1

Multimedia Networking

R. Yang

2

Outline

Admin. and review Introduction to multimedia

networkingAn architecture of stored multimediaNetwork support of multimedia app.Application adaptation

3

Preview: Multimedia NetworkingOur goals:

Understand the service requirements of networked multimedia applications

Learn the high level structures designed for multimedia networking

Deal with multimedia in a best-effort network

How the Internet might evolve to better support multimedia?

Schedule of the two classes:

Class 1: Multimedia applications; application adaptation in best-effort networks

Class 2: New architectures: IntServ and DiffServ

4

MM Networking Applications

Fundamental characteristics:

Typically delay sensitive end-to-end delay delay jitter

But loss tolerant: infrequent losses cause minor glitches

Antithesis of data, which are loss intolerant but delay tolerant.

Classes of MM applications:

1) Streaming stored audio and video

2) Streaming live audio and video

3) Real-time interactive audio and video

5

Streaming Stored Multimedia

Streaming: media stored at source transmitted to client streaming: client playout begins

before all data has arrived

timing constraint for still-to-be transmitted data: in time for playout

6

Streaming Stored Multimedia: Interactivity

VCR-like functionality: client can pause, rewind, FF, push slider bar 10 sec initial delay OK 1-2 sec until command effect

OK

7

Streaming Live Multimedia

Examples: Internet radio talk show Live sporting event

Streaming play can lag tens of seconds still have timing constraint (but shorter delay

required)Interactivity fast forward impossible rewind, pause possible!

8

Interactive, Real-Time Multimedia

end-end delay requirements: audio: < 150 msec good, < 400 msec OK

• includes application-level (packetization) and network delays• higher delays noticeable, impair interactivity

session initialization callee be able to advertise its IP address, port number, encoding

algorithms.

applications: IP telephony, video conference, distributed interactive worlds

9

Outline

Admin. and reviewA brief introduction to the physical

layerIntroduction to multimedia

networking An architecture of stored multimediaNetwork support of multimedia app.Application adaptation

10

Streaming from Web Server (1) Audio and video files

stored in Web servers Browser requests file with

HTTP request message Web server sends file in

HTTP response message Content-type header line

indicates an audio/video encoding

Browser launches media player, and passes file to media player

Media player renders file

• Major drawback: media playerinteracts with server throughintermediary of a Web browser

11

Streaming from Web Server (2)Alternative: set up

connection between server and player

Web browser requests and receives a meta file (a file describing the object) instead of receiving the file itself

Content-type header indicates specific audio/video application

Browser launches media player and passes it the meta file

Player sets up a TCP connection with server and sends HTTP request

Some concerns: Media player

communicates over HTTP, which is not designed with pause, ff, rewind commands

May want to stream over UDP

12

RTSP/RTCP/RTP

HTTP GET

mediaplayer

Webserver

mediaserver

Webbrowser

client server

presentation desc.

RTSP

RTCP

RTP

RTSP: Real-Time Signaling ProtocolRTCP: Real-Time Control ProtocolRTP: Real-Time Transport Protocol

13

Start: Using HTTP to get Presentation File

<title>Twister</title> <session> <group language=en lipsync> <switch> <track type=audio e="PCMU/8000/1" src =

"rtsp://audio.example.com/twister/audio.en/lofi"> <track type=audio e="DVI4/16000/2" pt="90 DVI4/8000/1"

src="rtsp://audio.example.com/twister/audio.en/hifi"> </switch> <track type="video/jpeg" src="rtsp://video.example.com/twister/video"> </group> </session>

Example presentation file

14

Control: RTSP, Out of Band ControlRTSP messages are sent out-of-band:

The RTSP control messages use a different port number (554) than the media stream, and are therefore sent out-of-band

RTSP can be sent over UDP or TCP Each RTSP message can be sent over a separate TCP

connection The media stream, whose packet structure is not defined

by RTSP, is considered “in-band” If the RTSP messages were to use the same port number

as the media stream, then RTSP messages would be said to be “interleaved” with the media stream

15

RTSP: Initialization and Control

HTTP GET

SETUP

PLAY

media stream (RTP)mediaplayer

Webserver

mediaserver

Webbrowser

clientserver

presentation desc.

TEARDOWN

PAUSE

RTCP

16

RTSP: Exchange Example C: SETUP rtsp://audio.example.com/twister/audio RTSP/1.0 Transport: rtp/udp; compression; port=3056; mode=PLAY

S: RTSP/1.0 200 1 OK Session 4231

C: PLAY rtsp://audio.example.com/twister/audio.en/lofi RTSP/1.0 Session: 4231 Range: npt=0-

C: PAUSE rtsp://audio.example.com/twister/audio.en/lofi RTSP/1.0 Session: 4231 Range: npt=37

C: TEARDOWN rtsp://audio.example.com/twister/audio.en/lofi RTSP/1.0 Session: 4231

S: 200 3 OK

17

Data: Real-Time Protocol (RTP)

RTP runs in the end systems RTP specifies a packet structure for packets

carrying audio and video data: RFC 1889 RTP packets are encapsulated in UDP segments Interoperability: If two Internet phone

applications run RTP, then they may be able to work together

18

RTP Header

- Payload Type - e.g. type 0: (Pulse Coded Modulation) PCM mu-law, 64 Kbps; type 3, GSM, 13 Kbps; type 33, MPEG2 videoQuestion: why include payload type in each packet?

- Sequence Number

- SSRC field- Identifies the source of the RTP stream. Each stream in a RTP session should have a distinct SSRC

- Timestamp field - Reflects the sampling instant of the first byte in the RTP data packet

8 16 32 32

19

RTP Header (2)

Timestamp field Reflects the sampling instant of the first byte in the RTP

data packet The timestamp is derived from a sampling clock at the

sender Example

• audio stream sampled every 125 usecs • audio application generates chunks consisting of 160 encoded

samples• then the timestamp increases by 160 for each RTP packet

when the source is active• the timestamp clock continues to increase at a constant rate

even when the source is inactive

8 16 32 32

20

H.323 Overview

Foundation for audio and video conferencing across IP networks

Targets real-time communication (rather than on-demand)

Umbrella recommendation from the ITU Broad in scope:

stand-alone devices (e.g., Web phones) applications in PCs

21

Overview

H.323 terminals Telephone: SS7, Inband

Internet PSTNGateway

Gatekeeper

22

H.232 Big Picture

H.3

23 te

rmin

als

Gatekeeper

Router

Internet

LAN = “Zone”

RAS

23

Gateway

H.3

23 te

rmin

als

Gatekeeper

Router Internet

LAN = “Zone”

RAS

Gateway

PSTN

• Bridge between IP Zone and PSTN (or ISDN) network.• Terminals communicate with gateways using H.245 and Q.931

24

Gatekeeper

• The gatekeeper is optional. Can provides to terminals:• address translation to IP addresses• bandwidth management: can limit amount of bandwidth consumed by real-time conferences

• Optionally, H.323 calls can be routed through gatekeeper. Useful for billing.• RAS protocol (over TCP) for terminal-gatekeeper communication.

H.3

23 te

rmin

als

Gatekeeper

Router

Internet

LAN = “Zone”

RAS

25

H.323 Terminal

26

H.323 Endpoints Must Support:

G.711 - ITU standard for speech compression

RTP - protocol for encapsulating media chunks into packets

H.245 - “Out-of-band” control protocol for controlling media between H.323 endpoints.

Q.931 - A signalling protocol for establishing and terminating calls.

RAS (Registration/Admission/Status) channel protocol - Protocol for communicating with a gatekeeper (if gatekeeper is present)

27

H.323 Encoding

Audio: H.323 endpoints must

support G.711 standard for speech compression. G.711 transmits voice at 56/64 kbps.

H.323 is considering requiring G.723 = G.723.1, which operates at 5.3/6.3 kbps.

Optional: G.722, G.728, G.729

Video Video capabilities for an

H.323 endpoint are optional.

Any video-enabled H.323 endpoint must support the QCIF H.261 (176x144 pixels).

Optionally supports other H.261 schemes: CIF, 4CIF and 16CIF.

H.261 is used with communication channels that are multiples of 64 kbps.

28

Generating Audio Packet Stream in H.323

AudioSource

Encoding:e.g., G.711 or G.723.1

RTP packetencapsulation

UDP socket

Internet orGatekeeper

29

H.245 Control Channel

H.323 stream may contain multiple channels for different media types.

One H.245 control channel per H.323 session.

H.245 control channel is a reliable (TCP) channel that carries control messages.

Principle tasks: Open and closing

media channels. Capability

exchange: before sending media, endpoints agree on encoding algorithm

Note: H.245 for multimedia conferencing is analogous with RTSP for media streaming

30

Information flows

H.323Terminal

H.323Terminal

Media Channel1

Media ControlChannel

Media Channel2

Call SignalingChannel

Call ControlChannel

TCP

UDP

31

Gatekeeper 2/2

H.323 terminal must register itself with the gatekeeper in its zone. When the H.323

application is invoked at the terminal, the terminal uses RAS to send its IP address and alias (provided by user) to the gatekeeper.

If gatekeeper is present in a zone, each terminal in zone must contact gatekeeper to ask permission to make a call.

Once it has permission, the terminal can send the gatekeeper an e-mail address, alias string or phone extension. The gatekeeper translates the alias to an IP address. If necessary, a

gatekeeper will poll other gatekeepers in other zones to resolve an IP address. Process varies by vendor.

32

Data Control: Real-Time Control Protocol (RTCP)

Works in conjunction with each real-time RTP session

Each participant in an RTP session periodically transmits RTCP control packets to all other participants, e.g., Receiver-report packets: fraction of packets lost, last

sequence number, average inter-arrival jitter Sender-report packets: the number of packets sent, the

timestamp and the wall-time of the last sent packet

RTCP attempts to limit its traffic to 5% of the session bandwidth

33

OutlineAdmin. and reviewA brief introduction to the physical

layerIntroduction to multimedia

networkingAn architecture of stored multimedia Network support of multimedia app.Application adaptation in best-effort

Internet

34

Multimedia in Networks:Fundamental Characteristics

Many have requirements on rate, i.e. long-term average bandwidth requirement (e.g. 13kbps phone conversion)

Many are delay and jitter sensitive jitter is the variability of packet delays within the same

packet stream. Many are loss tolerant

infrequent losses cause minor glitches that can be concealed but high loss rate makes the application useless

Antithesis of data (programs, banking info, etc.), which are loss intolerant but delay tolerant; multimedia is also called “continuous media”

rate rate

utility

35

Why Best-effort Internet May not Work Well?

Variable bandwidth dynamic bandwidth sharing

Potentially long delays Assume simple arrival and service processes

(Poisson). Suppose the utilization of a link is (<1), the delay is

Variable delay jitters

1

1

36

Multimedia Networking: Two Types of Approaches

Laissez-faire philosophy No reservations, no

change of the services provided by IP

Several tools As demand increases,

provision more bandwidth

Application adaptation Place stored content at

edge of network

Integrated/Differentiated services philosophy:

Extends the service provided by the Internet

Two approaches IntServ

• New protocols and router mechanisms so that each flow can reserve end-to-end bandwidth

DiffServ• Datagrams are

marked• User pays more to

send/receive 1st class packets

37

Outline

Admin. and review A brief introduction to the physical layer Introduction to multimedia networking An architecture of stored multimedia Network support of multimedia app. Application adaptation in best-effort

Internet dealing with variable delay dealing with packet loss dealing with variable bandwidth

38

Delay Distribution

In general, the distribution of delay is Bell-Shaped [MKT98]

39

Playout Buffer

Basic idea: delay playing out packets to accommodate delay jitter

Discussion: For what types of applications will playout buffer be the most effective?

packetsnumber

time

packetsgenerated

p p'

delay0

1

r

packets received

40

Example: Adaptive Playout Delay Estimation

TCP-like delay and timeout estimation; adjust playout delay at the beginning of each check point

41

OutlineAdmin. and reviewA brief introduction to the physical

layerIntroduction to multimedia

networkingAn architecture of stored multimediaApplication adaptation in best-effort

Internet dealing with variable delay dealing with packet loss dealing with variable bandwidth

42

Forward Error Correction (FEC): Piggyback A Lower Quality Stream on Next Packet

Send a lower resolution audio stream as theredundant information on the next packet, e.g. nominal stream at 64 kbps and redundant stream GSM at 13 kbps

2 3

43

Outline Admin. and review A brief introduction to the physical layer Introduction to multimedia networking An architecture of stored multimedia Application adaptation in best-effort

Internet dealing with variable delay dealing with packet loss dealing with variable bandwidth

44

Basic Idea: Change the Rate of an MM Application

Video and audio can generate different encoding rates by

controlling the encoding process Question: how do you change the rate of a

multimedia stream?

(262 KB)

45

Video Encoding: Block Transform Encoding

DCT

Zig-zag Quantize

Run-length Code

Huffman Code

011010001011101...

46

Discrete Cosine Transform

F[u,v] = 4C(u)C(v)

n2n-1 n-1

j=0 k=0

f(j,k) cos(2j+1)u

2n

(2k+1)v

2ncos

where

C(w) =

1

21

for w=0

for w=1,2,…,n-1

- Convert from pixel domain to frequency domain- DCT better at reducing redundancy than Discrete Fourier Transform but it is more computationally expensive

47

Basis Functions of DCT

48

Example: Block Encoding

139 144 149 153144 151 153 156150 155 160 163159 161 162 160

original image

DCT1260 -1 -12 -5-23 -17 -6 -3-11 -9 -2 2-7 -2 0 1

DC component

AC components

Quantize

79 0 -1 0-2 -1 0 0-1 -1 0 00 0 0 0

zigzag79 0 -2 -1 -1 -1 0 0 -1 0 0 0 0 0 0 0

run-lengthcode

0 791 -20 -10 -10 -12 -10 0

Huffmancode 10011011100011...

coded bitstream < 10 bits (0.55 bits/pixel)

49

Result of Coding/Decoding

139 144 149 153144 151 153 156150 155 160 163159 161 162 160

144 146 149 152156 150 152 154155 156 157 158160 161 161 162

original block reconstructed block

-5 -2 0 1-4 1 1 2-5 -1 3 5-1 0 1 -2

errors

50

Examples

Uncompressed(262 KB)

Compressed(22 KB, 12:1)

Compressed(6 KB, 43:1)

51

Encode Rate According to Available Bandwidth

Steps Estimate available bandwidth (how?) Encode the stream according to the

estimation of available bandwidth (how?)

52

Backup Slides

53

Layering

Put the stream into multiple layers Each layer is a refinement of the previous

layers Example:

Zigzag of the DCT matrix, put the coefficients into different layers

54

Alternative: Buffering Plus Layering

55

Bandwidth and Buffer Co-allocation