As a matter of fact, the type of modulation and coding technique used in a digital communication system is decided by the channel characteristics and application areas. Some important characteristics of a channel may be listed as under :
(i) power needed to achieve the desired SNR.
(ii) transmission bandwidth of channel.
(iii) amplitude and phase response of channel.
(iv) type of channel i.e., linear or nonlinear.
(v) effects of external interference on the channel.
1.12.1. Classification of Channels
          We can classify the digital communication channels into following five categories :
1.12.2. Telephone Channels
          A telephone channel is designed to provide service to voice signals such as telephones. The telephone channels are also used for the worldwide Internet connection. Therefore, the telephone channel is the best possible option for the data communication over long distances.

  1. Salient features of telephone channel

The salient features of a telephone channel may be listed as under
(i) band pass characteristics over 300 to 3400 Hz as shown in figure 1.14.
(ii) quite high signal to noise ratio of about 30 dB.
(iii) Almost linear response.
(iv) Figure 1.14 shows that the amplitude response is flat over the entire passband.
(v) However, no particular attention is given to the phase response because human ears are not very sensitive to the channel phase delay variations. But, the data and images (pictures) are strongly affected by the phase delay variations.
FIGURE 1.14 Characteristics of telephone channel
FIGURE 1.15 Use of equalizer.
Therefore, for digital transmission over the telephone channels, it is essential to use an equalizer as depicted in figure 1.15.
As shown in figure 1.16, the equalizer maintains a flat amplitude response and linear phase response over the passband.
FIGURE 1.16 Amplitude and phase response of telephone channel with equalizer.

  1. Transmission Rates

          If in digital communication system, the sophisticated modulation techniques analog with equalizer are used then it is possible to reach the transmission upto 16.8 kilobits/sec (Kb/s).
1.12.3. Co-axial Cables
1. Historical Background
Figure 1.17 shows the construction of co-axial cable. It consists of two concentric conductors separated by a dielectric material. The external conductor is metallic braid and used for the purpose of shielding. The co-axial cable may contain one or more co-axial pairs.
The co-axial cable was initially developed as the backbone of analog telephone networks where a single telephone cable would be used to carry more than 10,000 voice channels at a time. The digital transmission systems using the co-axial cable were developed in 1970s. These systems operated in the range of 8.5 Mb/s to 274 Mb/s.
FIGURE 1.17 Construction of a co-axial cable.
Another important application is a cable modem, with the cable modem termination system (CMTS). One more application is Ethernet LAN using the co-axial cable. The co-axial cable is used for its large bandwidth and high noise immunity.

  1. Salient Features

Some important features (characteristics) of a co-axial cable may be listed as under :
(i)      Two types of cables having 75 W and 50 W impedance are available.
(ii)     Due to the shield provided, this cable has excellent noise immunity.
(iii)    It has a large bandwidth and low losses.
(iv)    This cable is suitable for point to point or point to multi-point applications. In fact, this is the most widely used medium for local area networks.
(v)     These cables are costlier than twisted pair cables but they are cheaper than the optical fiber cables.
(vi)    It is essential to use closely spaced (after every 1 km) repeaters to achieve the data rates of 8.5 Mb/s to 274 Mb/s.
1.12.4. Optical Fiber Cables
          Figure 1.18 shows the construction of an optical fiber cable.
Basically, it consists of an inner glass core surrounded by a glass cladding which has a lower refractive index. Digital signals are transmitted in the form of intensity-modulated light signal which is trapped in the glass core. Light is launched into the fiber using a light source such as a light emitting diode (LED) or laser. It is detected on the other side using a photo detector such as a phototransistor. The optical fiber cables are costlier than the other two types but they have many advantages over the other two types.
FIGURE 1.18 Construction of an optical fiber cable.

  1. Salient Features

          The salient features (characteristics) of optical fiber cables may be listed as under :
(i)      Higher bandwidth, therefore, can operate at higher data rates.
(ii)     Reduced losses as the signal attenuation is low.
(iii)    Distortion is reduced hence better quality is assured.
(iv)    Optical fiber cables are immune to electromagnetic interferences.
(v)    Small size and light weight.
(vi)    Used for point to point communication.

  1. Applications

(i)      Optical fiber transmission systems are widely used in the backbone of networks. Current optical fiber systems provide transmission rates from 45 Mb/s to 9.6 Gb/s using the single wavelength transmission.
(ii)     The installation cost of optical fibers is higher than that for the
co-axial or twisted wire cables.
(iii)    Optical fibers are now used in the telephone systems.
(iv)    Optical fiber cables are used in the local area networks (LANs).
1.12.5. Satellite System
          As a matter of fact, satellite microwave systems transmit signals between directional parabolic antennas. They use low gigahertz frequencies and line of sight communication. These systems use satellites which are in the geostationary orbit (36000 km above the earth). The satellites act as repeaters with receiving antenna, transponder and transmitting antenna.
Satellite microwave systems can reach the most remote places on earth and communicate with mobile devices. This system works in a specified manner that the signal is sent through cable media to an antenna which beams the signal to the satellite. Then, the satellite transmits the shown in figure 1.19. Satellite microwave systems experience delays between the transmission of a signal and its reception back to the earth (540 m sec).
Salient Features
          Satellite microwave systems have the following salient features (characteristics) :
(i)      It uses frequency range between 11 and 14 GHz.
(ii)     It supports a bandwidth and data rate from 1 to 10 Mbps.
(iii)    Attenuation depends on frequency, power, antenna size and atmospheric condition.
(iv)    The signals are affected by EMI effect, jamming and eavesdropping.
(v)     The installation of satellites is extremely difficult and the alignment of earth station antennas must be perfectly aligned.
(vi)    The cost of building and launching is very very high.
1.12.6. RF Link (Microwave Link)
As a matter of fact, long form of RF link is radio frequency link. This is actually a type of point to point wireless communication. The radio frequencies used for RF links are in microwave range, therefore, RF links are also called as microwave links as shown in figure 1.20.
Although many wire communication systems use copper wires or optical fiber, some just send the signal into the air. This happens when infrared, lasers, microwaves and radio are used for the transmission of data, as they do not require any physical medium. For long distance communication, microwave radio transmission is widely used as an alternative to co-axial cable. The signal transmission takes place in the form of electromagnetic waves which have wavelengths of few centimeters.
As a matter of fact, parabolic antennas can be mounted on the towers to send a beam of waves to another antenna, tens of kilometers away. The transmitting and receiving antennas are highly directional to enable a point to point communication. This system is widely used for both telephone and television transmission. The higher the tower which holds the antenna, the greater is the range. With a 100 meter high tower, the distances of 100 km can be easily covered.
FIGURE 1.20 Illustration of microwave link.
Salient Features
          Some of the important salient features (advantages) of microwave link may be listed as under :
(i)      Installation of towers and associated equipments is cheaper than laying down a cable of 100 km length.
(ii)     Less maintenance as compared to cables.
(iii)    Repeaters can be used. So, effect of noise is reduced.
(iv)    No adverse effects such as cable breakage etc.
(v)     Due to the use of highly directional antenna, these links do not make any interference with other communication systems.
(vi)    Size of transmitter and receiver reduces due to the use of high frequency.
          (i)      Signal strength at the receiving antenna reduces due to multipath reception.
(ii)     The transmission will be affected by the thunderstorms and other atmospheric phenomenons.
Range of Frequencies
Generally, the microwave transmission takes place at frequencies between 2 and 40 GHz. This corresponds to a wavelength of 15 cm to 0.75 cm.