Specifications

Overview of Modem Interfaces
Logical Constructs in Modem Configurations
DC-61
Cisco IOS Dial Technologies Configuration Guide
For example, a laser printer is attached to line 10 of a Cisco 2511 router. Such a printer usually uses
XON/XOFF software flow control. Because the Cisco IOS software cannot receive an incoming
connection if the line already has a process, you must ensure that an EXEC session is not accidentally
started. You must, therefore, configure it as follows:
line 10
flowcontrol software
no exec
A host that wants to send data to the printer would connect to the router on TCP port 4008, send the data,
and then close the connection. (Remember that line number 10 octal equals 8 decimal.)
Signal and Flow Control Overview
The EIA/TIA-232 output signals are Transmit Data (TXDATA), Data Terminal Ready (DTR), and Ready
To Send (RTS—Cisco 2500 routers only). The input signals are Receive Data (RXDATA), Clear to Send
(CTS), and RING. The sixth signal is ground. Depending on the type of modem control your modem
uses, these names may or may not correspond to the standard EIA/TIA-232 signals.
Dialup modems that operate over normal telephone lines at speeds of 28800 bps use hardware flow
control to stop the data from reaching the host by toggling an EIA/TIA-232 signal when their limit is
reached.
In addition to hardware flow control, modems require special software configuring. For example, they
must be configured to create an EXEC session when a user dials in and to hang up when the user exits
the EXEC. These modems also must be configured to close any existing network connections if the
telephone line hangs up in the middle of a session.
The Cisco IOS software supports hardware flow control on its CTS input signal, which is also used by
the normal modem handshake.