BY PATRICK McLAUGHLIN
For decades, the use of shielded cabling systems in local area networks (LANs) within North America has been an oddity, reserved either for applications that never upgraded from the old IBM Type 1 system installed in the 1980s or for those facing extreme external noise sources that adversely affected signal transmission. Today, the specification of a shielded system for a LAN is no longer so rare, as several cabling vendors report increased demand for their shielded products among North American users.
Hitachi Cable Manchester (www.hcm.hitachi.com) recently reported that demand for its shielded Category 5e and Category 6 products has “skyrocketed” over the course of several months. And product engineers within Tyco Electronics’ AMP Netconnect group (www.ampnetconnect.com) have stated flatly that foiled twisted-pair cabling-a cable construction that includes a single foil shield over four otherwise unshielded twisted pairs-is a more viable alternative than unshielded twisted pair for running the latest twisted-pair-flavored Ethernet protocol, 10GBase-T.
“We’re finding that more and more companies are choosing our shielded products due to their inherent immunity from radio-frequency interference and electromagnetic interference,” said Hitachi Cable Manchester’s senior vice president Lynne Humenik. “They are also choosing our Category 6 ScTP [screened twisted-pair, an alternate name for foiled twisted-pair] because it supports 10-Gigabit Ethernet to 100 meters. As 10-Gigabit Ethernet electronics become more prevalent in the marketplace, we can expect demand for our Category 6 ScTP to continue to increase.”
AMP Netconnect recently introduced a half-day training course aimed at design and installation contractors who are certified to install the manufacturer’s cabling systems. The course specifically covers the use of shielded twisted-pair cabling systems for 10-Gigabit Ethernet applications.
“The objective is to educate certified contractors on the superior performance capabilities of shielded cabling, promote an understanding of how to specify, bid, properly install and test a shielded system, and verify the statistics that make shielded cabling a cost-competitive alternative to 10GbE UTP systems,” AMP Netconnect said in a release announcing the new course.
Added Wally Harvan, product training manager with the company, “With better Shannon Capacity, alien crosstalk, and EMI/RFI performance, shielded solutions are gaining more recognition. Still, there is a need to educate the industry about the advantages of using shielded cabling, including its smaller diameter that requires less space, its simplified installation due to innovations in connector technology, and elimination of time-consuming alien crosstalk testing.”
10GBase-T and alien crosstalk
The apparent re-emergence of shielded cabling results directly from the efforts that cable and connector manufacturers are facing to develop working systems that successfully transmit 10GBase-T signals. Although we are still some time away from 10GBase-T network switches being available on the market, cabling vendors know the performance parameters their systems will have to meet in order to handle 10GBase-T.
The most critical parameter in 10GBase-T transmission, mentioned by AMP Netconnect in their training announcement, is alien crosstalk-the electrical phenomenon that occurs when a signal from one cable acts as interfering noise to a signal running on another cable. It is a reality in twisted-pair copper LAN cabling because the cables typically are tightly bundled as they traverse the horizontal portion of the network, from the telecommunications room in which connecting hardware is housed to the user’s work area. The tightest bundles present a worst-case scenario of six cables around one, meaning the center cable could be adversely affected by the combined alien crosstalk emanating from the six surrounding cables.
While internal, pair-to-pair crosstalk has been a fact of life within UTP cabling systems as long as they have existed, digital signal processing technology built into transceivers has allowed the cancellation of much of that crosstalk. Alien-crosstalk cancellation through DSP technology is not possible. Furthermore, previous-generation twisted-pair-based Ethernet protocols 100Base-T (Fast Ethernet) and 1000Base-T (Gigabit Ethernet) each operated at a maximum frequency of 100 MHz. 10GBase-T (10-Gigabit Ethernet) operates at a maximum frequency of 500 MHz. 100-MHz signals do not bleed outside of the cable jacket into other cables; 500-MHz signals do.
Despite these challenges, cabling vendors have developed unshielded twisted-pair systems that meet the necessary parameters to transmit 10GBase-T.
Called Category 6A or Augmented Category 6, the cabling systems are regarded by some as “just barely” unshielded because of the cable’s elements. In addition to 23 AWG conductors (previous-generation UTP cables contained 24-AWG wires), Category 6A UTP includes separators and/or heavy outer sheaths, both designed primarily so that when multiple cables are bundled or grouped together, each cable’s conductors will be separated from every other cable’s conductors by as much space as possible.
Air, it turns out, has been the best defense against alien crosstalk within UTP systems. Some manufacturers developed Category 6A patch panels in what is called the “checkerboard” style, in which the panel takes up 2U of rack space, with ports alternating between the panel’s upper and lower halves so that they are always spaced diagonally across from each other.
One of the upshots of this need for increased distance between conductors is the cables are larger than ever-up to 0.375 inches in outer diameter. By comparison, a twisted-pair cable with a single foil around the four pairs typically has an outside diameter of 0.29 or 0.30 inches. And therein lies the opportunity for shielded/foiled cable to become a medium of choice in high-speed LANs.
Cabling-system standards, to which compliance is voluntary but widespread, require that the cables inside a conduit occupy no more than 40 percent of that conduit’s space. For years, shielded cable has been regarded as the bigger/bulkier cable that, in addition to being more expensive than UTP, was also more burdensome for reasons such as the standard-imposed conduit-fill limits. With the development of Category 6A those arguments have not been merely neutralized; they have been reversed. Proponents of shielded cabling say that from a total-cost-of-ownership standpoint, the medium is either extremely competitive with or even provides cost savings over UTP.
Additionally, advancements in connectors for shielded twisted-pair cables are bringing them on par with UTP concerning termination. For example, AMP Netconnect has developed a shielded connector that can be terminated in approximately 90 seconds.
Grounding and bonding
But one of the major drawbacks of shielded twisted-pair cabling systems has not changed, and may still scare many prospective users away: The need to properly ground a shielded twisted-pair cabling system.
It is commonly known that improper grounding of a shielded cabling system negates all the system’s noise-immunity benefits. In fact, improper grounding can cause the cable’s shield to act essentially as an antenna, attracting external signals.
Brian Davis, premises connectivity product manager, and Bob Zahr, systems engineering manager with Tyco Electronics/AMP Netconnect, point out that the grounding issue doesn’t have to be as intimidating as it is to many. “The procedure is straightforward and essentially no different than what is required for UTP systems. The grounding and bonding requirements specified in the current TIA J-STD-607-A Commercial Building Grounding (Earthing) and Bonding Requirements for Telecommunications are applicable to both [shielded] and UTP systems. It doesn’t matter which type of system you’re installing.”
For both shielded and unshielded systems, they point out, “Any metallic component in the telecommunications room that is attached to the cabling infrastructure must be bonded to the telecommunications grounding busbar per the TIA standard. Shielded patch panels, jacks, and cable have the grounding and bonding built in. So, once the cable is properly terminated to the jack and the jack mounted to the patch panel, bonding and grounding is complete.”
