Section VI: Network Hardware

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Cabling

Cable is the medium that ordinarily connects network devices. Cable's ability to transmit encoded signals enables it to carry data from one place to another. These signals may be electrical as in copper cable or light pulses as in fiber- optic cable.

A few networks don't use cable at all. Instead, data is carried through the air as microwave, infrared, radio frequency, or laser-produced visible light signals. These wireless networks are often expensive, and may require licenses from the Federal Communications Commission. When the cost of running cable is prohibitively high or a network must be mobile or temporary, a wireless LAN can make sense.

Three Choices

Network users have three basic cable choices: coaxial, twisted-pair, and fiber-optic. Coaxial and twisted-pair cables both use copper wire to conduct the signals; fiber-optic cable uses a glass or plastic conductor. Before the Ethernet standards for unshielded twisted-pair installations were approved in 1992, the majority of LANs used coaxial cable, but a high proportion of subsequent installations have used the more flexible and less costly unshielded twisted-pair medium. The use of fiber-optics in local networks is growing, albeit slowly. Fiber is most often used on the backbone network and is not commonly run to the desktop.

Originally, access protocols were tied to cable type. Ethernet and ARCnet ran on coaxial cable only. (Most of the installed coaxial cable is there because Ethernet and ARCnet have been around for so long.) However, these protocols have since been modified to run on shielded and unshielded twisted-pair, and fiber-optic cable. Cable type is no longer tied to the access method. ARCnet and Ethernet run on coaxial cable, unshielded twisted-pair, and fiber-optic cabling. Token Ring runs on unshielded and shielded twisted-pair and fiber-optic cabling.

A tradeoff between speed and distance exists, especially with copper cable. It is possible to increase the speed of data transmission, but this reduces the distance that data can travel without regeneration. Signal regenerating products like repeaters and amplifiers can help, but the physical properties of cable impose certain limitations.

Coaxial Cable

Coaxial cable, or coax, has a long history. If you have cable television in your home, you have coaxial cable. Broadband transmission uses the same principles as cable TV and runs on coax. Broadband and cable TV take advantage of coax's ability to transmit many signals at the same time. Each signal is called a channel. Each channel travels along at a different frequency, so it does not interfere with other channels.

Coax has a large bandwidth, which means it can handle plenty of traffic at high speeds. Other advantages include its relative immunity to electromagnetic interference (as compared to twisted-pair), its ability to carry signals over a significant distance, and its familiarity to many cable installers .

Coax cable has four parts (see Figure 1). The inner conductor is a solid metal wire surrounded by insulation. A thin, tubular piece of metal screen surrounds the insulation. Its axis of curvature coincides with that of the inner conductor, hence the name coaxial. Finally, an outer plastic cover surrounds the rest.


Figure 1: Coaxial cable, also called coax, is the oldest network cable. It is proven, easy to use. It has a large bandwidth and can support transmission over long distances.

Coax comes in several sizes. Standard Ethernet cable, the yellow stuff called thick Ethernet, is about the diameter of a man's thumb. Thin Ethernet, the black cable, is about as thick as a woman 's pinky finger. ARCnet uses RG/68 coax cable. Thicker coax is more robust, harder to damage, and transmits data over longer distances. It's also more difficult to connect.

Standard Ethernet requires a "vampire tap" and drop cable to connect a LAN device. This combination is bulky and expensive. Thin Ethernet uses a biconic (or BNC) connector, which is easier to install than vampire taps.

Twisted-Pair

Twisted-pair cable has been around a lot longer than coaxial, but it has been carrying voice, not data. Unshielded twisted-pair is used extensively in the nationwide telephone system. Practically every home that has telephones is wired with twisted-pair cable.

In the past few years , vendors have been able to transmit data over twisted-pair at reasonable speeds and distances. Some of the first PC LANs, such as Omninet or 10Net, used twisted-pair cable but could only transmit data at 1Mbit/sec. Token Ring, when it was introduced in 1984, was able to transmit data at 4Mbits/sec over shielded twisted-pair. In 1987, several vendors announced Ethernet-like technology that could transmit data over unshielded twisted-pair, but computers can only be about 300 feet apart, not the 2,000 feet allowed by thick coax. Recent developments in technology make it possible to run even 16Mbit/sec Token Ring and 100Mbit/sec FDDI traffic over unshielded twisted-pair.

Twisted-pair offers some significant benefits. It's lighter, thinner, more flexible, and easier to install than coax or fiber-optic cable. It's also inexpensive. It is therefore ideal in offices or work groups that are free of severe electromagnetic interference.

Although there are a variety of types of twisted-pair cable types, shielded (see Figure 2) and unshielded (see Figure 3) are the two most important. Shielded twisted-pair has an RF- insulating material wrapped around the two twisted wires. Unshielded twisted-pair (or ordinary telephone wire) does not. Shielded twisted-pair is more immune to interference, which usually translates into higher transmission speeds over longer distance-it is more expensive, however.


Figure 2: Shielded twisted-pair's shield increases its immunity to electromagnetic interference which allows it to transmit data over longer distances than unshielded twisted pair.

Figure 3: Unshielded twisted-pair is installed nearly everywhere. Besides being inexpensive and readily available, it is flexible and familiar to cable installers. It has become the cable of choice for the departmental network.

Unshielded twisted-pair is fast becoming the media of choice. By 1993, the market research firm Dataquest projected that 78 percent of Ethernet connections would be made via twisted-pair cables. Unshielded twisted-pair also gained in popularity for Token Ring networks, which were traditionally wired with shielded twisted-pair.

The most important result of the telephone industry's use of twisted-pair is modular cabling. A modular cabling system, built with patch panels, wiring closets, and connector jacks , makes it easier to move computers from one place to another without rewiring the LAN. A modular cabling system allows a company to prewire a building for its phone and data services. Once the wire is in place, people can move from office to office, and new cabling does not have to be run.

Fibrous Diet

Fiber-optics has been touted as the answer to all the problems of copper cable. It can carry voice, video, and data. It has enormous bandwidth and can carry signals for extremely long distances. Because it uses light pulses, not electricity to carry data, it is immune to electromagnetic interference. It is also more secure than copper cable, because an intruder cannot eavesdrop on the signals, but must physically tap into the cable. To get at the information inside, a device must be attached, and the light level will subsequently decrease.

Despite its many advantages, fiber-optic's deployment in the LAN has been slow. According to Dataquest figures, by 1993, fiber-optics held only 1.4 percent of the LAN market. Cable installer's experience and fiber's high cost is holding back its widespread installation. Very simply, installing fiber-optic cable is very difficult. Splicing fiber-optic cables together is even more difficult. Putting connectors on the fiber-optic cable is also harder than for copper cable. The expense of diagnostic tools is another problem. Time domain reflectometers, ohmmeters, voltmeters, and oscilloscopes can be easily connected to any type of copper cable. But such tools must be specifically designed or adapted for fiber-optics use.

Fiber-optics has enjoyed its greatest success as a backbone medium for connecting sub-networks. Its properties make it ideal for the heavy traffic, hostile environments, and great distances that characterize backbone networks. Its immunity to electrical interference makes it ideal for the factory floor, another popular application.

Fiber-optic cable itself is a core fiber surrounded by cladding (see Figure 4). A protective covering surrounds both. LEDs or light emitting diodes send the signals down the cable. A detector receives the signals and converts them back to the electrical impulses that computers can understand. While the bits are encoded into light in a number of ways, the most popular method is to vary the intensity of the light.


Figure 4: Fiber-optic cable offers tremendous bandwidth, tight security, immunity to electromagnetic interference, and can carry data over long distances. It is mostly used in backbones.

Fiber-optic cable can be multimode or single-mode. In single-mode cable, the light travels straight down the fiber, which means data can travel greater distances. But since single-mode cable has a smaller diameter than multimode cable, it is harder (more expensive) to manufacture. In multimode cable, the light bounces off the cable's walls as it travels down, which causes the signals to weaken sooner, and therefore data cannot travel great distances. Single-mode cable is most often used in the nationwide telephone system, and multimode cable is most often used in LANs, since data is not required to travel across the country.

Standards for fiber-optic LANs have been developed. ANSI's Fiber Distributed Data Interface (FDDI) describes a network that can transmit data at 100Mbits/sec. It also specifies a dual, counter-rotating ring, which makes it fault tolerant. The IEEE has also developed standards for fiber-optic Ethernet.

Imaging applications and the proliferation of networks will force installation of high capacity LANs. Fiber-optics has enormous potential. Its capacities are tremendous. When wiring a new building, the best strategy is to run fiber-optic backbones, with twisted-pair to the desktops.

This tutorial, number 7, was originally published in the February 1989 issue of LAN Magazine/Network Magazine.

 
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Network Tutorial
Lan Tutorial With Glossary of Terms: A Complete Introduction to Local Area Networks (Lan Networking Library)
ISBN: 0879303794
EAN: 2147483647
Year: 2003
Pages: 193

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