OUTLINE Multimedia Compression Requirements of Multimedia kernels CPU Scheduling Disk Scheduling Network Management An Example—ClineBlitz
MULTIMEDIA What Is Multimedia Media Delivery Media Characteristics Operating-System Issues
What Is Multimedia Multimedia describes a wide range of popular used application, which includes: Audio and video files—MP 3 audio files, DVD movies, short video clips and combination of them Live webcasts and webcams
A PowerPoint presentation used in a corporate office Virtual reality uses multimedia content VVO Multimedia-terminal in Germany
Multimedia Multimedia applications are delivered to Desktop computers Handheld devices—PDAs and smart cell phones
Media Rate Multimedia data are stored in the file system just like any other data, but it must be accessed at a specific rate Persistence of vision —the rate of the video is 24 ~ 30 frames per second, which keep it appear smooth to human eyes Continuous-media data—the video file must be accessed from the file system at a rate consistent with the rate at which the video is being displayed
Media Delivery -Local Playback Local playback: multimedia data are delivered from the local file system. Data comprise a regular file that is stored on local file system and played back from that system.
Media Delivery-Streaming Streaming: Technique that multimedia files are stored on a remote server and delivered to a client through a network. Progressive download — a media file containing audio or video is downloaded and stored on the client’s local file system. Real-time streaming — a media file is streamed to the client but is only played— and not stored—by the client.
Streaming Random access: both progressive download and real- time streaming allow user to move to different points in the stream, just as fast-forward and rewind operations on a DVD controller. Two types of real-time streaming: Live streaming — deliver an event, such as concert or a lecture, live as it is actually occurring. (random access is not allowed) On-demand streaming — deliver media streams such as full-length movies and archived lectures, which does not take place as the event is occurring. Well-known streaming media products include
Real-Time Streaming Live StreamingOn-demand Streaming
Live Streaming Example Tree 1 Tree 2 … … Video stream LS peers watching live video Video server
Media Characteristics Multimedia files are be quite large: 100-minute MPEG-1 video 1.125 GB storage 100 –minute HDTV 15 GB storage Continuous media may require very high data rate: a frame of color video is displayed at 800x600 resolution and using 24 bits to represent the color a single frame requires 800X600x24=11,520,000 bits of date the frames are displayed at a rate of 30 frames per second a bandwidth in excess 345 Mbps Multimedia application are sensitive to timing delays during playback
Operating-System Issues Quality of service (QoS): the specific rate and timing requirements that guarantee a computer system to deliver continuous-media data. Providing QoS requires several components in a computer system and influences related operating-system issues, such as CPU scheduling, disk scheduling, and network management. There are some examples: Compression and decoding may require significant CPU processing. Multimedia tasks must be scheduled with certain priorities to ensure meeting the deadline requirements of continuous media. File systems must be efficient to meet the rate requirements of continuous media. Network protocols must support bandwidth requirements while minimizing delay and jitter.
COMPRESSION Compression ratio Encoded/Decoded Algorithms
Compression Ratio Because of size and rate requirement of multimedia systems, multimedia files are often compressed from original form to a much smaller form, which will take up less storage space and more quick delivery. Compression ratio is the ratio of the original file size to the size of the compressed file.
Encoded/Decoded Algorithms Two compression algorithms: Lossy : some of the original data are lost when the file is decoded, which provides much higher compression ratios—images, audio, and video Lossless : ensures that the compressed file can always be restored to its original form—text files, computer programs(zip)
Lossy compression schemes —MPEG MPEG (Moving Picture Experts Group) refers to a set of file formats and compression standards for digital video. There are three major MPEG standards: MPEG-1 is used for digital video and its associated audio stream, which does not require high data rates and often used to download short video clips over the internet. MPEG-2 is used for compressing DVD movies and digital TV, which requires higher rates and is used for local playback. MPEG-4 is used to transmit audio, video and graphics including 2D and 3D animation layers, which provides a scalable level of quality and can be delivered to wireless devices.
REQUIREMETNS OF MULTIMEDIA KERNELS QoS levels Parameters defining QoS Admission control
QoS Levels Three QoS levels: Best-effort service: The system makes a best- effort attempt to satisfy the requirements, but no guarantees are made. Soft QoS: treats different types of traffic in different ways, giving certain traffic streams higher priority than other streams, but still no guarantees are made Hard QoS: The quality-of-service requirements are guaranteed.
Parameters Defining QoS Throughput is the total amount of work done during a certain interval—required data rate. Delay refers to the elapsed time from when a request is first submitted to when the desired result is produced. Jitter refers to delays that occur during playback of the stream, which is generally considered unacceptable for continuous-media applications Reliability refers to how errors are handled during transmission and processing of continuous media—lost packets in network or processing delays by CPU
Admission Control Admission control is simply the practice of admitting a request for service only is the server had sufficient resources to satisfy the request. Without admission control, the demands on the system might become so great that the system is unable to meet its QoS guarantees. Implement Admission Control: Using semaphores (Chapter 6) Resource reservations
Resource Reservation To use a resource, a client must make a reservation request for the resource in advance. If the request cannot be granted, the reservation is denied. An admission-control scheme assigns a resource manager to each type of resource. Requests for resources have associated QoS requirements. When a request for a resource arrives, the resource manager determines whether the resource can meet the QoS demands. If it cannot, the request may be rejected, or a lower level of QoS may be negotiated between the client and the server. If the request is accepted, the resource manager reserves the resources for the requesting client.
CPU SCHEDULING Real-time systems Static/Dynamic priority
Real-Time Systems Soft real-time systems simply give scheduling priority to critical processes. Hard real-time systems ensure that a critical task will be serviced within a guaranteed period of time.
Static/Dynamic Priority Static priority—the priority of a process will remain unchanged (real-time tasks) Dynamic priority—allow priorities to change over time (non-real-time tasks)
Disk Scheduling Network Management An Example-CineBlitz
Disk Scheduling Continuous-media files have two constraints that conventional data files do not have: Timing deadlines Rate requirements Disk scheduling algorithms must be optimized for these constraints and must be satisfied to preserve Qos guarantees. Continuous -media files typically require very high bandwidth rates to satisfy their data rate requirements
Disk Scheduling Contd.. Two disk scheduling algorithms that meet QoS requirements for continuous -media systems. Earliest Deadline- First Scheduling(EDF). SCAN-EDF Scheduling EDF can be used as a disk-scheduling algorithm which uses a queue to order requests according to the time each request must be completed (its deadline). It will serve requests according to deadline. Problem with this approach is servicing requests strictly according to deadline may result in higher seek times.
Scheduling Contd… Scan Scheduling Sort by cylinder# Moves in reverse direction
SCAN-EDF Scheduling SCAN-EDF is a hybrid algorithm that combines EDF and SCAN scheduling. It is similar to EDF ordering except that requests with the same deadline are ordered according to a SCAN policy
Scan-EDF Scheduling Several requests close together to deadlines? Scan EDF may batch requests,using scan ordering to service requests in the same batch. Suppose if deadlines are equally distributed, batches can be organized in groups of a certain size.
Unicasting & Multicasting There are three methods for delivering content from a server to a client across a network: Unicasting:The server delivers content to a single client.
Unicasting & Multicasting contd… Broadcasting: the server delivers the content to all clients, regardless whether they want the content or not.
Unicasting & Multicasting contd… Multicasting: The server delivers the content to a group of receivers that indicate they wish to receive the content.
Unicasting & Multicasting contd... Issues with Unicasting: Separate unicast session for each client. Waste for live real time streaming. Issues with Broadcasting: Not all clients wish to receive the same content. It is typically used in Local Area Network not across public Internet.
Real Time Streaming protocol Streaming media are delivered to clients from a standard web server using the HTTP protocol. Metafile is delivered using a continuous web server. The Problem with delivering streaming media from a standard web server is that HTTP is considered a stateless protocol. Difficult to pause or seek to random positions in the stream during playback.
RTSP Contd.. RTSP provides a stateful connection between the client and the server. Metafile is delivered from a streaming server using the RTSP protocol. Allows client to pause or seek to random positions in the stream during playback.
RTSP Commands RTSP defines several commands as part of its protocol: SETUP:The request specifies how a single media stream must be transported. This must be done before a PLAY request is sent.The server allocates resources for a client session. PLAY:The server delivers a stream to a client session. A range can be specified. If no range is specified, the stream is played from the beginning and plays to the end, or, if the stream is paused, it is resumed at the point it was paused. PAUSE:A PAUSE request temporarily halts one or all media streams, so it can later be resumed with a PLAY request RECORD:The RECORD request can be used to send a stream to the server for storage. TEARDOWN:A TEARDOWN request is used to terminate the session. It stops all media streams and frees all session related data on the server.
RTSP Contd... RTSP server may be in one of the three states: init ready playing Transitions between these three states are triggered when the server receives one of the RTSP commands from the client.
An Example : CineBlitz Cine Blitz supports both continuous media and conventional data Cineblitz guarantees to meet the rate requirements of real time clients by implementing an admission controller.
Disk Scheduling The cineBlitz disk schedulers services requests in cycles. The disk scheduler orders requests using C- SCAN order. Disk head moves to the beginning of the disk rather than reversing the direction.
Admission Control The network checks whether it has enough resources available to accept the connection, and then either accepts or rejects the connection request. The cineblitz admission controller admits a client only if there is enough disk bandwidth and buffer space to retrieve data for the clients at its required rate. During each cycle I, the server must,for each request Rj: Retrieve the data from disk to buffer(I mod 2). Transfer data from the ((I+1)mod 2) buffer to the client.
Admission control contd... Total buffer space required for N clients where client has rate requirement of r i r i is the required data rate for a given requested Ri
Admission control contd... Fundamental idea behind the admission controller in CineBlitz is to bound requests for entry into the queue according to the following criteria: The service time for each request is first estimated. A 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.
Admission control contd... The maximum latency incurred for servicing N requests is
Admission control contd... t-seek is worst case seek time. t-rot is rotational delay times. T * ri is Total data required for request Ri. b is size of disk block.
Admission Control contd.. The CineBlitz admission controller only admits a new client if there is at least 2 X T X ri bits of free buffer space and the following equation is satisfied: