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Transmission Media

Twisted Pair

Physical Description

Two insulared copper wires arranged in a regilar spiral pattern. A wire pair acts as a single communication link. Twisting of the individual pairs minimizes electromacnetigc interference between the pairs. The wires in a pair have thincknesses of from 0.016 to 0.036 in.

Uses

Most commonly used transmission medium for both analog and digital data.In the telephone system, individual telephone sets are connected to the local telephone exchange by twister-pair wire. These are referred to as "local loops".

Within an office building, telephone service is often provided by means of a private branch exchange (PBX).Within the building, the telephones are connected to the PBX via twisted pair.

For both of the systems just described, twisted pair has primarily been a medium for voice traffic between subscribers and their local telephone exchange office.

Digital traffic can also be carried over moderate distances. For modern digital PBX systems, data rates of about 64 kbps are achievable using digital signaling. When twisted pair is used for long distance trunking applications, data rate of 4 Mbps or more may be achieved.

Twisted pair cabling comes in two varieties. Category 3 twisted pair consist of two insulated wires gently twisted together. Four such pairs are typically grouped together in a plastic sheath for protection and to keep the eight wires together.  In 1988, category 5 twisted pairs were introduced. They are similar to category 3 pairs, but with more twists per centimeter and Teflon insulation, which results in less cross talk and a better quality signal over longer distances.

Transmission Characteristics

Wire pairs may be used to transmit both analog and digital signals. For analog signals, amplifiers are required about every 5 to 6 km. For digital signals, repeaters are used every 2 or 3 km.

Compared to other transmission media, twisted pair is limited in distance, bandwidth and data rate. The medium is quite susceptible to interference and noise because of its easy coupling with electromagnetic fields.  Impulse noise also easily intrudes into twisted pair.

Several measures are taken to reduce impairments. Shielding the wire with metallic braid or sheathing reduces interference. The twisting of the wire reduces low-frequency interference, and the use of different twist lengths in adjacent pairs reduces crosstalk.

For point-to-point analog signaling, a bandwidth of up to about 250 kHz is   possible. For voice transmission, such as the local loop, the attenuation is about 1 dB/km over the voice frequency range. A common standard for telephone lines is a maximum loss of 6dB; hence a 6-km section of line represents an iupper limit on the distance that can be covered. For digital point-to-point lines, data rates of up to a few Mbps are possible.


Coaxial Cable

Physical Description

Coaxial cable like twisted pair, consists of two conductors, but is constructed differently to permit it to operate over a wider range of frequencies. It consist of a hollow outer cylindrical conductor which surrounds a single inner wire conductor. The inner conductor can be either solid or stranded; the outer conductor can be either solid or braided. The inner conductor is held in place by either regularly spaced insulating rings or a solid dielectric material. The outer conductor is covered with a jacket or shield. A single coaxial cable has a diameter of from 0.4 to about 1 in.

Uses

Using frequency-division multiplexing (FDM), a coaxial cable can carry over 10,000 voice channel simultaneously. Also used for cable TV. A CATV (Community Antenna Television) system can carry dozens of TV channels at ranges up to a few tens of miles.

Coaxial cable can support a large number of devices with a variety of data and traffic types. Finally, caoxial cable is commonly used for short-range connections between devices. Using analog signaling, coaxial cable is used to transmit radio or TV signals. With digital signaling, coaxial cable can be uised to provide high-speed I/O channels on computer systems.

Transmission Characteristics

Coaxial cable has superior frequency characteristics to twisted pair, and can be used effectively at higher frequencies and data rates. It is much less suscpetivle to interference and crosstalk than twisted pair. The principal constraints on performance are attenuation, thermal noise and intermodulation noise.The latter is present only when several channels (FDM) or frequency bandiwdths are in use on the cable.

For long distance transmission of analog signals, amplifiers are needed every few km, with closer spacing required if higher frequencies are used. The usable spectrum for analog signaling extends to about 400 MHz. For digital signaling, repeaters are needed every km or so, with closer spacing needed for higher data rates. On experimental systems, data rates as high as 800 Mbops have been achieved with a repeater spacing of 1.6 km.


Optical Fiber

Physical Description

An optical fiber is a thin (2 to 125 Ám), flexible medium capable of conducting an optical ray. The lowest losses have been obtained using fibers of ultrapure fused silica. Ultrapuer fiber is difficult to manufacture; higher-loss multicomponents glass fibers are more economical and still provide good performance. Plastic fiber is even less costly and can be used for short-haul links, for which moderately high losses are acceptable.

An optical fiber cable has a cylindrical shape and consists of three concentric sections: the core, the cladding, and the jacket. The core is the innermost section and consists of one or more very thin strands, or fibers, made of glass or plastic. Each fiber is surrounded by its own cladding, a glass or plastic coating that has optical properties different from those of the core.

Uses

The following characteristics distinguish optical fiber from twisted pair and coaxial cable:

Five basic applications for optical fiber:

Transmission Characteristics

Optical fiber transmits a signal-encoded beam of light by means of total internal reflection.