Hello,
Please check the GSM interfacing projects and tutorial on this website and your problem will be solved. Pls note the GSM modem also sends some additional characters with the data like “OK”. They are some special characters like line feed, carriage return etc and hence there is garbage data.
There is a standard pattern of sending and reciveing data to the modem. When you recieve the data you need to remove/filter/reject/some of the bytes to discard the unwanted data and capture only the data you want. Similar is the case while sending the data to the modem. In sending you have to add the charchters.
Please go through the GSM based work on the website in the 8051 section.