ANALOG AND DIGITAL DATA TRANSMISSION
Data can be analog or digital. The term analog or digital corresponds, roughly, to continuous or discrete respectively. An example of analog data is the human voice.  when someone speaks, an analog wave is created in the air. This can be captured by microphone and converted into an analog signal. An example of digital data is data stored in the memory of a computer in the form of 0's and 1's.
 1 Analog Signals

Analog signals can be classified as simple or composite. A composite analog signal is composed of multiple sine wave. The sine wave is the most fundamental form of a periodic analog signal. A sine wave is shown in Figure

Period and frequency are shown in Figure.              

                            
Period is formally expressed in seconds. Frequency is formally expressed in hertz (HZ) as shown in Table 2.1.

The term phase describes the position of the waveform relative to time zero. Phase is measured in digrees or radians [360° is 2  rad; 1° is 27 /360 rad, and 1 rad is 360/(2n)]. A phase shift of 360° corresponds to a shift of a complete period; a phase shift of 180° corresponds to a shift of one-half of the period; a phase shift of 90° corresponds to a shift of one-quarter of a period. Relationships between different phases are shown in Figure.

The range of frequencies that a medium can pass is called its bandwidth. Because no medium can pass or block all frequencies, the bandwidth normally refers to the range of frequencies that a medium can pass without losing one-half of the power contained in that signal. The bandwidth is a range and is normally referred to as the difference between two numbers. For example, if a medium can pass frequencies: between 1000 and 4000 without loosing most of the power contained in this range, it, bandwidth is 4000 - 1000 = 3000.
So bandwidth is a property of medium. It is the difference between the highest and lowest frequencies that the medium can satisfactorily pass.

2 Digital Signals
Data can be represented by a digital signal. For example, a 1 can be encoded as E positive voltage and a 0 as zero voltage, which has been shown in Figure.

Most digital signals are a periodic and thus period or frequency is not appropriate Two new terms-bit interval (instead of period) and bit rate (instead of frequency; are used to describe digital signals. The bit interval is the time required to send one signal bit. The bit rate is the number of bit intervals per second. This means that the bit rate is the number of bits sent in 1 second, usually expressed in bits per second (bps). Figure 2.9 shows the bit rate and bit interval. If we are sending digital data through a medium, we are concerned with digital bandwidth (in bits per second). Digital bandwidth is the maximum bit rate that a medium can pass. Finally we come to this question: Should we use analog or digital signals? It really; depends on the situation and on the available bandwidth.
A channel or a link is either low-pass or band-pass. A low pass channel has a bandwidth with frequencies between o and f. The lower limit is 0, the upper limit call be any frequency (including infinity). On the other hand, a band-pass channel has a bandwidth with frequencies between fl and f2. Bandwidths of a low-pass channel and a band-pass channel are illustrated in Figure.

We have a low-pass channel only if the medium is dedicated to two devices (point to point) or shared between several devices in time (not in frequency). For example, in a wired local area network, a cable can be shared between stations. We can transmit data digitally in the system. So digital transmission needs a low-pass channel.
An analog signal requires a band-pass channel. A band-pass channel is more available than a low-pass channel. The bandwidth of a medium can be divided into several band-pass channels to carry several analog transmissions. For example, in analog cellular telephony, a limited bandwidth is divided between many telephone users. Each user has a bandwidth between 0 to 30 kHz with each signal shifted appropriately. So analog transmission can use a band-pass channel.

3 MODULATION AND DEMODULATION
As we all know, computers are digital devices. In fact, most computer communications such as terminal-to-computer or computer-to-disk transmission use digital signals. Most local area networks use digital signals. So where do analog signals enter the picture? The answer is: remote communications. Many people use PCs in their home to communicate with a computer at work. PCs also allow access to bulletin boards, stock quotations, airline reservation systems etc. In most cases there is no direct connection as such for a local area network. The physical connection uses the existing hardware of the telephone system. However, because the telephone is an analog device, the PC cannot communicate with it directly.
The solution to this problem is a device that converts PC digital signals into analog signals; a modem (short for modulation/demodulation as mentioned earlier). It fits between a PC and a telephone as shown in Figure.

The PC sends a digital signal out via its modem port, which the modem intercepts and converts (modulates) into an analog signal. This process is reversed at the receiving end (another PC). The analog signal comes through the phone line into the modem, and the modem converts it to a digital signal and sends it to the PC via the modem port, as shown in Figure .

Traditional telephone lines carry frequencies between 300 and 3300 Hz, giving them a bandwidth of 300 Hz. All this range is used for transmitting voice, where a great deal of interference and distortion can be accepted without loss of intelligibility. As we have seen, however, data signals require a high degree of accuracy to ensure integrity. For safety's sake, therefore, the edges of this range are not used for data communications. In general, we can say that the signal bandwidth must be smaller than the cable bandwidth. The effective bandwidth of a telephone line being used for data transmission is 2400 Hz, covering the range from 600 to 3000 Hz. Note that today some telephone lines are capable of handling more bandwidth than traditional lines. However modem design is still based on traditional capability. A telephone line has a bandwidth of almost 2400 Hz for data transmission. This bandwidth defines a baseband nature, which means we need to modulate if we want to use this bandwidth for data transmission. Devices that were traditionally used to do so are called modems.

a) Digital to Analog Conversion
It is essential to assign a group of one or more bit values to a particular analog signal.
There are three ways of varying an analog signal; by frequency, amplitude and phase
shift.

We discuss these conversion methods usually termed:
(1) Frequency modulation.
(2) Amplitude modulation.
(3) Phase modulation.

Frequency modulation, also called frequency shift keying, assigns a digital 0 to one
analog frequency and 1 to another. The frequency of the signal during each bit duration
is constant and its value depends on the bit (0 or 1): both peak amplitude and phase
remain constant.

Amplitude modulation, also called amplitude shift keying, is similar to frequency shift keying. The strength of the carrier signal is varied to represent binary 1 or 0. Both frequency and phase remain constant while the amplitude changes. Which voltage represents 1 and which represents 0 is left to the system. A bit duration is the period of time that defines one bit. For example, a binary 0 might be represented by a 1 volt signal and a binary 1 might be represented by a 5 volts signal.

Phase modulation, also called phase shift keying, is similar to the previous method. In phase shift keying, the phase of the carrier is varied to represent binary 1 or 0. Both peak amplitude and peak frequency remain constant as the phase changes. For example, if we start with a phase 0 degree to represent binary 0, then we can change the phase to 180 degrees to represent binary 1. The phase of the signal during
each bit duration is constant and its value depends on the bit (0 or 1).

b) Analog to Digital Conversion
Some analog to digital conversions are nothing more than the reverse of what we just now discussed. The modem examines incoming signals for amplitudes, frequencies and phase shifts and generates digital signals accordingly. These analog signals have constant characteristics at least over short intervals. What about analog signals where characteristics change continuously? The most obvious example may be analog signals produced by a sound such as voice. They are most complex than those generated by digital data and require alternative conversion techniques.

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