Hello Mr. Das,
I am carrying out similar type of project. It is very simple to implemet SMS based control system using 8051. You can carry out the project as follows
(i) Pick up any mobile handset from market.
(ii) Each handset has got a RS-232 serial interface. You needs to its connector diagram. Here you have two option
(a)If tell me mobile set model no then I can send you its serial port connection diagram. Most mobile outputs TTL logic level voltages for RS-232 signal. So if you are connecting this to 8051 no need for transrec/level trans like MAX 232. use/ buy appro connector for mobile u are using.
(b) PC serial interface adapter for most mobile are avail in Rs 200-Rs 250 range. Its level are as per RS-232 stn(+- 12 v). You can easily convert them to TTL for 8051(using MAX 232 , 1488 , 1489).
(3) Most mobile serial interface work like a modem ie if you type AT mobile will respond with OK. There is a complete set of SMS AT command set(that i will send you). Using this SMS AT command sey you can send & receieve SMS msg like AT <desn details> , <msg>
Using this SMS message you can send & receieve you own custom command to & from controller. like you can form you command synta struct like

<Commnd> , <para1> ,<para2>,........
<-------your cus cmd in form of SMS-->
for example
SENDDATA CHAN1 , CHAN2
ONRELAY RELAY1
remember SMS max string length is around 240 chara.

Anil