Here is a review of some recent developments from the TIA TR 42 committee meetings, held in Vienna, VA in May.
July 1, 2000
Held in Vienna, Virginia in May, the most recent TIA TR 42 committee meetings produced a variety of interesting information for structured cabling professionals.
Perhaps the most notable was a working draft of the TIA Category 6 standard that was reviewed within the committee. It was decided that there are still too many open issues in this document to go out for a first ballot at this time. The committee intends to clear up many of these issues at the forthcoming TIA TR 42.7 meeting in Vancouver on August 16, 2000.
The latest version of the Category 6 working draft contains a new transmission parameter called Insertion Loss Deviation, or ILD. It is important to get a good understanding of what it is and why it is so important.
GETTING ACQUAINTED WITH ILD
A channel, as specified in the current TIA/EIA 568-A standard, is made up of components that include cords, connectors and cables. In a worst case configuration, a channel can include up to four connectors (two at each end), an equipment cord, a patch cord, a horizontal cable, a furniture cable (part of horizontal) and a work area cord. All of these components can be characterized as having an insertion loss (also called attenuation) and an impedance.
Traditionally, the total insertion loss of a channel is determined by adding up the loss of all the components and calling it the channel insertion loss. All of the formulas for a Category 5 and Category 5e channel model in the TIA standard assume that the whole is equal to the sum of the parts. The problem is that it is only an approximation. It is a good first approximation, but the channel insertion loss is, in fact, higher than the just adding up the loss of each component. The additional losses are due to signal reflections and re-reflections at the boundaries between different components (as illustrated in Figure 1).
The greater the impedance mismatch between components, the higher the mismatch losses. What makes it even more interesting is that at some frequencies, these signal reflections can add in phase and at other frequencies they can add out of phase. Therefore, these mismatch losses are not uniform, and will vary depending on the length of patch cord, the number of connectors, the length of channel, etc. The difference between the actual Insertion Loss as measured on a channel and the Insertion Loss as determined by adding the component losses is called the ILD.
ILD is taken into account in the proposed Category 6 channel model. The value of ILD is about 0.3 dB at 100 MHz and 1.2 dB at 250 MHz.
What is the significance of ILD? ILD is caused by another signal that is superimposed or is riding on top of the receive signal. This superimposed signal appears as a noise source at the receiver. It is the level of noise in dB that when added (as a power sum) to the primary signal produces the observed value of ILD.
The relative magnitude of this noise source is quite significant when compared to other forms of noise such as NEXT and FEXT and can be a significant contributor to errors in digital transmission systems.
In addition to the Category 6 review, the ballot comments for next revision of the TIA-568-B.1 and B.2 were also reviewed in the respective subcommittees. A limited number of technical changes were made to the TIA-568-B.1 document and it was approved for a default ballot. This is usually the final stage before the standard is adopted.
Extensive changes were made to the TIA-568-B.2 document (copper cabling component specifications) and several task groups were formed to address technical comments dealing with measurement procedures. These task groups will complete their work by the end of June, and the TIA-568-B.2 document will be re-balloted in July.
TIA 606A ADMINISTRATION STANDARD
A working draft of the TIA 606-A administration standard was reviewed in the TR 42.6 subcommittee. This document is almost ready to go out for a first ballot, after an editorial review.
The proposed administration standard has been reorganized to make it easier to implement a labeling system for small office buildings, intermediate office buildings and large campuses. A good deal of positive feedback has been received on the proposed simplification of this standard.CS
Paul Kish is a senior Product Manager, IBDN Systems & Standards at NORDX/CDT, Pointe Claire, PQ. He is also Chair of the TR-42 engineering committee.
Disclaimer: The information presented is the author’s view and is not official TIA correspondence.