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Modem Signal and Line States
Signal and Line State Diagrams
DC-159
Cisco IOS Dial Technologies Configuration Guide
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:
Using the modem dtr-active command 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:
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.
Command Purpose
Router(config-line)# modem dtr-active
Configures a line to initiate automatic dialing.
Command Purpose
Router(config-line)# modem dialin
Configures a line to automatically answer a modem.