Networking

Know the facts about network cabling

Cabling is one of the most important aspects of network design, yet it's frequently an afterthought for many network administrators. In this Daily Drill Down, Scott Lowe gives you the facts about cable standards and installation.


When it comes to network design, one of the most important decisions you’ll make is choosing and installing the right cable for your network. Even if you hire professional cabling contractors, it is important for you to know what they’re doing and what to look for so that you can properly evaluate their work. In this Daily Drill Down, I’ll show how to choose the right cable and how to install it the right way the first time.

Fewer but better choices
If you’re running the cable for a network today, your choices are much clearer than they were just a few years ago. Not so long ago, coaxial cable was still in widespread use. Unshielded twisted-pair cabling was primarily based on Category 3 standards, running at a maximum of 10 Mbps. Category 5 cable was still out of reach for most network administrators’ budgets. Those with money to burn also had fiber to consider.

Standards were, at best, fuzzy. Several different standards were fighting it out for the supremacy of running 100 Mbps over unshielded twisted-pair. There was a lot of uncertainty about whether it was even possible to run data over copper at such speeds. Some critics claimed that 100 Mbps was only suitable for fiber.

Today, deciding on a particular type of cable isn’t as much a matter of which cable will eventually become the standard as it is a matter of what you need to do right now and for the next few years. Most of today’s network installations generally use some type of unshielded twisted-pair cabling, although some organizations are running fiber directly to their desktop machines.

Author’s note: Twisted-pair cabling
I’ll be focusing on twisted-pair cabling since that’s the prevalent technology today. You still have the same choices of fiber, shielded twisted-pair, and coax cable that you had in the past. However, because unshielded twisted-pair is so inexpensive and has such a large market share, it’s the first choice for most network administrators.

Many different kinds of unshielded twisted-pair (UTP) cabling are available, and you need to pick the one that will best serve your needs without breaking your budget. UTP cabling is generally rated by incremental “categories.” For example, when someone is talking about Category 3 cabling, they mean a type of cabling commonly found in telephone and other voice applications or low-speed data transmission, and that has a transmission frequency of 16 MHz.

Megahertz, not megabits
Please note that I’m using megahertz (MHz) and not megabits per second (Mbps) to describe the transmission frequency. The MHz rating directly affects the Mbps rating, but they’re not the same value.

Different kinds of UTP cabling are available. Each type runs at a different speed and has different uses. The key types of UTP cable you’ll encounter are:
  • Category 3: Cat 3 was the earliest successful implementation of UTP. It’s primarily used for voice and lower-speed data applications. It’s rated for a maximum of 10 Mbps.
  • Category 4: Cat 4 never achieved the popularity of Cat 3 or Cat 5. It’s primarily used for voice and lower-speed data at a maximum of 16 Mbps.
  • Category 5: As Fast Ethernet became a standard, Cat 5 became the basis for most high-speed data implementations. Cat 5 runs at a maximum of 100 Mbps.
  • Category 5e: With the need for higher speeds, Gigabit Ethernet has become the new replacement for Fast Ethernet. To make it work, Cat 5e extends the life of Cat 5 cable. It can run at a maximum of 1,000 Mbps.
  • Category 6: Cat 5e can run at gigabit speeds, but with 10-Gigabit Ethernet on the horizon, Cat 5e has stretched the Cat 5 standard to its limits. Cat 6 can currently run at 1,000 Mbps (1 Gbps). The Category 6 specification was released for publication very recently, however as designed, Category 6 cabling will be able to support speeds up to at least 10 Gbps.

For new installations, I highly recommend running a minimum of Category 5e cabling for both voice and data. In today’s environment, there’s no reason to use anything less. Cat 3 may be a little cheaper, but you’ll lose this savings in the costs of replacement when you find out it can’t go fast enough for you. Although you may get away with running regular Cat 5 cable, Cat 5e is only slightly more expensive. The incremental cost is well worth the speed advantage and future expandability you gain with Cat 5e.

Pay attention to the jacketing
UTP cabling is almost always sheathed in some type of plastic-like insulating material. Less expensive cabling uses PVC (polyvinyl chloride) as the jacket material. Not all environments can use this type of cabling, for both safety and legal reasons. When PVC burns, it gives off noxious dioxin fumes that are dangerous to those who breathe them. While a single burning PVC cable may not be deadly, firefighters who must enter burning buildings that have thousands of cables can be at risk.

Check your local laws—in many places, it’s illegal to use PVC cables in any air-handling spaces, such as in the ceiling or below a raised floor. In these situations, you need to use a cable with a more expensive but much safer jacket called plenum. If your cables are just run into the wall and you’re positive that they don’t run in air handling spaces, you should be fine with standard Cat 5 cable. If you don’t need it, don’t incur the expense of plenum cable, which can be two to three times the cost of PVC.

Follow the rules
Second only to choosing the appropriate cable is making sure it’s run throughout your organization in a manner consistent with standards that ensure the best possible performance. In fact, if you don’t follow a few basic tenets, you could end up with an installation that’s expensive but not functional. Here are a few basic rules you should follow when installing cable.

Watch the length of your cable runs
No cable run should be more than 100 meters (~327 feet) in length, including patch cables. The in wall/ceiling distance—also called the horizontal run distance—should be no longer than 90 meters. This allows for up to 10 meters for patch cables on either end of the connection.

Watch for interference
No cables should be run near devices that generate electromagnetic interference. This is one of the rules most often broken by amateur cabling installers. Devices that generate electromagnetic fields include heating/cooling units, printers, copiers, electrical wiring, video equipment, and much more. You should be very careful to keep UTP cabling at least 3 feet away from anything that can create an EM field. In addition, it’s critical to keep UTP cabling as far away from fluorescent lighting as possible since cables are very susceptible to interference from fluorescent lights.

Handle with care
Take care not to damage cable when installing it. Don’t exceed the bend radius of UTP cabling or it may not work as expected. It’s generally accepted that cable that is bent within a radius of four times the width of the cable is not run properly. Be very careful not to flatten cable with a hammer or a staple. Hammer and staple indentations on UTP cabling can create problems like changing the signal’s properties, resulting in a less efficient (or nonfunctional) network. If you’re tying a bundle of cables together, use a zip tie that is secure but leaves a little wiggle room. If you tie the cables too tightly together, you run the same risks as when you flatten it. Finally, when pulling cable through the ceiling or conduit, be careful about how hard you pull at the cable. Don’t exceed 25 pounds of pulling force in order to avoid stretching the cable, which can damage its electrical characteristics and render it out of compliance for high-speed data networks.

Use the right equipment
Wherever possible, make use of a ladder rack or a cabling tray with a solidly installed bottom. It will make your interconnections go much more smoothly.

Termination and testing of UTP: Category 5e
The next areas you need to focus on are properly terminating your cable and testing it to make sure that it’s within specifications. I’m going to concentrate on the proper termination of Category 5e cabling because it is the latest officially standardized UTP cabling currently available and it supports gigabit Ethernet installations.

Like every other phase of cable installation, a set of standards governs the termination phase in order to ensure that the plant will support high-speed data.

Terminate your network
For more information about termination considerations for patch panels and network jacks, see the Daily Drill Down “Use the right networking components for your next cabling job.”

One of the primary standards specifies exactly what should take place when the cable jacket is stripped back and the individual pairs untwisted to prepare them for termination. The twisting of Category 5e cabling is one of the characteristics that define the communication properties for the cabling and enable it to do its job. Removing too much twist from the cable results in an imperfect installation, and it can put the cable out of specification and possibly make it unable to support high-speed data transmission.

The current Category 5e specifications indicate that up to 13 mm of twist may be removed in order to support cable installation, and up to 40-60 mm of the jacket may be stripped away. This is the same at both ends of the horizontal run—i.e., at both the network closet patch panel and the wall jack—as well as for patch cables used with the system.

Testing equipment is important
If you’re a professional installing network cabling for a living, it’s critical to be able to certify that the cabling plants you install are up to the job and to prove that it is indeed Category 5e and gigabit ready. The Fluke OMNIScanner 2 is capable of doing this as well as producing reports that you can pass onto your clients. But be prepared to spend some money, as these devices start in the low $5,000 range, with options available. If you manage a very large cabling plant or are a cabling contractor, these are must-have devices.

Get to work
Installing cabling properly and for the long run is no walk in the park. Besides some very strict installation guidelines, you need to test each and every cable for specific parameters in order to make sure that your cable plant can support your requirements. Cabling isn’t as complicated as it sounds, but if you’re not careful, you can wind up wasting a lot of time and money. Follow these guidelines to do it right the first time.

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