We focused on the heterogeneity in the 4G(fourth generation) which allows a high speed and capacity.The main idea consists to track a mobile user moving from a WLAN( Wireless Local Area Network) to a 3G( third generation) network.The decision of the vertical handoff is based on the strength of the signal.In the other hand the handoff execution phase start once the user is tracked at or near the border of a cell. We showed the result through OMNET++.