uct a complete, new network expressly for the purpose of data trans- mission, and (2) to adapt the many existing telephone facilities for data transmission. The following paragraphs deal with the latter approach. Analog transmission facilities designed to handle voice traffic have characteristics that hinder the transmission of dc binary digits or bit streams. To permit the transmission of data over voice facilities (i.e., the telephone network), it is necessary to convert the dc data into a signal within the voice-frequency range. The equipment that performs the necessary conversion to the signal is generally called a modem, an acronym for modulator-demodulator. 10.9.2 Modulation-Demodulation Schemes A modem modulates and demodulates a carrier signal with digital data signals. In Section 2.4.3 the three basic types of modulation were introduced. These are listed as follows, along with their corresponding digital terminology: Modulation Type Corresponding Digital Terminology Amplitude modulation (AM) Amplitude shift keying (ASK) Frequency modulation (FM) Frequency shift keying (FSK) Phase modulation (PM) Phase shift keying (PSK) DATA COMMUNICATIONS In amplitude shift keying, the two binary conditions are represented by a tone on and a tone off. ASK used alone is not really a viable alternative in today's digital network. It is employed in many advanced hybrid modulation schemes. Frequency shift keying is still used for lower data rate circuits 1200 bps. The two binary states are each represented by a different frequency. At the demodulator the fre- quency is detected by a tuned circuit. So, of course, there are two tuned circuits, one for each frequency. Binary 0 is represented by the higher frequency and binary 1 by the lower frequency. Phase shift keying is widely used with data modems, both alone and with hybrid modulation schemes. The most elementary form is binary PSK. In this case there are just two phases which are separated one from the other by 180° phase difference. The demodulator is based on a synchro