Vertical handover or vertical handoff refers to a

network Network, networking and networked may refer to: Science and technology * Network theory, the study of graphs as a representation of relations between discrete objects * Network science, an academic field that studies complex networks Mathematics ...

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 A wireless LAN (WLAN) is a wireless computer network that links two or more devices using wireless communication to form a local area network (LAN) within a limited area such as a home, school, computer laboratory, campus, or office buildi ...

and a

cellular Cellular may refer to: *Cellular automaton, a model in discrete mathematics * Cell biology, the evaluation of cells work and more * ''Cellular'' (film), a 2004 movie *Cellular frequencies, assigned to networks operating in cellular RF bands *Cell ...

technology for

Internet The Internet (or internet) is the global system of interconnected computer networks that uses the Internet protocol suite (TCP/IP) to communicate between networks and devices. It is a '' network of networks'' that consists of private, pub ...

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 The data link layer, or layer 2, is the second layer of the seven-layer OSI model of computer networking. This layer is the protocol layer that transfers data between nodes on a network segment across the physical layer. The data link layer ...

technology used to access the network. Vertical handoffs between

WLAN A wireless LAN (WLAN) is a wireless computer network that links two or more devices using wireless communication to form a local area network (LAN) within a limited area such as a home, school, computer laboratory, campus, or office buildin ...

and

UMTS The Universal Mobile Telecommunications System (UMTS) is a third generation mobile cellular system for networks based on the GSM standard. Developed and maintained by the 3GPP (3rd Generation Partnership Project), UMTS is a component of the In ...

(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 Worldwide Interoperability for Microwave Access (WiMAX) is a family of wireless broadband communication standards based on the IEEE 802.16 set of standards, which provide physical layer (PHY) and media access control (MAC) options. The WiMAX ...

can be achieved using Media independent handover which is standardized as IEEE 802.21.

Related issues

Dual mode card

To support vertical

handover In cellular telecommunications, handover, or handoff, is the process of transferring an ongoing call or data session from one channel connected to the core network to another channel. In satellite communications it is the process of transfe ...

, a

mobile Mobile may refer to: Places * Mobile, Alabama, a U.S. port city * Mobile County, Alabama * Mobile, Arizona, a small town near Phoenix, U.S. * Mobile, Newfoundland and Labrador Arts, entertainment, and media Music Groups and labels * Mobile ( ...

terminal needs to have a dual mode card, for example one that can work under both

and

frequency Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from ''angular frequency''. Frequency is measured in hertz (Hz) which is eq ...

bands and modulation schemes.

Interworking architecture

For the vertical handover between UMTS and WLAN, there are two main interworking architecture: tight coupling and

loose coupling In computing and systems design, a loosely coupled system is one # in which components are weakly associated (have breakable relationships) with each other, and thus changes in one component least affect existence or performance of another comp ...

. 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 user. So the data transmitted through WLAN will not go through Cellular Networks.

Handover metrics

In traditional handovers, such as a handover between cellular networks, the handover decision is based mainly on RSS (Received 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 handover, 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 handover metrics in this situation should include RSS, user preference, network conditions, application types, cost etc.

Handover decision algorithm

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

Mobility management

When a mobile station transfers a user's session from one

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, Transport Layer, IP Layer, etc. The most common method is to use SIP (Session Initiation Protocol) and

Mobile IP Mobile IP (or MIP) is an Internet Engineering Task Force (IETF) standard communications protocol that is designed to allow mobile device users to move from one network to another while maintaining a permanent IP address. Mobile IP for IPv4 is des ...

Handoff procedure

The handover procedure specifies the control signalling used to perform the handover and is invoked by the handover decision algorithm.

Related standards

3GPP TS 23.234 “3GPP system to WLAN interworking; System description

* ttp://www.ieee802.org/21/index.html 802.21 Media independent handover * IEEE 802.21 *

Wireless networking Mobile telecommunications standards