modem communication

[s m]

hello guys,

please forgive me if my question is not related to this list but i don't
know where i should ask about it.

i have a freebsd8.2 and wanna talk my modem by com port. when i connect my
modem to com port, everything is ok.

for some reasons, i have to communicate with modem, run a command, close
the connection, connect again, run another command, close connection,... .
in the other words, in each communication i have to run just one ati
command and close connection.

my question is: are there any ati commands in modems that should be run
continuously (in the same communication)?  i mean running one command,
close connection, open it again and run another command would make any
wrong?

my program should work with all modems not a specific one.

any hints or comments are really appreciated.
sam