7. Handling Receiver Heterogeneity

Stream sharing techniques such as broadcast and multicast enable large-scale deployment of VOD applications. However, clients of such applications might use different forms of end devices ranging from simple palmtop personal digital assistants (PDA), to powerful PCs and high-definition television (HDTV) as receptors. Delivering a uniform representation of a video does not take advantage of the high-bandwidth capabilities nor does it adapt to the low-bandwidth limitation of the receivers. A heterogeneous technique that can adapt to a range of receivers is critical to the overall quality of service (QoS) of the VOD system.

One solution to the heterogeneity problem is the use of layered media formats. The basic mechanism is to encode the video data as a series of layers; the lowest layer is called the base layer and higher layers are referred to as enhancement layers [5][21]. By delivering various layers in different multicast groups, a user can individually mould its service to fit its capacity, independently of other users. In the Receiver-Driven Layered Multicast technique [21], a user keeps adding layers until it is congested, then drops the higher layer. Hence, clients look for the optimal number of layers by trying to join and leave different multicast groups.

Another approach is the use of bandwidth adaptors [13] between the server and receivers. This technique provides means to adapt to the receiving capabilities within the periodic broadcast framework. The main role of the adaptor is the conversion of incoming video segments into segments suitable for broadcast in the downstream at a lower data rate. Since the video arrives repeatedly at the adaptor at a higher speed than the data rate of the broadcast initiated at the adaptor, an As Late As Possible caching policy is used to ensure that an incoming segment is cached only if it will not occur again before it is needed for the broadcast in the downstream. Furthermore, an As Soon As Possible policy is used to cast out any segment after its broadcast if this segment will occur again before it will be needed for a future broadcast in the downstream. The adaptor, thus, stores only what it needs from a video, but never the video in its entirety. In this environment, clients can be routed to the right adaptor according to their capabilities. The adaptor sending the video data to the clients becomes their server in a transparent fashion. Compared to techniques relying on multi-resolution encoding, a major advantage of the Adaptor approach is that clients with lesser capability can still enjoy the same video with no loss of quality.

A different technique, called Heterogeneous Receiver-Oriented (HeRO) Broadcasting [14], proposes a trade-off between waiting time and service delay in a periodic broadcast environment. HeRO is derived from the observation that even though a periodic broadcast might be designed for a particular client bandwidth, a client, depending on its arrival time, might actually need less bandwidth than what the broadcast scheme was intended for. In fact, clients can do this most of the time in a carefully designed broadcast environment. In HeRO, data fragmentation is based on the geometric series [1, 2, 2², … 2^K^-¹], where K is the number of channels allocated to the video. Each channel i periodically broadcasts the segment of size 2ⁱ^-1. Users with different capacities are constrained to start their download at some specific times. That is, users with high bandwidth can start the download at the next occurrence of the first segment; but others with less bandwidth must wait for some specific time to start the download. This technique is illustrated in Figure 31.13, where the server repeatedly broadcasts 4 segments on the first set of channels. The numbers at the top indicate the minimum number of loaders a client needs to have in order to start the download at the beginning of that specific time slot. Clients with less than this number of loaders will have to wait for a future slot. For instance, a client with two loaders can only start its download at the beginning of time slots 2, 3, 4, 6, or 7, etc. in order to see a continuous playback. This pattern repeats every broadcast period of the largest segment. Two such periods are shown in Figure 31.13. To reduce service latency for less capable clients, HeRO provides the option to broadcast the longer segments on a second channel with a phase offset equal to half their size. The example in Figure 31.13 uses two such channels. We note that clients with only enough bandwidth for one or two loaders now have more possible slots to join the broadcast. For instance, a client with one loader can now join the HeRO broadcast at the beginning of slot 6. We observe that the HeRO approach, unlike the multi-resolution encoding techniques, does not reduce the playback quality of low-bandwidth clients.

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Figure 31.13: A Heterogeneous Receiver-Oriented (HeRO) broadcast example.