Someone once told me that if you stand still long enough, you'll witness just about everything. I don't remember who it was, but I'm pretty sure the person was not a motivational speaker hired by my employer to get my colleagues and me to work harder. I appreciated the comment for its ironic humor as the anti-motivationalspeech: There's no need to go out and conquer the world; just sit on the couch, grab a bag of chips and a cold beverage, and pretty much everything in the world will come to you ... eventually.
I like to think, however, that I did not take the statement seriously, nor follow it as advice. Furthermore, some of my most recent education about goings-on within the networking and cabling industries has led me to believe that if you keep moving forward, you're likely to see many things twice.
For example, in the article beginning on page 15, Valerie Maguire provides results and analysis from recent testing performed at her company's labs. Specifically, the tests concerned heat dissipation in cables carrying DC power via Power over Ethernet (PoE). Maguire describes the two sets of PoE specifications from the IEEE: 802.3af, the original Power over Ethernet, and 802.3at, the still-in-development Power over Ethernet Plus.
Something Maguire mentionsalmost in passing is that 802.3af PoE is being dubbed "Type 1" PoE and 802.3at "Type 2." The most seasoned of premises-cabling professionalsalready find that term, Type 1, in their personal dictionaries.
I can't tell you how many conversations on the evolution of structured cabling have included references to the old IBM Type 1 cabling systems. The shielded, proprietary system installed to support Big Blue's Token Ring protocol was the first near-ubiquitous system installed in customer-premises environments.
Go ahead and Google the term—IBM Type 1 cabling—and check out the nearly half-million results that come up. Well, don't check out all 448,000 results unless you have a lot of time on your hands. The point, I guess, is that Type 1 cabling is not our industry's version of the Dead Sea Scrolls; it is, in fact, a technology that is still alive for some users. I'm no longer surprised when I speak to cabling contractors and hear about how many of their customers still have some Type 1 circuits in their buildings. So, now the term Type 1 has two definitions, both relevant today to varying degrees.
The other term that will have two definitions relevant to structured cabling is "delay skew." The term initiallyappeared on the industry's radar about 12 years ago, when UTP cables were manufactured in such a way that not all of the copper pairs were insulated with the same material. In some cases, three of the four pairs were insulatedwith fluorinated ethylene propylene(FEP), while the fourth was insulatedwith polypropylene in a construction dubbed 3 x 1.
Similarly, some cables were constructed as 2 x 2—two pairs insulated with FEPand two insulated with polypropylene. Some of those cables exhibited performance anomalies because the different conductor-jacketing materials caused signals to propagate down the pairs at different rates. In some cases, those propagation differences were significant enough to distort the signal at the receiver. The situation was pervasive enough that the Telecommunications Industry Association (TIA) cabling-standards development group issued TSB-95, part of which addressed maximum allowable delay skew in twisted-pair cabling systems.
Much more recently, as other standards groups put together protocol specifications for Ethernet transmission at 40-and 100-Gbits/sec, the term "delay skew" has reared its head again—this time for optical rather than copper cabling.
While you will not see detailed technical information on delay skew in this month's issue, you will see it on our pagesshortly. Specifically, we are planning a contributed technical article for inclusion in our September issue that will detail the delay skew issue. That article will reference the use of the term more than a decade ago, as well as its similarities to/differences from differential mode delay—a multimode-fiber performance characteristic that came up during the development of the Gigabit Ethernet specifications, also about a decade ago.
In the meantime, the day after I write this, I am scheduled to interview an expert on this subject on-camera; it will be available for viewing in the Cabling Installation & Maintenance Video Library (www.cablinginstall.com).
So, get ready to wipe the dust off some familiar terms and begin using them again in conversation about the latestcabling-related technologies. I guess that's what happens when you stick with an industry that keeps moving forward.
By contrast, if you sat on the couch for a really long time, say 22 years, what would you have seen happen twice? Oh yeah, the Boston Celtics winning the NBA championship.
PATRICK McLAUGHLIN
Chief Editor
[email protected]