Ports
Modems
5-9
Ports that support V.32bis should be set to 19200 baud. Also, ports that use compression should be
set one or two speeds higher than the modem’s speed. For example, a port supporting a 2400 baud
modem with compression should be set to 4800 or 9600 baud.
2
Use the autobaud feature of the ETS; the ETS will adjust its speed to that of the modem.
Flow control isn’t strictly required in this configuration. Applications that cannot use flow control due
to limitations listed in the Parity, Character Size, and Stop Bits section on page 5-4 may still function
correctly, but if at all possible, flow control is still recommended.
Nonstandard transmission rates such as V.32bis (14,400 baud), error correction, and data compression
cannot be supported with this method. During a long data transfer, eventually either the ETS or the
modem will run out of buffer space and overflow the other, resulting in data loss.
3
If only one standard baud rate is to be supported, set the ETS to that rate and disable autobaud. Flow
control may be disabled, subject to the same caveats discussed above.
5.2.4 Modem Configuration
In dialout situations, it is possible for users to reconfigure the modem with normal “at” commands. It is also
possible for a user to misconfigure a modem such that it will be unavailable, unusable, or cause security
problems. Fortunately, many modems can reload their saved configuration each time DTR is dropped by
the ETS. The modem will always be configured correctly after each use provided the user doesn’t modify
the saved settings.
5.2.5 Modem Result Codes
If the modem is configured to return result codes or status messages, the ETS and the modem may start to
“babble.” A message from the modem will be interpreted as a user logging into the ETS, and the ETS will
send a prompt to the modem. The modem will interpret this prompt as a command and send an error message
to the ETS. This process causes the port and modem to become unavailable, and can be cleared temporarily
by logging the port out.
The best solution to this potential problem is to configure the modem so that it never returns result codes.
Unfortunately, the lack of status messages may make outgoing connections inconvenient. If the modem
supports displaying messages only during outbound connections, babbling can usually be avoided.
5.2.6 Modems with SLIP
If Modem Control is defined on a port set for SLIP autostart, SLIP is handled slightly differently. In this
case, SLIP does not start until the port sees assertion of the DSR signal, which generally occurs when the
attached local modem has formed a connection with a remote modem.
Until DSR is asserted, the port remains idle (in non-SLIP mode). It can be used as a service port until SLIP
is started. After SLIP is started, the SLIP session closes if the attached modem device de-asserts the DSR
signal. For more information on SLIP configuration, see SLIP on page 4-6.
