IEEE 802.21 specifications aim at defining a framework that supports information exchange between entities belonging to different access networks and helps in taking handoff decisions; they also define a set of functional components that have to execute those decisions. The proposed framework is built around the media independent handover function (MIHF), which lies between layer 2 and layer 3 and provides higher layers with abstracted services through a unified interface. Three services can be distinguished: the event service, the command service, and the information service.
Media independent event services (MIES): In general, handover is triggered by multiple events that may occur at the terminal or the network level. The MIES provides event classification, event filtering, and event reporting corresponding to dynamic changes in link characteristics, link status, and link quality. Figure 1 shows that events can be classified into link events and MIH events. Besides, the event model follows the subscription/notification procedure. More specifically, the MIHF registers link event notifications with the interface while any upper-layers entity may register for an MIHF event notification. Link events are generated by both the physical and the MAC layers and then sent to the MIHF which has to report them to any entity that has registered an MIH event or a remote MIH event. MIH events are generated by the MIHF and may be local or remote. Local events occur at the client level while remote events take place in the network entities. Both link and MIH events are classified into six categories, which are administrative, state change, linkparameter, predictive, link synchronous, and link transmission. The most common events include link up, link down, link parameters change, link going down, link handover imminent, etc. Upon being notified about certain events, the upper layer uses the command service to control the links to switch to a new PoA when needed.
Figure 1: Media-independent event services.
Media independent command service: The media independent command service (MICS) enables MIH users to manage and control link behavior relevant to handovers and mobility. More specifically, MICS commands aim at gathering information about the status of the connected links along with transferring higher-layer mobility and connectivity decisions to lower layers. As shown in Figure 1, upper layers generate MIH commands which may be local or become remote depending on the destination entity. These MIH commands are translated by the MIHF to link commands which differ depending on the access network being used. Examples of MIH commands include MIH poll, MIH scan, MIH configure, and MIH switch. These intend to poll connected links to get informed about their most recent status, to scan for newly discovered links, to configure the new links and then to switch between the available links.
Media independent information service: Mobile terminals need to discover the heterogeneous neighboring networks and communicate with their entities to optimize the handoff process and facilitate seamless handoff when roaming across these networks. To address this issue, media independent information service (MIIS) provides the capability for obtaining the necessary information for handovers. Such information includes neighbor maps, link-layer information, and availability of services related to a particular geographic area. MIIS enables all layers to share IEs required for making handoff decisions; it also specifies the information structure and its representation while implementing a query/response type mechanism. Information may be either static such as the names and providers of the mobile terminals neighboring networks or dynamic such as channel information, MAC addresses, security information, etc. MIIS fulfils quick data transfer understood by different access technologies with very little decoding complexity. In fact, reports can be transferred either by means of type-length value (TLV) or using a schema referred to as resource description framework (RDF) which is represented in eXtensible Markup Language (XML). As layer 2 information may not be available or may not be rich enough to make intelligent handoff decisions, higher-layer services may be consulted to assist in the mobility decision making process.
Service access points: service access points (SAPs) represent the API through which the MIHF can communicate with the upper-layer and the lower-layer entities using IEEE 802.1 primitives. More specifically, we distinguish three types of SAPs: lower-layer SAPs, upper-layer SAPs, and management plane SAPs. Lower-layer SAPs vary according to the network access type as defined by each MAC and PHY relative to the corresponding access technology. On the other hand, upper-layer SAPs define the interface between the MIHF and upper-layer mobility management protocols or policy engines implemented within a client or a network entity. Finally, management SAPs describe the interfaces between the MIHF and the management plane of various networks.
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