1.启动CMUX模式

通常模块发送AT＋CMUX命令来激活多路复用，该命令格式为：

命令类型	语法	返回
设置命令	AT+CMUX= [<mode>[,<subset>[,<port_speed>[,<N1>[,<T1>[,<N2>[,<T2>[,<T3>[,<k>]]]]]]]]]	OK
查询命令	AT+CMUX?	+CMUX:[<mode>[,<subset>[,<port_speed>[,<N1>[,<T1>[,<N2>[,<T2>[,<T3>[,<k>]]]]]]]]]   OK
测试命令	AT+CMUX=?	+CMUX:(<mode>),(<subset>s),( <port_speed>s),(<N1>s),(<T1>s),(<N2>s),(<T2>s),(<T3>s),(<k>s)    OK OK

 

参数定义：

参数	定义	取值	对取值的说明
	复用透传机制	0	基本选择
	控制通道的复用器建立的方式	0	仅使用UIH 包文
	传输速率，单位为bits/s	1	9600
		2	19200
		3	38400
		4	57600
		5	115200
		6	230400
		7	460800
	最大报文大小	1~255	默认：127
	接收timer，以10ms为单位	1-255	默认：10(100 ms)
	重传的最大数量	0-100	默认：3
	复用器控制通道的响应定时器，以10ms 为单位	2-255	默认：30
	唤醒响应定时器，以秒为单位	1-255	默认：10
	窗口大小，适用于有错误恢复选项的高级操作	1-7	默认：2

2.CMUX 帧结构 

多路复用有三种操作模式：基本模式、带错误恢复功能的高级模式、不带错误恢复功能的高级模式（具体可以参见GSM 07.10协议）。不同的GPRS模块也支持不同的模式。

 

其中，帧类型可分控制帧和信息帧。

CMUX双方通过控制帧协商用以构建、拆除虚拟链路，控制帧又为以下几种：

SABM：建立DLC

UA：响应SABM帧或DISC帧

DM：链路未成功建立时，对收到的DISC命令的响应

DISC：通知对端拆除链接，在DLC0发送DISC帧，等于退出MUX功能

 

信息帧：

UIH\UI\I：这三种是信息帧，也就是携带要传输数据的帧。

3.LINUX下的CMUX使用

3.1 挂载USB设备

在可移动设备，勾选QinHeng USB2.0-Serial

然后打开终端控制台，ls /dev/ttyUSB*查看USB设备，如下表示挂载成功。

3.2 CMUX虚拟串口生成

参考用例：https://www.kernel.org/doc/html/latest/driver-api/serial/n_gsm.html

3.2.1 测试实例gsmmux.h源代码

#ifndef _LINUX_GSMMUX_H
#define _LINUX_GSMMUX_H
 
struct gsm_config
{
unsigned int adaption;
unsigned int encapsulation;
unsigned int initiator;
unsigned int t1;
unsigned int t2;
unsigned int t3;
unsigned int n2;
unsigned int mru;
unsigned int mtu;
unsigned int k;
unsigned int i;
unsigned int unused[8];	/* Padding for expansion without
   breaking stuff */
};
 
#define GSMIOC_GETCONF	_IOR('G', 0, struct gsm_config)
#define GSMIOC_SETCONF	_IOW('G', 1, struct gsm_config)
 
struct gsm_netconfig {
unsigned int adaption;  /* Adaption to use in network mode */
unsigned short protocol;/* Protocol to use - only ETH_P_IP supported */
unsigned short unused2;
char if_name[IFNAMSIZ];	/* interface name format string */
__u8 unused[28];        /* For future use */
};
 
#define GSMIOC_ENABLE_NET      _IOW('G', 2, struct gsm_netconfig)
#define GSMIOC_DISABLE_NET     _IO('G', 3)
 
 
#endif

3.2.2 测试实例cmux.c 源代码

#include <stdlib.h>
#include <stdio.h>
#include <stdarg.h>
#include <string.h>
#include <sys/ioctl.h>
#include
#include
#include
#include
#include
#include
#include
#include
#include
#include
#include
/**
*	gsmmux.h provides n_gsm line dicipline structures and functions.
*	It should be kept in sync with your kernel release.
*/
#include "gsmmux.h"
 
/* n_gsm ioctl */
#ifndef N_GSM0710
# define N_GSM0710	21
#endif
 
/* attach a line discipline ioctl */
#ifndef TIOCSETD
# define TIOCSETD	0x5423
#endif
 
/* serial port of the modem */
//#define SERIAL_PORT	"/dev/ttyUSB0"
 
/* line speed */
#define LINE_SPEED	B115200
 
/* maximum transfert unit (MTU), value in bytes */
#define MTU	512
 
/**
* whether or not to create virtual TTYs for the multiplex
*	0 : do not create
*	1 : create
*/
#define CREATE_NODES	1
 
/* number of virtual TTYs to create (most modems can handle up to 4) */
#define NUM_NODES	4
 
/* name of the virtual TTYs to create */
#define BASENAME_NODES	"/dev/ttyGSM"
 
/* name of the driver, used to get the major number */
#define DRIVER_NAME	"gsmtty"
 
/**
* whether or not to print debug messages to stderr
*	0 : debug off
*	1 : debug on
*/
#define DEBUG	1
 
/**
* whether or not to detach the program from the terminal
*	0 : do not daemonize
*	1 : daemonize
*/
#define DAEMONIZE	1
 
 /* size of the reception buffer which gets data from the serial line */
#define SIZE_BUF	256
 
 
/**
*	Prints debug messages to stderr if debug is wanted
*/
static void dbg(char *fmt, ...) {
va_list args;
 
if (DEBUG) {
fflush(NULL);
va_start(args, fmt);
vfprintf(stderr, fmt, args);
va_end(args);
fprintf(stderr, "\n");
fflush(NULL);
}
return;
}
 
/**
*	Sends an AT command to the specified line and gets its result
*	Returns  0 on success
*	-1 on failure
*/
int send_at_command(int serial_fd, char *command, int max_retry) {
char buf[SIZE_BUF];
int r;
int retry = 0;
 
do{
/* write the AT command to the serial line */
if (write(serial_fd, command, strlen(command)) <= 0)
err(EXIT_FAILURE, "Cannot write to ttyUSB");
/* wait a bit to allow the modem to rest */
sleep(1);
 
/* read the result of the command from the modem */
memset(buf, 0, sizeof(buf));
r = read(serial_fd, buf, sizeof(buf));
if (r == -1)
{
dbg("read errno %d", errno);
retry++;
dbg("retry write time: %d", retry);
}
/* if there is no result from the modem, return failure */
if (r == 0) {
dbg("%s\t: No response", command);
retry++;
//return -1;
}
if(retry!= 0 && retry >= max_retry)
{
err(EXIT_FAILURE, "Cannot read ttyUSB");
}
else
{
break;
}
}while(1);
 
 
 
/* if we have a result and want debug info, strip CR & LF out from the output */
if (DEBUG) {
int i;
char bufp[SIZE_BUF];
memcpy(bufp, buf, sizeof(buf));
for(i=0; i
if (bufp[i] == '\r' || bufp[i] == '\n')
bufp[i] = ' ';
}
dbg("%s\t: %s", command, bufp);
}
 
/* if the output shows "OK" return success */
if (strstr(buf, "OK\r") != NULL) {
return 0;
}
return -1;	
 
}
 
 
/**
*	Gets the major number of the driver device
*	Returns  the major number on success
*	-1 on failure
*/
int get_major(char *driver) {
 
FILE *fp;
char *line = NULL;
size_t len = 0;
ssize_t read;
char device[20];
int major = -1;
/* open /proc/devices file */
if ((fp = fopen("/proc/devices", "r")) == NULL)
err(EXIT_FAILURE, "Cannot open /proc/devices");
 
/* read the file line by line */
while ((major == -1) && (read = getline(&line, &len, fp)) != -1) {
/* if the driver name string is found in the line, try to get the major */
if (strstr(line, driver) != NULL) {
if (sscanf(line,"%d %s\n", &major, device) != 2)
major = -1;
}
/* free the line before getting a new one */
if (line) {
free(line);
line = NULL;
}
 
}
 
/* close /proc/devices file */
fclose(fp);
 
return major;
}
 
/**
*	Creates nodes for the virtual TTYs
*	Returns the number of nodes created
*/
int make_nodes(int major, char *basename, int number_nodes) {
 
int minor, created = 0;
dev_t device;
char node_name[15];
mode_t oldmask;
 
/* set a new mask to get 666 mode and stores the old one */
oldmask = umask(0);
 
for (minor=1; minor
 
/* append the minor number to the base name */
sprintf(node_name, "%s%d", basename, minor);
/* store a device info with major and minor */
device = makedev(major, minor);
/* create the actual character node */
if (mknod(node_name, S_IFCHR | 0666, device) != 0) {
warn("Cannot create %s", node_name);
} else {
created++;
dbg("Created %s", node_name);
}
 
}
 
/* revert the mask to the old one */
umask(oldmask);
 
return created;
}
 
/**
*	Removes previously created TTY nodes
*	Returns nothing, it doesn't really matter if it fails
*/
void remove_nodes(char *basename, int number_nodes) {
 
int node;
char node_name[15];
 
for (node=1; node
 
/* append the minor number to the base name */
sprintf(node_name, "%s%d", basename, node);
/* unlink the actual character node */
dbg("Removing %s", node_name);
if (unlink(node_name) == -1)
warn("Cannot remove %s", node_name);
 
}
 
return;
}
 
/**
*	Function raised by signal catching
*/
void signal_callback_handler(int signum) {
remove_nodes(BASENAME_NODES, NUM_NODES);
return;
}
int main(int argc, char** argv) {
 
int serial_fd, major;
struct termios tio;
int ldisc = N_GSM0710;
struct gsm_config gsm;
char atcommand[40];
 
char* port;
 
if(argc != 2)
{
printf("usage\n ./cmux /dev/ttyUSBx\n");
exit(-1);
}
port = argv[1];
 
/* print global parameters */
dbg("SERIAL_PORT = %s", port);
 
/* open the serial port */
serial_fd = open(port, O_RDWR | O_NOCTTY | O_NDELAY);
if (serial_fd == -1)
{
dbg("openport ret = %d\n", errno);
err(EXIT_FAILURE, "Cannot open %s", port);
}
/* get the current attributes of the serial port */
if (tcgetattr(serial_fd, &tio) == -1)
err(EXIT_FAILURE, "Cannot get line attributes");
/* set the new attrbiutes */
tio.c_iflag = 0;
tio.c_oflag = 0;
tio.c_cflag = CS8 | CREAD | CLOCAL;
tio.c_cflag &= (~CRTSCTS);
tio.c_lflag = 0;
tio.c_cc[VMIN] = 1;
tio.c_cc[VTIME] = 0;
/* write the speed of the serial line */
if (cfsetospeed(&tio, LINE_SPEED) < 0 || cfsetispeed(&tio, LINE_SPEED) < 0)
err(EXIT_FAILURE, "Cannot set line speed");
/* write the attributes */
if (tcsetattr(serial_fd, TCSANOW, &tio) == -1)
err(EXIT_FAILURE, "Cannot set line attributes");
 
/**
*	Send AT commands to put the modem in CMUX mode.
*	This is vendor specific and should be changed
*	to fit your modem needs.
*	The following matches Quectel M95.
*/
//if (send_at_command(serial_fd, "AT+IFC=2,2\r") == -1)
//	errx(EXIT_FAILURE, "AT+IFC=2,2: bad response");
if (send_at_command(serial_fd, "AT+CGMM\r", 3) == -1)
warnx("AT+GMM: bad response");
if (send_at_command(serial_fd, "AT\r", 0) == -1)
warnx("AT: bad response");
sprintf(atcommand, "AT+CMUX=0,0,5\r");
if (send_at_command(serial_fd, atcommand, 0) == -1)
errx(EXIT_FAILURE, "Cannot enable modem CMUX");
 
/* use n_gsm line discipline */
sleep(2);
if (ioctl(serial_fd, TIOCSETD, &ldisc) < 0)
err(EXIT_FAILURE, "Cannot set line dicipline. Is 'n_gsm' module registred?");
 
/* get n_gsm configuration */
if (ioctl(serial_fd, GSMIOC_GETCONF, &gsm) < 0)
err(EXIT_FAILURE, "Cannot get GSM multiplex parameters");
 
/* set and write new attributes */
gsm.initiator = 1;
gsm.encapsulation = 0;
gsm.mru = MTU;
gsm.mtu = MTU;
gsm.t1 = 10;
gsm.n2 = 3;
gsm.t2 = 30;
gsm.t3 = 10;
 
if (ioctl(serial_fd, GSMIOC_SETCONF, &gsm) < 0)
err(EXIT_FAILURE, "Cannot set GSM multiplex parameters");
dbg("Line dicipline set");
/* create the virtual TTYs */
if (CREATE_NODES) {
int created;
if ((major = get_major(DRIVER_NAME)) < 0)
errx(EXIT_FAILURE, "Cannot get major number");
if ((created = make_nodes(major, BASENAME_NODES, NUM_NODES)) < NUM_NODES)
warnx("Cannot create all nodes, only %d/%d have been created.", created, NUM_NODES);
}
 
/* wait to keep the line discipline enabled, wake it up with a signal */
signal(SIGINT, signal_callback_handler);
signal(SIGTERM, signal_callback_handler);
/* detach from the terminal if needed */
if (DAEMONIZE) {
dbg("Going to background");
if (daemon(0,0) != 0)
err(EXIT_FAILURE, "Cannot daemonize");
}
 
dbg("catching sig");
 
pause();
/* remove the created virtual TTYs */
if (CREATE_NODES) {
remove_nodes(BASENAME_NODES, NUM_NODES);
}
 
/* close the serial line */
close(serial_fd);
 
return EXIT_SUCCESS;
}

3.2.3 生成cmux可执行文件

通执行gcc cmux.c -o cmux 把cmux.c编译成可执行文件

3.2.4 虚拟串口生成

通执行 ./cmux /dev/ttyUSB0 就会创建几个虚拟串口

下图中的ttyGSM1,ttyGSM2,ttyGSM3,ttyGSM4及为虚拟出的串口

4.常见问题处理

4.1 常见AT 错误处理

命令	错误情况	错误原因	处理
AT+CMUX	+CME ERROR: 980	参数输入错误	检查一下输入参数是否正确

4.2 VMware无法识别USB设备的解决方法

我的虚拟机上认不到USB设备，发现是VMware USB Arbitration Service服务没有启动，手动启动VMware USB Arbitration Service服务，再重启VMware，虚拟机就能识别出USB设备了。

具体步骤：
1.点击开始->运行，在对话框中输入"services.msc"，确定，打开windows服务管理器。

2.在服务列表中选中"VMware USB Arbitration Service"，双击打开属性对话框，再选择"启动"，就能启动VMware USB Arbitration Service服务了。

3.关闭VMware软件，并重新打开，启动一下虚拟机。

4.3 虚拟串口生成失败

如下图所示

出现原因是没有选择正确的USB口，我们720SL开发板上的开关要拨到UART2一端，出现上面的错误的原因是因为开关要拨到UART1一端了，把开关拨到UART2一端，重启一下模块，然后在做上述操作。