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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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, peerto-peer communications
 In multicast mode: 1 to n communications, or peer-tomultipeer
 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
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 In most modern networks: BER ~ 10 - 10 in fiber optics
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transmission systems, ~10 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-toend
 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 networkready
 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 T1 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
Interface
Interface
End system
Buffer
End system
Network
Buffer
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 realtime
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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-toend 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: oneway 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 endsystem 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
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