The development and proliferation of wireless and mobile technologies have revolutionized the world of communications. Such technologies are evolving towards broadband information access across multiple networking platforms in order to provide ubiquitous availability of multimedia services and applications.
Recent broadband wireless access systems include wireless local area networks (WLAN), broadband fixed wireless access (metropolitan networks) and wireless personal area networks (WPAN), as well as the widely used mobile access technologies, such as General Packet Radio Service (GPRS), Wide Code Division Multiple Access (WCDMA), Enhanced Data Rate for Global Evolution (EDGE), 3G and Beyond 3G (B3G) communications systems, Worldwide Interoperability for Microwave Access (WiMAX), and Bluetooth.
These wireless access technologies have characteristics that perfectly complement each other. Cellular systems and 3G provide wide coverage areas, full mobility and roaming, but traditionally offer low bandwidth connectivity and limited support for data traffic. On the other hand, WLANs provide high data rate at low cost, but only within a limited area, whereas WiMAX can supply mobile broadband for anyone, anywhere, whatever the technology and access mode. More specifically, WLANs are expected to provide access to IP-based services (including telephony and multimedia conferencing) at high data rates and reduced coverage in public and private areas.
In this context, several types of WLANs are emerging and become profusely used, allowing users to roam inside their home, enterprise or campus without interrupting their communication sessions. They are organized in form of hotspots, i.e. relatively small networks covering a particular location providing broadband and easy-to-use Internet access to their customers while supporting high traffic load. Classical hotspot examples are airports, hotels, dense urban areas, campuses, and private offices. Using hot-spots, providers can offer subscribers not only wide-area connectivity through the cellular infrastructure, but also increased bandwidth via Wi-Fi access points deployed in high concentration areas [1]. In this context, WLANs are often seen as a suitable complementary technology to the existing cellular radio access networks.
In order to fill the gap between WLANs and 3G wireless systems, IEEE 802.16 defines the Mobile WiMAX standard that provides the air interface specification for supporting broadband wireless access (BWA) in the context of wireless metropolitan area networks (WMAN). The Mobile WiMAX provides the capability to offer new wireless services, such as Voice over Internet Protocol (VoIP), videoconferencing, multimedia streaming, multiplayer interactive gaming, Web browsing, instant messaging, and media content downloading. Such services consume significant bandwidth, require short end-to-end delay, and may be offered with relatively low costs. In this context, the mobile terminal may be a wireless videophone, a laptop, or a PDA, and can be connected with a private or public network, from home to office. Combined with 3G, WiMAX offers high data rate services in addition to original voice services in hotspot areas. As a result, WiMAX and WLAN can be utilized as a powerful complement to 3G/B3G networks. In order to provide the mobile users with the requested multimedia services and corresponding quality of service (QoS) requirements, these radio access technologies will be integrated to form a heterogeneous wireless access network. Such a network will consist of a number of wireless networks, as illustrated below, and will form the 4th generation (4G) or next-generation of wireless networks. Heterogeneous wireless access, extensive support of IP-based traffic and excellent mobility support are among the main drivers for the architecture of such generation.
The 4G wireless networks will offer several advantages for both users and network operators. On one hand, users will benefit from the different coverage and capacity characteristics of each network throughout the integrated networks. In this way, a large set of available resources will allow them to seamlessly connect, at any time and any place, to the access technology that offers the best possible quality. For the network operators, the integration of all these technologies provides more efficient usage of the network resources, and may be the most economic and technologically diversified means of implementing the future anywhere, anytime, always-on visions, providing both universal coverage and broadband access. However, each access network provides different levels of QoS, in terms of bandwidth, mobility, coverage area and cost to the mobile users. As a result, when roaming across heterogeneous wireless access networks and experiencing vertical handoffs, high variability in the required QoS may be introduced, as the most optimal access network can dynamically change. In this case, choosing the correct time to initiate a vertical handoff request and select the best network to connect becomes important. Furthermore, vertical handoffs, i.e. handoffs between radio access networks using different technologies, require additional delay for reconnecting the mobile terminal to the new wireless access network, which may cause packet losses and degrade the QoS for real-time traffic. In this context, roaming and interworking between heterogeneous wireless access networks constitute important issues to the networking community.
