在上一节中我们知道3G卡设备驱动已经加载好了，并且和ttyUSB*已经绑定成功，意味着我们可以拨号了，和3G卡内部3G模块通信了，而我们知道3G模块通信是tty设备，通过串行设备，这里和3G卡的两种工作模式相吻合---moden模式.
首先我们这里先给出3G工作原理图
我们万事俱备只欠东风了，就是pppd拨号了，当然pppd的源码自己可以去网上下载最新的，自己编译（这里我们只说linux环境的）.安装好pppd程序后，我们运行pppd拨号，当然在拨号前我们需要配置下我们的pppd才能完美的工作，才能真正为我们对于的tty驱动和3g驱动运作起来.
主要看pppd运行参数文件/etc/ppp/options(这个参数文件名字，看开发者自己是可以修改的）该文件指定pppd运行的参数，若运行pppd时通过命令行指定参数同时出现时，则选择/etc/ppp/options中的配置.下面我们就看相关的参数：
#tty Options
/dev/ttyS1 #这里可以将/dev/ttyS1注释，然后使用命令行pppd /dev/ttyS1 运行来指定；
115200 #传输速率
lock #创建一个锁定文件，其他程序在发现存在这个文件后，就能得知相应的串口已经被使用。
modem #modem设备
crtscts #硬件流控为，无硬件流控为nocrtscts
#login
nodetach #若指定updetach则拨号成功后放入后台运行，若为nodetach,则在前台执行
debug #如果需要加入调试信息，就加入参数debug
#auth
require-pap
refuse-chap
name na_admin
connect "/usr/sbin/chat -v -E -V -f /etc/ppp/pstn-start-chat"
disconnect 'chat -s -v -f disconnect-chat'
...
当然这些信息我们都可以从pppd源码包的帮助文件中获取信息，比如readme等.
# 源码ppp-2.4.5/pppd/tty.c中的函数connect_tty()将会执行这个脚本。主要是对客户端的拨号的应答。
connect "/usr/sbin/chat -v -E -V -f /etc/ppp/pstn-start-chat"
配置信息中我只说明/dev/ttyS1 这个tty名字是我们自己设定，看我们3g驱动和哪一个tty设备绑定到哪一个作为通信用.电信或者联通（默认华为3G卡，evdo）则为ttyUSB0，因为我们在3G卡驱动识别过程中会见到3g attach ttyUSB*等信息. 还有就是在pppd运行开始，要初始化tty. 就是调用tty_init（）.
下面给出pppd相关代码:
/*
* Initialize each protocol.
*/
for (i = 0; (protp = protocols[i]) != NULL; ++i)
(*protp->init)(0);
/*
* Initialize the default channel.
*/
tty_init();
展开tty_init()：
void tty_init()
{
add_notifier(&pidchange, maybe_relock, 0);
the_channel = &tty_channel;
xmit_accm[3] = 0x60000000;
}
我们看到里面the_channel = &tty_channel;的操作,struct channel *the_channel;（main.c中）是一个全局变量.
/*
* This struct contains pointers to a set of procedures for
* doing operations on a "channel". A channel provides a way
* to send and receive PPP packets - the canonical example is
* a serial port device in PPP line discipline (or equivalently
* with PPP STREAMS modules pushed onto it).
*/
struct channel {
/* set of options for this channel */
option_t *options;
/* find and process a per-channel options file */
void (*process_extra_options) __P((void));
/* check all the options that have been given */
void (*check_options) __P((void));
/* get the channel ready to do PPP, return a file descriptor */
int (*connect) __P((void));
/* we're finished with the channel */
void (*disconnect) __P((void));
/* put the channel into PPP `mode' */
int (*establish_ppp) __P((int));
/* take the channel out of PPP `mode', restore loopback if demand */
void (*disestablish_ppp) __P((int));
/* set the transmit-side PPP parameters of the channel */
void (*send_config) __P((int, u_int32_t, int, int));
/* set the receive-side PPP parameters of the channel */
void (*recv_config) __P((int, u_int32_t, int, int));
/* cleanup on error or normal exit */
void (*cleanup) __P((void));
/* close the device, called in children after fork */
void (*close) __P((void));
};
extern struct channel *the_channel;
在ppp.h中定义，并声明.
下面看看如何初始化的
struct channel tty_channel = {
tty_options,
&tty_process_extra_options,
&tty_check_options,
&connect_tty,
&disconnect_tty,
&tty_establish_ppp,
&tty_disestablish_ppp,
&tty_do_send_config,
&tty_recv_config,
&cleanup_tty,
&tty_close_fds
};
同理这里我们只关注&connect_tty, 这个初始化在tty.c中.
/*
* connect_tty - get the serial port ready to start doing PPP.
* That is, open the serial port, set its speed and mode, and run
* the connector and/or welcomer.
*/
int connect_tty()
{
char *connector;
int fdflags;
#ifndef __linux__
struct stat statbuf;
#endif
char numbuf[16];
/*
* Get a pty master/slave pair if the pty, notty, socket,
* or record options were specified.
*/
strlcpy(ppp_devnam, devnam, sizeof(ppp_devnam));
pty_master = -1;
pty_slave = -1;
real_ttyfd = -1;
if (using_pty || record_file != NULL) {
if (!get_pty(&pty_master, &pty_slave, ppp_devnam, uid)) {
error("Couldn't allocate pseudo-tty");
status = EXIT_FATAL_ERROR;
return -1;
}
set_up_tty(pty_slave, 1);
}
/*
* Lock the device if we've been asked to.
*/
status = EXIT_LOCK_FAILED;
if (lockflag && !privopen) {
if (lock(devnam) < 0)
goto errret;
locked = 1;
}
/*
* Open the serial device and set it up to be the ppp interface.
* First we open it in non-blocking mode so we can set the
* various termios flags appropriately. If we aren't dialling
* out and we want to use the modem lines, we reopen it later
* in order to wait for the carrier detect signal from the modem.
*/
got_sigterm = 0;
connector = doing_callback? callback_script: connect_script;
if (devnam[0] != 0) {
for (;;) {
/* If the user specified the device name, become the
user before opening it. */
int err, prio;
...
这里我们要特别主要粗体注释部分. 就是在这里我们的ppp和tty联系了起来，才真正work. 当然还加载了前面的脚本文件，来获取必要的配置信息.
tty_init()完成后，就是来和ppp内核协议通信来建立必要的ppp网络接口，以供上层应用程序用
/*
* If we're doing dial-on-demand, set up the interface now.
*/
if (demand) {
/*
* Open the loopback channel and set it up to be the ppp interface.
*/
fd_loop = open_ppp_loopback();
set_ifunit(1);
/*
* Configure the interface and mark it up, etc.
*/
demand_conf();
}
在open_ppp_loopback()这个函数中调用 make_ppp_unit（）建立ppp0接口（当然这个接口名我们也可以自定义）
/*
* make_ppp_unit - make a new ppp unit for ppp_dev_fd.
* Assumes new_style_driver.
*/
static int make_ppp_unit()
{
int x, flags;
if (ppp_dev_fd >= 0) {
dbglog("in make_ppp_unit, already had /dev/ppp open?");
close(ppp_dev_fd);
}
ppp_dev_fd = open("/dev/ppp", O_RDWR);
if (ppp_dev_fd < 0)
fatal("Couldn't open /dev/ppp: %m");
flags = fcntl(ppp_dev_fd, F_GETFL);
if (flags == -1
|| fcntl(ppp_dev_fd, F_SETFL, flags | O_NONBLOCK) == -1)
warn("Couldn't set /dev/ppp to nonblock: %m");
ifunit = req_unit;
x = ioctl(ppp_dev_fd, PPPIOCNEWUNIT, &ifunit);
if (x < 0 && req_unit >= 0 && errno == EEXIST) {
warn("Couldn't allocate PPP unit %d as it is already in use", req_unit);
ifunit = -1;
x = ioctl(ppp_dev_fd, PPPIOCNEWUNIT, &ifunit);
}
if (x < 0)
error("Couldn't create new ppp unit: %m");
return x;
}
这里我们看到open函数打开了/dev/ppp ，然后发送一个ioctl给ppp协议
在ppp协议里
static struct file_operations ppp_device_fops = {
.owner = THIS_MODULE,
.read = ppp_read,
.write = ppp_write,
.poll = ppp_poll,
.ioctl = ppp_ioctl,
.open = ppp_open,
.release = ppp_release
};
ppp_ioctl调用ppp_unattached_ioctl(),在这个函数里
switch (cmd) {
case PPPIOCNEWUNIT:
/* Create a new ppp unit */
if (copy_from_user(&req, p, sizeof req))
break;
ppp = ppp_create_interface(req.unit, &err, req.ifname);
ppp_create_interface来真正创建这个接口，以供将来应用程序使用.
既然tty和ppp都初始化了，也建立了某种联系，并且能一起协同工作了，那么我们还犹豫什么？
当然是start_link了
start_link(0);
在这个函数里我们才真正调用我们上面长篇大论的connect_tty().
建立连接的过程分两个阶段：LCP，PCAP.
pppd建立连接过程中chat辅助程序起到不可磨灭的作用.
pppd运行后作为一个守护进程来运行，它必然有监控程序，而这个就是handle_events.
有人认为我这里说这么多是扯淡，我感觉不是，为什么呢，虽然我们一般人直接用pppd拨号就可以，但是很多人并不知道为什么，为什么就把pppd、ppp协议、tty、3G模块联系起来的.这里分析下我想功夫不会是白费的.如果一旦出现问题，那么我们很容易定位问题的所在，知其然还要知其所以然.以后我们应该讲讲ppp相关的数据包收发流程.
