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MPEG + RTP

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Previously, on CS5248

MPEG Compression Sequence, GOP, Picture, Slice,

Macroblock, Block, DC/AC Coefficient

I-Frame, P-Frame, B-Frame

NUS.SOC.CS5248Ooi Wei Tsang

Previously, on CS5248

RTP headers SSRC, Media Timestamp, Marker Bit,

Payload Type ..

Application-Level Framing

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You are Here

Network

Encoder

Sender

Middlebox

Receiver

Decoder

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Application-Level Framing

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How to send/recv?

Let the application decide, not protocol

stacks.

Tennenhouse + Clark

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Application Knows Best

How to reorder packetsWhether to ignore lossWhich packet to retransmit

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Application Data Unit (ADU)

Can be processed individually, even out-of-order

8-Bit PCM audio: 1 ADU = 1 ByteMPEG1 Video: 1 ADU =

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How to chop data into packets?

Every received packet should be useful (even in very lossy environment)

Ideally, 1 ADU in 1 packet

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RTP Payload Header

RTP HeaderRTP Payload

HeaderRTP Payload

•MPEG-1? 2?•Temporal Reference•I? P? B?•Begin of Slice? End of Slice?

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RTP Header

Media Timestamp: 32 bits the instant the first byte in this packet is

captured 90 kHz timestamp (90000 = 1 second)

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RTP Header

Marker Bit: 1 if contains the last byte of a frame

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RTP Header

Payload Type: 7 bits 32 for MPEG-1

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RTP Payload Header

MBZ (5 bits) Unused. Must be 0

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RTP Payload Header

T (1 bit) 1 if there is a MPEG-2 Extension Header

after this header.

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RTP Payload Header

Temporal Reference (10 bits) The ‘frame number’ of the current frame

within the GOP

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RTP Payload Header

AN bit and N bit Set to 0 for MPEG-1

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RTP Payload Header

S (1 bit) Is there a sequence header in this packet?

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RTP Payload Header

BS (1 bit) and ES (1bit) BS is 1 iff the ‘first’ byte of this payload is a slice

header ES is 1 iff the last byte of this payload is the end of a

slice

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RTP Payload Header

Picture Type (3 bits) I (1), P (2), B (3), D (4)

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RTP Payload Header

Motion Vectors Information Get from most recent picture header

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Fragmentation Rules

Sequence header: at the start of payload

GOP header: at the start of a payload (or follows Sequence header)

Picture header: at the start of a payload (or follows Sequence/GOP header)

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Fragmentation Rules

A slice must be either First data in the packet, or Follows integral number of slices

A slice may be fragmented if exceeds the size of a packet

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Packet Size

1 MTU is 1500 bytes IP HeaderUDP HeaderRTP HeaderRTP Payload HeaderPayload Size =

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Project 1

Packetize MPEG-1 Video into RTP Packets

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Goal

Read MPEG-1 video, output RTP packets with proper: RTP headers RTP payload headers fragmentation

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Mnt (Media Networking Toy)

C++Tcl/OTclUNIX

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Overview

C++

OTcl

MntComponents

MntData

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MntComponent

recv(MntData *data)push(MntData *data)

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MntPump

Subclass of MntComponent

No recv()pump_some( ) {

generate data push(data) pump_timer_.msched(time)

}

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Create and Link Components

set a [new MntA]set b [new MntB]

$a add_link_to $b$a instproc on_stop_pumping {} { puts “done!” exit}$a start_pumpingvwait forever

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Important Classes

MntComponent

MntRTPFileWriter

MntRTPFileReader

MntPump MntRTPPayloadDumper

MntMPEGFileRTPizer

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Important Classes

MntData

MntRTPMPEGPacket

MntRTPHdr MntRTPMPEGHdr

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MntRTPMPEGPacket

RTP HeaderRTP Payload

HeaderRTP Payload

MntRTPHdr *rtp_hdr_; MntRTPMPEGHdr *pl_hdr_; unsigned char *pl_data_; int pl_len_;

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Example Tcl Scripts

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Dealing with Bits

BitStream A buffer

buffer_ endDataPtr_ endBufPtr_

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Dealing with Bits

BitParser Read bits from BitStream

BitParser 1011101

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Basic Usage

#include “mnt_bit_parser.h”#include “mnt_bit_stream.h”

BitStream *bs = new BitStream(“a.mpg”);BitParser *bp = new BitParser();bp->wrap(bs);

int x;Bp_GetInt(bp, x);

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Dealing with MPEG Headers

MntMPEG1SeqHdrMntMPEG1GopHdrMntMPEG1PicHdrMntMPEG1SliceHdr

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Basic Operations

hdr->parse(bp)hdr->find(bp)hdr->skip(bp)hdr->dump(bp1,bp2)

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Example:MPEG Parsing

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Project Homepage

DescriptionsSkeleton CodeMnt Documentation IVLE Forums

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Advice

Use the source wiselyStart early