If it was plenum then, is it plenum now?

Oct. 1, 2005
The following represent some of the best Q&A fired at columnist Donna Ballast over the past year or so.

The following represent some of the best Q&A fired at columnist Donna Ballast over the past year or so. Remember, if you have a cabling-related question for Donna, you can e-mail her at: [email protected]

Q: We are trying to find out if the Cat 5 plenum-rated cable that was installed back in the late ‘80s would still be plenum-rated today. This isn’t abandoned cable; in fact, it’s still in use today. We currently have some original cable that we installed, and the questions came up: “Would the materials used in the plenum cable break down? If the materials that make the plenum cable ‘plenum rated’ would break down, what would happen during a fire? Also, what should happen to the cable as far as running our 10/100 Ethernet network as the cable ages? Should we expect to start seeing network problems down the road?”

A: Is the cable still CMP? You bet. It says so right there on the jacket. There is no requirement in NEC, or anywhere else that I’m aware of, to pull out one of those cables every now and then and send it to a testing lab to be incinerated in a Steiner Tunnel Test to see if things have changed since the cable was installed.

Is the cable exactly the same today as when it was installed? Probably not. In its pure form, polyvinyl chloride is rigid and brittle. Cable manufacturers add plasticizers to make the PVC flexible. Phthalates (pronounced THALaytes), which look like vegetable oil, are most often used in flexible PVC. Because the phthalates are not chemically bonded to the PVC, they tend to outgas over time, so to extend the life of the flexible PVC, stabilizers are also added. But PVC will produce a dense black smoke when burned; hence, PVDF was used as a cable jacket material in the late ‘80s. Some early CMP cable jackets were basically PVC with clay added-yes, they actually added very refined dirt. The ratio of PVC to dirt was such that the material would pass a Steiner Tunnel Test-enough dirt to pass the burn test with enough PVC to hold the dirt together.

If you are trying to sell your previous clients on recabling today, you are going to have to focus on the cable’s transmission performance, not the CMP rating. Network problems down the road depend on the application. If the 10Base-T and 100Base-T are working fine today, they will very likely continue to do so. But if your client plans to move to 1000Base-T, you will have to retest the installed base to see if it can support it.

1000Base-T uses all four pairs of bidirectional transmission, which requires measuring additional parameters-return loss and equal-level far-end crosstalk (ELFEXT). These tests would be performed with a Level III cable tester.

Should a link fail, there are five corrective actions that may solve the problem without recabling:

• Change patch cord with a Cat 5e cord;

• Change cross-connect to an interconnect;

• Change consolidation point with a Cat 5e consolidation point;

• Change work area connector to a Cat 5e connector;

• Change interconnect to a Cat 5e interconnect.

For beyond 1000Base-T (like 10-Gig), you will definitely need to install new cabling. But the jury is still out on exactly what that replacement cable should be.

Q: Like most data networks, mine is not “bulletproof.” This is most frequently evident to users when they can’t gain access some morning, or when their e-mail goes down during the workday. Executives in my company fear that with a VoIP telephone system, our phone service will be down as often as our data network, which they perceive as often. Are their fears justified, or is there some way I can calm them?

A: Public Switched Telephone Networks provide high-quality voice transmission between two or more parties. VoIP systems digitize and transmit analog voice signals as a stream of packets over a digital data network using standard analog, digital, and IP phones.

But VoIP is a delay-sensitive application. If you are going to run a delay-sensitive application on your existing IP network, it is imperative that you begin with a well-designed, end-to-end network, and only then fine-tune to adequately support VoIP. This fine-tuning will involve a series of steps (consult your network equipment manufacturer for specifics to improve quality of service-not the router running VoIP, but on all backbone routers and edge routers throughout your network). And since edge routers and backbone routers do not typically perform the same functions and steps necessary to improve quality of service for real-time, voice traffic will also differ.

Your executives are right to worry. The quality of VoIP telephony depends on the network that carries it. If your data network is not reliable, do not add the burden of VoIP traffic. You will only compound the problem. Instead, focus your efforts on resolving your existing network issues. To ignore something this basic is like trying to ride a donkey in a thoroughbred horse race. The pack will go whizzing by and leave you sitting on your…donkey. Hee-haw.

Q: I have a customer with an embedded base of Category 5e cabling verified to 400 MHz. If I installed matched Category 6 components all the way through the channel, with the exception of the cable, could I achieve Category 6 data transmission rates?

A: First, a word of caution. If your client requires a TIA-compliant Category 6 cabling system, then all of the components of the cabling system must be Category 6. Now that we have that disclaimer out of the way, the electrons traveling down the copper conductors cannot see the label on the outside of the cable jacket. So, if your client is only seeking Category 6 performance, then as long as the cable meets all the transmission and physical requirements of Category 6, electrically, it is Category 6 cable.

If you have a Category 5e cabling system in place using “400-MHz Category 5e” cable and you change to Category 6 connectors, and test with a Level III tester, you will, in all likelihood, pass Cat 6. But you may or may not have the same permanent link and channel performance that the TIA intended.

You see, the difference in your “super 5e” cable and Category 6 could be anywhere from significant to non-existent, depending on whether the “400-MHz Category 5e” cable will also meet the electrical balance requirements for Category 6. Electrical balance in the Category 6 standard is called longitudinal conversion loss (LCL), which is the difference between the differential and common mode voltage.

The differential voltage across the circuit pair is the desired signal, whereas the common voltage is an unwanted signal that may have been coupled into the transmission line. If the line is perfectly balanced, the common mode voltage cancels out.

Remember why there was a Category 5e in the first place? There were a few parameters that IEEE needed for 1000Base-T that Category 5 had not required to meet. Manufacturers did not want to guarantee that the cabling they already sold you would meet new requirements, and Category 5e was created.

Some Category 5 passed the new Category 5e requirements and some did not. To help users and installers determine what to do, TIA wrote a mitigation procedure-sort of, “change this and test, and if that does not pass, change that and test,” with the last resort being to change the cable.

But this will not be the case between Category 5e and Category 6. TR-42.7 polled members asking if their companies would support further characterization of Category 5e, and as expected, the overwhelming answer was “no.” Why? Because they now have a new Category 6 cabling system to sell you.

Bottom line-if you need 100% assurance of Category 6 performance, you will have to install Category 6 components, end to end.

DONNA BALLAST is BICSI’s standards representative, and a BICSI registered communications distribution designer (RCDD). Send your questions to Donna via e-mail: [email protected]

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