r example, ISDN instead of speech channels. Note that data transmission with a voice-band modem does not require anything other than just a modem at the end of the subscriber line and an analog voice-band circuit through the network. The voice-band modems that we have discussed in this section use the telephone network exactly the same way as ordinary telephones. A V.90 modem provides a 56-Kbps data rate to subscriber premises (downstream) and a lower 33.6-Kbps data rate transmission in the opposite direction. This device is not actually a voice-band modem because the down- stream data are not modulated to the speech channel and carried through analog telephone channels the same way as speech is. The source machine transmits data over a digital connection to the network and at the other end a PCM encoder converts the digital data stream into analog signals for sub- scriber loop from which the receiving device reproduces the data stream. Modulation to the analog speech channel is not carried out in this direction. In upstream transmission a voice channel is used and the data rate is restricted to 33.6 Kbps. The interfaces of a voice-band modem are shown in Figure 6.14. Exter- nal modems support a standardized physical interface (usually RS-232C, EIA-232D, or V.24) via which data are exchanged usually as asynchronous frames, as we saw in Section 6.1.3. A binary 0 corresponds to a voltage of +3V to +15V and binary 1 to -3V to -15V. Separate wires are dedicated to the control signals that are used to control data flow between devices. The two example control signals in Figure 6.14 are used for handshaking between modem (DCE) and terminal (DTE) in the following manner. When a terminal wants to send data, it indi- cates that by setting the request to send (RTS) signal on (+3V...+15V) and modem responses by setting the clear to send (CTS) signal on. If a terminal transmits data too fast, the modem sets CTS off and while it is off the termi- nal does not transmit. Many other control signals have been defined and reader m