31392678 Protocols in IPTV Manishdas

8/8/2019 31392678 Protocols in IPTV Manishdas

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Protocols in IPTVProtocols in IPTV

Manish DasManish Das


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deconstructiondeconstruction

dede--packetizingpacketizing

decodingdecoding


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Video encodingVideo encoding

Video in its most basic form is a sequence of images,Video in its most basic form is a sequence of images,which are displayed in a sequential order. The technicalwhich are displayed in a sequential order. The technical

term for one of these video images is a frame. Theterm for one of these video images is a frame. The

human eye in general can comfortably watch TV at aroundhuman eye in general can comfortably watch TV at around

25 frames per second (fps).25 frames per second (fps).

Devices called video encoders are used to compress videoDevices called video encoders are used to compress video

content contained in each of the frames, while maintainingcontent contained in each of the frames, while maintaining

a high level of picture quality.a high level of picture quality.

Standard practice is to use Motion Picture Expert GroupStandard practice is to use Motion Picture Expert Group

(MPEG) encoding.(MPEG) encoding. An uncompressed video signal is compressed and an aAn uncompressed video signal is compressed and an a

continuous realcontinuous real--time digital signal is outputted from thetime digital signal is outputted from the

encoder. This is known asencoder. This is known as Elementary StreamElementary Stream..


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MPEG uses three fundamental techniques to achieveMPEG uses three fundamental techniques to achieve

compression:compression:

SubsamplingSubsampling Reduces colour informationReduces colour information

which is less sensitive to eyewhich is less sensitive to eye

Spatial compressionSpatial compression Removes redundant informationRemoves redundant information

within frames using the propertywithin frames using the property

thatthat pixels within a singlepixels within a single

frame are related to theirframe are related to theirneigboursneigbours

Temporal compressionTemporal compression Removes redundant informationRemoves redundant information

between framesbetween frames


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Compression achieved:Compression achieved:

Coding versionCoding version SDTVSDTV HDTVHDTV

MPEGMPEG--22 3.5 Mbps3.5 Mbps 19.3 Mbps19.3 Mbps

MPEGMPEG--44 2 Mbps2 Mbps 8 Mbps8 Mbps


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VideoVideo packetisingpacketising

In order for the elementary streams to beIn order for the elementary streams to betransmitted over the digital network, eachtransmitted over the digital network, eachelementary stream is converted into anelementary stream is converted into aninterleaved stream of time stamped Packetizedinterleaved stream of time stamped PacketizedElementary Stream (PES) packets.Elementary Stream (PES) packets.

A PES packet may be a fixed (or variable) sizedA PES packet may be a fixed (or variable) sizedblock, with up to 65536 bytes per packet. Thisblock, with up to 65536 bytes per packet. Thisincludes an allocation of 6 bytes for the headerincludes an allocation of 6 bytes for the headerwith the remainder of the packet used to carrywith the remainder of the packet used to carrycontentcontent.


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MPEG Transport Stream (TS) constructionMPEG Transport Stream (TS) construction

The next layer deals with building a transportThe next layer deals with building a transport

stream, which consists of a continuous streamstream, which consists of a continuous stream

of packets.of packets.

These packets, commonly called TS packets,These packets, commonly called TS packets,are formed by breaking up the PES packetsare formed by breaking up the PES packets

into fixedinto fixed--sized TS packets of 188 bytessized TS packets of 188 bytes..

Each TS packet comprises 184 bytes ofEach TS packet comprises 184 bytes of

payload and a 4 byte header.payload and a 4 byte header.


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MPEG Transport Stream (TS)MPEG Transport Stream (TS)

HeaderHeader

(4 bytes)(4 bytes)Video Payload (184 bytes)Video Payload (184 bytes)

188 bytes


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Formation of IP PacketsFormation of IP Packets

Once the TS has been structured and formatted it isOnce the TS has been structured and formatted it ispassed down to either the transport layer (UDP) directlypassed down to either the transport layer (UDP) directlyor a layer that uses the Realor a layer that uses the Real--Time Transport ProtocolTime Transport Protocol(RTP).(RTP).

In a packetIn a packet--based network, it is quite common that thebased network, it is quite common that theroute for transporting the packets is not always theroute for transporting the packets is not always thesame; packets may arrive at different times and out ofsame; packets may arrive at different times and out oforder. The RTP protocol allows outorder. The RTP protocol allows out--ofof--order arrival of theorder arrival of thepackets. Since every RTP packet has a sequencepackets. Since every RTP packet has a sequencenumber, as long as the delay did not exceed the size ofnumber, as long as the delay did not exceed the size of

the receiving decoder buffer, the packet can bethe receiving decoder buffer, the packet can beprocessed and placed in the right position for decoding.processed and placed in the right position for decoding.


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RTP encapsulationRTP encapsulation

RTP HeaderRTP Header

(12 bytes)(12 bytes)

MPEGMPEG

TSTS

(188(188

bytes)bytes)

MPEGMPEG

TSTS

(188(188

bytes)bytes)

MPEGMPEG

TSTS

(188(188

bytes)bytes)

MPEGMPEG

TSTS

(188(188

bytes)bytes)

MPEGMPEG

TSTS

(188(188

bytes)bytes)

MPEGMPEG

TSTS

(188(188

bytes)bytes)

MPEGMPEG

TSTS

(188(188

bytes)bytes)


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Formation of IP Packets: Transport LayerFormation of IP Packets: Transport Layer

RTP packets form the input to the transport layer UDPRTP packets form the input to the transport layer UDPor TCP.or TCP.

It is also possible to map MPEGTS packets directly intoIt is also possible to map MPEGTS packets directly intothe transport layer protocol payload. This effectivelythe transport layer protocol payload. This effectivelyavoids the RTP layer completely. But a number ofavoids the RTP layer completely. But a number ofservice providers use RTP to address the inherentservice providers use RTP to address the inherentunreliability of the UDP protocol.unreliability of the UDP protocol.

In the context of IPTV, UDP is useful where theIn the context of IPTV, UDP is useful where thetransport network is career grade and is the mosttransport network is career grade and is the mostpopular transport level protocol employed by IPTVpopular transport level protocol employed by IPTV

service providers.service providers. An RTP header is identifiable with a value of 5004 in aAn RTP header is identifiable with a value of 5004 in a

User Datagram Protocol (UDP) header.User Datagram Protocol (UDP) header.


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Why UDP?Why UDP?

IPTV is a realIPTV is a real--time application and does not tolerate delays. TCP can oftentime application and does not tolerate delays. TCP can oftenintroduce latency into the delivery of IP video content due to fact that theintroduce latency into the delivery of IP video content due to fact that theprotocol employs flow control mechanisms.protocol employs flow control mechanisms.

UDP ensures that delivery of IPTV content is not delayed even if there isUDP ensures that delivery of IPTV content is not delayed even if there isdelayed or damaged packets contained in the network traffic, whereas indelayed or damaged packets contained in the network traffic, whereas in

using TCP, TV viewers are faced with a pause as they wait for a delayedusing TCP, TV viewers are faced with a pause as they wait for a delayedpacket or picture frame to arrive or wait for the damaged packet to bepacket or picture frame to arrive or wait for the damaged packet to bereplaced.replaced.

Low overhead: The size of the header is only 8 bytes when compared toLow overhead: The size of the header is only 8 bytes when compared tothe TCP header, which occupies 20 bytes of data.the TCP header, which occupies 20 bytes of data.

Speedy connection setup: The establishment and teardown of connectionsSpeedy connection setup: The establishment and teardown of connectionsbetween IPTVCDs (set top box) and IPTV data center networkingbetween IPTVCDs (set top box) and IPTV data center networkingcomponents takes place in a very short time period. Therefore, the deliverycomponents takes place in a very short time period. Therefore, the deliveryof video packets using the UDP protocol is generally quicker compared toof video packets using the UDP protocol is generally quicker compared tousing the TCP protocol.using the TCP protocol.


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The IP LayerThe IP Layer

Next task is IP encapsulation.Next task is IP encapsulation.

20 byte header is added to the payload.20 byte header is added to the payload.

IPv4IPv4

HeaderHeader


UDPUDP

HeaderHeader

(8 bytes)(8 bytes)

RTPRTP

HeaderHeader


OptionalOptional

7 X MPEG TS Packets (188 bytes)7 X MPEG TS Packets (188 bytes)


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To RecapitulateTo Recapitulate


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Now the delivery mechanism (contd.)Now the delivery mechanism (contd.)

In order to preserve bandwidth over the final linkIn order to preserve bandwidth over the final linkto the house, IPTV systems are designed toto the house, IPTV systems are designed to

deliver only the requested channel to the STB.deliver only the requested channel to the STB.

In order to change channels, special commandsIn order to change channels, special commandsare sent into the Access network requesting aare sent into the Access network requesting a

change of channel.change of channel.

Thus in IPTV systems the channel change isThus in IPTV systems the channel change is

made in the network and not on the local STBmade in the network and not on the local STB.


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Now the delivery mechanism (contd.)Now the delivery mechanism (contd.)

IP communication is normally one to oneIP communication is normally one to one unicastunicast..

This method of communications is not effective forThis method of communications is not effective for

delivering traditional broadcast style channels becausedelivering traditional broadcast style channels because

the duplication of pointthe duplication of point--toto--point sessions wouldpoint sessions would

overwhelm the network.overwhelm the network. So IPTV uses multicast to provide a single broadcast TVSo IPTV uses multicast to provide a single broadcast TV

channel to multiple clients simultaneously.channel to multiple clients simultaneously.

The primary purpose of multicast technologies is toThe primary purpose of multicast technologies is to

ensure that end users are able to instantaneously andensure that end users are able to instantaneously andreliably switch channels during a TV viewing sessionreliably switch channels during a TV viewing session.


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Features of IP Multicast in IPTV networkFeatures of IP Multicast in IPTV network

Only a single copy of every video stream needs to beOnly a single copy of every video stream needs to besent to a router, which in turn makes a copy of thatsent to a router, which in turn makes a copy of thatstream for the requesting devices.stream for the requesting devices.

Multicast not only reduce the bandwidth requirements ofMulticast not only reduce the bandwidth requirements ofthe network but the processing power of the contentthe network but the processing power of the content

server can also be kept relatively low because it onlyserver can also be kept relatively low because it onlytransmits one copy of an IPTV stream at a time.transmits one copy of an IPTV stream at a time.

But it increases the workload and processingBut it increases the workload and processingrequirements of routers as they handle additional tasksrequirements of routers as they handle additional taskssuch as replicating video streams and keeping track ofsuch as replicating video streams and keeping track ofmultiple copies of video packets. Processing the variousmultiple copies of video packets. Processing the varioustasks associated with IP multicasting adds a significanttasks associated with IP multicasting adds a significantburden to the workload of IP routers.burden to the workload of IP routers.


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A simple exampleA simple example

multiple unicasts IP multicast

S S

R

R

R

R

R

R


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Channel zapping in IPTV

In an IPTV network, each broadcast televisionIn an IPTV network, each broadcast televisionchannel is an IP multicast groupchannel is an IP multicast group.

Internet Group Management Protocol (IGMP) isInternet Group Management Protocol (IGMP) is

used for zapping channels.used for zapping channels.

IGMP v2 is widely used in IPTV.IGMP v2 is widely used in IPTV.

Whenever an end user changes a channel, theWhenever an end user changes a channel, the

SetSet--top BOX responds by sending twotop BOX responds by sending two

commands to the central equipment:commands to the central equipment:(1) To leave the existing video multicast.(1) To leave the existing video multicast.

(2) To join the desired new video multicast.(2) To join the desired new video multicast.


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IGMP DevicesIGMP Devices

Basic IGMP operation involves two devices:

IGMP host is a server or client, which issues messages

to join or leave a multicast group. The client also

responds to queries from the multicast router. A set-topbox is an example of an IGMP host.

IGMP router (or multicast router) responds to the join

and leave messages to determine if multicast groupsshould be forwarded out an interface.


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IGMP message formatIGMP message format


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Encapsulation of IGMP packetEncapsulation of IGMP packet


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Typical IPTV System: Selection of first channelTypical IPTV System: Selection of first channel

TransportHeadend Access Home

Transport

Network xDSL CPE/FTTH

Computer

STB

IGMP

Router

Digital and

Analog

Receivers

MPEGEncoders

AppsS

ervers

IGMPDSLAM

VideoServers

OtherContent

Sources

VoIP

Subscriberpresses

channelnumberThe IP set-top box

accepts the channel

request commandand sends this

instruction in theform of an IGMPjoin

request to theDSLAM.

IGMP JOIN

DSLAM examines therequest to see if the

requested channelalready exists at one

of its ports. If this isthe case, then the

DSLAM simply copiesthe stream and sends

to the requestingdevice.

If the requested channelis not available in the

DSLAM, the request issent to the upstream

routers.

IGMP JOIN

Router examines therequest to see if the

requested channelalready exists at one

of its ports. If this isthe case, then the

Router simply copiesthe stream and sends

to the requesting

device

If the requestedchannel is not

available in the

Router, the request

is sent to theupstream routers.

IGMP JOIN

Video Stream

The request for thechannel finally ends

up at the IPTV datacenter where all

broadcast channelsare available. The IP

address of thesubscribers IP set-top

box get added to the

multicast list. Thechannel is then copied

and sent onward to

the IP set-top box.


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Channel selection: Possible steps that may occurChannel selection: Possible steps that may occur

across a DSL based IPTV networkacross a DSL based IPTV network

Once a subscriber wishes to select a channel they press a channelOnce a subscriber wishes to select a channel they press a channelnumber on their remote control or select from an EPG application. Thesenumber on their remote control or select from an EPG application. These

commands are received by the infrared receiver.commands are received by the infrared receiver.

The IP setThe IP set--top box accepts the channel changing command and sendstop box accepts the channel changing command and sends

this instruction in the form of an IGMPthis instruction in the form of an IGMPjoinjoin request to the DSLAM. Therequest to the DSLAM. The

DSLAM sees the request and will either pass onward or else examine theDSLAM sees the request and will either pass onward or else examine the

request to see if the requested channel already exists at one of its ports. Ifrequest to see if the requested channel already exists at one of its ports. Ifthis is the case, then the DSLAM simply copies the stream and sends tothis is the case, then the DSLAM simply copies the stream and sends to

the requesting device.the requesting device.

If the requested channel is not available in the DSLAM, the request is sentIf the requested channel is not available in the DSLAM, the request is sent

to the upstream routers.to the upstream routers.

When the router located at the regional office receives a request that hasWhen the router located at the regional office receives a request that has

not been serviced by the various downstream network components, it alsonot been serviced by the various downstream network components, it also

has two options, namely, to copy the stream to the correct interface orhas two options, namely, to copy the stream to the correct interface or

pass the request upstream to the distribution router in the event that thepass the request upstream to the distribution router in the event that the

channel is unavailable at its downstream interface.channel is unavailable at its downstream interface.


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Channel selection: Possible steps that may occurChannel selection: Possible steps that may occur


The request for the channel finally ends up at the IPTV data center whereThe request for the channel finally ends up at the IPTV data center whereall broadcast channels are available. It is important to note that theall broadcast channels are available. It is important to note that the

channel is generally identified by an IP address. The IP address of thechannel is generally identified by an IP address. The IP address of the

subscribers IP setsubscribers IP set--top box get added to the multicast list. The channel istop box get added to the multicast list. The channel is

then copied and sent onward to the IP setthen copied and sent onward to the IP set--top box.top box.

The setThe set--top receives the new IP stream. It then buffers in memory, andtop receives the new IP stream. It then buffers in memory, and

waits for an Iwaits for an I--frame to arrive before decoding starts. The Iframe to arrive before decoding starts. The I--frame containsframe containsall of the necessary information required to reconstruct the original pictureall of the necessary information required to reconstruct the original picture

frame. Once the Iframe. Once the I--frame is received the IP setframe is received the IP set--top box, which can taketop box, which can take

between a half and two full seconds, can start the process of displayingbetween a half and two full seconds, can start the process of displaying

the channel.the channel.

The first picture frame of the new channel is displayed on the TV display.The first picture frame of the new channel is displayed on the TV display.

Note that the ITUNote that the ITU--T FG IPTV group is recommending that the time takenT FG IPTV group is recommending that the time takento acquire a broadcast channel should not exceed 2.5 s.to acquire a broadcast channel should not exceed 2.5 s.


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Typical IPTV System: Change of channelTypical IPTV System: Change of channel

TransportHeadend Access Home

Transport

Network xDSL CPE/FTTH

Computer

STB

IGMP

Router

Digital and

Analog

Receivers

MPEGEncoders

AppsS

ervers

IGMPDSLAM

VideoServers

OtherContent

Sources

VoIP

Subscriberchanges

channelnumber

IGMP JOIN

Video Stream

IGMP LEAVE

The set-top box issuesan IGMP Leave

message to terminate

the stream associatedwith the old stream.

Thetermination of

the streamtakes place in

the DSLAM.

A join

message isthen sent tostart viewing

the nextchannel.


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Channel change: Possible steps that may occurChannel change: Possible steps that may occur


The process involved in changing channels in the middle of the TVviewing experience is quite similar to selecting a channel. There is

however a couple of extra steps:

When a viewer wants to change to another IPTV signal the remote

control button is pressed and the instruction is accepted by the IPset-top box.

The set-top box issues an IGMP Leave message to terminate the

stream associated with the old stream.

The termination of the stream takes place in the DSLAM.

A join message is then sent to start viewing the next channel.


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SessionSession Announcement Protocol (SAPAnnouncement Protocol (SAP))

Session Announcement Protocol (SAPSession Announcement Protocol (SAP) is used to) is used toinform multicast enabled receivers periodically aboutinform multicast enabled receivers periodically about

programs currently being multicast on a network.programs currently being multicast on a network.

The most important part of a SAP message is theThe most important part of a SAP message is the

multicast address of the multicast stream. Once the usermulticast address of the multicast stream. Once the userdevice has this address, it can send a request to thedevice has this address, it can send a request to the

network to join that multicast.network to join that multicast.

By default, SAP communications always take place onBy default, SAP communications always take place on

the multicast group address 224.2.127.254 on port 9875.the multicast group address 224.2.127.254 on port 9875.

Specialized software on the user device converts theSpecialized software on the user device converts the

information received from SAP into a list of choices frominformation received from SAP into a list of choices from

which the user can select.which the user can select.


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MULTICASTING IPTV content across IPV6MULTICASTING IPTV content across IPV6

networksnetworks

IPv6 deployment requires service providers touse a multicasting signaling protocol called

Multicast Listener Discovery (MLD) when

delivering broadcast TV channels to IPTV end

users.

MLD has been derived IGMP.


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Multicast TVMulticast TV

DSLAM # 1 DSLAM # 2 DSLAM # 3 DSLAM # 4

T2 T2

T1

PC PC

TVTV

IGMP

JOIN

PC

BNG

SETTOPB

OX

ADSLMODEM

Video Server


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TimeTime--shift TVshift TV


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Video on Demand (Video on Demand (VoDVoD))

VoD is unicast delivery and uses RTSP.

The Real Time Streaming Protocol (RTSP) is aprotocol for use in streaming media systems whichallows a client to remotely control a streaming media

server, issuing VCR-like commands such as "play" and"pause", and allowing time-based access to files on aserver

RTSP requests are based on HTTP requests

In RTSP we can distinguish following requests:DESCRIBE, SETUP, PLAY, PAUSE, RECORD,TEARDOWN.

RTSP servers use RTP among other things the transportprotocol for the actual audio/video data and RTCP tomonitor quality of streaming.


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Example:RTSPExample:RTSP with UDPwith UDP--Based RTP DeliveryBased RTP DeliveryDESCRIBErtsp://192.168.1.25:554/mp

eg4/movies/3idiots.mpg

RTSP/1.0

CSec: 101Accept: application/sdp

RTSP/1.0 200OK

CSec: 101

Content-Base:rtsp://192.168.1.25:554/mpeg4/

movies/3idiots.mpg

Content-Type:application/sdp

Content-Length:320

OPTIONSrtsp://192.168.1.25:554/mp

eg4/movies/3idiots.mpgCSec: 102

RTSP/1.0200 OK

CSec: 102Public:

DESCRIBE,OPTIONS,

PAUSE,PLAY,

SETUP,TEARDOWN,

ANNOUNCE

SETUPrtsp://192.168.1.25:554/mp

eg4/movies/3idiots.mpgCSeq: 103

Transport:RTP/UDP;unicast;client_po

rt=4042-4043

RTSP/1.0200 OK

CSeq: 103Session:

1234567891;timeout=10

Transport:RTP/UDP;u

nicast;mode=play;

client_port=4042-4043;

server_port=5072-5073

PLAYrtsp://192.168.1.25:554/m

peg4/movies/3idiots.mpgRTSP/1.0

CSeq: 104Session: 1234567891

RTSP/1.0200 OK

CSeq: 104Session:

1234567891

PAUSErtsp://192.168.1.25:554/m

peg4/movies/3idiots.mpgCSeq: 105

Session: 1234567891TEARDOWN

rtsp://192.168.1.25:554/m

peg4/movies/3idiots.mpgRTSP/1.0

CSeq: 106

Session: 1234567891RTSP/1.0200 OK

CSeq: 106Session:

1234567891


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Thank You!Thank You!

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31392678 Protocols in IPTV Manishdas