Coaxial Cable. Coaxial cable is popular as a medium for LANs because it's inexpensive and provides the greatest flexibility in installation; it can be folded and kinked with minimal signal loss. The coaxial design, where the center conductor is shielded by a copper or aluminum mesh or foil, provides a relatively secure connection and a high bandwidth. However, from a security perspective, it's virtually impossible to determine if an eavesdropper has tapped a run of coaxial cable. In addition, unlike fiber, it's possible for someone with a sensitive receiver and antenna to remotely pick up signals traveling through coaxial cable, amplify them, and decode the digital stream. This is especially true in coaxial cable designs in which the outer shield is formed by a copper or aluminum wire mesh, which provides incomplete shielding of the inner wire compared to cable made with a solid foil outer shield.
Fiber. As summarized in Figure 3-10, of the most popular media used in networks, glass fiber
provides the greatest bandwidth, highest level of security, greatest range, and resistance to electrical noise. Although fiber provides a working range of up to several kilometers with standard electronics, it's less flexible to install compared to copper cable. For example, unlike twisted pair or coaxial cable, fiber can't be snaked through very tight turns because the glass fiber is more fragile than the copper or aluminum wire used in the coaxial cable, twisted pair, or power line cable.
From a security perspective, fiber is the superior medium because, unlike the other copper cables or wireless, there is no radio frequency signal that can be intercepted by a nearby receiver. A wire run in parallel with a twisted pair or coaxial cable acts as an antenna to pick up the signals traversing through the cable that can be amplified and interpreted. In contrast, the light in a fiber cable is confined to the optical fiber, which is additionally shielded by a tough sheath. Furthermore, whereas coaxial cable or twisted pair can be tapped without detection, tapping into a fiber strand results in a marked, detectable drop in signal level because of the loss associated with a physical tap.
Twisted Pair. Twisted pair cable, the wiring used in virtually every office and residence for telephone communications, is a comprise between cost, bandwidth, security, and availability. It's more affordable than coaxial cable or fiber, but the bandwidth isn't as great, and security is a much greater concern. When used with radio frequency network signals, twisted pair cables don't perfectly cancel out the signals traversing the two wires, but act as antennas. As a result, not only are signals in the cable more readily intercepted, but the twisted pair cable is more susceptible to electrical noise in the environment. For this reason, twisted pair may not be able to be used in laboratory settings in which electronic equipment may interfere with the network signals, or in which the radiated network signals may interfere with sensitive laboratory equipment. One option is to use shielded twisted pair cable, but this usually involves running the special cable in walls because standard telephone twisted pair cable is unshielded.
Power Line Cable. Power line cable is a low-cost, low-bandwidth solution to networking. Although it may be suitable for exchanging text-only e-mails and other small files, the limitations of the medium prevent it from being a serious network medium for bioinformatics applications. It may be a viable as part of a redundant backup network system, however.
Ether. As a conduit for light or radio frequency signals, the ether provides the greatest flexibility of the options listed here, but also presents the greatest security risk. Typical internal installations for wireless LANs are limited to the same floor in a building. However, within that space, users may have complete mobility with laptops or desktop workstations that are frequently moved. Optical LANs, based on infrared (IR) links are line-of-sight only, and are limited to a single work area.
Radio frequency communications are also commonly used between buildings, in the form of
microwave links. These links tend to be line-of-sight and limited to perhaps 30 miles, depending on terrain and buildings that may interfere with line-of-sight communications. Unlike the radio frequency technology used with LANs, the bandwidth of these links is on the same order as coaxial cable.
Similarly, radio frequency satellite links that extend thousands of miles support high-bandwidth transmission rates comparable to that provided by coaxial cable and fiber media.
Note that the media characteristics summarized in Figure 3-10 reflect the physical properties of the media as well as the current state of the art in network electronics. For example, although wireless LANs are limited to a range of about 200 meters because of legal restrictions on the power of the electronics, the ether is capable of supporting communications across virtually infinite distances, and satellite-based wireless Internet connectivity is a viable alternative to wire, fiber, and cable in remote areas. Similarly, although glass fiber is less expensive than coaxial cable, the associated electronics and connectors are more expensive and more difficult to use.
The type of media used for Internet access depends primarily on the types of service available, and secondarily on the bandwidth, security, and cost constraints. For example, the TV cable companies that offer Internet service use coaxial cable to feed cable modems. Conversely, DSL companies provide access to the Internet through the same type of twisted pair used by the telephone
by the distance from a telephone switching station, and the maximum bandwidth diminishes with distance from the station. Many academic institutions and some well-funded biotech firms have access to the Internet through high-bandwidth, secure fiber.
In contrast to the media used for Internet access, the choice of media that can be used to support an internal LAN is more a function of cost, bandwidth requirements, security, ease of installation, and type of existing wiring, if any. For example, many older buildings have spare twisted pair cables running throughout their structure from the telephone service. In some of these buildings, running cables through asbestos or concrete structures many be prohibitively expensive or time-consuming, making wireless the only viable media. Another option is to use the power wiring as a data network medium. However, because the wire isn't twisted but is run parallel, it's more susceptible to noise than the other common types of media, resulting in a significantly lower maximum bandwidth.
Network Electronics
The media running from office to office and across the country become a useful communications channel with the addition of electronics capable of sending and receiving signals through the media. These electronics serve a variety of functions, including:
● Generating signals destined for a recipient somewhere in the network
● Coordinating signals through media in order to minimize interference
Amplifying and conditioning signals so that they can continue error-free to their destination ● Blocking signals from certain paths to minimize interference in those paths
● Routing signals down the quickest or least-expensive route from source to destination
● Translating signals originally designed to work with one protocol so that they are compatible
with networks designed to support other protocols ● Connecting different networks
● Monitoring the status of the network, including the functioning of network electronics and the
amount of data on segments of the network
Although there are hundreds of devices on the market that transmit, receive, manage, convert, block, redirect, and monitor signals on the network, most fit into the categories listed in Table 3-3. Several of the network devices listed in Table 3-3 are illustrated in context in Figure 3-11 on page 130. However, it's important to note that the physical layout of the network depicted in this figure may have little relation to the logical functioning of the network electronics. For example, even though the workstations or clients are connected directly to the printer, all printing requests or jobs may be directed to the print server, which manages the printing queue and buffers printing requests, freeing the processors in the workstation clients to handle other computations instead of devoting machine cycles to managing individual print jobs.