“…However, the vertical handoff from cellular network to WLAN is regarded as a network selection process, because mobile terminals are in a wireless overlay area where both cellular access and WLAN access are available to mobile terminals at same time. Seamless vertical handoffs face challenges caused by the gap between different QoS levels in cellular network and in WLAN Shafiee et al, 2011): UMTS cellular networks provide wide coverage with high QoS provisioning for voice service, but limited-rate data service. However, WLANs support high-rate data service, but lack of universal roaming ability and suffer from low QoS level for voice service, due to their original real-time constraints.…”