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In terms of simulation, fog computing essentially translates to having compute capability at the edge (or) at the intermediate devices. 

As one example let's say you have a sensor network where data is being sent to the server. Rather than do the processing at the server, in this example, compute operations are done at the gateway/router.

Let us take an example where sensors sends readings of the temperature through the LoWPAN Gateway. The Gateway will forward the packets if the sensed parameter (reading by Sensor) is greater than a specific threshold else it will drop it. A random value is used for the temperature readings; for packet drops, we set the packet TTL to zero.

This example can be done in NetSim by:

1. Modifying the application payload of the data being sent by the sensor (See https://tetcos.freshdesk.com/solution/articles/14000041020-how-do-i-change-the-payload-of-a-packet-in-netsim-for how to change the payload)

2. Using an input function for the value of this payload data. For example, a random number say between MIN and MAX

3.  Once the above is done and working correctly, go to _declspec(dllexport) int fn_NetSim_IP_Run() in IP.c. Write the condition in Network IN Event that if the device ID matches that of the gateway/router, and then check the payload. 

4. If the payload value is less than THRESHOLD then drop else forward

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Procedure

1. Change the Payload value on the basis of Random numbers:

The payload of the packet is added to the application layer while generating the packet.

Let's Suppose the temperature varies from 0 to 99. So, we use a random value generation function giving the value between 0 to 99. We have to copy that value to our payload.

To change the payload the following steps can be followed:

Step 1: Go to NetSim Home > Click on Your Work > Workspace Options > Open Code  Go to copy_payload() function present in Application.c file inside Application project.

Step 2: Inside the function copy_payload(), make the modifications as given below (highlighted in red) :

Step 3: Now make modification highlighted in red in int fn_NetSim_Add_DummyPayload(NetSim_PACKET* packet,APP_INFO* info) in Application.c

2. Find the LoWPAN Gateway ID

In order to ensure that the computing happens only in the Gateway, we use the device type and interface count parameters to differentiate Gateway node from the sensor nodes.

To find the Gateway Id the following steps can be followed:

Step 1: Go to fn_NetSim_IP_Run() function present in IP.c file inside IP project.

Step 2: Inside the function fn_NetSim_IP_Run(),make the modifications as given below (highlighted in red) :

declspec(dllexport) int fn_NetSim_IP_Run()

{

int i = 0; int GatewayDeviceId = -1;

for (i = 0; i < NETWORK->nDeviceCount; i++)

{

    if (DEVICE(i+1)->nDeviceType == 6 && DEVICE(i + 1)->nNumOfInterface == 2 && 

            DEVICE(i + 1)->ppstruInterfaceList[0]->pstruMACLayer->nMacProtocolId == MAC_PROTOCOL_IEEE802_15_4)

            GatewayDeviceId = NETWORK->ppstruDeviceList[i]->nDeviceId;

    }

switch (pstruEventDetails->nEventType)

{

case NETWORK_OUT_EVENT:

3. Add condition to forward or drop the packet and Store the details of each packet in a file 

Now whether the packet is forwarded or dropped by the LoWPAN Gateway depends on the payload value (Temperature reading) and threshold value. In this case the threshold value is set to 50. So packet will be forwarded by the Gateway if payload value is greater than equals to 50, else will get dropped. These Modification are added in the IP module. It is also ensured that the payload is verified only for data packets and not for control packets. In NetSim all control packets have the id  0. So if packet Id is zero then there is no check done.

In order to obtain a log file containing details such as PayLoad value, Sender Id, Packet Status(Forwarded or Dropped) and packet Id, the following steps are to be followed:

Step 1: Open fn_NetSim_IP_Init() function present in IP.c file inside IP project.

Step 2: Inside the function fn_NetSim_IP_Init() make the modifications as given below (highlighted in red) :

{

FILE* fp;

char filename[BUFSIZ] ;
sprintf(filename, "%s\\TemplogFile.csv", pszIOLogPath); 

fp = fopen(filename, "w+");

if (fp)

{

fprintf(fp, "PAYLOAD,STATUS,SENDER_ID,PACKET_ID,"); 

fclose(fp);

}

NETSIM_ID loop;

if (nVersion_Type / 10 != VERSION)

{

printf("IP---Version number mismatch\n Dll Version=%d\n NetSim Version=%d\n FileName=%s\n Line=%d\n", VERSION, nVersion_Type / 10, __FILE__, __LINE__);

exit(0);

}

To add the condition following steps can be followed:

Step 1: Open fn_NetSim_IP_Run() function present in IP.c file inside IP project.

Step 2: Inside the function fn_NetSim_IP_Run(), make the modifications as given below (highlighted in red) :

switch (pstruEventDetails->nEventType)
{
case NETWORK_OUT_EVENT:
{
ptrIP_FORWARD_ROUTE route = NULL;
 NetSim_PACKET* packet = pstruEventDetails->pPacket;

FILE* fp;

char filename[BUFSIZ];

sprintf(filename, "%s\\TemplogFile.csv", pszIOLogPath);

 int reading = 0;

  if (pstruEventDetails->pPacket->nPacketId != 0)

     {

        fp = fopen(filename, "a+");                

        reading = atoi(pstruEventDetails->pPacket->szPayload->packet);

      }

 int thresholdvalue = 50;

 if (pstruEventDetails->nDeviceId == GatewayDeviceId && (pstruEventDetails->pPacket->nPacketId != 0) && reading < thresholdvalue)

        {

              if (fp)

                {

                    fprintf(fp, "\n%d,%s,%u,%lld,", reading, " Dropped ", pstruEventDetails->pPacket->nSourceId,pstruEventDetails->nPacketId);

                    fclose(fp);

                }

                fprintf(stderr, "\nPacket dropped. Payload is %d\n", reading);

                packet->pstruNetworkData->nTTL = 0;

      }

            else if (pstruEventDetails->nDeviceId == GatewayDeviceId && pstruEventDetails->pPacket->nPacketId != 0)

            {

                if (fp)

                {

                    fprintf(fp, "\n%d,%s,%u,%lld,", reading, " Forwarded ", pstruEventDetails->pPacket->nSourceId, pstruEventDetails->nPacketId);

                    fclose(fp);

                }

                fprintf(stderr, "\nPacket forwarded. Payload is %d\n", reading);

            }

NETWORK_LAYER_PROTOCOL nLocalNetworkProtcol;

nLocalNetworkProtcol = fnGetLocalNetworkProtocol(pstruEventDetails);

if (nLocalNetworkProtcol)

{

fnCallProtocol(nLocalNetworkProtcol);

return 0;

}

After the source code is modified, rebuild the IP module by right-clicking on the IP project and selecting rebuild. Similarly, rebuild the Application project. 

Simulation Results and Analysis:

The steps involved may vary across versions of NetSim. Please refer to the respective section below:

Create a simple scenario in the IoT module and configure traffic from sensor nodes to a Wired Node connected externally using a router. 

Packets can be captured using Wireshark to analyze the payload. Set the Wireshark to online in the Destination wired node where you can observe the packet payload.

Upon running the simulation Wireshark will pop up automatically as shown below where you can check the packet payload:

NetSim Console will print the payload of those packets that are dropped at the Gateway as shown below:

The TempLogFile.csv can be accessed from NetSim Result Dashboard under Log Section  as shown below:

Create a simple scenario in the IoT module and configure traffic from sensor nodes to a Wired Node connected externally using a router. 

Packets can be captured using Wireshark to analyze the payload. Set the Wireshark to online in the Destination wired node where you can observe the packet payload.

Upon running the simulation Wireshark will pop up automatically as shown below where you can check the packet payload:

NetSim Console will print the payload of those packets that are dropped at the Gateway as shown below:

The TempLogFile.csv can be accessed from NetSim Result Dashboard under Log Section  as shown below: