Electrical Signals

Introduction to Transmission

Transmission may be defined as the electrical transfer of a signal, message, or other form of intelligence from one location to another. Traditionally, transmission has been one of the two major disciplines of telecommunication. Switching is the other principal specialty. Switching establishes a connection from user X to some distant user Y. Simplistically, we can say that transmission is responsible for the transport of the signal from user X to user Y. In the old days of telephony, these disciplines were separate with strong demarcation between one and the other. Not so today. The demarcation line is fast disappearing. For example, under normal circumstances in the PSTN, a switch provides network timing that is vital for digital transmission.

What we have been dealing with so far is baseband transmission. This is the transmission of a raw electrical signal described in Section 2.3.2. This type of baseband signal is very similar to the 1s and 0s transmitted electrically from a PC. Another type of baseband signal is the alternating current derived from the mouthpiece of a telephone handset (subset). Here the alternating current is an electrical facsimile of the voice sound wave impinging on the telephone microphone.

Baseband transmission can have severe distance limitations. We will find that the signal can only be transmitted so far before being corrupted one way or another. For example, a voice signal transmitted from a standard telephone set over a fairly heavy copper wire pair (19 gauge) may reach a distant subset earpiece some 30 km or less distant before losing all intelligibility. This is because the signal strength is so very low that it becomes inaudible.

To overcome this distance limitation, we may turn to carrier or radio transmission. Both transmission types involve the generation and conditioning of a radio signal. Carrier transmission usually implies (not always) the use of a conductive medium such as wire pair, coaxial cable, or fiber-optic cable to carry a radio or light-derived signal. Radio transmission always implies radiation of the signal in the form of an electromagnetic wave. We listen to the radio or watch television. These are received and displayed or heard as the result the reception of radio signals.

Modulation

At the transmitting side of a telecommunication link a radio carrier is generated. The carrier is characterized by a frequency, described in Section 2.3.3. This single radio frequency carries no useful information for the user. Useful information may include voice, data, or image (typically facsimile or television). Modulation is the process of impinging that useful information on the carrier, and demodulation is the recovery of that information from the carrier at the distant end near the destination user.

The IEEE defines modulation as “a process whereby certain characteristics of a wave, often called the carrier, are varied or selected in accordance with a modulating function.” The modulating function is the information baseband described above.

There are three generic forms of modulation:

1. Amplitude modulation (AM)

2. Frequency modulation (FM)

3. Phase modulation (PM).

Item 1 (amplitude modulation) is where a carrier is varied in amplitude in accordance with information baseband signal. In the case of item 2 (frequency modulation), a carrier is varied in frequency in accordance with the baseband signal. For item 3 (phase modulation) a carrier is varied in its phase in accordance with the information baseband signal.

Figure 2.9 graphically illustrates amplitude, frequency, and phase modulation. The modulating signal is a baseband stream of bits: 1s and 0s. We deal with digital transmission (e.g., 1s and 0s)

Prior to 1960, all transmission systems were analog. Today, in the PSTN, all telecommunication systems are digital, except for the preponderance of subscriber access lines. These are the subscriber loops described in Chapter 1. Let us now distinguish and define analog and digital transmission.

Analog Transmission

Analog transmission implies continuity as contrasted with digital transmission that is concerned with discrete states. Many signals can be used in either the analog or digital sense, the means of carrying the information being the distinguishing feature. The information content of an analog signal is conveyed by the value or magnitude of some characteristic(s) of the signal such as amplitude, frequency, or phase of a voltage, the amplitude or duration of a pulse, the angular position of a shaft, or the pressure of a fluid. Typical analog transmission are the signals we hear on AM and FM radio and what we see (and hear) on television. In fact, television is rather unique. The video itself uses amplitude modulation; the sound subcarrier uses frequency modulation, and the color subcarrier employs phase modulation. All are in analog formats.

Digital Transmission

The information content of a digital signal is concerned with discrete states of the signal, such as the presence or absence of a voltage (see Section 2.3.2); a contact is the open or closed position, or a hole or no hole in certain positions on a card or paper tape. The signal is given meaning by assigning numerical values or other information to the various combinations of the discrete states of the signal. We will be dealing extensively with digital transmission as the argument in this text proceeds.

Binary Digital Signals

In Section 2.4.1, we defined a digital waveform as one that displayed discreteness. Suppose we consider the numbers 0 through 9. In one case, only integer values are permitted in this range, no in-between values such as 3.761 or 8.07. This is digital where we can only

assign integer values between 0 and 9. These are discrete values. On the other hand, if we can assign any number value between 0 and 9, there could be an infinite number of values such as 7.01648754372100. This, then, is analog. We have continuity, no discreteness.

Consider now how neat it would be if we had only two values in our digital system. Arbitrarily, we’ll call them a 1 and a 0. This is indeed a binary system, just two possible values. It makes the work of a decision circuit really easy. Such a circuit has to decide on just one of two possibilities. Look at real life: A light is on or it is off; two values, on and off. A car engine is running or not running, and so on. In our case of interest, we denominate one value a 1 and denominate the other a 0. We could have a condition where current flows, and we’ll call that condition a 1; no current flowing we’ll call a 0.

Of course we are defining a binary system with a number base of 2. Our day-to-day numbers are based on a decimal number system where the number base is 10. There is a review of binary arithmetic in Appendix B.

The basic key in binary digital transmission is the bit, which is the smallest unit of information in the binary system of notation. It is the abbreviation of the term binary digit. It is a unit of information represented by either a “1” or a “0.”

A 1 and a 0 do not carry much information, yet we do use just one binary digit in many applications. One of the four types of telephone signaling is called supervisory signaling. The only information necessary in this case is that the line is busy or it is idle. We may assign the idle state a 0 and the busy state a 1. Another application where only a single binary digit is required is in built-in test equipment (BITE). In this case, we accept one of two conditions: A circuit, module, or printed circuit board (PCB) is operational or it is not. BITE automates the troubleshooting of electronic equipment.

 To increase the information capacity of a binary system is to place several bits (binary digits) contiguously together. For instance, if we have a 2-bit code, there are four possibilities: 00, 01, 10, and 11. A 3-bit code provides eight different binary sequences, each 3 bits long. In this case we have 000, 001, 010, 011, 101, 110, and 111. We could assign letters of the alphabet to each sequence. There are only eight distinct possibilities, so only eight letters can be accommodated. If we turn to a 4-bit code, 16 distinct binary sequences can be developed, each 4 bits long. A 5-bit code will develop 32 distinct sequences, and so on.

As a result, we can state that for a binary code of length n, we will have 2n different possibilities. The American Standard Code for Information Interchange (ASCII) is a 7-bit code (see Section 10.4); it will then have 27, or 128, binary sequence possibilities. When we deal with pulse code modulation (PCM) (Chapter 6) as typically employed on the PSTN, a time slot contains 8 bits. We know that an 8-bit binary code has 256 distinct 8-bit sequences (i.e., 28 = 256).

Consider the following important definitions when dealing with the bit and binary transmission. Bit rate is defined as the number of bits (those 1s and 0s) that are transmitted per second. Bit error rate (BER) is the number of bit errors measured or expected per unit of time. Commonly, the time unit is the second. An error, of course, is where a decision circuit declares a 1 when it was supposed to be a 0, or declares a 0 when it was supposed to be a 1.