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Lecture 2: Introduction to Multimedia Lecture 3: Multimedia Networks

Multimedia Communications 3. Lecture 2: Introduction to Multimedia Lecture 3: Multimedia Networks Important performance parameters in multimedia networking Roles in distributed multimedia communications

kerry-grimes
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Lecture 2: Introduction to Multimedia Lecture 3: Multimedia Networks

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  1. Multimedia Communications 3 • Lecture 2: Introduction to Multimedia • Lecture 3: Multimedia Networks • Important performance parameters in multimedia networking • Roles in distributed multimedia communications • Distributed multimedia: distribution of multimedia information between different geographical locations • Transporting multimedia information across a communications network

  2. Distributed Multimedia Applications 3 • Digitization and networking  a distributed information society • Applications of distributed multimedia: many & various • Each application places specific performance requirements on the network

  3. Distributed Multimedia Applications 3

  4. Peer-to-Peer and Multipeer Communications 3 • Two basic modes of multimedia communications: unicast and multicast • In unicast mode: two communicating partners, or peers, peer-to-peer communications • In multicast mode: 1 to n communications, or peer-to-multipeer • Broadcast mode: 1 to all communications

  5. Peer-to-Peer and Multipeer Communications 3 • Client-to-server applications such as home-shopping, online banking, video-on-demand, or multimedia email: unicast • Distance learning or teleseminar: peer-to-multipeer • Teleconferencing: multipeer-to-multipeer

  6. Peer-to-Peer and Multipeer Communications 3 • Multiparty Interactive Multimedia (MIM): multipeer communications • Computer Supported Collaborative Work (CSCW): distributed sharing of a multimedia workspace (a common set of files, graphical displays, and a distributed whiteboard, applications such as spreadsheets, editors, and drawing programs) • Collaborative workers at different locations solve large design or engineering problems in real-time

  7. Peer-to-Peer and Multipeer Communications 3 • MIM interactions: dynamic or static • Dynamic interactions: all participants are allowed to exchange information at any time, such as a multimedia teleconference; CSCW; a Virtual Café (Internet Chat Session) • Static interactions: only a prescribed subset of participants are allowed to present information, such as in multicast mode, information passed from a central source to many receivers; in monitoring, information sent from many sources to a single receiver; teleteaching

  8. Network Performance Parameters for Multimedia 3 • Key network performance parameters for multimedia communications: • Bit rate • Throughput • Error rate • Delay • Each plays a vital role in transport of audiovisual signals over a digital network

  9. Network Performance Parameters for Multimedia 3 • Throughput: effective bit rate, or effective bandwidth • Equals to the physical-link bit rate minus the various overheads needed by transmission technologies • In high speed networking such as employing ATM technology over SONET (Synchronous Optical Network), the network carrier’s provisioned bit rate 155.52 Mbps

  10. Network Performance Parameters for Multimedia 3 • Principal overheads: 3% for SONET, 9.5% for ATM • The maximum throughput: 136 Mbps • Other factors: network congestion, bottlenecks, node or line faults

  11. Error rate 3 • Bit Error Rate (BER): ratio of average number of error bits to total number of transmitted bits • Packet error rate (PER): ratio of average number of error packets to total number of transmitted packets in data communications • Packet: in data communications, a data unit belonging to level 3 of ISO reference model

  12. Error rate 3 • ISO: International Standards Organization, in Geneva, developing industrial standards in numerous fields including computing & data communications • Frame Error Rate: applied to ATM networks, ratio of average number of error frames to total number of transmitted frames

  13. Error rate 3 • Frame: in data communications, a data unit belonging to level 2 of ISO reference model • In most modern networks: BER ~ 10-9 - 10-12 in fiber optics transmission systems, ~10-7 in satellite digital circuit • ~ One bit error per frame in digital video transmission • In interbanking: a single error bit might be catastrophic

  14. Delay 3 • End-to-end delay: time to transmit a block of data from sending to receiving end system • Transmit delay: a physical parameter for propagation time to send one bit from one site to another • Limited by speed of light & distance traversed • Significant in satellite links

  15. Delay 3 • Transmission delay: time to transmit a block of data end-to-end • Limited by bit rate of network and processing time in intermediate nodes (routing, buffering, etc.)

  16. Delay 3 • Network delay: composed of transit and transmission delay • Interface delay: waiting time from sender-ready to network- ready • In connection-oriented networks (an end-to-end circuit) & token ring LANs (a free token)

  17. Round-Trip Delay 3 • Round-trip delay: total time for sender to send a block of data through network and receive an acknowledgement of block correctly received • Gives a better picture of network performance than end-to-end delay when networks very congested • Plays a role in TCP networks running on top of connectionless IP networks

  18. Delay Variation or Jitter 3 • Uniform latency not guaranteed by most of today’s networks • Variations in delay referred to as jitter: imperfection in hardware or software, traffic conditions • Upper limit on permissible jitter in designing a multimedia network

  19. Characteristics of Multimedia Traffic Sources 3 • Multimedia traffic often caused by long streams of video/audio data • Even if broken up into packets or frames for network transport, the integrity of streams must be observed, placing constraints on network performance parameters

  20. Characteristics of Multimedia Traffic Sources 3 • How do network performance parameters affect multimedia traffic? • Multimedia traffic: audio, video, data, bit-mapped images, line drawings, 3D graphics • Audio/video: continuous • Others: usually discrete

  21. Characteristics of Multimedia Traffic Sources 3 • Multimedia data streams characterized by • throughput variation with time • time dependence • bidirectional symmetry

  22. Throughput Variation with Time 3 • Multimedia traffic characterized as constant bit rate (CBR) or variable bit rate (VBR) • Constant Bit-Rate Traffic: Many multimedia applications such as CD-ROM applications generate output at CBR • For real-time applications, it is important for network to transport these data streams at CBR

  23. Throughput Variation with Time 3 • Otherwise, extensive buffering at each end system • Many networks such as ISDN: CBR data transports • Variable Bit-Rate Traffic: A data rate various with time in bursts or spurts • Bursty traffic: Random periods of relative inactivity interspersed with bursts of data • A bursty traffic source generates varying amounts of data at different time periods

  24. Throughput Variation with Time 3 • A good measure of burstiness: Ratio of peak traffic rate over mean traffic rate over a given period of time • Recent advances in video coding technology  VBR traffic streams • In a slow-moving scene: No need to retransmit, from frame to frame, static parts of the scene • In a motion video scene: New data for motion of objects generated by compression algorithm

  25. Throughput Variation with Time 3 • VBR: To conserve transmission capacity or to control display quality • VBR video streams: Inherently bursty but can be adapted to CBR data networks • VBR traffic: Relatively new in multimedia communications

  26. Time Dependency 3 • In applications such as video conferencing, the traffic generated is in real-time: End-to-end latency must be kept low • For videoconferencing, the delay must be at most 150 ms • For multimedia email, the traffic not required to be real-time

  27. Bidirectional Symmetry 3 • When two end-systems connected by a network, traffic over connection is often asymmetric • In a cable network serving video-on-demand application: Video data sent to the client on forward data channel, and selection request by the client sent on reverse (control) channel • Peer-to-peer teleconferencing traffic: Symmetric

  28. Factors Affecting Network Performance 3 • Network performance parameters: Throughput, error rate, delay, and delay jitter • Throughput of most networks, whether LAN or WAN, varies with time • Throughput can change very quickly due to • node or link failures • congestion • bottlenecks • buffer capacity • flow control

  29. Node or Link Failures 3 • Operation interruption in network nodes or transmission links congestion in other nodes and links in immediate vicinity • Packet delays or loss, file transfer errors, or even total loss of connectivity • Failure rates of network nodes or links are usually low, but failures do occur • Measures must be taken to guard against such incidences

  30. Network Congestion 3 • Congestion due to heavy traffic or bottlenecks • Capacity of a network usually designed to accommodate average traffic demands • At certain times of the day or in emergency situations, demand for network capacity > availability: throughput decreases due to: • many datagram networks drop packets as node buffers overflow • network management procedures take effect to decrease traffic on certain links • heavily loaded nodes become bottlenecks

  31. Bottlenecks 3 • Bottlenecks due to node or link failures, or due to inadequate link or node capacity • TransAtlantic satellite links connect data networks in North America to those in Europe • Many of them: A throughput of 128 kbps • When these links connect two high-speed networks such as T-1 or E-1 on opposite sides of the Atlantic: A significant bottleneck • Internet users experience

  32. Buffer Capacity 3 • For each end-to-end connection, there is a limited amount of buffer memory at the end-systems and at the network interfaces Interface Interface End system End system Network Buffer Buffer Buffering in End-to-End Connections

  33. Buffer Capacity 3 • Data temporarily store in those buffers when sending to or receiving from the network • In transmission of large files such as video frames, buffer capacity is very often inadequate to send or receive in real-time

  34. Flow Control 3 • When buffer capacity at either end is a problem, flow control protocols are often invoked • Flow control (an end-to-end protocol) limits the rate of data transmission between two end-systems connected through a network • When the receiving end-system does not have sufficient buffer capacity to accommodate all data sender wishes to transmit, the protocol is invoked to limit or meter the data rate from sender to prevent data loss at the receiving end-system

  35. Flow Control 3 • End-to-end throughput affected as flow control in operation • Flow control not a network performance parameter • Invoked by end-system buffer limitations

  36. Issues in Network Error Performance 3 • Errors: a major concern in packet-switching networks • individual bits in packets inverted or lost • packets lost in transmission • packets dropped or delayed • packets arrived out-of-order • Missing packets • lost in transit (inadvertent error) • dropped by intermediate node (deliberate error)

  37. Issues in Network Error Performance 3 • Error performance depends on communications protocols • connection-oriented networks: best for stream traffic • connectionless: good for short messages

  38. Individual Bit Errors 3 • With quality of today’s data transmission networks (e.g., fiber optics networks), bit errors are rare • Bit errors occur due to noise in lines or packet switches • Error detection codes in most packet switches detect presence of a bit error in the packet and can request retransmission of faulty packet • Retransmission handled in intermediate nodes or on an end-to-end bases

  39. Packet Loss 3 • In connection-oriented networks: Packets having bit errors or being lost or dropped, detected by the receiving end-system • But the receiving end-system does not always have precise information about which packets having such problems

  40. Packet Loss 3 • In connectionless networks: packet loss or dropped packets are hard to detect • Packets being lost or dropped in high-speed networks due to insufficient buffer space at the receiving end-system by congestion

  41. Out of Order Packets 3 • When a long file or stream of data transmitted, individual packets in the stream numbered in sequence • The receiving end-system shall arrange received packets according to the numerical order • Otherwise, received packets out of order

  42. Issues in Network Delay Performance 3 • Some network delay inevitable • Two end-systems communicating via satellite connection: one-way transit delay ~ 0.25 sec • Other delays: due to bit rate of link • Certain delays unpredictable: congestion, transmission errors, physical problems in lines and switching nodes, all called random delays

  43. Issues in Network Delay Performance 3 • Use of buffers can smooth out delay problems • A long video stream would be much less jitter if buffered before playback • Very desirable to have a constant, non-varying delay to the end-systems • With constant delay or zero jitter, buffer resources could be allocated in advance, received audio/video could have much higher quality

  44. Multimedia Traffic Requirements for Networks 3 • Expressed in terms of network performance characteristic: Throughout, reliability (error), latency, multicast communications

  45. Throughput Requirements 3 • High transmission bandwidth requirement • High storage bandwidth requirement • Streaming requirement: • a multimedia network must be able to handle long streams of traffic • must have sufficient throughput capacity to ensure availability of high bandwidth channels for extended periods of time

  46. Throughput Requirements 3 • For example, insufficient for a network to offer a user a 5-second time-slot at 1.5 Mbps if the user needs to send a stream of traffic of 30 Megabits • The streaming requirement met if the continuous availability of a 1.5 Mbps channel to the user • If there are many streams on the net at any one time, the network must have available throughput capacity equal to or greater than the aggregate bit rate of the streams

  47. Reliability (Error Control) Requirements 3 • Hard to quantify error control requirements for multimedia networks since multimedia applications are, to certain extent, tolerant of transmission errors • Visual and auditory senses in a human not equally tolerant of errors • Dropped packets more noticeable in audio stream than in video stream • Dropped packets more noticeable in text stream than in audio/video stream

  48. Reliability (Error Control) Requirements 3 • Hard to quantify error control requirements due to contradiction between error control and end-to-end latency • Error-control: detection and retransmission of packet in error or lost • In some cases, retransmission carried out on an end-to-end basis, significantly increasing delay • For real-time video/audio, delay is a more important performance issue than error rate

  49. Delay Requirements 3 • Multimedia data in form of multiple streams of data (video/ audio streams), different but interrelated parts of video scenes • In real-time applications, video/audio streams must be transmitted through network with min delay and synchronized with help of buffering

  50. Delay Requirements 3 • Asynchronous: latency can be any value • Synchronous: multiple streams traverse the network at essentially the same bit rate and arrive at destination end-system at the same time, a fixed, predictable delay over the transit delay • Isochronous: upper and lower bound of latency and small difference between

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