Chapter 20 Embedded Systems

20 CHAPTER Multimedia Systems Exercises 20.1 Provide examples of multimedia applications that are delivered over the Internet. Answer: Examples include: streaming audio and video of current events as they are taking place, realtime video conferencing, and voice over IP. 20.2 Distinguish between progressive download and real-time streaming. Answer: Progressive download is the situation where a media file is downloaded on demand and stored on the local disk. The user is able to play the media file as it is being downloaded without having to wait for the entire file to be accessed. Realtime streaming differs from progressive downloads in that the media file is simply streamed to the client and not stored on the client disk. A limited amount of buffering might be used to tolerate variances in streaming bandwidth, but otherwise the media file is played and discarded without requiring storage. 20.3 Which of the following types of real-time streaming applications can tolerate delay? Which can tolerate jitter? • Live real-time streaming • On-demand real-time streaming Answer: Both delay and jitter are important issues for live real-time streaming. The user is unlikely to tolerate a large delay or significant jitter. Delay is not an important issue for on-demand real-time streaming as the stream does not contain live clips. These applications can also tolerate jitter by buffering a certain amount of data before beginning the playback. In other words, jitter can be overcome by increasing delay, 131 132 Chapter 20 Multimedia Systems and since delay is not an important consideration for this application, the increase in delay is not very critical. 20.4 Discuss what techniques could be used to meet quality of service requirements for multimedia applications in the following components of a system: • Process scheduler • Disk scheduler • Memory manager Answer: The process scheduler can use the rate monotonic scheduling algorithm to guarantee that the processor is utilized for meeting the quality of service requirements in a timely manner. In this scheme, processes are modeled to be periodic and require a fixed processing time every time they are scheduled. The disk scheduler needs to schedule requests in a timely manner and also minimize the movement of the disk head in order to minimize disk seek overheads. One option is to use a technique that combines the disk SCAN technique with the earliest-deadline-first strategy. Tasks are processed in the order of deadlines. When a batch of requests have the same or related deadlines, then the SCAN technique is used to satisfy the batch of requests. The memory management needs to ensure that unexpected page faults do not occur during playback of media files. This can be guaranteed if the required pages are swapped into physical memory before the multimedia application is scheduled. 20.5 Explain why the traditional Internet protocols for transmitting data are not sufficient to provide the quality of service guarantees required for a multimedia system. Discuss what changes are required to provide the QoS guarantees. Answer: Internet protocols such as IP and TCP do not typically reserve resources during connection setup time at the intermediate routers. Consequently, during congestion, when the buffers in routers fill up, some of the packets might be delayed or lost, thereby violating any quality of service requirements. Congestion losses could be avoided if the Internet uses ciruit switching and reservation of buffer resources to ensure that packets are not lost. 20.6 Assume that a digital video file is being displayed at a rate of 30 frames per second; the resolution of each frame is 640 × 480, and 24 bits are being used to represent each color. Assuming that no compression is being used, what is the bandwidth necessary to deliver this file? Next, assuming that the file has been compressed at a ratio of 200 : 1, what is the bandwidth necessary to deliver the compressed file? Answer: he bandwidth required for uncompress data is 30×640× 480 × 3 bytes per second assuming 640 × 480 frames at a rate of 30 frames per second. This works out to about 26 MB/s. If the file is compressed by a ratio of 200 : 1, then the bandwidth requirement drops to 135 KB/s. Exercises 133 20.7 A multimedia application consists of a set containing 100 images, 10 minutes of video, and 10 minutes of audio. The compressed sizes of the images, video, and audio are 500 MB, 550 MB, and 8 MB, respectively. The images were compressed at a ratio of 15 : 1, and the video and audio were compressed at 200 : 1 and 10 : 1, respectively. What were the sizes of the images, video, and audio before compression? Answer: The sizes of the images, video, and audio before compression are 33.33 MB, 2.75 MB, and 0.8 MBrespectively. 20.8 Assume that we wish to compress a digital video file using MPEG-1 technology. The target bit rate is 1.5 Mbps. If the video is displayed at a resolution of 352 × 240 at 30 frames per second using 24 bits to represent each color, what is the necessary compression ratio to achieve the desired bit rate? Answer: The bit rate required to support uncompressed video is 352*240*30*3/(1024*1024) MB/s = 7.2509 MB/s. The required compression ratio is therefore 7.2509*8/1.5:1 = 38:1. 20.9 Consider two processes, P1 and P2 , where p1 = 50, t1 = 25, p2 = 75, and t2 = 30. a. Can these two processes be scheduled using rate monotonic scheduling? Illustrate your answer using a Gantt chart. b. Illustrate the scheduling of these two processes using earliestdeadline-first (EDF) scheduling. Answer: If P1 were assigned a higher priority than P2 , then the following scheduling events happen under rate monotonic scheduling. P1 is scheduled at t = 0, P2 is scheduled at t = 25, P1 is scheduled at t = 50, and P2 is scheduled at t = 75. P2 is not scheduled early enough to meet its deadline. If P1 were assigned a lower priority than P2 , then the following scheduling events happen under rate monotonic scheduling. P2 is scheduled at t = 0, P1 is scheduled at t = 30, which is not scheduled early enough to meet its deadline. The earliest deadline schedule performs the following scheduling events: P1 is scheduled at t = 0, P2 is scheduled at t = 25, P1 is scheduled at t = 55, and so on. 20.10 The following table contains a number of requests with their associated deadlines and cylinders. Requests with deadlines occurring within 100 milliseconds of each other will be batched. The disk head is currently at cylinder 94 and is moving toward cylinder 95. If SCAN-EDF disk scheduling is used, how are the requests batched together, and what is the order of requests within each batch? 134 Chapter 20 Multimedia Systems request deadline cylinder R1 57 77 R2 300 95 R3 250 25 R4 88 28 R5 85 100 R6 110 90 R7 299 50 R8 300 77 R9 120 12 R10 212 2 Answer: Batch 1 (R1, R4, R5), batch 2 (R6, R9), and batch 3 (R2, R3, R7, R8, R10). Within batch 1, requests are scheduled as: (R5, R1, R4) Within batch 2, requests are scheduled as: (R9, R6) Within batch 3, requests are scheduled as: (R2, R8, R7, R3, R10) 20.11 Repeat the preceding question, but this time batch requests that have deadlines occurring within 75 milliseconds of each other. Answer: The batches are as follows: Batch 1: (R1), Batch 2: (R4, R5, R6, R9), Batch 3: (R10), Batch 4: (R2, R3, R7, R8) 20.12 Contrast unicasting, multicasting, and broadcasting as techniques for delivering content across a computer network. Answer: Unicasting is the situation where a server sends data to only one client. If the content is required by multiple clients and if unicast was the only mechanism available, then the server would have to send multiple unicast streams to reach the different clients. Broadcasting allows a server to deliver the content to all of the clients irrespective of whether they wish to receive the content or not. This technique could result in unnecessary work for those clients that do not need the content but still get the broadcast data. Multicasting is a reasonable compromise where the server can send data to some subset of the clients and requires support from the network router to intelligently duplicate the streams at those points in the network where the destination clients are attached to different sub-networks. 20.13 Describe why HTTP is often insufficient for delivering streaming media. Answer: HTTP is a stateless protocol where the server does not maintain any information regarding the clients. This is sufficient for accessing static data, but for streaming media where the clients might require the ability to pause the stream and resume it at a later time, a stateless protocol is insufficient. The server would not be able to keep track of the current position within the stream for a client and therefore would not be able to resume the streaming at a later point in time. Exercises 20.14 135 What operating principle is used by the CineBlitz system in performing admission control for requests for media files? Answer: Cineblitz differentiates clients into two classes: those that require realtime service and those do not. The resources are allocated such that a fraction of it is reserved for realtime clients and the rest are allocated to non-realtime clients. Furthermore, when a client requires realtime service enters the system, it is admitted into the system only if there are sufficient resources to service the new client. In particular, when a client makes a request, the system estimates the service time for the request and the request is admitted only if the sum of the estimated service times for all admitted requests does not exceed the duration of service cycle T.