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Modem Signal and Line States
Signal and Line State Diagrams
DC-163
Cisco IOS Dial Technologies Configuration Guide
Configuring a Line to Disconnect Automatically
To configure automatic line disconnect, use the following command in line configuration mode:
The autohangup command causes the EXEC facility to issue the exit command when the last
connection closes. This feature is useful for UNIX-to-UNIX copy program (UUCP) applications because
UUCP scripts cannot issue a command to hang up the telephone. This feature is not used often.
Supporting Reverse Modem Connections and Preventing Incoming Calls
In addition to initiating connections, the Cisco IOS software can receive incoming connections. This
capability allows you to attach serial and parallel printers, modems, and other shared peripherals to the
router or access server and drive them remotely from other modem-connected systems. The Cisco IOS
software supports reverse TCP, XRemote, and local-area transport (LAT) connections.
The specific TCP port or socket to which you attach the device determines the type of service that the
Cisco IOS software provides on a line. When you attach the serial lines of a computer system or a data
terminal switch to the serial lines of the access server, the access server can act as a network front-end
device for a host that does not support the TCP/IP protocols. This arrangement is sometimes called
front-ending or reverse connection mode.
The Cisco IOS software supports ports connected to computers that are connected to modems. To
configure the Cisco IOS software to function somewhat like a modem, use the following command in
line configuration mode. This command also prevents incoming calls.
Figure 29 illustrates the modem callout process. When the Cisco IOS software receives an incoming
connection, it raises the DTR signal and waits to see if the CTS signal is raised to indicate that the host
has noticed the router DTR signal. If the host does not respond within the interval set by the modem
answer-timeout line configuration command, the software lowers the DTR signal and drops the
connection.
Command Purpose
Router(config-line)# autohangup
Configures automatic line disconnect.
Command Purpose
Router(config-line)# modem callout
Configures a line for reverse connections and prevents incoming
calls.