The Internet Protocol (IP) was developed in the early 1980s with the aim of supporting connectivity within research networks. However, in the last decade IP has become the leading networking protocol. It is the basic tool for a plethora of client/server and peer-to-peer networks; it is predominant in both wired and wireless worlds, and now the current scale of deployment is straining many aspects of its more than 30 year old design. To overcome the limitations inherent in the original IP design, IPv6 has been proposed as the new protocol that will provide a firmer base for the continued growth of today's complex networks.
More people will access the Internet via wireless rather than via wired connections, and each user will have a set of wireless devices interconnected that will be accessing a great variety of IP-based services. There are over 3 billion mobile subscriptions in the world, and this number continues to grow, one for every two inhabitants. In the light of this growth, IPv4 faces many problems related to its limited address space and mobility capabilities applied to the mobile world.
Every mobile device is potentially capable of accessing IP services, Wi-Fi networks are becoming ubiquitous; the growth in the hotspot market is being accompanied by similar growth in other wireless technologies (e.g., Bluetooth, Ultra Wideband, and satellite), posing the urgent need for a new identification scheme and an adequate support for mobility.
Whereas the main thrust of IPv6 is to solve the addressing problem, it also provides important functions to enable mobility (e.g., scaling and ease-of-configuration). IPv4 has difficulties managing mobile terminals for several reasons such as address configuration and location management. However, in order to drive the evolution in the current mobile world and avoid the humongous effort to migrate all computers and equipments from IPv4 to IPv6, Charles Perkins, from Nokia Labs, originally proposed Mobile IPv4 (MIPv4). This protocol extension was projected to enable IPv4 devices to support micro and macro mobility. MIPv4 has disadvantages in comparison with its successor, Mobile IPv6 (built as a natural part of IPv6 protocol, and less of an extension, as in the case of IPv4), but neither Mobile IPv4 nor Mobile IPv6 were intended to support seamless roaming in heterogeneous environments such as 4G Networks.
The rapidly growing demand for "anywhere, anytime" high-speed access to IP-based services is becoming one of the major challenges for operators. As the demand for mobility increases, mobile terminals need to roam freely across heterogeneous systems forming the present wireless landscape. Currently, Mobile IP stands as the de facto solution for mobility management in 4G networks.
This work targets the user-driven evaluation of mobility in 4G networks, using MIPv4 as the underlying support protocol (Figure 1). In particular, it is of interest to evaluate the impact of vertical handovers (i.e. network handovers) and terminal mobility on user perception. A network handover occurs when a mobile device changes its point of attachment to the Internet and the former point belongs to a different wireless technology; for example, a mobile phone handing off from the cellular network to a public hotspot. A brief introduction to the basic concepts of Mobile IPv4 is included next to introduce readers to its basic concepts.
Mobile IP Version 4
Mobile IPv4 defines mechanisms that add support to a terminal (i.e. the Mobile Node) to change its point of attachment to the Internet whilst remaining reachable through a permanent address (the Home Address, HoA) and preserving all the active connections it had before travelling to its new location. While a Mobile Node (MN) is connected to its Home Network (i.e. the network its Home Address belongs to), no special mode of operation is needed, and packets are forwarded (using normal IP routing) between the Mobile Node and any other node it is communicating with (the Correspondent Node, CN). When a MN is connected to a network other than its Home Network (i.e. it is visiting a Foreign Network), the MN acquires an IPv4 address belonging to the address space of the Foreign Network it is visiting (supported by the Foreign Agent, FA), called the Care-of Address (CoA).
The MN announces its CoA (by sending a Binding Update message, BU) to a special entity, called the Home Agent (HA) that is located in the MN's Home Network. The HA intercepts the packets addressed to the MN's Home Address while the MN is away from its Home Network and establishes a bidirectional tunnel with the MN's CoA, in order to redirect these packets to the MN's current point of attachment to the Internet. The MN also uses this tunnel to send its traffic to the Correspondent Node, avoiding in this way any ingress filtering.
Latency on MIPv4-enabled links can be very high, especially for interactive applications that require real-time response. Therefore, the research community is working on mechanisms that decrease this latency as much as possible, at least to levels that support real-time applications. Two of the most significant proposals are Fast Handovers for Mobile IPv6 (FMIPv6) and Hierarchical Mobile IPv6 (HMIPv6).
FMIPv6 aims to decrease the total latency to almost only the Layer 2 handover time. This approach has been shown to perform well in intra-technology (i.e. horizontal) handovers. The HMIPv6 approach is designed to reduce the degree of signaling required and to improve handover speed for mobile connections by managing local mobility in a more efficient way. Previous work has analyzed which of these approaches (i.e. FMIPv6 and HMIPv6) performs better, the conclusion being that a combined approach would be optimal. However, given the implementation complexity that this would require, the FMIPv6 optimization by itself is good enough.
Future 4G networks, where heterogeneity will be more the rule rather than the exception, have challenging characteristics when performing vertical handovers (also known as inter-technology). The present work focuses on the impact of this new type of mobility on the user perception. The enhancement (better performance, lower latencies, etc) of the underlying protocols (i.e. MIPv4) is not in the scope of this research.
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