You can think of a symbol as a simple 'thing' used to represent a more complicated thing. We humans use number symbols to represent the more complicated concept of 'how many' or quantities. The quantity 'four' (or * * * * ) is represented by the symbol '4'. We use letters and combinations of letters to represent the sounds made when speaking. Letters and letter combinations are symbols for sounds.
Any group of symbols used together as a set are referred to as a symbol system. The alphabet is a symbol-system. The Arabic numeral system is a symbol system. Egyptian hieroglyphs are also a symbol system.
Modems use symbols to transmit data faster. The analog phone line is sampled by the phone system 8,000 times per second to create 8,000 digital symbols. While that might seem fast, if you transmit only 1 bit per sample, you get 8,000 bits per second, or 8 kilobits per second. Modems can transmit at speeds up to 56 kilobits per second by using symbols. The computer sends any one of 128 voltage values every sampling, each representing 7 bits of digital data. That allows the computer to send up to 7 bits per sampling at each of 8,000 intervals each second for a total of 56 bits (7 x 8,000 = 56,000) per second.
Your cable modem uses 256 different symbols at each sampling interval, but the sampling interval on the cable company system is much, much higher (about 69 million) than the phone systems. However, keep in mind that the cable system is shared, so
Pulse-codes are also a type of symbol system used by computers and communications equipment. Human voice is sampled (quantized) and converted to a pattern of digital zeroes and ones. Each digital zero or one is referred to as a 'bit'. Each chunk of sound obtained from a single sample is converted to an eight-bit pattern that represents that sound by converting it to a symbol. Each eight bit pattern is called a 'pulse code' which represents a symbol.
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