nt requires the RS-449 interface on all data com- munications products that it acquires. Industry has been sluggish about adopting the improved standard for commercial applications. 2.5 MODEM FEATURES Modems are available with several useful features that benefit spe- cific applications. Most high-speed modems are available with a mul- tiport feature, to accommodate two or more (typically four) terminals or lines with aggregate data rates that do not exceed the modem's maximum data rate. MODEMS AND MULTIPLEXERS 21 Modem RS-232 Connector Transmit Receive Fig. 2-3. Modems are supplied with a simple cable terminal with spade lugs, which are either two- or four-wire depending on network type. The telephone company provides a ter- minal block or an 829 four-wire terminating set. The interface to the terminal is RS-232 or RS-449. A reverse channel is an economical alternative to full-duplex com- munications on a four-wire line to support message acknowledge- ment. The narrowband simplex channel eliminates turnaround time for message acknowledgement with the economy of a two-wire line. A secondary channel is a full-duplex narrowband channel that typ- ically supports data rates up to 150 bps and shares the bandwidth of a four-wire line with the main channel. The out-of-band channel is useful for several applications. For example, it can be used as a data channel for a low-speed terminal to eliminate the cost of another line. Alternate voice data eliminates extra charges for voice-coordi- nated transmissions. The feature includes a telephone handset and dialer with switch selection of voice or data. 22 TELECOMMUNICATIONS Auto answer automatically answers incoming calls over the DDD network for unattended sites. Dial backup restores communications interrupted by a failure on a dedicated facility by reestablishing com- munications over the DDD network. The dial-up connection is es- tablished at the central site via a conventional telephone; remote site equipment automatically answers an incoming call and switches the modem 