 same or better QoS as the current POA but at a lower price; e.g. transitioning a voice call from a cellular phone service to a VoIP service when moving into range of an 802.11 hotspot. New service A new application is started that requires a level of service that requirement is not supported by the current POA; e.g. downloading a large file may be able to take advantage of a higher data rate available on another network. The next step in providing uninterrupted connectivity to the mobile user is to be able to make similar seamless transitions across multiple wireless networks of different types. These so-called heterogeneous transitions might involve a single user connection successively handing-off from an 802.11 WLAN to a cellular phone service and then to a WiMAX MAN and finally back to a WLAN via a new access point. As for homogeneous transitions, there are a number of reasons why mobile users may want to make this type of heterogeneous transition, as shown in Table 14-2. The challenge of enabling uninterrupted, QoS guaranteed, heterogeneous hand-offs is being addressed in the IEEE 802.21 Working Group, which started work in March 2004. 802.21 defines media independent handover (MIH) mechanisms that enable networks such as Wi-Fi, WiMAX and cellular phone networks to co-operate at the Network and Data Link layers of the OSI protocol stack (see Figure 14-1). The MIH function is a unified technology-independent interface that provides inputs and context to the upper layers to assist in handover decision-making. In turn the MIH gathers the necessary information on link parameters such as uplink /downlink rate, signal strength and range, and link capabilities such as QoS and security. This information is Leading Edge Wireless Networking Technologies Applications: Multimedia streaming, Internet, e-mail, voice MIH Messages : inter-radio handoff 802.11 WiMAX Cellular Network selection protocol protocol protocol Manager stack stack stack Network discovery 802.21 Smart triggers Wi-Fi WiMAX WWAN/Cellular Figure 14-1: M