Thanks Akash for such a detailed answer !

It is a very interested question

Thank you Jumaa Salman. This way I was able to changed some nodes into Fog Routers while the others were normal routers.

An additional questions on this itself, how do I let changes affect only certain 'link's? And also what is the difference between a link and an interface in NetSim?

Abhishek,

Each link is referred using a unique ID which is the Link_ID.

Each interface of a device has a unique ID which is the Interface_ID.

For Eg: If a router is connected to three devices, it will have three interfaces each referred using a unique ID.

In NetSim, the properties of links can be accessed from the NETWORK variable as follows: NETWORK->ppstruNetSimLinks[LINK_ID]

Where, 

LINK_ID is the unique ID of a link

Hence you make changes to the particular link using this.

The properties of each interface of a specific device can be accessed from the NETWORK variable as follows:

NETWORK->ppstruDeviceList[DEVICE_ID]->ppstruInterfaceList[INTERFACE_ID]

Where,

DEVICE_ID is the unique ID of the device for which the interface details needs to be accesssed

INTERFACE_ID is the unique ID of the interface of device with ID DEVICE_ID which is to be accessed