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Heterogeneous Networks, IEEE 802.21, Media Independent Handover, Mobility.
The standard  specifies a Media Independent Information Service (MIIS) server supporting various information elements that provide network information within a geographical area. Based on information from several access networks and operators, a Mobile Node (MN) can take an optimized handover decision. The information available via the MIIS can be categorized as:
General Information and Access Network Specific Information: presents a general overview about the networks covering a specific area such as network type, operator identifier, QoS, security, cost and roaming partners.
Link connection point information: provides information about Points of Attachment (PoA) for each available access network and categorizes aspects such as geographical location, data rate, channel configuration, and so on.
There have been proposals - taking into consideration the IEEE 802.21 MIIS service for network discovery. Current literature considers the existence of only one MIIS server in the network which responds with neighborhood information. However, the number of network entities and supported communication technologies directly impact the amount of MIIS information sent from the network to the MN, causing handover performance delay.
There are many shortcomings related to the specification of a single MIIS server: 1) too much information to store when hundreds of access networks and dozens of operators exist; 2) can represent a single point of failure and 3) high discovery delay if the MIIS server is located many hops away from the MN. Its clear that the usage of a single MIIS server for a large city or a whole country with several operators is not desirable. Moreover, this centralized architecture is not scalable when information queries from the MN are highly frequent. Finally, inaccurate or unrelated network information results in sub-optimal handovers. Therefore, in this letter, we propose a hierarchical neighbor discovery scheme which enhances the performance of the MN in terms of experienced throughput and discovery response time.
This section describes our proposed scheme and how it supports an optimized MN mobility performance. We argue that a solution considering multiple networks and operators has to contemplate a hierarchical splitting of the existing information. This is due to the fact that the amount and detail of information pertaining to specific PoAs of a single access network, and the combination of all these details for a number of access networks and different operators, may be very large.
The IEEE 802.21 allows the MN to restrict the response message size by optionally setting the MaxResponseSize parameter in the query message. When the response message exceeds the maximum size, some information must be removed from the MIIS response. Clearly, this is not suitable for the user. Removing important information may cause a sub-optimal handover decision. Considering this, and in order to improve the MIIS response in quality, we propose a hierarchical neighbor discovery scheme in which the network coverage area is divided into mobility zones, managed by different MIIS servers as illustrated in Fig. 1.