Networking: A Beginner's Guide Fifth Edition- P15

Networking: A Beginner’s Guide Fifth Edition- P15:I have run into many people over the years who have gained good evenimpressive working knowledge of PCs, operating systems, applications,and common problems and solutions. Many of these people are wizards withdesktop computers.
Nội dung trích xuất từ tài liệu:
Networking: A Beginner’s Guide Fifth Edition- P1552 Networking: A Beginner’s Guide Level or Category Rated Performance Level 1 Not performance rated Level 2 1 Mbps Category 3 10 Mbps Category 4 16 Mbps Category 5 100 Mbps to 1 Gbps Category 6 >1 Gbps Table 4-3. Twisted-Pair Performance Designations Coaxial Cable Many older networks (those built prior to circa 1992) still have coaxial cable installed. Most of this coaxial cable is the thin variety, which is RG-58, and is used with Thin Ethernet. A few may also use the thicker RG-8 cable for Thick Ethernet, but this is rare. Thin Ethernet cabling is wired in a bus arrangement, where each network segment starts with a terminator that connects to the end of the cable, runs to each node in turn, and ends with another terminator on the other end. The terminators contain special 50-ohm resistors, and the network cable will not work unless both are installed. All the connectors in a Thin Ethernet system are BNC connectors, a quick-release bayonet-style connector, both reliable and easy to use. BNC connectors come in a variety of different styles to enable you to make just about any network connection you need along the bus. T-connectors have two female BNC connectors on each side of the crossbar of the T and a male BNC connector at the end of the shaft of the T. The two female connectors are used for the RG-58 cable coming into and out of a node, while the male connector attaches to a female BNC connector on the node’s Ethernet card. Barrel connectors have two female connectors that are used to connect two Thin Ethernet wires together. Barrel connectors are also available in different shapes, including an elbow bend and a U-shaped bend, but usually the simple straight barrel connector is used. Figure 4-2, earlier in the chapter, shows the various parts of a Thin Ethernet BNC cable system. Coaxial cable has a central conductor, which can be either a solid, single copper wire or a stranded set of wires. A white plastic material surrounds the central conductor, which is surrounded by a metal foil and then a braided wire shield. The shield is finally wrapped in a plastic cable sheath. CAUTION Cable types must not be mixed in any coaxial network. If the network uses, say, RG-58A/U, then that is what you must always use—not any other coaxial cable. Not mixing RG-58A/U and RG-58/U is also a good idea because they have ever-so-slightly different signaling characteristics. (A/U cable uses a stranded center conductor, while /U—sometimes called C/U— uses a solid center conductor.) Chapter 4: Understanding Network Cabling 53 Plenum Versus Nonplenum Cable In a building, the area between the ceiling of the rooms and the roof of the building is called the plenum space. Most buildings use ducts (big, flexible hoses) to provide conditioned air to the rooms in the building, and they use the open plenum space for air returned from the rooms. Typically, the air returned from the rooms is partially reused by the air conditioning units to save energy because it’s already cooled or heated as appropriate. Occasionally, a building uses ducts for the return air, but the standard for office space is simply to use the plenum space. Why is this discussion of office building air handling important in a chapter about cables? Because to run network cable through the ceiling of a building that uses the plenum for return air, you must either install the cable inside special piping, called conduit piping (which is extremely expensive), or use plenum-grade cable. The difference between nonplenum cable and plenum cable is that the plastics used in plenum cable do not give off toxic fumes in case of a fire. Because most office buildings reuse the air in the plenum space, the last thing you would want to happen is to have the cables redistributing toxic fumes if a fire broke out somewhere in the building’s roof or plenum space. A fire in a very small area could cause the fumes from the burning cable to be distributed to a very large area of the building because of how these ventilation systems work—most definitely a Bad Thing. Make sure to check with your cabling contractor for details about the municipality in which you are installing network cable, but virtually all local codes in the Unit ...