simple_source模块的功能是按照配置产生包，包括不同的包格式和产生速率，也是比较简单的进程模型。
参考了《opnet process model：simple_source分析》http://hi.baidu.com/ebinghaus/blog/item/c7cade9228876c83a877a405.html
该模块有4个local statistic，注意有5个model attribute，分别是Packet Interarrival Time，Packet Size，Packet Format，Start Time，Stop Time，这些是在使用该模块时需要配置的，当然也可以使用默认配置。
状态机也很简单：
在header block中定义了
/* Include files. */
#include <oms_dist_support.h> //注意该模块使用了外部文件《oms_dist_support》和《oms_string_support》
/* Special attribute values. */
#define SSC_INFINITE_TIME -1.0 //定义无穷大时间为-1
/* Interrupt code values. */ //注意这里的中断代码是自定义的，为了在产生自中断是使用
#define SSC_START 0
#define SSC_GENERATE 1
#define SSC_STOP 2
/* Node configuration constants. */
#define SSC_STRM_TO_LOW 0 //发送的stream index，默认的，该模块只有一个stream连接，所以不需要动态获取stream index
/* Macro definitions for state transitions.*/ //转换条件为中断代码是自定义的常量：
#define START (intrpt_code == SSC_START)
#define DISABLED (intrpt_code == SSC_STOP)
#define STOP (intrpt_code == SSC_STOP)
#define PACKET_GENERATE (intrpt_code == SSC_GENERATE)
/* Function prototypes. */
static void ss_packet_generate (void); //产生包的函数，在function block中定义
一，INIT的入口代码：
/* At this initial state, we read the values of source attributes */
/* and schedule a selt interrupt that will indicate our start time */
/* for packet generation. */
/* Obtain the object id of the surrounding module. */
own_id = op_id_self (); //首先得到surrounding objid，应该是自己的objid，后面根据objid获取对象的属性
/* Read the values of the packet generation parameters, i.e. the */
/* attribute values of the surrounding module. */
//用op_ima_obj_attr_get()得到对象的属性的值，是产生包的参数。注意interarrival_str和size_str是临时变量，还是char数组类型。
op_ima_obj_attr_get (own_id, "Packet Interarrival Time", interarrival_str);
op_ima_obj_attr_get (own_id, "Packet Size", size_str);
op_ima_obj_attr_get (own_id, "Packet Format", format_str);
op_ima_obj_attr_get (own_id, "Start Time", &start_time);
op_ima_obj_attr_get (own_id, "Stop Time", &stop_time);
/* Load the PDFs that will be used in computing the packet */
/* interarrival times and packet sizes. */
//装入PDF，用于前两个参数，得到包产生和包大小的分布，函数是oms_dist_load_from_string()，这样很方便的使用string类型的参数。
//获得的是分布函数的句柄，注意两个句柄都是state variable，类型为Omst_dist_handle。
interarrival_dist_ptr = oms_dist_load_from_string (interarrival_str);
pksize_dist_ptr = oms_dist_load_from_string (size_str);
/* Verify the existence of the packet format to be used for generated packets. */
if (strcmp (format_str, "NONE") == 0) //检查是否是无格式包
{
/* We will generate unformatted packets. Set the flag. */
generate_unformatted = OPC_TRUE;
}
else
{
/* We will generate formatted packets. Turn off the flag. */
generate_unformatted = OPC_FALSE; //不是无格式包
/* Get the list of all available packet formats. */
//用prg_tfile_name_list_get()得到所有可用的包格式
pk_format_names_lptr = prg_tfile_name_list_get (PrgC_Tfile_Type_Packet_Format);
/* Search the list for the requested packet format. */
format_found = OPC_FALSE;
for (i = prg_list_size (pk_format_names_lptr); ((format_found == OPC_FALSE) && (i > 0)); i--)
{
/* Access the next format name and compare with requested format name */
found_format_str = (char *) prg_list_access (pk_format_names_lptr, i - 1);
if (strcmp (found_format_str, format_str) == 0)
format_found = OPC_TRUE; //找到了需求的包格式
}
if (format_found == OPC_FALSE)
{
/* The requested format does not exist. Generate unformatted packets. */
generate_unformatted = OPC_TRUE;
/* Display an appropriate warning. */
op_prg_odb_print_major ("Warning from simple packet generator model (simple_source):",
"The specified packet format", format_str,
"is not found. Generating unformatted packets instead.", OPC_NIL);
}
/* Destroy the lits and its elements since we don't need it */
/* anymore. */
prg_list_free (pk_format_names_lptr);
prg_mem_free (pk_format_names_lptr);
}
/* Make sure we have valid start and stop times, i.e. stop time is not earlier than start time. */
if ((stop_time <= start_time) && (stop_time != SSC_INFINITE_TIME)) //开始/结束时间不合法，设置开始仿真时间为无穷大
{
/* Stop time is earlier than start time. Disable the source. */
start_time = SSC_INFINITE_TIME;
/* Display an appropriate warning. */
op_prg_odb_print_major ("Warning from simple packet generator model (simple_source):",
"Although the generator is not disabled (start time is set to a finite value),",
"a stop time that is not later than the start time is specified.",
"Disabling the generator.", OPC_NIL);
}
/* Schedule a self interrupt that will indicate our start time for */
/* packet generation activities. If the source is disabled, */
/* schedule it at current time with the appropriate code value. */
//产生自中断，以指示开始产生包的时间，如果停止产生包时刻小于开始产生包的时刻（这时开始产生包的时间是无限大）在当前仿真时刻产生中断号为SSC_STOP中断
if (start_time == SSC_INFINITE_TIME)
{
op_intrpt_schedule_self (op_sim_time (), SSC_STOP); //马上给出自中断
}
else
{
op_intrpt_schedule_self (start_time, SSC_START); //自中断，在开始时间，中断号为SSC_START
/* In this case, also schedule the interrupt when we will stop */
/* generating packets, unless we are configured to run until */
/* the end of the simulation. */
if (stop_time != SSC_INFINITE_TIME)
{
op_intrpt_schedule_self (stop_time, SSC_STOP); //当结束时间不是无限大时，在结束时间产生自中断，中断号为SSC_STOP
}
next_intarr_time = oms_dist_outcome (interarrival_dist_ptr); //通过分布函数句柄计算产生下一个包的时间间隔，这里是state variable
/* Make sure that interarrival time is not negative. In that case it will be set to 0.*/
if (next_intarr_time <0)
{
next_intarr_time = 0.0; //保证间隔时间不小于0
}
}
/* Register the statistics that will be maintained by this model. */
//注册局部统计量，返回局部统计量的句柄
bits_sent_hndl = op_stat_reg ("Generator.Traffic Sent (bits/sec)", OPC_STAT_INDEX_NONE, OPC_STAT_LOCAL);
packets_sent_hndl = op_stat_reg ("Generator.Traffic Sent (packets/sec)", OPC_STAT_INDEX_NONE, OPC_STAT_LOCAL);
packet_size_hndl = op_stat_reg ("Generator.Packet Size (bits)", OPC_STAT_INDEX_NONE, OPC_STAT_LOCAL);
interarrivals_hndl = op_stat_reg ("Generator.Packet Interarrival Time (secs)", OPC_STAT_INDEX_NONE, OPC_STAT_LOCAL);
二，由于初始状态是非强制状态，进程执行完入口代码后，停留在该状态等待中断触发转移状态,仿真时刻到10s（属性start_time设置的值）时将产生中断号为SSC_START中断，中断产生后将首先执行当前状态的出口代码，初始状态的出口代码如下：
intrpt_code = op_intrpt_code (); //得到当前的中断代码
这时条件START满足，进程将从初始状态转换到generate状态，转移时首先执行转移函数ss_packet_generate (void)，在function block中定义的转移函数如下：
static void ss_packet_generate (void)
{
Packet* pkptr;
double pksize;
/** This function creates a packet based on the packet generation **/
/** specifications of the source model and sends it to the lower layer. **/
FIN (ss_packet_generate ());
/* Generate a packet size outcome. */
pksize = (double) ceil (oms_dist_outcome (pksize_dist_ptr)); //通过分布函数句柄得到包大小，再通过ceil()函数向上舍入，得到包大小
/* Create a packet of specified format and size. */
if (generate_unformatted == OPC_TRUE)
{
/* We produce unformatted packets. Create one. */
pkptr = op_pk_create (pksize); //使用op_pk_creat()函数产生无格式包
}
else
{
/* Create a packet with the specified format. */
pkptr = op_pk_create_fmt (format_str); //使用op_pk_creat_fmt()函数产生有格式包
op_pk_total_size_set (pkptr, pksize); //置有格式包的总大小
}
/* Update the packet generation statistics. */ //通过局部统计量句柄更新统计量，注意xxx/sec的量都是在更新后马上清零
op_stat_write (packets_sent_hndl, 1.0);
op_stat_write (packets_sent_hndl, 0.0);
op_stat_write (bits_sent_hndl, (double) pksize);
op_stat_write (bits_sent_hndl, 0.0);
op_stat_write (packet_size_hndl, (double) pksize);
op_stat_write (interarrivals_hndl, next_intarr_time);
/* Send the packet via the stream to the lower layer. */
op_pk_send (pkptr, SSC_STRM_TO_LOW); //发送包到定义好的stream index，默认为0
FOUT;
}
三， 执行完转移函数后将跳到generate状态，并首先执行入口代码，如下：
/* At the enter execs of the "generate" state we schedule the */
/* arrival of the next packet. */
next_intarr_time = oms_dist_outcome (interarrival_dist_ptr); //通过分布函数句柄得到下一个包的产生时间间隔
/* Make sure that interarrival time is not negative. In that case it will be set to 0.*/
if (next_intarr_time <0)
{
next_intarr_time = 0;
}
//在当前时间+间隔时间后，产生自中断，中断号为SSC_GENERATE，注意返回值为Evhandle类型
next_pk_evh = op_intrpt_schedule_self (op_sim_time () + next_intarr_time, SSC_GENERATE);
四，执行完入口代码后，进程将停留在generate状态等待中断，当下一个中断发生，进行状态转换，首先执行generate状态的出口代码：
intrpt_code = op_intrpt_code (); //得到中断号
然后判断转移条件。
如果是STOP，执行stop状态的入口代码：
/* When we enter into the "stop" state, it is the time for us to */
/* stop generating traffic. We simply cancel the generation of the */
/* next packet and go into a silent mode by not scheduling anything */
/* else. */
if (op_ev_valid (next_pk_evh) == OPC_TRUE) //如果下一个事件仍然有效，就取消该事件，使用事件句柄
{
op_ev_cancel (next_pk_evh);
}