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1.5.2 Known and Unknown Variations
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A continuous media streaming system comprises many system components. At the highest level we have the media server and a number of media clients (Figure 1.10). For on-demand streaming servers the media data are typically stored in local storage devices such as hard disks, and then retrieved into memory for processing, and nally transmitted through the network interfaces into the network to the clients. The media client will receive the data arriving through a network interface, temporarily store them in memory buffers while waiting for decoding and playback to the user.
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Once playback begins, the playback schedule for all media data are set.
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Figure 1.9 Relation between start-up delay and the playback schedule
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Scalable Continuous Media Streaming Systems
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Media data playback of media may consume data at varying data-rate. Media codec encoding/compressing media may result in varying data-rate. Storage subsystem the I/O bandwidth may varies w.r.t. the workload. Network the bandwidth available may vary from time to time. Processor the available processor time may vary due to multi-tasking.
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Figure 1.10 Known and unknown variations in a continuous media streaming system
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If the many system components all operate in a clockwork manner with xed and known processing time, then there will be little problem in ensuring performance such as delay limit and playback continuity. In reality, these system components seldom, if at all, operate in a clockwork manner. Instead, the performance of the individual system components can and does vary with time. Some of these variations are known while others are unknown and often unpredictable. Let us begin by considering the preparation of the media data. Data compression is almost always needed to reduce the data rate of the raw media data (e.g., video in hundreds of Mbps) to practical levels (e.g., compressed into a few Mbps). This compression process, which we will cover in more detail in the next chapter, can introduce variations in terms of the amount of data needed for decoding each fragments of the media. Take video, for example, while the raw video frames are of the same resolution and size, the compressed video frames often consume varying amount of bytes for storage and transmission (cf. Section 2.4). Note that for stored video these variations are known and thus can be used in scheduling the media delivery process. Once stored in the media server s storage, these compressed media data will need to be retrieved in the memory buffer for processing and transmission. The most common storage devices in media servers are hard disks. Unlike memory-based storage, where I/O throughput and access time are relatively constant, disk through and access time can vary substantially depending on the device characteristics, I/O access pattern, as well as the scheduler used in managing the device. These issues will be addressed in 3. After successful retrieval from storage, the media server will then process the data (e.g., by adding control information in packet headers) and send them over the network. Depending on the network infrastructure, the time it takes for the media data to traverse the network to arrive at the media client can also vary. The variations are typically caused by temporary congestion
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Introduction
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Data packet loss
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Figure 1.11 Variations in the system can disrupt continuous media playback
in the network or changes in the path the data took to reach the client. For networks that support quality-of-service (QoS) guarantees, the delay and delay variations can be known (and controlled) a priori. Otherwise, for public networks such as the Internet, the delay and delay variations are often unknown and unpredictable. Finally, the media client, upon receiving the media data from the network, will buffer them in local memory to wait for decoding and playback. The decoding process typically takes a variable amount of time, and even the playback speed may sometimes vary. This is especially signi cant in the PC environment where the decoder/player is implemented in software and has to compete for processor time with other multi-tasking applications. The above discussions illustrate the fact that variations in a media streaming system are the norm rather than the exception. Any one of these variations can lead to media data arriving too late for playback, i.e., losing playback continuity as depicted in Figure 1.11. In the subsequent chapters we will illustrate some ways to compensate for these variations to ensure continuous media playback.