Cisco Systems DC-157 User Manual
Modem Signal and Line States
This chapter describes modem states in the following section:
• Signal and Line State Diagrams
To identify the hardware platform or software image information associated with a feature, use the
Feature Navigator on Cisco.com to search for information about the feature or refer to the software
release notes for a specific release. For more information, see the “Identifying Supported Platforms”
section in the “Using Cisco IOS Software” chapter.
For a complete description of the modem support commands in this chapter, refer to the Cisco IOS
Modem Command Reference. To locate documentation of other commands that appear in this chapter,
use the command reference master index or search online.
Signal and Line State Diagrams
The following signal and line state diagrams accompany some of the tasks in the following sections to
illustrate how the modem control works:
• Configuring Automatic Dialing
• Automatically Answering a Modem
• Supporting Dial-In and Dial-Out Connections
• Configuring a Line Timeout Interval
• Closing Modem Connections
• Configuring a Line to Disconnect Automatically
• Supporting Reverse Modem Connections and Preventing Incoming Calls
Cisco IOS Dial Technologies Configuration Guide
DC-157
Signal and Line State Diagrams
Ready
c
S1201a
The diagrams show two processes:
• The “create daemon” process creates a tty daemon that handles the incoming network connection.
• The “create EXEC” process creates the process that interprets user commands. (See Figure 25
In the diagrams, the current signal state and the signal the line is watching are listed inside each box.
The state of the line (as displayed by the show line EXEC command) is listed next to the box. Events
that change that state appear in italics along the event path, and actions that the software performs are
described within ovals.
Figure 25 illustrates line states when no modem control is set. The DTR output is alw ays high, and CTS
and RING are completely ignored. The Cisco IOS software starts an EXEC session when the user types
the activation character. Incoming TCP connections occur instantly if the line is not in use and can be
closed only by the remote host.
Figure 25 EXEC and Daemon Creation on a Line with No Modem Control
Modem Signal and Line States
through Figure 29.)
DTR high
Network
onnection
closed
DTR high
Ready and active
Incoming
network
connection
Create
daemon
User-typed
activation
character
Create
EXEC
Exit
DTR high
Ready and active
Ringing
Cisco IOS Dial Technologies Configuration Guide
DC-158
Modem Signal and Line States
Configuring Automatic Dialing
With the dialup capability, you can set a modem to dial the phone number of a remote router
automatically. This feature of fers 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
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.
Configures a line to initiate automatic dialing.
Automatically Answering a Modem
Signal and Line State Diagrams
You can configure a line to answer a modem automatically. Y ou also can conf igure 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 in put
(pin-22), then use the following command in line configuration mode:
Command Purpose
Router(config-line)# modem dialin
You can turn on modem hardware flow control independently to respond to the status of router CTS
input. Wire CTS t o what ever signal the modem uses for hardware flow control. If the modem expects to
control hardware flow in b oth directions, you mig ht also need to wir e modem flo w cont rol input t o 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 in put of an idle lin e, 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.
Configures a line to automatically answer a modem.
Cisco IOS Dial Technologies Configuration Guide
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