Bad Cabling Can Terminate a System
One of the most important components of any IP network is also the one that is most overlooked – the cabling. Without proper cable, and cable installation, the network just won’t function right.
Obviously if a cable is cut or connected in the wrong place, it won’t work at all. Unfortunately most cabling issues aren’t that obvious. Most of the time problems manifest themselves as transient, annoying issues that defy quick troubleshooting. Examples include intermittent packet loss and slow network speeds, or the mysterious dropping and reconnecting of the user’s PC to the network.
You’ve probably heard the old saying, “Prior planning prevents poor performance.” Nowhere is this more applicable than in designing and implementing an IP video network cable infrastructure.
Cable Basics: UTP and Categories
The cabling used today for networking, and rapidly being used for just about everything else, is UTP or unshielded twisted pair. UTP consists of four pairs of copper cable twisted together for the length of the cable, sheathed inside a jacket. The twists in each pair of cables provide resistance against outside noise interference, and keep the electrical signals from hopping between pairs and causing even more data corrupting problems.
STP, or shielded twisted pair, cable is also available and is primarily used for areas where there is a great deal of EMI (electromagnetic interference) and RFI (radio frequency interference) close to the cable. A good example would be when the cable has to run very close to electrical motors or generators, very high power lighting, or parallel to high voltage cable. It is best to avoid these situations completely, but for those cases where avoidance isn’t an option, STP might get the job done.
UTP cable is further divided up into categories (see sidebar on page 2). These categories were created by the EIA/TIA (Electronic Industries Alliance/Telecommunications Industry Association) to separate the types of cable by the bandwidth and data carrying capacity of each type. How they achieve these speeds is really what the standards determine. Each new category specifies a lower and lower amount of crosstalk and interference that the cable can be susceptible to. The lower the interference and crosstalk, the more and faster data can be carried.
Currently, most high speed networks have been upgraded to Category-5e, which can do speeds up to 1,000Mbps, or Cat-6, which is more stable and resilient at that same 1,000Mbps, or gigabit, speed. Most new installations are going in with Cat-6 cable. A newer standard, Cat-6a (augmented) achieves almost a 100-percent reduction in crosstalk noise compared to Cat-6 and therefore can be used to achieve 10-gigabit speeds.
Another important thing to remember is that not all cable of the same category is created equal. Cheaper offshore brands may technically, although barely, meet the standard’s requirements, but cheap materials and low quality control will make for headaches later on.
In fact, a relatively new organization called the CCCA (Communications Cable and Connectivity Association) has raised the alarm about cheap offshore cable not meeting the flame resistance requirements of plenum cable that is approved for use in air handling spaces about ceilings.
Doing It Right the First Time
Improper or shoddy installation practices will nullify any benefits you achieve from high performance cable and can also cause intermittent, annoying issues.
While copper cable isn’t quite as fragile as the glass in fiber-optic cable, it must still be treated with respect. Copper that is bent too far over or flexed repeatedly can break, even inside the jacket. There may be no visible sign that there is anything wrong with the cable.
Longer runs may require the services of a time domain reflectometer, or TDR. This device sends a signal down a cable and measures the time it takes for that signal to be reflected back from the first break. Short of pulling out the whole cable run, this tool may be the only way to troubleshoot a small, nonvisible break. An OTDR, or optical TDR, performs the same test on fiber.
Along these lines, an important factor to observe when installing any cable is the bend radius, or the limit on how far you can bend a certain type of cable before damage to the copper center conductor occurs.
Minimum bend radius is usually part of a cable’s specification sheet. For instance, a West Penn Wire cable No. 4245, which is a Cat-5e UTP cable, has a minimum bend radius of four times the cable’s outside diameter, which is .202 inches. In contrast, No. 815, which is an RG59U coaxial cable, has a minimum bend radius of 2.5 inches. So the Cat-5e can safely bend to about an inch, but the coax can’t go nearly that tight.
Another important specification for any cable is the amount of tension that can be applied when pulling it, sometimes called pull weight. Using the same cables as above, the 815 coax has a maximum pull tension of 41 pounds, while the Cat-5e can only be pulled with 25 pounds of tension, maximum. So while the Cat-5e is more flexible and can be bent tighter for short periods of time, it is more sensitive to pulling damage.
While pulling a cable too hard might not actually break the copper conductor, it does have the ability to change electrical characteristics of the cable, such as capacitance, which can alter the performance.
One more important thing to remember about cable, and network cable especially, is the maximum distance. Most network folks can recite the maximum distance for Cat-5 or -6 UTP: 100 meters (100m) or about 328 feet.
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