You need to install some application to the node that generates packets. The UdpEchoClient has an "interval" attribute you can set to 1s to send a packet every second. To get a response you install a UdpEchoServer on some node and then set the UdpEchoClient RemoteAddress attribute to the server node's IP address. In this case, you will get 1 message per second from client to server, and one response from server to client.
The OnOffApplication can also create packets with some interval. It is paired with a PacketSInk application on the node whose address is set as the client's Remote attribute. I find the UdpEchoClient/Server easier to setup though. Look through the examples for those applications for code showing how to set them up.
Jared.