f the telephone network is used for data transmission. The frequency band of the voice channel is 300 to 3,400 Hz and the baseband digital information is transferred to this band through CW modu- lation. The CW modulation methods used in voice-band modems are exactly the same as those used for radio transmission (see Chapter 4). As we know, CW modulation may vary three characteristics of a car- rier: amplitude, frequency, or phase. The corresponding basic modulation methods are AM, FM, and PM. All these basic modulation methods are used in the voice-band modems. As we see in Figure 6.13, the only analog section in the connection through a modern telecommunications network is the subscriber line of the local-access network. The fastest standardized voice-band modems can sup- port data rates up to 33.6 Kbps. The maximum user data rate is of the order of 30 Kbps even though the transmission rate inside the PSTN is 64 or 56 Kbps (data rate of PCM coded voice channel). Half of the end-to-end data capacity is wasted because of analog subscriber lines that perform A/D and D/A conversions at both ends. New modems with essentially higher capacity will not be standardized because voice-band modems are already quite close to the theoretical Data Communications Binary data 2-wire analog local access Local Modem = Modulator + Demodulator network exchange Modem converts the digital signal into an Modem analog signal for transmission through 64/128 the voice channel of PSTN. Customer Premises The amplitude, frequency, and/or phase Analog of the carrier wave carry the values of 300-3,400 Hz Public switched the digital signal to the receiving end. telephone network, PSTN The demodulator of the modem converts amplitude, frequency, and/or phase into Binary data 2-wire analog the values of the digital signal. local access :network Local DTE = Data terminal equipment Modem exchange DCE = Data circuit terminating equipment or data communications equipment Customer Premises Analog PCM = Pulse code modulation 300-3,400 Hz Figure 6.13 Mo