Power-Efficient Interface Selection Scheme using Paging of WWAN for WLAN in Heterogeneous Wireless Networks SuKyoung Lee* and Nada Golmie † *Yonsei University, Seoul, Korea. Email:sklee@cs.yonsei.ac.kr † National Institute of Standards and Technology, Gaithersburg, MD, USA. Email:nada.golmie@nist.gov Abstract— In heterogeneous wireless networks, there have been several efforts aimed at having mobile devices equipped with multiple interfaces connect optimally to the access network that minimizes their power consumption. However, a study of existing schemes notes that in the idle state, a device with both a WLAN and a WWAN interface need to keep both interfaces “on” in order to receive periodic beacon messages from the AP (WLAN) and downlink control information from the base station (WWAN), resulting in significant power consumption. Therefore, in this paper, we propose a Power-efficient Communication Protocol that includes turning off the WLAN interface after it enters the idle state and using the existing paging of WWAN in order to wake up the WLAN interface when there is incoming long-lived multimedia data. Further, we propose turning on the WLAN interface when the number of packets in the radio network controller’s buffer reaches a certain threshold level in order to avoid repeatedly turning on and off WLAN interfaces, that consumes a significant amount of power. The tradeoffs between the power saving and the number of packets dropped at the buffer are investigated analytically. Simulation results for scenarios of interest are also provided. I. I NTRODUCTION Most recently, a significant number of telecommunication carriers are migrating towards heterogeneous wireless net- works where Wireless Local Area Networks (WLANs) based on IEEE 802.11 standards and third-generation Wireless Wide Area Networks (3G WWANs) such as CDMA2000 and UMTS are interconnected in order to offer Internet access to end users with better Quality of Service (QoS). These trends are set by the well-known fact that the two technologies have characteristics that complement each other perfectly. However, before a cost-effective and seamless integration of heteroge- neous wireless networks is realized, a number of issues have to be resolved. There are several research and standards group activities including the recently formed IEEE 802.21 Working Group focused on this integration of networks. In particular, since most mobile terminals are battery pow- ered, it is a challenge to design new techniques that allow mobile terminals to maintain their active connection as they move across different types of wireless networks, that is known as “vertical handoff”, while minimizing their power consumption [1]. There have been several efforts aimed at hav- ing mobile devices equipped with multiple interfaces, switch their connection to the access network that provides the best coverage. The authors of [1] introduced several performance metrics that can be used in the handoff decision. In [2], various network layer based inter-network handover techniques are evaluated for a realistic heterogeneous network testbed . As for a potential integration architecture for WLAN and 3G WWAN, the authors of [3] describe a loosely-coupled architecture in the form of an IEEE 802.11 gateway and a corresponding service access client software. Here, it is worth mentioning that a large portion of the power consumption in a wireless interface corresponds to the power consumed while the interface is idle. In most existing vertical handoff management schemes [1]-[2], a mobile node must turn on both WLAN and WWAN interfaces even in the idle state in time to receive the periodic beacon signals from the AP and the signal through the downlink Paging Channel (PCH) from the Base Station (BS), resulting in significant power consumption. Therefore, in this paper, we propose a Power-efficient Communication Protocol (PCP) for heterogeneous wireless networks. This scheme assumes that if a certain time expires just after the WLAN interface enters the idle state, the interface is turned off without any periodic wake-up. In the remainder of this paper, this state will be referred to as the ”inactive state”. In addition, we use the existing paging in cellular networks in order to turn on the WLAN interface due to incoming data from long-lived multimedia traffic. Our goal is to keep the WLAN interface, which consumes a significant amount of power in the idle mode, off for a longer period of time. Therefore, we propose using the relatively lower-power PCH for waking up the WLAN interface on an as-needed basis. If the WLAN interface spends considerable amount of time in the idle state, as in the case of Internet access and long multimedia downloads, there are obvious benefits for entering the inactive state and limiting the power consumption. Further, our proposed scheme is designed to avoid repeat- edly turning on and off WLAN interfaces, that consumes a significant amount of power. We propose turning on the WLAN interface when the number of packets in the Radio Network Controller (RNC)’s buffer reaches a certain threshold level. The remainder of this article is structured as follows. Section II discusses the proposed PCP in details. Section III provides an analysis of the power consumption during non- communication state for a typical WLAN node, as well as the proposed PCP. Section IV provides simulation results and a discussion of the results. Conclusions are offered in Section V. II. POWER-EFFICIENT COMMUNICATION PROTOCOL When connected to the WLAN, a WLAN interface card is usually in the idle mode for around 70% of the overall time