准备：移植好pppd软件工具（可见下面的参考文章）、ppp拨号脚本、能上网的SIM卡、使能复位好EC20模块、串口初始化、读写的接口（因为CPU的USB接口给占用了，因此这里使用串口与模块通信）
识别、适应运营商原理：每个运营商的SIM卡会有相应规则的IMSI号码。通过AT指令(AT+CIMI)和EC20模块通信，读取SIM卡的IMSI号，然后判断CIMI号是哪个运营商的SIM卡，最后根据运营商的拨号码传递给拨号脚本进行拨号。
判断运营商的接口：根据获取到的IMSI判断运营商
static int get_IMSI(char *IMSI){char s2[128]="AT+CIMI\r\n";char s1[1024]={0};tcflush(ec20_fd,TCIOFLUSH);UART0_Send(ec20_fd, s2,strlen(s2));usleep(500000);int count=UART0_Recv(ec20_fd,5,0,s1,1024);int i=0;#if 0printf("count=%d\n",count);while(count-->0){printf("%c",s1[i++]);}printf("\n");#endifchar *p=s1;while(*p++){// printf("%c ",*p);if(*p>='0'&&*p<='9'){memcpy(IMSI,p,15);break;}}//printf("IMSI:%s\n",IMSI);if(IMSI[0]=='4'&&IMSI[1]=='6'&&IMSI[2]=='0'){if(IMSI[3]=='0' && IMSI[4]=='0' || IMSI[3]=='0' && IMSI[4]=='2' || IMSI[3]=='0' && IMSI[4]=='7'||IMSI[3]=='2'&&IMSI[4]=='0'){printf("中国移动\n");return 0;}if(IMSI[3]=='0' && IMSI[4]=='1' || IMSI[3]=='0' && IMSI[4]=='6' || IMSI[3]=='0' && IMSI[4]=='9' ){printf("中国联通\n");return 1;}if(IMSI[3]=='0' && IMSI[4]=='3' || IMSI[3]=='0' && IMSI[4]=='5' || IMSI[3]=='1' && IMSI[4]=='1' ){printf("中国电信\n");return 2;}printf("code[%c] [%c]\n",IMSI[3],IMSI[3]);return 3;}sleep(1);return 0;}
PPP拨号接口：根据获取到的运营商向PPP拨号脚本传递对应运营商的拨号号码进行拨号，并且执行pppd拨号程序
拨号号码：
移动:ATD991#或981#
联通:ATD*99#
电信:#777（ATD#777）
#define SHELL_PPPD_PATH "/etc/ppp/peers/pppd.sh"#define USER "user"#define PASSWORAD "password"#define EC20 "/dev/ttyPS1"static int ppp_ec20(){char IMSI[128]={0};char buf[1024]={0};int ret=get_IMSI(IMSI);if(ret==0)//ret=execlp("pppd","pppd","call","provider",NULL);{sprintf(buf,"%s %s %d cmnet %s %s ATD*99***1# &",SHELL_PPPD_PATH,EC20,921600,USER,PASSWORAD);}if(ret==1){sprintf(buf,"%s %s %d 3gnet %s %s ATD*99# &",SHELL_PPPD_PATH,EC20,921600,USER,PASSWORAD);}if(ret==2){sprintf(buf,"%s %s %d ctnet %s %s ATD#777 &",SHELL_PPPD_PATH,EC20,921600,USER,PASSWORAD);}change_baud(921600);//AT指令修改模块波特率。因为使用的是串口通信，所以默认的115200速率上网较慢，这里直接修改到最大波特率sleep(1);ret=system(buf);}
手敲的拨号脚本pppd：可传递4个参数进来
#!/bin/shecho "devname baud apn user password"#默认的参数设置DEVNAME=/dev/ttyPS1BAUD=115200APN=ctnetUSER=userPASSWORD=passwordNUM=ATD#777if [ $# -ge 1 ]; thenDEVNAME=$1echo "devname $DEVNAME"fiif [ $# -ge 2 ]; thenBAUD=$2echo "BAUD $BAUD"fiif [ $# -ge 3 ]; thenAPN=$3echo "apn $APN"fiif [ $# -ge 4 ]; thenUSER=$4echo "user $USER"fiif [ $# -ge 5 ]; thenPASSWORD=$5echo "password $PASSWORD"fiif [ $# -ge 6 ]; thenNUM=$6echo "NUM $NUM"fiCONNECT="'chat -s -v ABORT \"BUSY\" ABORT \"NO CARRIER\" ABORT \"NO DIALTONE\" ABORT \"ERROR\" ABORT \"NO ANSWER\" \"\" AT OK ATE0 OK ATI OK AT+CSUB OK AT+CSQ OK AT+CPIN? OK AT+COPS? OK AT+CGREG? OK AT+CGDCONT=1,\\\"IP\\\",\\\"$APN\\\",,0,0 OK $NUM CONNECT'" DISCONNECT="'chat -s -v ABORT ERROR ABORT OK ABORT DELAYED ABORT \"NO CARRIER\" ABORT \"NO DIALTONE\" SAY \"\nSending break to the modem\n\" \"\" +++ \"\" +++ \"\" +++ SAY \"\nGood bay\n\"'"pppd $DEVNAME $BAUD user "$USER" password "$PASSWORD" connect "'$CONNECT'" disconnect "'$DISCONNECT'" nodetach lock \modem \persist \crtscts \usepeerdns \noauth \noipdefault \novj \novjccomp \noccp \defaultroute \ipcp-accept-local \ipcp-accept-remote
编写、执行测试程序调试：
1.先查看SIM卡信号（AT+CSQ指令），良好
2.查看运营商，正确
3.拨号测试，正确识别运营商和成功拨号；输入ifconfig命令后可以看到已经生成ppp0网卡，添加ppp0默认路由后ping外网测试成功。
注意点： 1.不同运营商有不同的apn和拨号号码，要正确选择，否则拨号不成功 2.拨号前用AT指令检查能否接受到基站信号 3.用户名(user)和密码(password)可有可无，不重要。 4.ping测试识别不了域名的时候在/etc/resolv.conf上加上nameserver8.8.8.8/114.114.114.114
参考：
/zhaoxd08501/article/details/72637695
/blog_xu/article/details/99676812
/zhuangquan-p-12191214.html
