Multimedia Services
Tarik Cicic
University of Oslo
December 2001
Overview
• A framework for multimedia
communications in the Internet
– terminology
– problems
– what is being done
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Multimedia
• “multi + media”: using, involving, or
encompassing several media (Webster)
• usually meant as doing so by using
advanced electronic equipment, primarily
computers
• we are mostly interested in audio and video
perception media
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Multimedia terminals
• “Everybody” has a multimedia terminal
today
• some laptops play DVD movies with
acceptable quality
• but it is in the nature of the media to be
communicated to the others …
• … which is still difficult to realize (except
the telephony)
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Multimedia production example
Video player
Video capture
subsystem, ADC
Compression,
encoding
Video card
Analog signals
(e.g. S-video)
Disk storage
Memory storage
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Processing of MM data
• Selection of resolution, sampling rate, encoding
scheme and compression algorithm
• Reproduction and synchronization of different
media through the computer system components
• the final result is as good as the “worst”
component in the production chain
permits!
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Dilemma
• Since
– “everybody” has a multimedia terminal
– “everybody” is connected to the Internet
? why not send the multimedia over the
Internet ?
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Answer
• Yes, we (will) do it!:
– cost effectiveness
– new applications
– fun
• but …
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Unfortunately ...
… multimedia and the Internet as we know it are a
poor match:
– the Internet is based on the best effort principle
– no assumptions about the underlying hardware
– multimedia communications depend on knowledge of
available resources and must advance in a timely
fashion
• The Internet quickly becomes the mentioned
“worst component” in the multimedia system
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Network multimedia
Network cloud
Multimedia server
(or live source)
Data flow
Network router
Multimedia client
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Problems and solutions
• We would like our multimedia computers to
cooperate smoothly over the internet
• we may skip the IP and use another, more
multimedia friendly technologies, e.g. ATM
• OR we can modify and upgrade the IP
protocol family
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What does Internet offer …
• Not much:
– a promise that it will do its best to carry our
data packet to a destination
• fine for letters (e-mail), bad for IP telephony
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… and does not offer
• “Quality of Service” (QoS) guarantees
• an agreement with the network service
provider is needed about the quality of the
service
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QoS definition
• QoS: “The set of those quantitative and
qualitative characteristics of a
communication system that are necessary to
achieve the required functionality” (Vogel)
• many other definitions, e.g. “a measure of
the user’s satisfaction with the system”
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QoS levels
• Different levels of QoS specification:
– user level (“good/bad quality”)
– application level (“25 frames per second, 1600
* 900 pixel)
– network level (vector of peak BW, average BW
and max burst size)
• mapping between these
• reminds of the OSI layers?
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Network resources
• QoS provisioning deals with the network
resource allocation and use
• example:
– bandwidth: “Data quantity transferable per a
time unit”. Units: [kb/s], [MByte/s].
– buffer space in the communication devices
– router CPU time
• time is an absolute resource!
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Resources and QoS
• Mapping the QoS requirements to resources is
difficult (might be too difficult):
– inter-layer transitions are nontrivial
– QoS comparisons are often orthogonal
– an universal function f: QoS ! resource reservation
does not exist
• example: max delay
– an usual QoS parameter
– maps to network (link) resources, buffering and priority
queuing in the routers etc.
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Example: Max delay
•
•
•
•
•
•
Distance: 2000 km
9 hops, store and forward
Constant rate
100 Mb/s links
1000 Byte packets
Max delay: 100 ms
• ~ 1 MB buffer place per
interface should suffice
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Example: Audio transmission
• An audio application digitalizes speech at sampling
rate of 8kHz, 8 bit
• to transport this raw data we need a 64 kb/s CBR
stream,
• often transported as 25 pps, 320 Byte (40 ms audio)
• in addition, delay of <150 ms is a requirement
• in sender, 40 ms delay + , say, 10 ms on processing
• 2 packet buffering ! 80ms in receiver
• 150 ms – 130 ms = 20 ms allowed in the network!
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Example: QoS Routing
• Standard IP routing: one path
• QoS source routing:
– many paths
– remove those with too long delay (!)
– chose the one with the least traffic
• reject the request if unavailable BW or too
long delay
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Routing (Example)
I
• Networks are built of
nodes and links
• how to find way from
a sender to a receiver?
• we need a mechanism
to “teach” each node
where to forward data
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Forwarding (Example)
I
• The process of finding
way (“the outgoing
interface”) in each
node in the network
• important to
distinguish from the
routing
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A router’s task list
1.
Build and maintain a routing table containing the forwarding
decisions
2.
forward the incoming packets
3.
buffer the incoming packets, if processing capacity needs it
4.
buffer the outgoing packets, if the outgoing link needs it
5.
drop the packets as needed, perform the flow control
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Routing algorithms (Example)
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• Simple:
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– Prerequisite: enough network
information
– regard the network as a graph
– use Dijkstra’s Shortest Path
algorithm
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• more complex:
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– QoS routing
– traffic engineering
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Routing algorithms (Example)
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• Complex example:
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• link metrics are used instead of
the number of hops
• each link has 10ms delay
• delay constraint <= 50ms
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Routing algorithms (Example)
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• Complex example:
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• link metrics are used instead of
the number of hops
• each link has 10ms delay
• delay constraint <= 50ms
• rightmost path does not satisfy the
delay constraint
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Routing algorithms (Example)
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• Complex example:
• link metrics are used instead of
the number of hops
• each link has 10ms delay
• delay constraint <= 50ms
• rightmost path does not satisfy the
delay constraint
• two paths: 5/20 and 4/31
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Routing algorithms (Example)
I
1
• Complex example:
• link metrics are used instead of
the number of hops
• each link has 10ms delay
• delay constraint <= 50ms
• rightmost path does not satisfy the
delay constraint
• two paths: 5/20 and 4/31
• 5/20 is chosen, despite more hops
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Resource allocation & reservation
• Resource allocation deals with real
resources, while resource reservation is
analogous to booking
• mechanisms for admission and policy
control must exist in addition to the
resource reservation
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Problems
• Bandwidth itself is “unproblematic”. Its fair
distribution is more difficult and demands resource
reservation mechanisms
• some of the problems met by distributed
multimedia applications:
– continuous media over packet-switched networks
– fair treatment of many flows and users
– QoS transitions, lack of QoS support on some levels,
policy and admission control
– effective implementation of QoS routing
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A journey through the layers
• Distributed Internet
multimedia application
• QoS guarantees
needed
• protocol stack as
shown
RTP
UDP
IP
ATM
Physical
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Layer 1
• Bandwidth is plentiful at the physical layer
• of-shelf fiber optic carriers achieve
bandwidth of size order of 1 Gb/s
• achievable BW is up to 50 Tb/s
• electrical–optical conversion is a problem
• bottlenecks inside computer systems
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Layer 2
• Can provide basic QoS support
• example: ATM with Variable Bit Rate
• other approach: no QoS support, simple and
effective
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Layer 3
•
•
•
•
The network layer and IP have become synonyms
IP does not provide any QoS support by itself
additional mechanisms (e.g. RSVP) needed
IPv6:
– provides no QoS support
– the new priority and flow label fields simplify the QoS
support
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Higher layers
• UDP is often used for IP multimedia
• RTP (Real-time Transport Protocol, RFC
1889) provides transport for audio and
video data
• no resource reservation or guaranteed QoS
in RTP
• other protocols on top of IP, e.g. RSVP
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Where to locate QoS support?
• Factors:
– single layer (L3) should perform the routing
– QoS support is based on interaction with the
routing
– possibility for simple and effective lower layers
… talk in favor of Layer 3
• fine flow granularity favorites Layer 2
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World of plenty
• Another view on QoS provisioning says that
we do not need it!:
– amount of available bandwidth seems to be
ever-increasing
– everybody can use as much as needed
• this view contradicts with the experiences
of the human civilization so far
• what about different types of traffic?
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Restrictions
• Communication resources will persist as
restricted
• guaranteed services will be paid for
• “Busy signal” must be present in the
networks with limited resources and
guaranteed services
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Conclusion
• Different media types will converge to a
single network technology
• probably IP based, but much extended
• many unsolved questions
• tremendous progress in recent years
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Discussion: ATM vs. IP QoS
• ATM is by far the most sophisticated fastline network technology today
• ATM includes advanced QoS features
• ATM looses ground to IP.
WHY?
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