VARIABLE TIME-SCALE AUDIO STREAMING OVER 802.11 INTER-VEHICULAR AD-HOC NETWORKS Antonio Servetti and Juan Carlos De Martin Dipartimento di Automatica e Informatica / IEIIT–CNR Politecnico di Torino Corso Duca degli Abruzzi 24, 10129 Torino, Italy Phone: +39-011-564-7036 / -5421, fax: +39-011-564-7099 E-mail: [antonio.servetti juancarlos.demartin]@polito.it ABSTRACT This paper presents an analysis of audio streaming in an inter-vehicular network based on 802.11b wireless devices. In such a scenario characterized by strong link availability variations, we investigate the performance of an adaptive packet scheduling policy that adapts the inter-packet trans- mission interval to the channel conditions. Network simu- lations are used to evaluate the effects of varying the trans- mission time scale between zero, when the connection is not available, to as fast as possible when the channel is available and reliable. Results show that the proposed approach en- sures high quality audio streaming among the nodes of the inver-vehicular network by heavily reducing the percentage of lost packets and with only a limited increase in the delay and jitter. 1. INTRODUCTION IEEE 802.11 Wireless LAN (WLAN) products have be- come widely used because of their simple set-up and mod- erate cost. Potential uses of such equipment range from WLAN hot spots to direct connectivity of devices in ad-hoc mode. Ad-hoc networks are a key factor in the evolution of wireless communications enabling data exchange between wireless hosts in absence of a centralized fixed infrastruc- ture. In an inter-vehicular scenario, for instance, vehicles can operate as a pure ad-hoc network in which each indi- vidual vehicle broadcasts data to other vehicles. Due to the relative novelty of the application, few ef- fort have been devoted to study and simulate 802.11 inter- vehicular transmissions [1][2], and, to the best of our knowl- edge, exploitation of inter-vehicular 802.11 communica- tions for real-time multimedia services received even less attention. In [3] performance of video communications This work was supported in part by MIUR, Project FIRB PRIMO, http://primo.ismb.it has been evaluated while driving two cars equipped with 802.11b standard devices in urban and highway scenarios. The experiments show that each scenario presents pecu- liar characteristics in terms of average link availability and SNR which can be exploited to develop more efficient inter- vehicular applications. The strong link availability varia- tions experienced in the highway scenario suggest, in fact, that a variable time-scale transmission policy may be inves- tigated for multimedia streaming to mitigate the effect of frequent disconnection between the mobile stations. Streaming implementations developed and tuned for wired connections or wireless environments with limited mobility are usually designed to cope only with limited vari- ations in network latency and bandwidth [4]. Before start- ing the playout, a pre-roll delay is then used to fill the re- ceiver buffer. Buffering, in fact, reduces system sensitiv- ity to short-term fluctuations in the data arrival rate by ab- sorbing variation in end-to-end delay. However, if the rate offered by the channel falls below that of the source, the buffer will soon underflow. In this case rate adaptive algo- rithms are used to adapt the source rate to the current state of the network so as to generate only the bandwidth that the network is capable of carrying. The assumption is, in fact, that the distortion introduced by lowering the source coding rate is smaller than the expectedly larger distortion due to packet losses. To deal with the fast changing inter-vehicular wireless scenario, where the connection to other mobile nodes is fre- quently lost and the streaming flow is interrupted, in addi- tion to the foregoing techniques appropriate streaming algo- rithms must be implemented so that the receiver has enough data to continue the playback until the connection is re- established. Then the buffer needs to be refilled to a level that provides sufficient protection for a subsequent discon- nection [5]. In this paper we analyze the performance on audio streaming of an adaptive packet scheduling (APS) technique