SIM900发送AT+SAPBR=1,1返回异常！

SIM900发送AT+SAPBR=1,1返回错误！！！
用单片机给SIM900发送指令和结果返回如下，到第三条AT+SAPBR=1,1时，就返回+CME ERROR: operation not allowed，求高手赐教啊，在线等！！！
AT

OK
AT+SAPBR=3,1,"Contype","GPRS"

OK
AT+SAPBR=3,1,"APN","CMNET"

OK
AT+SAPBR=1,1

+CME ERROR: operation not allowed
AT+SAPBR=2,1

+SAPBR: 1,3,"0.0.0.0"

OK
AT+HTTPINIT

OK
AT+HTTPPARA="CID",1

OK
AT+HTTPPARA="URL","www.baidu.com"

OK
AT+HTTPACTION=0

OK

+HTTPACTION:0,601,0
AT+HTTPREAD

OK

------解决方案--------------------
建议先执行AT+CSTT AT+CIICR AT+CIFSR后在执行AT+SAPBR=1,1
我的代码如下，可以获得数据
AT+CSQ

AT+CREG?

AT+CGATT?;

AT+SAPBR=3,1,"CONTYPR","GPRS"

AT+SAPBR=3,1,"APN","CMNET"

AT+SAPBR=1,1

AT+HTTPINIT

AT+HTTPPARA="URL","www.baidu.com/images/favicon.png"

AT+HTTPACTION=0 
  
AT+HTTPREAD