levels (100Base-T), to five signal levels in the case of 1000Base-T. This means that there is more noise, coming from more places, in a situation where things need to be quieter.

UNVEILING PERFORMANCE PARAMETERS

To accommodate multipair, bidirectional applications like 1000Base-T, additional parameters have been specified (not only for CAT 6 but for CAT 5 and CAT 5e, as well):

* Skew is the difference in propagation delay between the slowest and fastest pair in the cable or channel. It is important in "reassembling" the data being transmitted on the four pairs.

* Far end crosstalk (FEXT) is a measure of the amount of signal from a device on one end of the cabling system that couples into an adjacent receive circuit in a device on the other end of the cabling system. Equal level far end crosstalk (ELFEXT) takes into consideration the attenuation of the cabling system.

* Power sum refers to the additional noise created when more than one transmit circuit is present in one four-pair channel. Power sum NEXT and ELFEXT (PSNEXT, PSELFEXT) represent the addition of crosstalk noise from each group of three pairs onto the fourth pair.

* Return loss is a measure of the signal that is reflected back toward the transmitter from impedance mismatches caused by connectors or irregularities within the cable itself.

Secondary signal reflections--those headed away from the transmitter--are a significant, additional source of noise not accounted for in other parameter measurements. The effect of these reflections shows up as differences between the actual insertion loss (attenuation) of a link or channel and the insertion loss as determined from adding the component losses. This insertion loss deviation (ILD) is taken into consideration in the Category 6 standard.

Another new performance parameter involved in the CAT 6 standard is balance. Balance refers to the ability of the cabling system to cancel out ambient (common mode) noise picked up from the environment. The requirements for balance are still under discussion and are represented by placeholders in the draft standard. Two laboratory measurements will determine the balance of twisted pair cables and connectors: longitudinal conversion loss (LCL) and longitudinal conversion transfer loss (LCTL). Both LCL and LCTL measure the amount of common mode voltage converted to differential voltage by the cabling system.

WHAT'S THE MORAL OF THE STORY?

Where does the industry stand in the Category 6 development process? The good news is that system performance requirements for CAT 6 have been fairly stable throughout the development process. Prestandard Category 6 systems should do a good job of supporting CAT 6 applications. Other issues, such as balance and interoperability, have yet to be resolved.