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Multimedia Communications
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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
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Distributed Multimedia Applications
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Digitization and networking a distributed information society
Applications of distributed multimedia: many & various Each application places specific performance
requirements on the network
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Distributed Multimedia Applications
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Work in office Work on home PC Telecommute/desktop collaborationBoard games Electronic games Multiplayer interactive gamesLibrary research CD-ROM research Online services researchStore shopping CD-ROM shopping Internet shopping
Physical Electronic Networked
“Content” “Digitize” “Network”
Evolution of Networked Services
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Peer-to-Peer and Multipeer Communications
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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
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Peer-to-Peer and Multipeer Communications
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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
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Peer-to-Peer and Multipeer Communications
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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
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Peer-to-Peer and Multipeer Communications
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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
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Network Performance Parameters for Multimedia
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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
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Network Performance Parameters for Multimedia
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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
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Network Performance Parameters for Multimedia
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Principal overheads: 3% for SONET, 9.5% for ATM The maximum throughput: 136 Mbps Other factors: network congestion, bottlenecks, node
or line faults
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Error rate
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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
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Error rate
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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
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Error rate
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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
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Delay
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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
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Delay
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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.)
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Delay
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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)
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Round-Trip Delay
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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
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Delay Variation or Jitter
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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
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Characteristics of Multimedia Traffic Sources
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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
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Characteristics of Multimedia Traffic Sources
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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
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Characteristics of Multimedia Traffic Sources
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Multimedia data streams characterized by throughput variation with time time dependence bidirectional symmetry
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Throughput Variation with Time
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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
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Throughput Variation with Time
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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
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Throughput Variation with Time
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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
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Throughput Variation with Time
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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
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Time Dependency
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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
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Bidirectional Symmetry
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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
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Factors Affecting Network Performance
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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
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Node or Link Failures
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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
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Network Congestion
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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
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Bottlenecks
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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
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Buffer Capacity
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For each end-to-end connection, there is a limited amount of buffer memory at the end-systems and at the network interfaces
End systemInterface
Buffer
InterfaceEnd system
BufferNetwork
Buffering in End-to-End Connections
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Buffer Capacity
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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
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Flow Control
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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
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Flow Control
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End-to-end throughput affected as flow control in operation
Flow control not a network performance parameter Invoked by end-system buffer limitations
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Issues in Network Error Performance
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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)
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Issues in Network Error Performance
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Error performance depends on communications protocols connection-oriented networks: best for stream traffic connectionless: good for short messages
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Individual Bit Errors
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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
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Packet Loss
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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
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Packet Loss
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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
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Out of Order Packets
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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
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Issues in Network Delay Performance
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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
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Issues in Network Delay Performance
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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
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Multimedia Traffic Requirements for Networks
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Expressed in terms of network performance characteristic: Throughout, reliability (error), latency, multicast communications
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Throughput Requirements
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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
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Throughput Requirements
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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
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Reliability (Error Control) Requirements
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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
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Reliability (Error Control) Requirements
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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
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Delay Requirements
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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
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Delay Requirements
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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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Quality of Service (QoS)
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QoS: how well a network performs in dealing with a multimedia application
Individual applications have different expectations of network performance, expressed by QoS parameters
QoS parameters: max allowable delay, delay jitter, throughput,error rates
In real-time conferencing: Latency and throughput
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Quality of Service (QoS)
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QoS parameters can be defined explicitly A basis to determine if a network is able to meet QoS
requirement for a given application New QoS concepts due to multimedia communications:
resource reservation and scheduling resource negotiations admission control guaranteed QoS