Basically, we shall consider following channels:
(i) telephone channels
(ii) optical fibers
(iii) mobile radio channels, and
(iv) satellite channels.
These four different channels have been chosen because they play important roles in modern telecommunications environment.
(i) Telephone Channels : A telephone network makes use of a switching mechanism. This switching mechanism is known as circuit switching and it is used to establish and end –to –end communication link on a temporary basis. Infact, the primary purpose of the network is to ensure that the telephone transmission between a speaker at one end of the link and a listener at the other end, is an acceptable replacement for face- to –face conversion.
In this form if communication, the message source is the sound produced by the speaker’s voice and the destination is the listener’s ear. However, it may be noted that the telephone channel supports only the transmission of electrical signals.
Because of this reason, appropriate transducers are used at the transmitting and receiving ends of the system.
Further, the telephone channel is essentially a linear, bandwidth limited channel. A speech signal is essentially limited to a band from 300 to 3100 Hz in the sense that frequencies in lying outside this band do not contribute much to articulation efficiency. Hence, the particular frequency band may be viewed as a rough guideline for the passband of a telephone channel that provides satisfactory services.
(ii) Optical Fibers : The second communication channel, an optical fiber, is a dielectric waveguide which transports light signal from one place to another just as a metallic wire pair or a co-axial cable, transports electrical signals.
An optical fiber consists of a central core within which the propagating electromagnetic field is confined and which is surrounded by a cladding layer, which is itself surrounding by a thin protective jacket. Basically, the core and cladding are both made of pure silica glass, whereas, the jacket is made of plastic. Optical fibers have unique characteristics that make then highly attractive as a transmission medium. Optical fibers offer the following unique advantages:
(iii) Mobile Radio Channels : The third communincation channel, mobile radio channel, extends the capability of the public telecommunications network by introducing mobility into virtue of its ability to broadcast.
The term ‘mobile radio’ is usually meant to encompass terrestrial situations where a radio transmitter or receiver is capable of being moved, regardless of whether it actually moves or not. Basically, there is no ‘line of sight’ path for communication, rather, radio propagation takes place mainly by way of scattering from the surfaces of the surrounding buildings and by diffraction over and / or around them.
Therefore, the energy reaches the receiving antenna via more than one path. Therefore, in a mobile radio environment, we face a problem of multipath phenomenon in the sense that the various incoming radio waves reach their destination from different directions and with different time delays. Because of this, the received signal strength varies with location in a vary complicated manner and therefore, a mobile radio channel may be viewed as a linear time varying channel that is statistical in nature.
(iv) Satellite Channel
A satellite channel provides broad-area coverage in a continental as well as intercontinental
sense. Moreover, access to remove areas not covered by conventional cable or fiber communications is also a distinct feature of satellites.
In almost all satellite communication systems, the satellites are placed in geostationary orbit. For the orbit to be geostationary, it has to satisfy two requirements. First, the orbit is geosynchronous, which requires the satellite to be at an altitude of 22,300 miles, a geosynchronous satellite orbits the earth in exactly 24 hours. Second, the satellite is placed in orbit directly above the equator on an eastward heading.
Viewed from the earth, a satellite in geostationary orbit appears to be stationary in the sky.
Communications satellites in geostationary orbit offer the following unique system capabilities:
a. broad-area coverage
b. reliable transmission links
c. wide transmission bandwidth
In terms of services, satellites can provide fixed point to point links extending over long distances and into remote areas, communication to mobile platforms (e.g., aircraft, ships), or broadcast capabilities.
In a typical satellite communication system, a message signal is transmitted from an earth station via an uplink to a satellite, amplified in a transponder on board the satellite. It is then retransmitted from the satellite via a downlink to another earth station. In effect the satellite acts as a powerful repeater in the sky. The most popular frequency band for satellite communications is 6 GHz for the uplink and 4 GHz for the downlink.
In fact, the use of this frequency band offers the following advantages:
a. relatively inexpensive microwave equipment
b. low attenuation due to rainfall
c. insignificant sky background noise.
A single transponder can carry at least one color television signal, 1200 voice circuits, or digital data at a rate of 50 Mb/s.
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