Tài liệu Transmission Facilities

Thảo luận trong 'Thiết Kế Web' bắt đầu bởi Thúy Viết Bài, 5/12/13.

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    Electric currents, electromagnetic waves, and optical energy carry messages on
    transmission facilities. The availability of ubiquitous transport is a prerequisite for
    the operation of the networks described in earlier chapters. It is tempting for managers
    to fantasize about owning all the communication facilities needed to support an
    enterprise. However, it soon becomes apparent that transmission equipment is
    expensive, sites are difficult to obtain, and maintenance by enterprise employees is
    virtually impossible. Consequently, most transport outside corporate buildings uses
    facilities owned and operated by common carriers. In this chapter, I describe some
    of the systems likely to be provided by the telephone companies and other entities.
    Because these facilities work together, all companies providing transport services
    operate compatible equipment.
    7.1 Twisted Pairs
    Twisted pairs are major components of the public telephone network. They are the
    dominant bearers in the local loop. In addition, twisted pairs are used extensively
    for on-premises wiring for enterprise installations.
    A twisted pair is two insulated wires twisted together and contained in a cable
    of many pairs. Known as tip and ring, neither of the wires is connected directly to
    the ground. The twist keeps the conductors balanced with respect to themselves, the
    cable shield, and other pairs. Often, twisted pairs are called cable pairs. A paired
    cable is a cable whose conductors are twisted pairs
     

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