Fundamentals of Transmission
Telecommunication systems use electromagnetic waves to transfer information. Electromagnetic waves can travel through transmission media like copper wires, fiber optics or as radio waves. They can also travel in vacuum. Wireless communication uses electromagnetic waves for transmission of information. The transmission media through which the waves propagate are not perfect. As a result, the waves propagated via the transmission media get attenuated and distorted.
The information to be transmitted does not always exist in a form that is compatible with the transmission medium. Waves that are compatible with the transmission medium must be generated to carry information. A signal is a wave that is suitable for carrying information over a transmission medium. Signals can be electric signals, light signals, electromagnetic signals or radio signals. Electric signals are used to carry information through copper wires, light signals for fiber optic cables, and radio signals for carrying information in free space. Electrical signals have limited bandwidth and cannot be used in long distance communication.
They need to be amplified or regenerated. Light signals have a high bandwidth and are suited for long distance communication.
Analog and Digital Signals
Information carrying signals are of two types—(a) analog signal, and (b) digital signal.
· Analog Signal: An analog signal is a wave that continuously changes its information carrying properties over time. The wave may vary in amplitude or frequency in response to changes in sound, light, heat, position, or pressure etc. For example a telephone voice signal is analog. The intensity of the voice causes electric current variations. At the receiving end, the signal is reproduced in the same proportion.
· Digital Signal: A digital signal is a wave that takes limited number of values at discrete intervals of time. Digital signals are non-continuous, they change in individual steps. They consist of pulses or digits with discrete levels or values. The value of each pulse is constant, but there is an abrupt change from one digit to the next. Digital signals have two amplitude levels called nodes. The value of which are specified as one of two possibilities such as 1 or 0, HIGH or LOW, TRUE or FALSE, and so on.
· Analog and digital signals are compared on the basis of—(1) impact of noise, (2) loss of information, and (3) introduction of error.
· Analog signal has the potential for an infinite amount of signal resolution. Another advantage with analog signals is that they can be processed more easily than their digital equivalent. The primary disadvantage of the analog signals is the noise. The effects of noise create signal loss and distortion, which is impossible to recover, since amplifying the signal to recover attenuated parts of the signal, also amplifies the noise. Even if the resolution of an analog signal is higher than a comparable digital signal, the difference can be overshadowed by the noise in the signal. In digital systems, degradation cannot only be detected, but corrected as well.
· Amplifier is any device or a circuit that changes, usually increases, the amplitude of an analog signal.
· Repeater is an electronic device that receives a signal and retransmits it at a higher level and/or higher power, so that the signal can cover longer distances. With physical media like Ethernet or Wi-Fi, data transmissions can only span a limited distance before the quality of the signal degrades. Repeaters attempt to preserve signal integrity and extend the distance over which data can safely travel. Actual network devices that serve as repeaters usually have some other name. Active hubs, for example, are repeaters. Active hubs are sometimes also called “multiport repeaters,” but more commonly they are just “hubs.”
Modulation and Demodulation
· Modulation: Signals consist of two components—the information signal and the carrier signal. The transmission of any signal over some communication medium usually involves modulation of a carrier. Prior to their transmission the information signal and the carrier signal are combined and the process of combining these two signals is called modulation. Characteristics of the carrier signal are varied in proportion to the amplitude of the information-carrying signal. Modulation results in the transfer of the signal information to higher frequency carrier signal. In simple English terms, the information signal sits on top of the carrier signal and rides on it from the receiver to the transmitter.
· Need for Modulation: Let’s understand the need for modulation by using a simple example. Stereophonic radio signal consist of frequency ranges from 30 Hz (Hertz) to 15 KHz (Kilo Hertz). Hence they need a bandwidth of 15 KHz. If ten different radio stations start transmitting their voice signals between 30 Hz and 15 KHz frequencies, then a combination of these signals would only create noise and the receiver would not be able to discriminate between the signals of each radio station. To overcome this, usually the FM broadcast band, used for broadcasting FM radio stations, goes from 87.5 to 108.0 MHz (Mega Hertz). For example a radio channel-1 may be broadcast using a carrier signal of 102 MHz and would typically use band of frequencies between 101.9 to 102.1 MHz. Radio channel2 using a carrier signal of 102.2 MHz would use band of frequencies between 102.1 and 102.3 MHz. Similarly for other channels, the same method of allocation would be followed. This eliminates the problem of discrimination and decoding signals of each of the radio stations at the receiving end.
There are three primary reasons which necessitate modulation:
1. To make efficient use of the lines or media used for communication
2. To make radio communications feasible: The lower the frequency of signal, the larger is the size of the antenna needed for transmission and reception. A signal of 10 KHz would require an antenna whose dimensions are in the range of a few kilometres.
3. To simplify signal processing: It is simpler to design electronic systems for narrow frequency bands.
· At the sending side (transmitter), the signal is superimposed on the carrier wave, which results in a modulated carrier wave. The modulated carrier wave is transmitted. At the receiving end, the receiver is configured to recognize the carrier that the sender is using. The receiver detects the modulation of the carrier wave and reconstructs the data signal.
· The process of segregating the data signal and the carrier signal from the modulated carrier wave is called demodulation. At the receiving end, the carrier wave is discarded after the data signal has been reconstructed.
· Modulation technique did not originate for data communication for computers, but has long been used for radio, television, and telephone communication. For long distance transmission, computer networks use modulation, whether the signals are transmitted over wires, optical fibers, microwave or radio frequency.
· Modulation is of three kinds, which are defined as follows:
Amplitude Modulation—The amplitude of the carrier wave is modified in proportion to the data signal. The frequency and phase of the carrier signal remains unchanged.
Frequency Modulation—The frequency of the carrier signal is modified in proportion to the data signal. The amplitude and phase of the carrier signal remains unchanged.
Phase Shift Modulation—The phase of the carrier signal is modified in proportion to the data signal. The amplitude and frequency of the carrier signal remains unchanged.
For computer networks, generally phase shift modulation is used. Figure shows the different kinds of modulation of a carrier wave with signal.
(a) Carrier wave with signal (b) Amplitude modulation (c) Frequency modulation (d) Wavelength modulation
Modem is a device that has both a modulator and a demodulator. Modulator accepts data signals from the computer and modulates the carrier wave accordingly. Demodulator accepts modulated carrier wave and regenerates the original data signal from it. During data communication, modem is attached to the computer, both at the sender and the receiver side. Modems are used with all transmission media like RF modem for RF transmission and optical modem for transmission through fiber optics.
Modems