simple_source模塊的功能是按照配置產生包，包括不同的包格式和產生速率，也是比較簡單的進程模型。
參考了《opnet process model：simple_source分析》http://hi.baidu.com/ebinghaus/blog/item/c7cade9228876c83a877a405.html
該模塊有4個local statistic，注意有5個model attribute，分別是Packet Interarrival Time，Packet Size，Packet Format，Start Time，Stop Time，這些是在使用該模塊時需要配置的，當然也可以使用默認配置。
狀態機也很簡單：
在header block中定義了
/* Include files. */
#include <oms_dist_support.h> //注意該模塊使用了外部文件《oms_dist_support》和《oms_string_support》
/* Special attribute values. */
#define SSC_INFINITE_TIME -1.0 //定義無窮大時間爲-1
/* Interrupt code values. */ //注意這裏的中斷代碼是自定義的，爲了在產生自中斷是使用
#define SSC_START 0
#define SSC_GENERATE 1
#define SSC_STOP 2
/* Node configuration constants. */
#define SSC_STRM_TO_LOW 0 //發送的stream index，默認的，該模塊只有一個stream連接，所以不需要動態獲取stream index
/* Macro definitions for state transitions.*/ //轉換條件爲中斷代碼是自定義的常量：
#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); //產生包的函數，在function block中定義
一，INIT的入口代碼：
/* 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，應該是自己的objid，後面根據objid獲取對象的屬性
/* Read the values of the packet generation parameters, i.e. the */
/* attribute values of the surrounding module. */
//用op_ima_obj_attr_get()得到對象的屬性的值，是產生包的參數。注意interarrival_str和size_str是臨時變量，還是char數組類型。
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. */
//裝入PDF，用於前兩個參數，得到包產生和包大小的分佈，函數是oms_dist_load_from_string()，這樣很方便的使用string類型的參數。
//獲得的是分佈函數的句柄，注意兩個句柄都是state variable，類型爲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) //檢查是否是無格式包
{
/* 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; //不是無格式包
/* 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)) //開始/結束時間不合法，設置開始仿真時間爲無窮大
{
/* 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. */
//產生自中斷，以指示開始產生包的時間，如果停止產生包時刻小於開始產生包的時刻（這時開始產生包的時間是無限大）在當前仿真時刻產生中斷號爲SSC_STOP中斷
if (start_time == SSC_INFINITE_TIME)
{
op_intrpt_schedule_self (op_sim_time (), SSC_STOP); //馬上給出自中斷
}
else
{
op_intrpt_schedule_self (start_time, SSC_START); //自中斷，在開始時間，中斷號爲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); //當結束時間不是無限大時，在結束時間產生自中斷，中斷號爲SSC_STOP
}
next_intarr_time = oms_dist_outcome (interarrival_dist_ptr); //通過分佈函數句柄計算產生下一個包的時間間隔，這裏是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; //保證間隔時間不小於0
}
}
/* Register the statistics that will be maintained by this model. */
//註冊局部統計量，返回局部統計量的句柄
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，由於初始狀態是非強制狀態，進程執行完入口代碼後，停留在該狀態等待中斷觸發轉移狀態,仿真時刻到10s（屬性start_time設置的值）時將產生中斷號爲SSC_START中斷，中斷產生後將首先執行當前狀態的出口代碼，初始狀態的出口代碼如下：
intrpt_code = op_intrpt_code (); //得到當前的中斷代碼
這時條件START滿足，進程將從初始狀態轉換到generate狀態，轉移時首先執行轉移函數ss_packet_generate (void)，在function block中定義的轉移函數如下：
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)); //通過分佈函數句柄得到包大小，再通過ceil()函數向上舍入，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()函數產生無格式包
}
else
{
/* Create a packet with the specified format. */
pkptr = op_pk_create_fmt (format_str); //使用op_pk_creat_fmt()函數產生有格式包
op_pk_total_size_set (pkptr, pksize); //置有格式包的總大小
}
/* Update the packet generation statistics. */ //通過局部統計量句柄更新統計量，注意xxx/sec的量都是在更新後馬上清零
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); //發送包到定義好的stream index，默認爲0
FOUT;
}
三， 執行完轉移函數後將跳到generate狀態，並首先執行入口代碼，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); //通過分佈函數句柄得到下一個包的產生時間間隔
/* 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;
}
//在當前時間+間隔時間後，產生自中斷，中斷號爲SSC_GENERATE，注意返回值爲Evhandle類型
next_pk_evh = op_intrpt_schedule_self (op_sim_time () + next_intarr_time, SSC_GENERATE);
四，執行完入口代碼後，進程將停留在generate狀態等待中斷，當下一箇中斷髮生，進行狀態轉換，首先執行generate狀態的出口代碼：
intrpt_code = op_intrpt_code (); //得到中斷號
然後判斷轉移條件。
如果是STOP，執行stop狀態的入口代碼：
/* 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) //如果下一個事件仍然有效，就取消該事件，使用事件句柄
{
op_ev_cancel (next_pk_evh);
}