2.5
INTRODUCTION TO TRANSPORTING ELECTRICAL SIGNALS
31
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 possi-
bilities: 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 2
n
different
possibilities. The American Standard Code for Information Interchange (ASCII) is a 7-bit
code (see Section 10.4); it will then have 2
7
, 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., 2
8
= 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 (Ref. 4).
2.5
INTRODUCTION TO TRANSPORTING ELECTRICAL SIGNALS
To transport electrical signals, a transmission medium is required. There are four types
of transmission media:
1. Wire pair
2. Coaxial cable
3. Fiber-optic cable
4. Radio
2.5.1
Wire Pair
As one might imagine, a wire pair consists of two wires. The wires commonly use a
copper conductor, although aluminum conductors have been employed. A basic impair-
ment of wire pair is loss. Loss is synonymous with attenuation. Loss can be defined