Previously, on CS5248MPEG CompressionSequence, GOP, Picture, Slice, Macroblock, Block, DC/AC Coefficient

I-Frame, P-Frame, B-Frame

MPEG Frame SizesConstant Bitrate (CBR) vs. Variable Bitrate (VBR)

Previously, on CS5248RTP headersSSRC, Media Timestamp, Marker Bit, Payload Type ..

Application-Level Framing

You are HereNetworkEncoderSenderMiddleboxReceiverDecoder

NUS.SOC.CS5248-2009Roger Zimmermann (based in part on slides by Ooi Wei Tsang)

Application-Level Framing

How to send/recv?

Let the application decide, not protocol stacks.Tennenhouse + Clark

Application Knows BestHow to reorder packetsWhether to ignore lossWhich packet to retransmit

Application Data Unit (ADU)Can be processed individually, even out-of-orderUnit of error-recoveryIf part of an ADU is lost, the whole ADU is considered lost

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

How to chop data into packets?Every received packet should be useful (even in very lossy environments)

Ideally, 1 ADU in 1 packet

RTP Payload HeaderRTP HeaderRTP Payload HeaderRTP PayloadMPEG-1? 2?Temporal ReferenceI? P? B?Begin of Slice? End of Slice?

RTP HeaderMedia Timestamp: 32 bitsthe instant the first byte in this packet is captured90 kHz timestamp (90000 = 1 second)

RTP HeaderMarker Bit:1 if contains the last byte of a frame

RTP HeaderPayload Type: 7 bits32 for MPEG-1

RTP Payload HeaderMBZ (5 bits)Unused. Must be 0

RTP Payload HeaderT (1 bit)1 if there is a MPEG-2 Extension Header after this header.

RTP Payload HeaderTemporal Reference (10 bits)The frame number of the current frame within the GOP

RTP Payload HeaderAN bit and N bitSet to 0 for MPEG-1

RTP Payload HeaderS (1 bit)Is there a sequence header in this packet?Repetition of sequence header is useful for resynchronization

RTP Payload HeaderBS (1 bit) and ES (1bit)BS is 1 iff the first byte of this payload is a slice headerES is 1 iff the last byte of this payload is the end of a slice

RTP Payload HeaderPicture Type (3 bits)I (1), P (2), B (3), D (4)

RTP Payload HeaderMotion Vectors InformationGet from most recent picture header

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

Fragmentation RulesA slice must be eitherFirst data in the packet, orFollows integral number of slices

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

Packet Size1 MTU is 1500 bytesIP HeaderUDP HeaderRTP HeaderRTP Payload HeaderPayload Size =

NUS.SOC.CS5248-2009Roger Zimmermann (based in part on slides by Ooi Wei Tsang)

ProjectPacketize MP3 or Speex audio into RTP Packets

GoalUse the Yima Personal Edition streaming media server codeModify the yimasplit utility, which creates data blocks containing pre-computed RTP packetsServer reads data blocks and sends out RTP packetsModify client to decompress and play audio

Project HomepageDescriptionsYima Personal Edition Code on CD-ROMDocumentationIVLE Forums

AdviceThe Yima PE source code is not very well documentedSelect your own project, or do the suggested oneEither way: Start early