Coaxial Cable

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 consists of a hollow outer cylindrical conductor that surrounds a single inner wire conductor (Figure 4.2b). 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 1 to 2.5 cm. Coaxial cable can be used over longer distances and support more stations on a shared line than twisted pair.

Applications: Coaxial cable is a versatile transmission medium, used in a wide variety of applications.The most important of these are
• Television distribution
• Long distance telephone transmission
• Short run computer system links
• Local area networks
Coaxial cable is widely used as a means of distributing TV signals to individual homes—cable TV. From its modest beginnings as Community Antenna Television (CATV), designed to provide service to remote areas, cable TV reaches almost as many homes and offices as the telephone. A cable TV system can carry dozens or even hundreds of TV channels at ranges up to a few tens of kilometers.
            Coaxial cable has traditionally been an important part of the long distance telephone network.Today, it faces increasing competition from optical fiber, terrestrial microwave, and satellite. Using frequency division multiplexing (FDM, see Chapter 8), a coaxial cable can carry over 10,000 voice channels simultaneously.
            Coaxial cable is also commonly used for short range connections between devices. Using digital signaling, coaxial cable can be used to provide high speed I/O channels on computer systems.
 
Transmission Characteristics: Coaxial cable is used to transmit both analog and digital signals. As can be seen from Figure 4.3b, coaxial cable has frequency characteristics that are superior to those of twisted pair and can hence be used effectively at higher frequencies and data rates. Because of its shielded, concentric construction, coaxial cable is much less susceptible 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 bands are in use on the cable.
             For long distance transmission of analog signals, amplifiers are needed every few kilometers,with closer spacing required if higher frequencies are used.The usable spectrum for analog signaling extends to about 500 MHz.For digital signaling, repeaters are needed every kilometer or so,with closer spacing needed for higher data rates.

Twisted Pair Cable

GUIDED TRANSMISSION MEDIA
For guided transmission media, the transmission capacity, in terms of either data rate or bandwidth, depends critically on the distance and on whether the medium is point to point or multipoint. Table 4.1 indicates the characteristics typical for the common guided media for long distance point to point applications; we defer a discussion of the use of these media for LANs to Part Four.
The three guided media commonly used for data transmission are twisted pair, coaxial cable, and optical fiber (Figure 4.2).We examine each of these in turn.
Twisted Pair
The least expensive and most widely used guided transmission medium is twisted pair.
Physical Description
  A twisted pair consists of two insulated copper wires arranged in a regular spiral pattern.A wire pair acts as a single communication link.
Typically, a number of these pairs are bundled together into a cable by wrapping them in a tough protective sheath. Over longer distances, cables may contain hundreds of pairs.The twisting tends to decrease the crosstalk interference between adjacent pairs in a cable. Neighboring pairs in a bundle typically have somewhat different twist lengths to reduce the crosstalk interference. On long
distance links, the twist length typically varies from 5 to 15 cm.The wires in a pair have thicknesses of
from 0.4 to 0.9 mm.
Applications :
By far the most common guided transmission medium for both analog and digital signals is twisted pair. It is the most commonly used medium in the telephone network and is the workhorse for communications within buildings.
In the telephone system, individual residential telephone sets are connected to the local telephone exchange, or “end office,” by twisted pair wire. These are referred to as subscriber loops.Within an office building, each telephone is also connected to a twisted pair, which goes to the in house private branch exchange (PBX)
system or to a Centrex facility at the end office.These twisted pair installations were designed to support voice traffic using analog signaling. However, by means of a modem, these facilities can handle digital data traffic at modest data rates.
Twisted pair is also the most common medium used for digital signaling. For connections to a digital data switch or digital PBX within a building, a data rate of 64 kbps is common. Twisted pair is also commonly used within a building for local area networks supporting personal computers. Data rates for such products are typ ically in the neighborhood of 100 Mbps. However, twisted pair networks with data rates of to 10 Gbps have been developed, although these are quite limited in terms of the number of devices and geographic scope of the network. For long distance applications, twisted pair can be used at data rates of 4 Mbps or more.
Twisted pair is much less expensive than the other commonly used guided transmission media (coaxial cable, optical fiber) and is easier to work with.
Transmission Characteristics:
Twisted pair may be used to transmit both analog and digital transmission. For analog signals, amplifiers are required about every 5 to 6 km. For digital transmission (using either analog or digital signals), repeaters
are required every 2 or 3 km.
Compared to other commonly used guided transmission media (coaxial cable, optical fiber), twisted pair is limited in distance, bandwidth, and data rate.As Figure 4.3a shows, the attenuation for twisted pair is a very strong function of frequency. Other impairments are also severe for twisted pair. The medium is quite susceptible to interference and noise because of its easy coupling with electromagnetic fields. For example, a wire run parallel to an ac power line will pick up 60 Hz energy. 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 1 MHz is possible. This accommodates a number of voice channels. For long distance digital point to point signaling, data rates of up to a few Mbps are possible; for very short distances, data rates of up to 10 Gbps have been achieved in commercially available products.