Efficient packetisation scheme for Bluetooth video transmission R. Razavi, M. Fleury, E. Jammeh and M. Ghanbari The Bluetooth wireless link is likely to be the last hop in the delivery of an encoded streamed video clip. It is shown that it is preferable to optimally map arriving IP packets onto Bluetooth packets than to preserve the stream’s internal synchronisation structure. Video quality improves and latency reduces, even when there is cross traffic on the piconet. Introduction: For efficient transport across an IEEE 802.15.1 (Blue- tooth) wireless link [1], the slice structure of an MPEG-2 encoded video stream must be mapped onto the Bluetooth (B=T) packet structure. In the widely-deployed MPEG-2 codec, a slice consists of up to one row of macroblocks but unlike an MPEG-1 slice should not extend beyond one row. The main reason for defining slices is to prevent channel error propagation by means of synchronisation markers. Assuming a slice arrives at a B=T base station encapsulated in an IP packet then it must be formed into a B=T packet, the maximum sizes of which are quantised by the B=T channel time- slot structure. This Letter demonstrates that allowing a slice to be split between two B=T packets, dynamic packetisation, brings significant advantages when the receiver’s buffer size is limited. The advantages occur in terms of reduced overall delay and packet loss through buffer overflow, and improved peak signal-to-noise ratio (PSNR). The dynamic packetisation scheme achieved higher received video quality, despite the potential loss of synchronisation at the decoder due to the arrival of packets bearing partial slices. Only in conditions of very low SNR will dynamic packetisation possibly lose its attraction, in which case static slice allocation schemes may be considered. The results have implications for other codecs. Methodology: AB=T data frame in asymmetric mode across an Asynchronous Connection-Less (ACL) link consists of a packet from the master occupying one, three or five time slots and at least a single slot reply by a slave. In Table 1, the maximum user data rates are defined for a B=T v. 2.0 Extended Data Rate (EDR) ACL link at a gross air-rate of 3.0 Mbit=s. In this Letter, the assumed B=T controller behaviour is that, given a maximal B=T packetisation scheme, for example 3-DH5 or 3-DH3, then packets up to the maximum user payload will be formed. However, if the arriving packets do not justify the pre-set maximal scheme then a reduced scheme is applied. For example, the controller swaps from 3-DH5 down to 3-DH3 or even 3-DH1. Table 1: Packet types showing user payload and bitrates for B=T v. 2.0 EDR Packet type User payload in bytes Asymmetric max. rate in kbit=s 3-DH1 0–83 531.2 3-DH3 0–552 1776.4 3-DH5 0–1021 2178.1 Length and master to slave bitrates, for a single ACL master-slave logical link, DH ¼ data high rate, 3- indicates a gross air rate of 3.0 Mbit=s The experiments assume a per link buffer size of about 100 KB, equivalent to 50 packets. Reduced receiver buffer sizes are likely on low-cost mobile devices [2] and these must match sender buffer sizes to avoid further overflows. In [3]. it was concluded for B=T v. 1.2 basic rate, 732.2 Mbit=s, that, in an Additive White Gaussian Noise (AWGN) channel, if packets are fully-filled then DH5 packets, not DH3 or DH1, are optimal for E s =N 0 > 13.52 dB. For B=T v. 2.0, under the same setup as [3], simulation revealed that a maximal 3-DH5 packet scheme is preferable for an AWGN when E s =N 0 > 15.10 dB, as well as when packet loss only occurs through buffer overflow, as reported below. In conditions of low SNR, one of B=T v. 2.0’s lowergross air-rates is to be preferred rather than change the packet time-slot structure. In simulations, a European-formatted Standard Definition (SD) MPEG-2 encoded video clip, with group of picture structure of N ¼ 12, M ¼ 3 for duration of 40 s, generated data at an average rate of 1.77 Mbit=s. The clip contained moderate motion and, hence, a moderate bitrate for the given quality and size. IP packets were formed on a per-slice basis, each slice consisting of a row of macro-blocks. At 18 slices per frame, the IP packet arrival rate at the B=T master was 450 packet=s. This research employed the University of Cincinatti Bluetooth (UCB=T) extension to the well-known ns-2 network simulator (v. 2.28 used). The UCB=T extension has the advantage that it supports B=T EDR but is also built on the air models of previous B=T extensions such as BlueHoc from IBM and Blueware. Results: Fig. 1 shows the loss rate against packet size for selected input rate of constant-bit rate (CBR) traffic arriving at an error-free 3.0 Mbit=s EDR ACL link. It becomes clear that choice of packet length has a significant effect on the goodput. For example, at 1.7 Mbit=s input rate there are very low packet loss rates at a packet size starting around 875 B and extending just beyond 1000 B or again at around 1450 B, corresponding respectively to selection of a single 5-slot packet or one 5-slot packet and one 3-slot packet. 400 600 800 1000 1200 1400 1600 0 0.2 0.4 0.6 0.8 packet size, bytes lost packets/total packets Fig. 1 Packet loss rate against selected CBR input rate and packet length for 3.0 Mbit=s ACL link —s— input rate ¼ 1.77 Mbit=s —.— input rate ¼ 2.2 Mbit=s — þ— input rate ¼ 2.6 Mbit=s The distribution of slice sizes (equivalently arriving IP packet sizes) for the sample video clip in the simulations is shown in Fig. 2. The majority of the packet sizes fall within the range 50–850 bytes. This implies an inefficient B=T packetisation scheme in terms of the output data rate, as the B=T controller will select one or three slot packets for most of the packets. Sending any packets with a slightly larger size than the maximum of the defined packet sizes in Table 1 is inefficient as well. The controller will send a full packet for the first portion of the data but the remaining part will be sent in a partially filled single slot. 0 500 1000 1500 0 100 200 300 400 500 slice size, byte number of slices Fig. 2 Distribution of slice sizes for SD MPEG-2 encoded video clip A simple measure of burstiness was calculated by dividing the peak by the average instantaneous packet throughput and various allocations of slices to packets. Table 2 demonstrates that if the arriving IP burstiness is 3.459 then a simple single slice per packet scheme results in a bursty input at the receiver and a relatively low average bitrate, principally as a result of inefficient packetisation. Double-slice alloca- tion brings some improvement but most gain comes from dynamic packetisation. ELECTRONICS LETTERS 28th September 2006 Vol. 42 No. 20