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CHAPTER 7: MULTIMEDIA NETWORKING

CHAPTER 7: MULTIMEDIA NETWORKING. Multimedia Applications Streaming VoIP Traffic Policing Quality of Service. MULTIMEDIA AND QOS. In data communications, multimedia consists of networked video and audio, i.e., applications that require “continuous” media.

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CHAPTER 7: MULTIMEDIA NETWORKING

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  1. CHAPTER 7: MULTIMEDIA NETWORKING • Multimedia Applications • Streaming • VoIP • Traffic Policing • Quality of Service

  2. MULTIMEDIA AND QOS In data communications, multimedia consists of networked video and audio, i.e., applications that require “continuous” media. Quality of Service (QoS) for networks is a set of mechanisms for ensuring high-quality performance for critical applications. These mechanisms help network administrators use existing resources efficiently to ensure the required level of service without reactively expanding their networks. Network Characteristics Managed by QoS • Traditionally, quality in networks meant that all traffic was treated equally. • All network traffic receives the network’s best effort. • No performance guarantees. • One bandwidth-intensive application can result in poor performance for all apps. The QoS concept views the requirements of some applications as more critical than others, so some traffic needs preferential treatment. CS 447 Chapter 7 Page 179

  3. MULTIMEDIA NETWORKING APPLICATIONS Classes of Multimedia Applications • stored streaming • Media stored at source, transmitted to client • Client playout begins before all data has arrived 2) live streaming • Streaming playback can lag tens of seconds after transmission • Examples: Internet radio talk show, live sporting event 3) interactive, real-time • End-to-end delay-sensitive (application packetizing, network traversal, etc.) • Examples: VoIP, videoconferences, distributed interactive worlds Fundamental Characteristics typically delay sensitive, e.g., end-to-end delay, delay jitter loss tolerant, i.e., infrequent losses cause minor glitches Opposite of other data, which is loss intolerant, but delay tolerant. CS 447 Chapter 7 Page 180

  4. BEST-EFFORT SERVICE TCP, UDP, and IP only provide “best effort” service, with no guarantees regarding delay or loss. These forces today’s Internet multimedia applications to use Application Level techniques to improve service. There are three common philosophies regarding the future of multimedia networking. Laissez-Faire Make no major changes Provide more bandwidth when needed Have the Application Layer handle content distribution, multicast, etc. Integrated Services Make fundamental changes to the Internet so apps can reserve end-to-end bandwidth This would require new, complex software in hosts & routers Differentiated Services • Requiring fewer changes to the Internet infrastructure, this would just provide 1st and 2nd class service CS 447 Chapter 7 Page 181

  5. MUSIC COMPRESSION Several Application Layer standards have evolved regarding the compression and encoding of musical audio files. (Used in MP3) Digitally encoding music by sampling the sound stream (Used in MIDI) Record physical input into a synthesized instrument controller (Used in MPEG-4) Specify signal-processing elements, like oscillators and filters, for synthesizing sound CS 447 Chapter 7 Page 182

  6. STREAMING AUDIO & VIDEO Workstation running Web browser Streaming multimedia server containing audio and video clips Web server with streaming multimedia metafiles 2. Media player is launched on workstation & URL is contacted on multimedia server 3. The multimedia server negotiates with the workstation regarding connection rate and sends the appropriate version of the audio or video file 5. The received packets are stored on a workstation buffer which, when full, sends them to the mediaplayer for decompression and execution 1. Request for metafile containing URL for audio or video file and instructions for Web browser to launch media player 4. The compressed audio or video file is packetized and sent via UDP, which does not accommodate retransmissions CS 447 Chapter 7 Page 183

  7. VOICE OVER IP Implementing voice communication on the Internet, while desirable, faces several problems that are still being resolved via the Session Initiation Protocol (SIP) and the Real-Time Transport Protocol (RTP): Delay Jitter Packet Loss Echo Security SIP sets up and terminates real-time multimedia sessions. RTP handles the end-to end transmission service, including payload type identification, sequential numbering of packets, timestamp and delivery monitoring. CS 447 Chapter 7 Page 184

  8. VoIP PACKET LOSS RECOVERY One means of recovery from packet loss is Forward Error Correction, in which redundant information is transmitted in order to make at least partial recovery possible. FEC Option 1 For every N packets, send an (N+1)st packet which is the XOR of the previous N. If one packet is lost, its contents can be determined from the other N. FEC Option 2 Piggyback a low-quality version of the audio from each packet onto the subsequent packet. If one packet is lost, the low-quality version of its contents can be determined from the next packet. CS 447 Chapter 7 Page 185

  9. REAL-TIME TRANSPORT PROTOCOL (RTP) V P X CSC M PT Sequence Number Running between the Application Layer and the Transport Layer (usually UDP), RTP formats packets to facilitate real-time interactive applications. V P X CSC M PT Sequence Number Timestamp Timestamp Synchronization Source Identifier Synchronization Source Identifier Contributing Source Identifiers (0 or more) Contributing Source Identifiers (0 or more) Version: 2-bit version number of version of RTP being used. Padding Flag: Indicates that payload is padded (# of padded bytes is last byte of padding). Extension Flag: Indicates that the fixed header is followed by one extension header. Contributing Source Count: # of Contributing Source Identifiers after fixed header. Marker Flag: Indicates that frame boundaries are marked in the packet stream. Payload Type: 7-bit identifier of payload format (JPEG, MPEG, Pulse Code Modulation, etc.). Sequence Number: 16-bit packet number to enable packet loss identification. Timestamp: Application-level sampling instant of the RTP packet’s first byte. Synchronization Source Identifier: Randomly chosen ID number for the current stream. Contributing Source Identifier: Additional SSRC Identifiers that contributed to the combined stream (e.g., a mixer-combined audio conference). CS 447 Chapter 7 Page 186

  10. LEAKY BUCKET ALGORITHM packet packet packet packet packet packet packet packet packet packet packet packet packet packet packet packet One approach to policing traffic restricts the rate at which endstations can place packets onto the network to a particular range of values Trying to send slower than agreed? NO PROBLEM! Trying to send slightly faster than agreed? NO PROBLEM (within limits)! Sending significantly faster than agreed? BIG PROBLEM! Interface throttles back on the transmissions! NETWORK CS 447 Chapter 7 Page 187

  11. TOKEN BUCKET ALGORITHM packet packet packet packet packet packet T T T T packet packet packet packet packet packet packet An alternative approach accommodates bursty traffic better by producing “tokens” at a steady rate, allowing packets to be released only if a corresponding token is available. If an endstation suddenly sends a large burst of packets, it’s allowed to transmit them until its reservoir of tokens has been depleted If an endstation sends at a slow rate for a while, tokens start to stack up (until the bucket is full - then tokens are discarded) NETWORK NETWORK CS 447 Chapter 7 Page 188

  12. DIFFERENTIATED SERVICES One of the two main QoS models for the Internet is Differentiated Services (DiffServ), which uses the “provisioned” approach of setting up network nodes to service multiple classes of traffic and varying QoS requirements. DiffServrequires end stations to use the old IPv4 Precedence and Type-of-Service fields to indicate one of 64 DiffServ service levels. Each internal node in the network is then configured to treat all incoming packets with the same service level in the same way. CS 447 Chapter 7 Page 189

  13. DIFFSERV ARCHITECTURE Since only the boundary nodes between adjacent DiffServ “clouds” must deal with traffic classification, shaping, and policing, the rest of the network nodes are free to concentrate on routing. DiffServ Problems: • Routers handle ToS fields inconsistently, making end-to-end service unpredictable • Serious congestion problems are resolved via… dropped packets! PHB: Per-Hop Behavior; LLQ: Low Latency Queuing; WRED: Weighted Random Early Detect CS 447 Chapter 7 Page 190

  14. INTEGRATED SERVICES In Integrated Services (IntServ), the second major Internet QoS model, an end-user application first classifies the level of service that it wants its packets to enjoy. IP then uses the Resource ReSerVation Protocol (RSVP) to establish a route with the desired QoS by means of each router’s Admission Control (i.e., does the router have sufficient resources?) and its Policy Control (i.e., does the end user have the administrative authority to make a reservation?). IntServ Problems: • Every device on the network must know and be able to handle RSVP • Routers must keep track of all paths currently using them, which means this approach isn’t really scalable to Internet-sized networks CS 447 Chapter 7 Page 191

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