Cisco Systems DC-157 manual Configuring Automatic Dialing, Automatically Answering a Modem, DC-159

Modem Signal and Line States

Signal and Line State Diagrams

Configuring Automatic Dialing

With the dialup capability, you can set a modem to dial the phone number of a remote router automatically. This feature offers cost savings because phone line connections are made only when they are needed—you pay for using the phone line only when there is data to be received or sent.

To configure a line for automatic dialing, use the following command in line configuration mode:

Command	Purpose
Router(config-line)# modem dtr-active	Configures a line to initiate automatic dialing.

Using the modem dtr-activecommand causes a line to raise DTR signal only when there is an outgoing connection (such as reverse Telnet, NetWare Asynchronous Support Interface (NASI), or DDR), rather than leave DTR raised all the time. When raised, DTR potentially tells the modem that the router is ready to accept a call.

Automatically Answering a Modem

You can configure a line to answer a modem automatically. You also can configure the modem to answer the telephone on its own (as long as DTR is high), drop connections when DTR is low, and use its Carrier Detect (CD) signal to accurately reflect the presence of carrier. (Configuring the modem is a modem-dependent process.) First, wire the modem CD signal (generally pin-8) to the router RING input (pin-22), then use the following command in line configuration mode:

Command	Purpose
Router(config-line)# modem dialin	Configures a line to automatically answer a modem.

You can turn on modem hardware flow control independently to respond to the status of router CTS input. Wire CTS to whatever signal the modem uses for hardware flow control. If the modem expects to control hardware flow in both directions, you might also need to wire modem flow control input to some other signal that the router always has high, such as the DTR signal.

Figure 26 illustrates the modem dialin process with a high-speed dialup modem. When the Cisco IOS software detects a signal on the RING input of an idle line, it starts an EXEC or autobaud process on that line. If the RING signal disappears on an active line, the Cisco IOS software closes any open network connections and terminates the EXEC facility. If the user exits the EXEC or the software terminates because of no user input, the line makes the modem hang up by lowering the DTR signal for 5 seconds. After 5 seconds, the modem is ready to accept another call.

Cisco IOS Dial Technologies Configuration Guide

DC-159