Vertical handover

Vertical handover or vertical handoff refers to a network node changing the type of connectivity it uses to access a supporting infrastructure, usually to support node mobility. For example, a suitably equipped laptop might be able to use both a high speed wireless LAN and a cellular technology for Internet access. Wireless LAN connections generally provide higher speeds, while cellular technologies generally provide more ubiquitous coverage. Thus the laptop user might want to use a wireless LAN connection whenever one is available, and to 'fall over' to a cellular connection when the wireless LAN is unavailable. Vertical handovers refer to the automatic fallover from one technology to another in order to maintain communication. This is different from a 'horizontal handover' between different wireless access points that use the same technology in that a vertical handover involves changing the data link layer technology used to access the network. Vertical handover or vertical handoff refers to a network node changing the type of connectivity it uses to access a supporting infrastructure, usually to support node mobility. For example, a suitably equipped laptop might be able to use both a high speed wireless LAN and a cellular technology for Internet access. Wireless LAN connections generally provide higher speeds, while cellular technologies generally provide more ubiquitous coverage. Thus the laptop user might want to use a wireless LAN connection whenever one is available, and to 'fall over' to a cellular connection when the wireless LAN is unavailable. Vertical handovers refer to the automatic fallover from one technology to another in order to maintain communication. This is different from a 'horizontal handover' between different wireless access points that use the same technology in that a vertical handover involves changing the data link layer technology used to access the network. Vertical handoffs between WLAN and UMTS (WCDMA) have attracted a great deal of attention in all the research areas of the 4G wireless network, due to the benefit of utilizing the higher bandwidth and lower cost of WLAN as well as better mobility support and larger coverage of UMTS. Vertical handovers among a range of wired and wireless access technologies including WiMAX can be achieved using Media independent handover which is standardized as IEEE 802.21. To support vertical handover, a mobile terminal needs to have a dual mode card, for example one that can work under both WLAN and UMTS frequency bands and modulation schemes. For the vertical handover between UMTS and WLAN, there are two main interworking architecture: tight coupling and loose coupling.The tight coupling scheme, which 3GPP adopted, introduces two more elements: WAG (Wireless Access Gateway) and PDG (Packet Data Gateway). So the data transfers from WLAN AP to a Corresponding Node on the internet must go through the Core Network of UMTS.