There are many types of handover systems existing today, which can be partitioned in different ways. Several dimensions can be used in partitioning the handover systems. These are, e.g., regarding the domain, the system, the overlay and the technology.
• For instance, handover systems can be partitioned with reference to technology, which can be similar or different. In the first case we have homogeneous handovers and in the second case we have heterogeneous handovers.
• Handover systems can be also partitioned with reference to the place of the access points, which can be within the same network or in different ones. The first case refers to horizontal handover and the second case to vertical handover. The vertical handover can in turn be of two classes, which are the upward handover and the downward handover.
• Another dimension is the domain. Handover systems can in this case be of two classes, namely intra-domain handover and inter-domain handover. Intradomain handover means that the mobile node can roam within the same network domain. Inter-domain handover means that the mobile node can cross from one domain to another one.
• Finally, the last dimension is the system. An inter-system handover refers to the case that a mobile node hands off between two independent systems controlled by different operators. An intra-system handover refers to the situation where the both domains are deserved by the same system.
When discussing about the seamless handover situation today, its important to note that there are three possibilities to handle movement, namely at the link layer (L2), network layer (L3) and application layer (L5). Most of the existent solutions attempt to solve the handover at L2 (access and switching) and L3 (IP) with particular consideration given to L4 (transport). Some of the most important requirements are on seamless handover, efficient network selection, security, flexibility, transparence with reference to access technologies and provision of QoS.
The basic idea of L2/L3 handover is using Link Event Triggers (LET) fired at Media Access Control (MAC) layer, and sent to a handover management functional module such as L3 Mobile IP (MIP) or L3 Fast MIP (FMIP) or IEEE 802.21 Information Server (IS). LET is used to report on changes with regard to L2 or L1 conditions, and to provide indications regarding the status of the radio channel. The purpose of these triggers is to assist IP in handover preparation and execution.
The type of handover (horizontal or vertical) as well as the time needed to perform it can be determined with the help of neighbor information provided by the Base Station (BS) or Access Point (AP) or the IEEE 802.21 Media Independent Handover Function (MIHF) Information Server (IS).
Given the extreme diversity of the access networks, the initial model was focused on developing common standards across IEEE 802 media and defining L2 triggers to make Fast Mobile IP (FMIP) work well. Connected with this, media independent information needs to be defined to enable mobile nodes to effectively detect and select networks. Furthermore, appropriate ways need to be defined to transport the media independent information and the triggers over all 802 media.
In reality, however, the situation is much more challenging. This is because of the extreme diversity existent today with reference to access networks, standard bodies and standards as well as architectural solutions. Other problems are because of;
o the lack of standards for handover interfaces
o lack of interoperability between different types of vendor equipment,
o lack of techniques to measure and assess the performance (including security),
o incorrect network selection,
o increasing number of interfaces on devices and
o the presence of different fast handover mechanisms in IETF, e.g., MIPv4, Fast MIPv6 (FMIPv6), Hierarchical MIPv6 (HMIPv6), Fast Hierarchical MIPv6 (FHMIPv6).
IETF anticipated L2 solutions in standardized form (in the form of triggers, events, etc), but today the situation is that we have no standards and no media independent form. Other problems are related to the use of L2 predictive trigger mechanisms, which are dependent of L1 and L2 parameters. Altogether, the consequence is in form of complexity of the existent solutions and dependence on the limitations of L1, L2 and L3. The existent solutions are simply not yet working properly, which may result in service disruptions. Because of this, it is important to develop cross-layer architectural solutions where cooperation is established between L2 and L3 to assist the IP handover process and to improve the performance. Even better would be to develop architectural solutions where IP has control over specific L2 handover-related actions.
Today, user mobility across different wireless networks is mainly user centric, thus not allowing operators a reasonable control and management of inherently dynamic users. This is the reason for why the IEEE 802.21 Working Group is doing an effort to ratify the Media Independent Handover (MIH) standard, to enhance the user centric mobility handovers and enable network controlled handovers across heterogeneous networks.
In parallel to this, IETF addresses the IP level support for mobile heterogeneous access like, e.g., the Working Group (WG) on ”The Mobility for IP: Performance, Signaling and Handoff Optimization (MISHOP)”. This WG regards the delivery of information for MIH services at L3 or above. The L3 discovery component is also defined. The target is to enable MIH services even in the absence of the corresponding L2 support. The security issue is addressed as well. IEEE 802.21 defines a framework to support information exchange regarding mobility decisions, which is irrespective of media. The goal is to facilitate handovers among heterogeneous access networks. Handover decisions are taken based on information collected from both mobile nodes and network, e.g., link type, link identifier, link availability, link quality.
The core of the IEEE 802.21 framework is the Media Independent Handover Function (MIHF), which provides abstracted services to higher layers by means of a unified interface. This unified interface provides service primitives that are independent of the access technology. This interface is called Service Access Point (SAP).
Furthermore, it is important to point out that the traditional TCP/IP protocol stack was not designed for mobility but for fixed computer networks. This is particularly shown by the fact that the responsibility of individual layers is ill-defined with reference to mobility. The main consequence is that problems in lower layers related to mobility may create bigger problems in higher layers. Higher layer mobility schemes are therefore expected to better suit Internet mobility.
Better prediction mechanisms and especially some form of movement prediction would definitely improve the handover performance in the sense that this could compensate for errors connected with delay in the handover process and the associated service disruptions.
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