Untangling network wiring
We explain the different levels -- er, categories -- of cabling and highlight the pros and cons of going with either copper or fiber optic
Over the past year I've written about emerging higher-speed networks, notably Gigabit Ethernet technology, that are pushing the limits of existing cable solutions. What we haven't explored is cabling infrastructure, and what you need to do to take advantage of both gigabit and asynchronous transfer mode (ATM) technologies. The natural assumption may be that fiber optic cable will dominate the high-speed realm, but there are still plenty of good reasons to use copper cable. This article reviews industry efforts to ensure that the wiring solutions you purchase are up to the task -- whether you choose copper or fiber optic. (2,400 words)
Table 1 outlines the basic definitions of performance for each level. Typical wiring in place today is of the Category 3 type, though very few network managers are currently installing anything below Category 5 -- which, up until recently, was thought to be the highest we'd have to go. Category 5 was rated to 100 megabits per second (Mbps) and up, which was more than sufficient before ATM, Gigabit Ethernet, and full-duplex technologies entered the networking scene. Today's networking technologies have exposed defects in the wiring, connectors, and occasionally the installation of these components. This has led to the definition of even higher levels, designed to ensure that today's networks will perform as expected.
|1||POTS (plain old telephone) service|
|2||Up to 1.2 Mbps terminal and Arcnet services|
|3||10 Mbps Ethernet and 4- to 16 Mbps Token Ring|
|4||16 Mbps Token Ring up to 20 MHz|
|5||Copper version of the fiber distributed data interface (FDDI) up to 100 MHz|
|6||Performance up to 155 MHz with four-pair performance up to 100 MHz|
|7||Performance up to 200 MHz, higher four-pair performance and Gigabit Ethernet out to 100 meters|
Problems in the plant
In moving up to Category 5, new standards had to be developed for installation of the cable and all supporting components. The makeup of a cabling plant includes the cable itself, the connectors at each end of the cable, the wall plates or patch panels, and the patch cords. Each component in the plant must be certified to Category 5 performance, but that doesn't guarantee that the network will perform as expected. For example, if the cable sheath is stripped more than 1.5 inches, or the twists of the cable are undone more than 1 inch, or the individual wire sheath is stripped more than .5 inches, the network may not perform at Category 5 levels and cannot be certified as such. In addition, environmental elements like temperature, humidity, or an electromagnetic source, can affect the performance characteristics of a cable system.
Such tight tolerances require trained installation personnel and sophisticated test equipment to install properly. At each stage of installation it's necessary to test the assembled components, ensuring that they maintain Category 5 standards. The testing concludes with a complete end-to-end test, which certifies the entire path of the network, from the workstation to the hub. In the beginning, additional labor significantly increased the cost of network installation, causing many users to forego Category 5 installations, but market demand has since pushed prices down to an acceptable rate.
Market demand has created no less than 105 Category 5 plenum cable designs and 33 Category 5 nonplenum designs. All of these cables meet the minimum standards set out for Category 5 certification, but with varying levels of performance. This performance ranges from cable that just meets the standards, provided absolutely everything in the cable plant is correctly installed and functioning, to cable that will maintain Category 5 performance even if one part of the plant drops in performance. It's important to remember though, that any component that drops below the minimum Category 5 standard will take that entire segment of the network with it, so a hub-to-hub cable or a patch panel cable can reduce network performance to no better than 10 Mbps.
Levels 6 and 7
ATM has raised the bar of performance to 155 Mbps. A number of the high-end Category 5 cables and supporting components certify at that level, so it's possible to find existing installations that support this level of networking. But what about 622 Mbps ATM or 1-gigabit-per-second (Gbps) Ethernet? These new network technologies demand even higher levels of performance, for which no standards have been set. This is where Anixter has once again stepped in to lead the industry to consistency, with industry study groups like the TIA forming to develop new standards using the Anixter levels as a foundation.
Level 6 cable is intended to address full-duplex and ATM technologies at the 100- to 155 Mbps range. At Level 6, cable with subsequent components and installation should handle many of the high-speed networks planned today. But 622 Mbps ATM and Gigabit Ethernet have pushed the demand for Level 7 performance standards.
Networking applications have increased the need for new standards. Initially only signal-to-noise ratios and maximum noise thresholds had to be met. Now ATM requires NEXT measurements (to measure the level of cross-talk between cable pairs), and 100BaseT and 100VG (100 Mbps) networks require pair-skew and total-cable-propagation-delay performance standards. These new measures are necessary because signals are moving across more or all of the pairs in the cable, and the signals need to remain intact and synchronous with each other -- even though the physics of the structure work against this.
Why use copper instead of fiber?
Good question, with a reasonable answer. It's clear from the limitations placed on copper wiring solutions that fiber optic cabling systems are needed in a number of applications. But it's still relative cost or distance that determines the choice of physical plant.
Some basic factors that can be used to differentiate the choice are:
A quick fiber primer
Fiber optic cables come in two basic designs, a multimode fiber and a single-mode fiber. The overall operating characteristics of these two types are significantly different (noted in Table 2). The single-mode fiber is the better performer of the two. The frequency of light used to propagate the network signal will travel differently based on the cable type, and multimode cables don't have the same distance capabilities that single-mode fibers do. So the use of each type is determined by the distance and network equipment interface requirements.
|Backbone cable length||3000 m|
|Horizontal cable length||Not Recommended|
FDDI, ATM (1.2 Gbps)
|Backbone cable length||2000 m|
|Horizontal cable length||100 m|
E-net, TR, FDDI
155 Mbps ATM
What's air-blown fiber?
A recent advance in installation technologies, the air-blown fiber concept, has reduced overall installation and growth costs associated with fiber systems. In the past it was necessary to lay down dark fiber at initial installation time. This was because the work involved in properly installing fiber included placing pull strings into the cable path then pulling the fiber through the path. This method frequently broke the fiber, requiring new cable to be pulled or splices to be made on the broken cable. Additional fibers, called dark fiber, were often put in place for future expansion. When expansion became necessary, either the dark fiber was used or new fiber had to be pulled. However, this wasn't an option when the conduit or plenum spaces were full.
With the air-blown fiber concept, a large conduit is installed to house the fiber. Then air is blown through this conduit, allowing the fiber to be drawn into it and gently carried to lengths of up to 2 kilometers. Additional fibers can be installed in the same conduit space, and fibers that become damaged can be removed with the same technology. Systems like these reduce the installation and maintenance costs, bringing fiber almost on par with copper network systems.
Fiber systems may also be a better bet to install due to the lack of electrical components required to propagate the signal. Instead of expensive hubs in various wire closets, fibers can be run from the desktop to a passive wire closet, where the cable is patched into a riser cable that carries the network connection directly back to the electronics in the main network center. This design approach reduces the environmental costs associated with copper networks today.
Many organizations can't afford to completely drop copper for fiber today, nor can they afford to replace copper everywhere to support the increased standards for higher speed networks. As such, it is necessary for network planners to approach networks in terms of areas or zones. By looking at the bandwidth required in each area, planners can tailor a network to match the existing infrastructure and minimize the amount of new technology required.
As an example, imagine you have four departments spread out over three floors (a basement and two stories) of one building and a second building 300 meters away. You need to install a Gigabit Ethernet network between systems located in each department. Working within the budget and building constraints, the basement location can't be reached with a fiber-capable conduit. Mixing technologies, however, allows a fiber conduit to be installed between buildings using an air-blown fiber design (thus paving the way for future growth) while a Level 7 cable is installed in the basement department. The other two departments will rely on fiber connections to the central network center. Each desktop can then be upgraded, as needed, to either fiber or copper based on limitations in the cable path. Add to the design some switched Ethernet capability to augment lower speed connections, and you've substantially increased overall network performance with minimum investment.
About the author
Robert E. Lee is a technology consultant, speaker, columnist, and author who has been in the computer industry for 20 years. He specializes in networking, Internet strategies, systems analysis, and design activities, and has participated in the Windows NT and Internet Information Server betas since their respective beginnings. In addition to several other recent feature stories, Rob wrote the June 1997 SunWorld news story, "Cisco throws its support behind Microsoft's directory service vaporware."
You can learn more about Robert E. Lee's The ISDN Consultant: A Stress-Free Guide to High-Speed Communications and Serving the Net: Using the Power of Microsoft Internet Information Server at Amazon.com Books. Reach Robert at email@example.com.
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