Download - CHAPTER 7: MULTIMEDIA NETWORKING
CHAPTER 7:MULTIMEDIA NETWORKING
• Multimedia Applications• Streaming• VoIP• Traffic Policing• Quality of Service
MULTIMEDIA AND QOS
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In data communications, multimedia consists of networked video and audio, i.e., applications that require “continuous” media.
QUALITY OF SERVICE (QOS) FOR NETWORKS IS A SET OF MECHANISMS FOR ENSURING HIGH-QUALITY PERFORMANCE FOR CRITICAL APPLICATIONS. THESE MECHANISMS HELP NETWORK ADMINISTRATORS USE EXISTING
RESOURCES EFFICIENTLY TO ENSURE THE REQUIRED LEVEL OF SERVICE WITHOUT REACTIVELY EXPANDING THEIR NETWORKS.
TRADITIONALLY, QUALITY IN NETWORKS MEANT THAT ALL
TRAFFIC WAS TREATED EQUALLY.• ALL NETWORK TRAFFIC RECEIVES
THE NETWORK’S BEST EFFORT.• NO PERFORMANCE GUARANTEES.• ONE BANDWIDTH-INTENSIVE
APPLICATION CAN RESULT IN POOR PERFORMANCE FOR ALL APPS.
• The rate at which the network carries traffic.Bandwidth
• The delay in data transmission from source to destination.
Latency
• The variation in latency.Jitter
• The percentage of packets discarded by a router.
Reliability
THE QOS CONCEPT VIEWS THE REQUIREMENTS OF SOME APPLICATIONS AS MORE CRITICAL THAN OTHERS, SO SOME TRAFFIC NEEDS PREFERENTIAL
TREATMENT.
Network Characteristics Managed by QoS
MULTIMEDIA NETWORKING APPLICATIONS
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FUNDAMENTAL CHARACTERISTICS• TYPICALLY DELAY SENSITIVE, E.G., END-TO-END DELAY, DELAY JITTER • LOSS TOLERANT, I.E., INFREQUENT LOSSES CAUSE MINOR GLITCHES • OPPOSITE OF OTHER DATA, WHICH IS LOSS INTOLERANT, BUT DELAY TOLERANT.
CLASSES OF MULTIMEDIA APPLICATIONS1) STORED STREAMING
– MEDIA STORED AT SOURCE, TRANSMITTED TO CLIENT– CLIENT PLAYOUT BEGINS BEFORE ALL DATA HAS ARRIVED
2) LIVE STREAMING– STREAMING PLAYBACK CAN LAG TENS OF SECONDS AFTER TRANSMISSION– EXAMPLES: INTERNET RADIO TALK SHOW, LIVE SPORTING EVENT
3) INTERACTIVE, REAL-TIME– END-TO-END DELAY-SENSITIVE (APPLICATION PACKETIZING, NETWORK
TRAVERSAL, ETC.) – EXAMPLES: VOIP, VIDEOCONFERENCES, DISTRIBUTED INTERACTIVE WORLDS
BEST-EFFORT SERVICE
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TCP, UDP, and IP only provide “best effort” service, with no guarantees regarding delay or loss.
These forces today’s Internet multimedia applications to use Application Level techniques to improve service.
There are three common philosophies regarding the future of multimedia networking.
Integrated Services
• Make fundamental changes to the Internet so apps can reserve end-to-end bandwidth
• This would require new, complex software in hosts & routers
Differentiated Services
• Requiring fewer changes to the Internet infrastructure, this would just provide 1st and 2nd class service
Laissez-Faire• Make no major
changes• Provide more
bandwidth when needed
• Have the Application Layer handle content distribution, multicast, etc.
MUSIC COMPRESSION
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Several Application Layer standards have evolved regarding the compression and encoding of musical audio files.
(Used in MP3)Digitally encoding music by sampling the sound stream
(Used in MIDI)Record physical
input into a synthesized instrument controller
(Used in MPEG-4)Specify signal-
processing elements, like oscillators and
filters, for synthesizing sound
STREAMING AUDIO & VIDEO
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1. Request for metafile containing
URL for audio or video file and
instructions for Web browser to launch
media player
2. Media player is launched on workstation & URL is contacted on multimedia server
3. The multimedia server negotiates with the workstation regarding connection rate and sends the appropriate version
of the audio or video file
4. The compressed audio or video file is packetized and sent via UDP, which does not accommodate retransmissions
5. The received packets are stored on a workstation
buffer which, when full, sends them to the media player
for decompression and execution
VOICE OVER IP
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Implementing voice communication on the Internet, while desirable, faces several problems that are still being resolved via the Session Initiation Protocol (SIP) and the Real-Time Transport Protocol (RTP):
Delay Jitter Packet Loss Echo Security
SIP sets up and terminates real-time multimedia sessions.
RTP handles the end-to end transmission service, including payload type identification, sequential numbering of packets, timestamp and delivery monitoring .
VoIP PACKET LOSS RECOVERY
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One means of recovery from packet loss is Forward Error Correction, in which redundant information is transmitted in order to make at
least partial recovery possible.
FEC Option 1For every N packets, send an
(N+1)st packet which is the XOR of the previous N.
If one packet is lost, its contents can be determined from the other
N.
FEC Option 2Piggyback a low-quality version of the audio
from each packet onto the subsequent packet.
If one packet is lost, the low-quality version of its contents can be determined from the
next packet.
REAL-TIME TRANSPORT PROTOCOL (RTP)
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Running between the Application Layer and the Transport Layer (usually UDP), RTP formats packets to facilitate real-time interactive
applications.
Version: 2-bit version number of version of RTP being used.Padding Flag: Indicates that payload is padded (# of padded bytes is last byte of padding).Extension Flag: Indicates that the fixed header is followed by one extension header.
V P X CSC M PT Sequence Number
Contributing Source Count: # of Contributing Source Identifiers after fixed header.Marker Flag: Indicates that frame boundaries are marked in the packet stream.
V P PTX CSC M Sequence NumberTimestampTimestamp
Synchronization Source IdentifierSynchronization Source IdentifierContributing Source Identifiers (0 or more)Contributing Source Identifiers (0 or more)
Payload Type: 7-bit identifier of payload format (JPEG, MPEG, Pulse Code Modulation, etc.).Sequence Number: 16-bit packet number to enable packet loss identification.Timestamp: Application-level sampling instant of the RTP packet’s first byte.Synchronization Source Identifier: Randomly chosen ID number for the current stream.Contributing Source Identifier: Additional SSRC Identifiers that contributed to the combined stream
(e.g., a mixer-combined audio conference).
LEAKY BUCKET ALGORITHM
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One approach to policing traffic restricts the rate at which endstations can place packets onto the network to a particular range of values
NETWORK
packet
packet
packet
packet
packet
packet
packet
packet
packetpacket
packetpacket
packet
packet
packetpacket
Trying to send slower than agreed?NO PROBLEM!
Trying to send slightly faster than agreed?
NO PROBLEM (within limits)!
Sending significantly faster than agreed?
BIG PROBLEM! Interface throttles back on the transmissions!
TOKEN BUCKET ALGORITHM
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NETWORK
An alternative approach accommodates bursty traffic better by producing “tokens” at a steady rate, allowing packets to be released
only if a corresponding token is available.
NETWORK
If an endstation sends at a slow rate for a
while, tokens start to stack up (until the bucket is full - then
tokens are discarded)
If an endstation suddenly sends a large
burst of packets, it’s allowed to transmit
them until its reservoir of tokens has been
depleted
packet
packet
packet
packet
TTT T
packet
packet
packet
packet
packet
packet
packet
packet
packet
DIFFERENTIATED SERVICES
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One of the two main QoS models for the Internet is Differentiated
Services (DiffServ), which uses the “provisioned” approach of setting up
network nodes to service multiple classes of traffic and varying QoS
requirements.
Each internal node in the network is then configured to treat all incoming packets with the same service level
in the same way.
DiffServ requires end stations to use the old IPv4 Precedence and Type-of-Service fields to indicate one of 64
DiffServ service levels.
DIFFSERV ARCHITECTURE
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Since only the boundary nodes between adjacent DiffServ “clouds” must deal with traffic classification, shaping, and policing, the rest of the network nodes
are free to concentrate on routing.
DiffServ Problems:• Routers handle ToS fields
inconsistently, making end-to-end service unpredictable
• Serious congestion problems are resolved via… dropped packets!
PHB: Per-Hop Behavior; LLQ: Low Latency Queuing; WRED: Weighted Random Early Detect
INTEGRATED SERVICES
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In Integrated Services (IntServ), the second major Internet QoS model, an end-user application first classifies the level of service that
it wants its packets to enjoy.
IP then uses the Resource ReSerVation Protocol
(RSVP) to establish a route with the desired QoS by means of each router’s Admission Control (i.e., does the router have
sufficient resources?) and its Policy Control (i.e., does
the end user have the administrative authority to
make a reservation?).
IntServ Problems:• Every device on the network must know and
be able to handle RSVP• Routers must keep track of all paths currently
using them, which means this approach isn’t really scalable to Internet-sized networks