Deadline-Aware Video Delivery in a Disrupted Bluetooth Network R. Razavi ESE Dept., University of Essex Colchester CO4 3SQ, U.K. rrazav@essex.ac.uk M. Fleury ESE. Dept., University of Essex Colchester CO4 3SQ, U.K. fleum@essex.ac.uk M. Ghanbari ESE Dept., University of Essex Colchester CO4 3SQ, U.K. ghan@essex.ac.uk Abstract Adaptive ARQ is applied to a wireless network, altering re- transmission policy according to content importance. By a video deadline-aware buffer discard policy, repeated trans- mission of expired packets and buffer overflow are miti- gated, helping to maintain video quality. Multiple dead- lines are applied according to picture type. The combined policy is demonstrated for a Bluetooth piconet achieving fast recovery after disruption. INTRODUCTION Wireless networks are susceptible to interference and other disruptions, which have a severe impact on delay-sensitive services such as video streaming. There exist two deadline types for encoded video: 1) a display deadline, herein set by the playout buffer size; and 2) a decode deadline rela- tive to the duration of a Group of Pictures (GOP). Packets forming an anchor frame (I- and P-picture) are still of value in retroactive decoding of subsequent P- or B-pictures [7]. However, if disruption is prolonged retained packets con- tribute to increasing buffer fullness, which may lead to excessive delay and, in severe cases, to send buffer over- flow. These circumstances lead to a need for a deadline- aware buffer (DAB) [8]. In the face of disruption, the DAB discards packets according to content importance. If Auto- matic Repeat Request (ARQ) is available, and in Bluetooth (B/T) [6] (IEEE 802.15.1) it comes for free by virtue of slave polling, then it too should be content adaptive. In this paper, DAB and adaptive ARQ are combined to allow fast recovery from disruption, which in our examples takes the form either of prolonged error bursts across a single B/T interconnect or while dynamic reconfiguration of a B/T piconet [12] occurs. Dynamic reconfiguration is applied to maximize overall piconet throughput. DAB with adaptive ARQ work together to avoid late packet arrivals. Otherwise, in conditions of heavy traffic and/or high bit error rates (BER), as mentioned, the send buffer may over- flow. The combination of the two techniques in a form that is relatively simple to implement on a B/T piconet is the paper’s main contribution. DEADLINE-AWARE BUFFER For reasons of error resilience, encoded video is transmit- ted as a repeating sequence of Group of Pictures (GOP), with the start of each GOP formed by an Intra-coded or I- picture. An I-picture is the basis for prediction of all other pictures in the GOP (usually 12 to 15 pictures in all) and, hence, its loss has drastic consequences for all other pic- tures in the GOP. Other predictive pictures or P-pictures also are essential for the reconstruction of some bi- directionally predicted or B-pictures within the GOP, though, in some cases, anchor pictures retained in the de- code buffer can form the basis for error concealment. Lastly, the third type of picture, the B-picture, has no pre- dictive value but is dependent on macro-blocks from adja- cent anchor pictures. The picture type is identifiable through the bit-stream header without decoding. In the conservative sender buffer discard policy of this pa- per, all packets of whatever picture type have a display deadline which is the size of the playout buffer expressed as a time beyond which buffer underflow will occur. In a conservative policy, in which there is no need for playout buffer fullness updates, the deadline is set as the maximum time that the playout buffer can delay the need for a packet. Playout buffers are normally present to smooth out jitter across a network path (if the B/T master was also an access point) and in this paper the size is assumed to be constant. In the discard policy, the propagation delay is assumed (optimistically) to be constant without retransmissions. In addition to the display deadline, all I-picture packets have a decode deadline, which is the display time remain- ing until the end of the GOP. Thus, for a 12 frame GOP, this is the time to display 11 frames, i.e. 0.44 s at 25 frame/s. For P-picture packets, the time will vary depend- ing on the number of frames to the end of the GOP. For B- pictures the decode deadline is set to zero. The decode deadline is added to the display deadline and a packet is discarded from the DAB after its total deadline expires. By storing the GOP end time, an implementation performs one subtraction to find each decode deadline. Account has been taken of I- B- P-picture reordering, Fig. 1, at encode and send buffer output, which has an effect on buffer full- ness. Reordering is introduced to ensure that reference pic- tures arrive and can be decoded before the dependent B- pictures. ADAPTIVE-RETRY B/T employs variable-sized packets up to a maximum of five frequency-hopping slots of 625 μs duration. Due to the risk of RF noise, by default, Bluetooth provides repeat packet transmissions, if a packet is not acknowledged within a pre-set time. For every B/T packet transmitted from a B/T traffic source, the receiver replies with a packet occupying at least one slot. Therefore, a single-slot packet