Multimedia Systems

What is Multimedia?

• Compression

• Requirements of Multimedia Kernels

• CPU Scheduling

• Disk Scheduling

• Network Management

• An Example: Cineblitz

Objectives
To identify the characteristics of multimedia data

• To examine several algorithms used to compress multimediadata

• To explore the operating system requirements of multimedia

• data, including CPU and disk scheduling and network management

What is Multimedia?
Multimedia data includes
- audio and video clips (i.e. MP3 and MPEG files)
- live webcasts
Multimedia data may be delivered to
- desktop PC’s
- handheld devices (PDAs, smart phones

Media Delivery

• Multimedia data is stored in the file system like other ordinary data

• However, multimedia data must be accessed with specific timing

requirements

• For example, video must be displayed at 24-30 frames per second. Multimedia video data must be delivered at a rate which guarantees 24-30 frames/second
• Continuous-media data is data with specific rate requirements

Streaming

• Streaming is delivering a multimedia file from a server to a client - typically the deliver occurs over a network connection

• There are two different types of streaming:

      1. Progressive download - the client begins playback of the multimedia file
           as it is delivered. The file is ultimately stored on the client computer
      2. Real-time streaming - the multimedia file is delivered to - but not stored
          on - the client’s computer
Real-time Streaming
There are two types of real-time streaming:
(1) Live streaming - used to deliver a live event while it is occurring
(2) On-demand streaming - used to deliver media streams such as movies,
archived lectures, etc. The events are not delivered in real-time

Multimedia Systems Characteristics
Multimedia files can be quite large

• Continuous media data may require very high data rates

• Multimedia applications may be sensitive to timing delays during playback of the media Operating

Compression
Because of the size and rate requirements of multimedia systems,
multimedia files are often compressed into a smaller form

• MPEG Compression:

     (1) MPEG-1 - 352 X 240 @ 30 frames/second
     (2) MPEG-2 - Used for compressing DVD and high-definition television
           (HDTV)
     (3) MPEG-4 - Used to transmit audio, video, and graphics. Can be delivered
           over very slow connections (56 Kbps)


Operating Systems Issues
The operating system must guarantee the specific data rate and timing
requirements of continuous media

• Such requirements are known as Quality-of-Service (QoS) guarantees

QoS Guarantees
Guaranteeing QoS has the following effects in a computer system:
(1) CPU processing
(2) Scheduling
(3) File systems
(4) Network protocols

Requirement of Multimedia Operating Systems
There are three levels of QoS
(1) Best-effort service - the system makes a best effort with no QoS
guarantees
(2) Soft QoS - allows different traffic streams to be prioritized, however
no QoS guarantees are made
(3) Hard QoS - the QoS rquirements are guaranteed

Parameters Defining QoS

• Throughput - the total amount of work completed during a specific time interval

• Delay - the elapsed time from when a request is first submitted to when the desired result is produced

• Jitter - the delays that occur during playback of a stream

• Reliability - how errors are handled during transmission and processing of continuous media

Further QoS Issues

• QoS may be negotiated between the client and server
• Operating systems often use an admission control algorithm that admits a request for a service only if the server has sufficient resources to satisfy the request.

Resources on a file server

CPU Scheduling

• Multimedia systems require hard realtime scheduling to ensure critical tasks will be serviced within timing deadlines

• Most hard realtime CPU scheduling algorithms assign realtime processes static priorities that do not change over time

Disk Scheduling

• Disk scheduling algorithms must be optimized to meet the timing deadlines and rate requirements of continuous media

• Earliest-Deadline-First (EDF) Scheduling

• SCAN-EDF Scheduling

Disk Scheduling (Cont)

• The EDF scheduler uses a queue to order requests according to the time it must be completed (its deadline)

• SCAN-EDF scheduling is similar to EDF except that requests with the same deadline are ordered according to a SCAN policy.