Power Aware Network Interface Management for Streaming Multimedia Davide Bertozzi Luca Benini DEIS - University of Bologna Bologna, ITALY 40136 Bruno Ricco’ Abstract—A significant fraction of the power in portable devices (handheld terminals, PDAs, laptops, etc.) is drawn by the wireless network interface card (NIC). We test the viability of a novel power management technique, based on the exploitation of the NIC off- mode while a client-controlled streaming multimedia application is in progress. The basic idea is to switch off the card while frames are being played back, until a low-threshold level is reached in the client buffer. In this paper, pessimistic assumptions are made on the timing and power overheads associated with recovering the card from off-mode, but nevertheless a power saving of about 25% is achieved over the average power consumption incurred by the standard IEEE 802.11 mechanism. We also provide design curves showing the minimum buffer size that makes our technique effective, as a function of the network bandwidth and of the card characteristics. I. I NTRODUCTION The main issue that has to be addressed in the design of a portable device is power consumption, due to the serious problem of limited battery lifetime [7]. It is well known that a significant fraction of the total power in a wireless station is drawn by the network interface transceiver [5], in particular by listening to the radio channel [8]. In [10] is shown that the parameter that mainly impacts the energy consumed by a wearable playback device for a wireless audio streaming and decoding is the cost for receiving the stream of compressed data from the wireless link. That is why the heavily-compressed low bit rate format of MP3 turns out to be less power-consuming of a lightly compressed one like MP1 or MP2, despite the bigger decoding overhead. Typically, the interface device is designed so to be able to work in different power states. The IEEE 802.11 draft Standard for WLANs itself comes with a power management mechanism [9]. When there is no wireless activity on the channel, the card is put in a less consuming state, called doze mode. IEEE 802.11 does not address the need for power management at the system level. The work done in [6] goes in this direction: a ”power manager” is defined that decides when and how to turn the NIC completely off according to the workload of the system. The power state transition is however scheduled when the user is not actively communicating via the card. In this paper we introduce an approach that aims to go a step further, as we try to power off the NIC while a streaming multimedia application is in progress, and precisely for the time the level of the playback buffer at the client is above a certain threshold. This approach comes with a buffer management policy that accommodates both the need of the NIC to minimize its costly switching activity and the need of the audio playback chip to never run out of data. Playback buffers are an interesting research topic, often investigated to deal with bandwidth and delay problems [2] [3]. In this paper we approach the playback buffer issue also considering its impact on power, thus evaluating the power efficiency of its configuration (buffer size and low-water mark level LWM). In section II our strategy will be explained more in detail. Section III addresses the relevant issue of the off-mode shortcomings, showing how our method deals with them. In section IV we describe the hardware setup. Results are reported in sections V and VI. In section VII conclusions are drawn. II. CLIENT- CONTROLLED STREAMING MULTIMEDIA Our approach targets the utilization of the NIC as well as the playback buffer design in a client portable device. Fig. 1 compares our technique with the normal behaviour of an IEEE 802.11-compliant portable wireless client. Throughout this paper, the simulations refer to an uncompressed audio stream, that enables us to focus on the efficient handling of data across the wireless link interface at the client side. We assume that streaming multimedia is always initiated by the client request, that triggers buffering of data at the playback buffer. In the standard approach, when the buffer is full and playback starts, the card enters doze mode and wakes up periodically to listen to the beacons coming from the AP. Both the turnaround time and the current drain associated with this operation may result in a sometimes hardly tolerable overhead. When the buffer level reaches the LWM, the card is activated to request more data from the server and refill the buffer. Our approach switches off the NIC when the buffering phase is over and recovers it from the power down mode when the LWM is crossed. In this way, the power overhead due to doze mode and to listening to the beacons is completely removed (see the lower part of Fig. 1). In doing this, we make the assumption that the streaming multimedia is client-controlled: the mobile host acts as a master and tells the server when the data transmission can occur. This enables the client to tolerate being disconnected from the network while it is off, and to wake up when it 0-7803-7376-6/02/$17.00 (c) 2002 IEEE. 922 926