To get the most out of your network infrastructure, pay attention to details — such as cabling. There are always areas in which a good engineer can economize, but buying cheap, non-rated network cabling or failing to install it properly can increase maintenance costs down the road.
Cable categories
Let's assume that your network is Ethernet. These networks invariably use unshielded, twisted-pair (UTP) cable. UTP cable manufacturers classify their products according to several categories and frequently label them CAT-3, CAT-5, CAT-6E and so on. As a facility engineer, you don't have to understand the subtleties of CAT ratings, but there are several things you should know. First, there is a difference between rated and nonrated cable. Rated cable has guaranteed performance characteristics that meet or exceed IEEE Ethernet specifications. This is important because Ethernet network interface cards (NICs) are designed to work within these specifications. If the cabling in your facility does not meet the required specifications, it may cause errors on the network. The wire's performance may be so bad that the link between two Ethernet devices fails entirely, even though an ohmmeter indicates the connection is good. Second, the CAT ratings generally describe the type of service for which you can use the cable, and the maximum frequency or data rate the cable can handle. For example, CAT-3 wire is typically used for telephone service. CAT-5 is appropriate for Ethernet and works in 10Base-T networks. CAT5-E (the E stands for extended) has been certified for use at data rates up to 1000Mb/s.
CAT-5 cable
A CAT-5 cable consists of four pairs of wires (eight wires total) twisted together in a specific way. The IEEE Ethernet specifications specify the twist direction (left-hand or right-hand), twist per inch and so on, so that the cable meets certain requirements for crosstalk, return loss, etc. If you think that crosstalk and return loss sound like parameters that apply to a transmission line, you are correct. Design engineers use transmission line models when designing their cables. As for the color of the wires, Standard 568 of the Electronic Industry Association/Telecommunications Industry Association (EIA/TIA) standardizes the colors of the wires in Ethernet cables. Each pair is color-coded; one wire in a pair is a solid color and the other is striped. For example, in one pair, one wire is solid blue while the other wire is blue/white. This helps greatly when assembling connectors or troubleshooting a wiring problem. Figure 1 shows how the EIA/TIA Standard 568 assigns specific pair colors.
Cable connectors
CAT-X cables are invariably terminated in an RJ-45 connector (RJ stands for registered jack). The telecommunications industry has standardized several registered jacks. The two most common are the RJ-11 connector, which is used with telephones, and the RJ-45, which is used with Ethernet UTP cable.
Roll your own
You can easily make up your own RJ-45 cables. All you need are some connectors, a crimp tool, some patience and this article. As Figures 2 and 3 on page 44 illustrate, the EIA/TIA 568B RJ-45 wiring scheme standard specifies that pair two connects to pins one and two, pair three connects to pins three and six, pair one connects to pins four and five, and pair four connects to pins seven and eight. For the cable to work correctly, you must follow the specific colors in the illustrations. Be sure that the wires are all fully seated in the connector before crimping. Also, many connectors include a strain relief that crimps the jacket of the cable near the back of the connector. Be sure that you trim the wires such that the strain relief engages the jacket.
Plenum, please
One option you have when considering different network cables is whether to buy plenum-rated cable. Plenum-rated cable is designed for use inside an air plenum (duct). It's unlikely that you'll run your computer cables though an air duct. But consider this. Most equipment racks in a post-production or television facility are cooled by forcing cool air into the bottom of the rack and exhausting it out the top. (You can do it the other way around, but you would be fighting the tendency of hot air to rise.) In the author's locality, the fire marshals consider the racks part of an air plenum system. Therefore, we must use plenum-rated cable. The author is not an expert on cable jackets but believes that plenum-rated cable is designed to be less toxic if the wires inside the jacket overheat and cause a fire. The most important thing project engineers need to know about plenum-rated cable is that it is expensive. In fact, plenum-rated cable can be more than double the price of non-plenum-rated cable.
CAT-rated jumpers
Frequently, installers run Ethernet cables from patch panels to computer equipment. They use patch cords to jumper between the patch panel and an Ethernet switch. This type of installation is flexible, and can give years of trouble-free service. But beware of non-CAT-rated jumpers. If you use the wrong cable for jumpers, you can run into all sorts of difficulties. For example, the author was involved with a facility where the installation team had used flat (nontwisted) ribbon cable for jumpers. Everything worked fine initially. But, after the installation was complete, some computers began to experience symptoms of network congestion. We spent a great deal of time trying to locate the source of congestion but, of course, we were unable to find a computer that was generating an inappropriate amount of network traffic. Ultimately, we discovered that several of the flat jumper cables were degrading the Ethernet signal to the point where the NIC cards were unable to function properly. We replaced all jumpers with CAT-5-certified cables and the problem went away.
Cable test sets
Finally, you should know that compact, full-featured LAN-cable test sets are available. If you are going to be doing LAN cabling on a regular basis, you should definitely purchase one of these. It can save you hours of troubleshooting time.
Brad Gilmer is president of Gilmer & Associates, executive director of the Advanced Authoring Format Association, executive director of the Video Services Forum, and editor in chief of the “File Interchange Handbook.”
Send questions and comments to:brad_gilmer@primediabusiness.com
