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Twisted-wire Pairs

 
  
 
would be fewer hassles when it came time to figure out such things as line speeds, line capacities, variations in line distortion, and so on. However, there area number of  types,  ranging  in  cost  and  capabilities.  In  the following paragraphs, we examine the advantages and disadvantages of twisted-wire pairs, baseband and broadband coaxial cabling, and fiber optic cabling. Twisted-wire   Pairs Twisted-wire pairs, also known as twisted-pair wire or cable, is by far the least expensive transmission media. It consists of two insulated wires twisted around each other so that each wire faces the same amount of interference (noise) from the environment (see fig. 1-9). Unfortunately, this noise becomes part of the signal being transmitted. Twisting the wires together reduces but does not eliminate the noise. Twisted-pair  wire  comes  in  a  wide  range  of  gauges and pairs. Wire has an American Wire Gauge (AWG) number based on its diameter. For network purposes, 22- and 24-gauge wires are the two most common types of twisted-pair media. Some local-area networks use the  same  inexpensive,  unshielded  twisted-pair  cables telephone companies use. Others require a higher data grade quality. It’s not uncommon to have several hundred pairs (and, in some cases, thousands) of wires placed in a single cable. Normally, each twisted-wire pair in a cable can accommodate a single phone call between two people or between hardware devices. The advantages of using telephone wires are their relative   low   cost   and   their   availability.   Their disadvantages  include  susceptibility  to  signal  distortion errors and the relatively low transmission rates they provide over long distances. Twisted wire can handle a data flow of up to approximately one megabit per second (Mbps) over several hundred feet. For a small local-area network with a limited number of users, twisted-pair  is  an  ideal  choice  because  it  is  both inexpensive and easy to install. A phenomenon called Figure  1-9.—Twisted-wire  pairs  (2  wire  pairs  shown). crosstalk  exists  in  twisted-wire  pairs  whenever transmission occurs at a high rate of speed. Crosstalk is taking place whenever you can hear someone else’s conversation in the background; say Mr. Frost telling Mrs. Christmas what a great recipe he has for southern fried chicken, or Mrs. Brush telling Mr. Smith what a large fish she caught in the Gulf of Mexico, while you’re trying to carry on a conversation with your party. With voice communications this really isn’t a problem; however,   crosstalk   can   inhibit   the   high-speed transmission required for data communications. Twisted-wire pairs used in data communications are either private or public lines.  Private lines  are  those provided by the user. Public lines are those provided by a common carrier such as American Telephone and Telegraph (AT&T). Generally, public lines are used whenever  distances  are  great  or  the  terrain  or  other environmental factors prohibit the use of private lines. Public lines may be either switched lines or leased lines. Switched lines are used whenever the amount of data to be transmitted is short in duration or when many locations must be contacted for relatively short periods of time. There is a drawback. The telephone company cannot guarantee you exactly which path or switching equipment such a connection will use. Therefore, the speed and quality of the switched connection are questionable. Leased lines come into play when the connection time between locations A and B is long enough to cover the cost of leasing, or if higher speeds than those available with switched lines must be attained. Leased lines can also be conditioned by the telephone company to lower the error rate and increase transmission speeds. Conditioned leased lines typically operate at speeds of up to 64,000 bits per second (bps). Very-high-speed connections are also available from the common carrier. These are designated T1, T2, T3, and T4, and offer transmission rates of 1.5, 6.3, 46, and 281 million bits per  second  (Mbps),  respectively. Coaxial  Cables Coaxial (or coax) cable, the medium used by most cable  television  companies,  was  developed  primarily because  of  the  crosstalk  in  twisted-wire  pairs  when transmission occurs at a high rate of speed. While coax is more expensive than twisted-pair, it can transmit data significantly faster, over much longer distances, and with less electrical interference. Coaxial cable is made up of one or two central data transmission wires composed of copper surrounded by 1-16

   


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