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Fiber is intrinsically secure, as it is virtually impossible to place a physical tap without detection. As no light is radiated outside the cable, physical taps are the only means of signal interception. Additionally, the fiber system supports such a high volume of traffic that it is difficult to intercept and distinguish a single transmission from the tens of thousands of other transmissions that might ride the same cable system. The digital nature of most fiber, coupled with encryption techniques frequently used to protect transmission from interception, make fiber highly secure. In fact, a number of U.S. government agencies recently have lobbied Congress and the FCC to require that the carriers place physical taps on existing fiber optic systems in order to ease their ability to place wiretaps, assuming that an enabling court order is issued.
While the acquisition, deployment, and rearrangement costs of fiber are relatively high ([ge]130% of Cat 5 copper), the immense bandwidth can outweigh that cost in bandwidth-intensive applications. At Gbps speeds, a single set of fibers can carry huge volumes of digital transmissions over longer distances than alternative systems, thereby lowering the transport cost per bit and cost per conversation to fractions of a penny per minute.
While fiber certainly does not have the break strength of copper or coax, it does have the same tensile strength as steel of the same diameter. When covered by a protective jacket or armored, fiber can be treated fairly roughly without damage. Additionally, fiber is more resistant to temperature extremes and corrosion. However, and in consideration of the huge number of conversations supported over a typical fiber optic cable, a train derailment, earthquake or other traumatic event can have consequences of disruptive, and even catastrophic, proportions.
As one would expect, cost-effective applications for fiber optic transmission systems are those which are bandwidth-intensive. Such applications include backbone carrier networks, international submarine cables, backbone LANs (FDDI), interoffice trunking, computer-to-computer or cabinet-to-cabinet (e.g., mainframes and PBXs) distribution networks (e.g., CATV and Information Superhighway), and fiber to the desktop (e.g., CAD or Computer Aided Design).
While each transmission medium/system has its own unique properties and applications, it is clear that digital fiber optic cable offers the most potential. As it also is costly and fragile, it, however, is not always the ideal approach. In fact, the most appropriate transmission medium depends of issues mentioned at the beginning of this chapter. Namely, those considerations include bandwidth, error performance, throughput, distance between elements, propagation delay, security, mechanical strength, physical dimensions, and a number of cost factors. In fact, a given long-haul conversation typically will traverse a number of transmission systems, perhaps both wired and wireless, and typically including twisted pair and fiber optics, at a minimum.
The true concept of a hybrid transmission system, however, generally involves a local loop connection deployed in a well-planned convergence scenario. Such a scenario involves a single provider, or multiple providers, developing a communications grid designed to deliver voice, data and entertainment information to the premise. Hybrid systems (Figure 3.9) usually are described as involving Fiber-to-the-Neighborhood (FTTN), Fiber-to-the-Curb (FTTC), or Fiber-to-the-Home (FTTH). Although conventional wisdom suggests that cost considerations will dictate that the last link of such a hybrid network involve either coaxial cable or twisted copper pair, various wireless technologies are currently challenging that concept.
Figure 3.9 Hybrid network: The convergence scenario.
While many of the traditional telephone carriers and CATV providers have made dramatic announcements about their plans to install fiber optic cable to the curb or even to the premise, their ardor has cooled as the economics of such an approach have become apparent. It is likely that fiber will be deployed to the neighborhood, with coaxial cable deployed to the premise. Additionally, it is likely that the telcos will extend the life of the embedded twisted pair cable as long as possible, through the use of new local loop technologies such as Asymmetric Digital Subscriber Loop (ADSL). In the future, it is possible that Wireless Local Loop (WLL) technology will seriously challenge the traditional wired approach.
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