ase 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 Os) 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 SIGNALS CONVEY INTELLIGENCE as the dissipation of signal strength as a signal travels along a wire pair, or any other transmission medium for that matter. Loss or attenuation is usually expressed in decibels (dB). In Appendix C the reader will find a tutorial on decibels 