Vertical handoff

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 'fail over' to a cellular connection when the wireless LAN is unavailable. Vertical handoffs refer to the automatic failover from one technology to another in order to maintain communication. This is different from a 'horizontal handoff' between different wireless access points that use the same technology in that a vertical handoff involves changing the data link layer technology used to access the network.

Vertical handoffs between WLAN and UMTS (CDMA2000) 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 handoffs 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.

Related Issues

Dual Mode Card

To support vertical handoff, a mobile terminal needs to have a dual mode card that can work under both WLAN and UMTS frequency bands and modulation schemes.

Interworking Architecture

For the vertical handoff 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.

Loose coupling is more used when the WLAN is not operated by cellular operator but any private users. So the data transmitted through WLAN will not go through Cellular Networks.

Handoff Metrics

In traditional handoffs, such as a handoff between cellular networks, the handoff decision is based mainly on RSS (Relative Signal Strength) in the border region of two cells, and may also be based on call drop rate, etc. for resource management reasons.In vertical handoff, the situation is more complex. Two different kinds of wireless networks normally have incomparable signal strength metrics, for example, WLAN compared to UMTS. In, WLAN and UMTS networks both cover an area at the same time.The Handoff Metrics in this situation should include RSS, user preference, network conditions, application types, cost etc.

Handoff Decision Algorithm

Based on the handoff metrics mentioned above, the decision about how and when to switch the interface to which network will be made.Many papers have give their reasonable flow chart based on the better service and lower cost, etc. while some others, using fuzzy logic or neuron network to solve the problem.

Mobility Management

When a mobile station transfers a user's session from one network to another, the IP address will change. In order to allow the Corresponding Node that the MS is communicating with to find it correctly and allow the session to continue, Mobility Management is used.The Mobility Management problem can be solved in different layers, such as the Application Layer, TCP Layer, IP Layer, etc. The most common method is to use SIP (Session Initiation Protocol) and Mobile IP.

Handoff Procedure

The handoff procedure specifies the control signalling used to perform the handoff and is invoked by the Handoff Decision Algorithm.

Related Standards

* [http://www.3gpp.org/ftp/Specs/html-info/23234.htm 3GPP TS 23.234 “3GPP system to WLAN interworking; System descriptionGPRS]
* [http://www.3gpp.org/ftp/Specs/html-info/23228.htm IMS TS 23.228 IP Multimedia Subsystem]
* [http://www.ieee802.org/21/index.html 802.21] Media independent handover