unfortunately, i haven't been able to find some valid solution for this problem..
i managed to make my program work by giving large time delay between command and reading the answer.. as you can guess this solution is rather slow.. also, problem is because i have to send few files to ftp server, and for each file sim900 is connecting to server ( password, user name, etc ), and its large waste of time... so for 4 files ( 1 download (30B) , 4 upload (together not more then 200B)) it takes more then 1 and half minute ( and sometimes even 3 or 4).. It could be much faster if these timeouts between commands and answer are shorter, but then the code is very much unreliable. Maybe the solution is not to use FTP commands when you are working with multiple files ( because the connection is established for each file.. ), but it seems complicated do make normal TCP connection ( it takes two different connection, one for data, and other for commands .. )