Mobile computing for Personal Communication Service (PCS) enables users with reliable, bi-directional internet access anytime, anywhere. Mobile IP, as a network layer protocol, addresses mobile computing needs. Terminal mobility and service area changes make efficient IP mobility crucial in minimizing packet loss and handover latency in heterogeneous networks. This work presents an in-depth analysis of mobile IP protocols and evaluates the GTP protocol's performance in a customized heterogeneous network, focusing on handoff. We propose an overlay-based architecture for heterogeneous networks, including 2G/2.5G/3G/WLAN/WiMax, and a crossover switch to manage vertical handoff and interlayer mobility. Simulations using NS-2 show that the proposed GWON (Global Wireless Overlay Network) architecture enables vertical handoff between networks with minimal delay and no call drops. The results demonstrate improved handoff delay compared to existing protocols.