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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