Adam has installed and configured his wireless network. He has enabled numerous security features such as changing the default SSID, enabling WPA encryption, and enabling MAC filtering on his wireless router. Adam notices that when he uses his wireless connection, the speed is sometimes 16 Mbps and sometimes it is only 8 Mbps or less. Adam connects to the management utility wireless router and finds out that a machine with an unfamiliar name is connected through his wireless connection. Paul checks the router's logs and notices that the unfamiliar machine has the same MAC address as his laptop.

Which of the following attacks has been occurred on the wireless network of Adam?