Introduction

A number of wireless technologies have evolved rapidly during the past decade. Mobile devices and gadgets (e.g., cellular phones, personal digital assistants (PDAs), laptops) supported by some of these technologies are becoming more and more important in people's everyday life. Wireless local area networks (WLANs) and cellular networks are two paradigms of such technologies in the present wireless realm.

WLAN, which is based on the IEEE 802.11 standards, is able to provide services with high data rate up to 11 Mbps (802.11b) or 54 Mbps (802.11a/g) at a relatively low access and deployment cost. Moreover, 802.11n, which is still under development, promises to offer a maximum data rate of up to 700 Mbps. However, the coverage area of WLAN is typically less than 100 meters, making it only suitable for hotspot regions such as hotels, libraries, airports, and coffee shops.

Compared to the WLAN, cellular networks cover a much larger area that provides ubiquitous access over several kilometers. Nevertheless, the supported service data rate of cellular networks such as GSM (Global System for Mobile Communications), GPRS (General Packet Radio Service), UMTS (Universal Mobile Telecommunication System), or CDMA2000 (Code Division Multiple Access 2000) only ranges from a few kbps to 2.4 Mbps. Furthermore, the cost of accessing and deploying cellular networks is much higher than that of the WLANs.

Driven by the complementary characteristics of these two wireless technologies (high-rate, low-cost, small coverage area of WLAN versus low-rate, high-cost, large coverage area of cellular network), a strong trend of combining them into one integrated system has emerged during the past years [1]-[6].