User guide

A-40 Cisco 2500 Series Access Server User Guide
Modems
Hardware Flow Control (RTS/CTS)
Hardware flow control is used between the access server and the modem to start and stop
data transfer on the TX/RX wire pair. Hardware flow control is used to prevent the loss of
data when buffers are full. Hardware flow control is controlled on the RTS/CTS wire pair.
The conditions that must be met when setting hardware flow control are as follows:
• Both the access server asynchronous port and the modem must be configured for
hardware flow control.
• Software flow control must be turned off for packet data, because it can cause
communications to stop unexpectedly.
• No flow control on either end will cause loss of data.
Modem Control (DTR/DCD)
Modem control is used between the access server and the modem to initiate and end calls.
Modem control is performed on the DTR/DCD wire pair. The conditions that must be met
when setting modem control are as follows:
• The modem should be configured so that if DTR is dropped and then raised, it will
terminate any calls and return to its stored settings. This configuration is the standard
for EIA/TIA-232 operations.
• The modem should be configured to only send the DCD signal to the access server when
an active call has been negotiated and is established.
Configuring the Asynchronous Port
This section describes how to configure the Cisco 2500 series access server for use with a
modem, and includes information on configuring the line and security commands.
Full configuration of an access server asynchronous port requires the configuration of two
entities—the line and the asynchronous interface. The asynchronous interface is created
and configured for support of Serial Line Internet Protocol (SLIP) and PPP. (See the section
“Asynchronous Protocols” later in this appendix.)