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


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


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


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


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


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


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)


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


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


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


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


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


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

CHAPTER 7: MULTIMEDIA NETWORKING

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