Transmission medium is the physical path between transmitter and receiver in a data transmission system. Transmission media can be classified or unguided. In both cases, communication is in the form of electromagnetic waves. With guided media, the waves are guided along a solid medium, such as copper twisted pair, copper coaxial cable, and optical fiber. The atmosphere and outer space are examples of unguided media that provide a means of transmitting electromagnetic signals but do not guide them; this form of transmission is usually referred to as wireless transmission systems.
Fig 13.1: Transmission medium & physical layer.
In telecommunication, transmission media can be divided into two broad categories which are as shown in Fig 13.2.
Fig 13.2 : Classes of transmission media.
The characteristics and quality of a data transmission are determined both by the characteristics of the medium and the characteristics of the signal. In the case of guided media, the medium itself is more important in determining the limitations of Transmission.
For unguided media, the bandwidth of the signal produced by the transmitting antenna is more important than the medium in determining transmission characteristics. One key property of signals transmitted by antenna is directionality. In general, signals at lower frequencies are omni directional; that is, the signal propagates in all directions from the antenna. At higher frequencies, it is possible to focus the signal into a directional beam.
In considering the design of data transmission systems, a key concern, generally, is data rate and distance: the greater the data rate and distance, the better. A number of design factors relating to the transmission medium and to the signal determine the data rate and distance:
1. Bandwidth: All other factors remaining constant, the greater the bandwidth of a signal, the higher the data rate that can be achieved.
2. Transmission impairments: Impairments, such as attenuation, limit the distance. For guided media, twisted pair generally suffers more impairment than coaxial cable, which in turn suffers more than optical fiber.
3. Interference: Interference from competing signals in overlapping frequency bands can distort or wipe out a signal. Interference is of particular concern for unguided media, but it is also a problem with guided media. For guided media, interference can be caused by emanations from nearby cables. For example, twisted pair are often bundled together, and conduits often carry multiple cables. Interference can also be experienced from unguided transmissions. Proper shielding of a guided medium can minimize this problem.
4. Number of receivers: A guided medium can be used to construct a point-to-point link or a shared link with multiple attachments. In the latter case, each attachment introduces some attenuation and distortion on the line, limiting distance and/or data rate.
Guided transmission media
# 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 multi point, such as in a local area network (LAN). Table 3.1 indicates the type of performance typical for the common guided medium for long distance point-to-point applications; The three guided media commonly used for data transmission are twisted pair,
coaxial cable, and optical fiber (Figure 13.3). We examine each of these in turn.
# Twisted Pair:
The least-expensive and most widely-used guided transmission medium is twisted pairs. A twister pair consists of two conductors (normally copper) .Each with its own plastic insulation, twisted together as shown in fig 13.3.
a) Twisted-pair cable
b) Coaxial cable
c) Optical fiber
Fig 13.3: Guided transmission media.
# 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 two to six inches. The wires in a pair have thicknesses of from 0.016 to 0.036 inches.
By far the most common transmission medium for both analog and digital signals is twisted pair. It is the most commonly used medium in the telephone network as well as being 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 typically in the neighborhood of 10 Mbps.
However, recently, twisted-pair networks with data rates of 100 Mbps 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. It is more limited in terms of data rate and distance.
# Coaxial Cable:
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 13.3b). 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. Because of its shielded, concentric construction, coaxial cable is much less susceptible to interference and crosstalk than is twisted pair. Coaxial cable can be used over longer distances and supports more stations on a shared line than twisted pair.
# Applications of Coaxial cable:
Coaxial cable is perhaps the most versatile transmission medium and is enjoying widespread use 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 spreading rapidly 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 will eventually reach 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 miles.
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 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 110 channels on computer systems. Another application area for coaxial cable is local area networks. Coaxial cable can support a large number of devices with a variety of data and traffic types, over distances that encompass a single building or a complex of buildings.