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Ver 1.0 MPEG-2 DVB GUIDE 02/02/12

1 System Engineering Group

MPEG-2 DVB POCKET GUIDE

Revision History

Date Revision Description Author

22nd .Oct 2001 1.0 First Draft Version. Athif

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Preface

For the next few years, the opportunities for internetwork professionals will be immense. However, the challenge ofpossessing the proper knowledge base and skill set will be faced by every internetworking professional. From anInternetwork education perspective, the MPEG and DVB understanding provide a valuable insight into howcompression technologies operate. A broader goal of this document is to demonstrate the power of MPEG-2 andDVB technologies. In sum, I hope this document assists in developing the readers MPEG-2 and DVB analysisskills.

TABLE OF CONTENTS

MPEG-2

Summary of MPEG Compression Capability

InteractionThe MPEG-2 StandardMPEG-2 Video Compression

Temporal redundancySpatial redundancy

Basic Operation of an MPEG-2 EncoderMPEG-2 ProfilesMPEG-2 Audio CompressionMPEG-2 DataA Quick History of MPEG-2

MPEG-2 Transmission

Building the MPEG Bit StreamElementary Stream (ES)

Packetized Elementary Stream (PES)MPEG-2 MultiplexingMPEG Program StreamMPEG Transport Stream

MPEG Transport StreamsTransmission of the MPEG-TSSingle and Multiple Program Transport StreamsStreams supported by the MPTS

Signalling TablesDVB Signalling Tables and Transport Layer PIDs

MPEG-2 Signalling TablesProgramme Service Information (SI) provided by MPEG-2 and used by DVB

DVB Signalling TablesService Information (SI) provided by DVB

Format of a Transport Stream Packet

Option Transport Packet Adaption FieldDVB Satellite

MPEG Transport Service Encoding Specified by DVB-SDigital Storage Media Command and Control (DSM-CC)

DVB: Digital Video BroadcastingFlexibility - a key design goal of DVB

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Open DesignSummary of DVB Features

DVB Transmission

Typical Streams carried in a DVB Terrestrial Transport MultiplexDVB Bearer NetworksDigital Satellite TV (DVB-S)Digital Terrestrial TV Network (DVB-T)

DVB Receivers & Transmission

DVB ReceiversMultimedia Home Platform (MHP)Wide-Screen FormatProgramme Guides

Event Schedule Guide (ESG)Electronic Programme Guide (EPG)

DSM-CC for Software DownloadData Transmission using MPEG-2 and DVB

IntroductionForward Data Transmission

Typical configuration for providing Direct to Home (DTH) Internet delivery using DVBData PipingData StreamingMulti-Protocol Encapsulation (MPE)Data CarouselsObject Carousels

Digital Storage Media Command and Control

Digital Storage Media Command and Control (DSM-CC) Packet DownloadDSM-CC Multi Protocol EncapsulationReturn Channel SystemsReturn Data Transmission (Interaction Channel) via Satellite

Protocol architecture for (a) Outbound link to client and (b) return link from clientMPEG-2 Encoders and Decoders

MPEG-2 DecodersSoftware MPEG-2 DecodersPC-Based MPEG-2 AcceleratorsComputer MPEG-2 Decoders

Network Computers / Thin Clients The MPEG-2 Set-Top Box (STB)

MPEG-2 Consumer Equipment

Delivery of MPEG-2Streaming applicationsBuffered ApplicationsIssues

Programming APIs for MPEG-2 Computer CardsBroadband Multimedia Satellite Systems

What is Broadband Multimedia?MultimediaMulti-Platform delivery

Broadband Multimedia Content - Is Content King?DVB Satellite Return Channel (DVB-RCS)

One arrangement of the User Terminal Protocol StackTerminal OperationSimplified Transponder Usage by a Group of DVB-RCS User Terminals

Broadband Interactive Service ( BBI )

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TCP over Satellite

SPECIFIC ISSUES THAT IMPACT THE SATELLITE SERVICEMPEG-2 and DVB Standards

MPEG-2 StandardsDVB StandardsDAVIC Specifications

Glossary

Appendix ADigital VideoFirst There was AnalogDefining Digital VideoFour Factors of DVFrame RateColor ResolutionSpatial ResolutionImage Quality

Need For CompressionFactors Effecting Compression

Real-Time Versus Non-Real-TimeSymmetrical versus AsymmetricalCompression RatiosLossless versus LossyInterframe Versus IntraframeBit Rate ControlSelecting a CompressionMPEG OverviewThe Status of MPEGReference Frames and RedundancyInside an MPEG StreamStep 1, Finding the Macro Block ( Motion Compensation)Step 2, Tracking the Changes ( Spatial Redundancy)

Applications for MPEG-1Video KioskVideo On DemandVideo Dial ToneTrainingCorporate PresentationsVideo LibraryInternet and Intranet

Applications for MPEG-2DVD-ROMDVD VideoCATVDBS

HDTVMPEG Playback System ConfigurationMPEG Displayed on Television MonitorMPEG Video in a Window SVGAVideo CD with MPEG VideoConclusion

Questions and AnswersWhat is the difference between MPEG-1 and MPEG-2 ?

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MPEG-2MPEG is an encoding and compression system for digital multimedia content defined by the Motion Pictures ExpertGroup (MPEG). MPEG-2 extends the basic MPEG system to provide compression support for TV qualitytransmission of digital video. To understand why video compression is so important, one has to consider the vast

bandwidth required to transmit uncompressed digital TV pictures.Phase Alternate Line (PAL) is the analogue TV transmission standard used in India, and throughout many parts ofthe world. An uncompressed PAL TV picture requires a massive 216 Mbps, far beyond the capacity of most radiofrequency links. The U.S. uses an analogue TV system called NTSC. This system provides less precise colorinformation, and a different frame rate. An uncompressed NTSC signal requires slightly less transmission capacityat 168 Mbps. The situation becomes much more acute, when one realizes that high definition TV is just around thecorner. AHigh Definition TV(HDTV) picture requires a raw bandwidth exceeding 1 Gbps (1000 Mbps).MPEG-2 provides a way to compress this digital video signal to a manageable bit rate. The compression capabilityof MPEG-2 video compression is shown in the table below:

Summary of MPEG Compression Capability

Because the MPEG-2 standard provides good compression using standard algorithms, it has become the standard fordigital TV. It has the following features:

y Video compression which is backwards compatible with MPEG-1

y Full-screen interlaced and/or progressive video (for TV and Computer displays)y Enhanced audio coding (high quality, mono, stereo, and other audio features)

y Transport multiplexing (combining different MPEG streams in a single transmission stream)

y Other services (GUI, interaction, encryption, data transmission, etc)

The list of systems which now (or will soon) use MPEG-2 is extensive and continuously growing: digital TV (cable,satellite and terrestrial broadcast), Video on Demand, Digital Versatile Disc (DVD), personal computing, card

payment, test and measurement, etc.

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Interaction

One of the potential new services which may be provided by MPEG-2 is the ability to use a return channel to allow

the user to control the content or scheduling of the transmitted video / audio / data. This is known as interaction, andis seen by many as the key discriminator between traditional video and MPEG-2. MPEG-2 defines an interactionchannel using DSM-CC (explained later).Interaction channels may be used for diverse services including:

y Display and control of small video clips to promote products / future programming

y Ability to select and pay for Video on Demand (VOD)

y Access to remote information servers

y Access to remote databases / systems providing home shopping, banking, etc.

y Internet AccessAt the moment, nobody is quite sure whether, when everything falls into place, the bulk of users will interactthrough their TV s or their Personal Computers - and exactly how much interaction the users want or will pay for.

The MPEG-2 StandardMPEG-2 is a standard from the Motion Pictures Expert Group (MPEG), which defines a series of standards forcompression of moving picture information. TheMoving Picture Coding Experts Group was established in January1988 with the mandate to develop standards for coded representation of moving pictures, audio and theircombination.The following MPEG standards exist:

y MPEG-1, a standard for storage and retrieval of moving pictures and audio on storage media.

y MPEG-2, a standard for digital television.

y MPEG-4, a standard for multimedia applications.

y MPEG-7, a content representation standard for information search , to allow fast and efficient searching forthe material that is of interest to the user

MPEG-1 and MPEG-2 have been standardized, whereas MPEG-4 and MPEG-7 are currently being developed. The

MPEG-2 standard has been extended by various groups including:y Digital Video Broadcasting (European)

y U.S. Advanced Televisions Systems Committee (ATSC)

y Digital Audio Visual Council (DAVIC)

y Digital Versatile Disk (DVD)

MPEG-2 Video Compression

The MPEG-2 video compression algorithm achieves very high rates of compression by exploiting the redundancy invideo information. MPEG-2 removes both the temporal redundancy and spatial redundancy that are present inmotion video.

Temporal redundancy arises when successive frames of video display images of the same scene. It is

common for the content of the scene to remain fixed or to change only slightly between successiveframes.

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Spatial redundancy occurs because parts of the picture (calledpels) are often replicated (with minorchanges) within a single frame of video.

Clearly, it is not always possible to compress every frame of a video clip to the same extent - some parts of a clip

may have low spatial redundancy (e.g. complex picture content), while other parts may have low temporalredundancy (e.g. fast moving sequences). The compressed video stream is therefore naturally of variable bit rate,where as transmission links frequently require fixed transmission rates. The key to controlling the transmission rateis to order the compressed data in a buffer in order of decreasing detail. Compression may be performed byselectively discarding some of the information. A minimal impact on overall picture quality can be achieved bythrowing away the most detailed information, while preserving the less detailed picture content. This will ensure theoverall bit rate is limited while suffering minimal impairment of picture quality.The basic operation of the encoder is shown below:

Basic Operation of an MPEG-2 Encoder

MPEG-2 includes a wide range of compression mechanisms. An encoder must therefore which compressionmechanisms are best suited to a particular scene / sequence of scenes. In general, the more sophisticated the encoder,the better it is at selecting the most appropriate compression mechanism, and therefore the higher the picture quality

for a given transmission bit rate. MPEG-2 decoders also come in various types and have varying capabilities(including ability to handle high quality video, ability to cope with errors) and connection options.

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MPEG-2 Profiles

A number oflevels andprofiles have been defined for MPEG-2 video compression. Each of these describes a usefulsubset of the total functionality offered by the MPEG-2 standards. An MPEG-2 system is usually developed for acertain set of profiles at a certain level. Basically:

y Profile = quality of the video

y Level = resolution of the video

The basic system is known asMain Profile Main Level(MP@ML) which covers video compression to 1-15 Mbps.There are other levels such as: High Level, High Level-1440, and Low Level; just as there are other profiles:Simple, SNR, Spatial, 4:2:2 and High.Typical decoder specifications are:

y 720 x 576 x 25 fps (PAL CCIR 601)

y 352 x 576 x 25 fps (PAL Half-D1)

y 720 x 480 x 30 fps (NTSC CCIR 601)

y

352 x 480 x 30 fps (NTSC Half-D1)Most decoders will also support MPEG-1:

y 352 x 288 x 25 fps (PAL SIF)

y 352 x 240 x 30 fps (NTSC SIF)

MPEG-2 Audio Compression

The MPEG-2 standard defines an audio encoding scheme. Digital audio may be encoded in a number of otherencoding formats at various bit rates.

MPEG-2 Data

MPEG-2 also provides support for data transmission. MPEG-2 differentiates two types of data:

y Service Information - Information about the video, audio, and data streams carried by the MPEG-2transmission.

y Private Data - Information for one or more particular user (or receiver equipment).

A Quick History of MPEG-2

y 1970's Work in digital compression lead to specification of Discrete Cosine Transform algorithms.

y 1988, Motion Picture Expert Group (MPEG) formed.

y 1992, MPEG-2 (TV) and MPEG-3 (HDTV) combined.

y 1993, MPEG-2 Main profile defined (compatible with MPEG-1).

y 1993,ETSI DVB Project set up to extend MPEG-2 system details

y 1994, ISO 13818-1 MPEG-2 Systems Definition.

y

1996,Standardization of the 4:2:2 video format.y 1996, Set of Digital Video Broadcast (DVB) standards published by ETSI.

y 1996, HDTV (1250/50) demonstrated in 16:9 (widescreen) format

y 1996, 2 Million MPEG-1 video disk players in China

y 1996,U.S. adopts a Digital TV (DTV) standard based on MPEG-2

y 1997, Extended CPU graphics instruction sets able to decode MPEG on a PC

y 1997, First interactive Digital Video Broadcast service using MPEG-2

y 1998, Digital Versatile Disk (DVD) using MPEG-2

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y 1998, Active Movie API allowing MPEG-2 to be played on a PC

y 1998, Launch of DVB-T terrestrial TV service throughout the UK

y 1998,Launch of DTV service in U.S.A

y 2000, Definition of the DVB Multimedia Home Platform (MHP)

MPEG-2 Transmission

The MPEG-2 standards define how to format the various component parts of a multimedia programme (which mayconsist of: MPEG-2 compressed video, compressed audio, control data and/or user data). It also defines how thesecomponents are combined into a single synchronous transmission bit stream. The process of combining the steams isknown as multiplexing.The multiplexed stream may be transmitted over a variety of links, standards / products are (or will soon be)available for:

y Radio Frequency Links (UHF/VHF)

y Digital Broadcast Satellite Links

y Cable TV Networks

y Standard Terrestrial Communication Links (PDH, SDH)

y Microwave Line of Sight (LOS) Links (wireless)

y Digital Subscriber Links (ADSL family)

y Packet / Cell Links (ATM, IP, IPv6, Ethernet)Many of these formats are being standardized by the DVB project.

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Building the MPEG Bit Stream

To understand how the component parts of the bit stream are multiplexed, we need to first look at each componentpart. The most basic component is known as anElementary Stream in MPEG. A programme (perhaps most easilythought of as a television programme, or a Digital Versatile Disk (DVD) track) contains a combination ofelementary streams (typically one for video, one or more for audio, control data, subtitles, etc).

Elementary Stream (ES)

EachElementary Stream (ES) output by an MPEG audio, video and (some) data encoders contain a single type of(usually compressed) signal. There are various forms of ES, including:

y Digital Control Data

y Digital Audio (sampled and compressed)

y Digital Video (sampled and compressed)

y Digital Data (synchronous, or asynchronous)For video and audio, the data is organized into access units, each representing a fundamental unit of encoding. For

example, in video, an access unit will usually be a complete encoded video frame.Packetized Elementary Stream (PES)

Each ES is input to an MPEG-2 processor (e.g. a video compressor or data formatted) which accumulates the datainto a stream ofPacketised Elementary Stream (PES) packets. A PES packet may be a fixed (or variable) sized

block, with up to 65536 bytes per block and includes a 6 byte protocol header. A PES is usually organized to containan integral number of ES access units.The PES header starts with a 3 byte start code, followed by a one-byte stream ID and a 2-byte length field.The following well-known stream IDs are defined in the MPEG standard:

1. 110x xxxx - MPEG-2 audio stream number x xxxx.2. 1110 yyyy - MPEG-2 video stream number yyyy.3. 1111 0010 - MPEG-2 DSM-CC control packets.

The next field contains thePES Indicators. These provide additional information about the stream to assist thedecoder at the receiver. The following indicators are defined:

y PES_Scrambling_Control - Defines whether scrambling is used, and the chosen scrambling method.

y PES_Priority - Indicates priority of the current PES packet.

y data_alignment_indicator - Indicates if the payload starts with a video or audio start code.

y copyright information - Indicates if the payload is copyright protected.

y original_or_copy - Indicates if this is the original ES.A one-byte flags field completes the PES header. This defines the following optional fields, which if present, areinserted before the start of the PES payload.

y Presentation Time Stamp (PTS) and possibly aDecode Time Stamp (DTS) - For audio / video streams thesetime stamps which may be used to synchronize a set of elementary streams and control the rate at whichthey are replayed by the receiver.

y Elementary Stream Clock Reference (ESCR)

y Elementary Stream rate - Rate at which the ES was encoded.

y

Trick Mode - indicates the video/audio is not the normal ES, e.g. after DSM-CC has signalled a replay.y Copyright Information - set to 1 to indicated a copyright ES.

y CRC - this may be used to monitor errors in the previous PES packet

y PES Extension Information - may be used to support MPEG-1 streams.The PES packet payload includes the ES data. The information in the PES header is, in general, independent of thetransmission method used.

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MPEG-2 Multiplexing

The MPEG-2 standard allows two forms of multiplexing:

y MPEG Program Stream A group of tightly coupled PES packets referenced to the same time base. Suchstreams are suited for transmission in a relatively error-free environment and enable easy software

processing of the received data. This form of multiplexing is used for video playback and for some networkapplications.

y MPEG Transport Stream Each PES packet is broken into fixed-sized transport packets forming a generalpurpose way of combining one or more streams, possibly with independent time bases. This is suited fortransmission in which there may be potential packet loss or corruption by noise, or / and where there is aneed to send more than one programme at a time.

Combining Elementary Streams from encoders into a Transport Stream (red) or a Programme Stream (yellow). TheService Information (SI) component on the transport stream is not shown.The Programme Stream is widely used in digital video storage devices, and also where the video is reliablytransmitted over a network (e.g. video-clip down load).Digital Video Broadcast (DVB) uses the MPEG-2 TransportStream over a wide variety of under-lying networks. Since both the Program Stream and Transport Stream multiplexa set of PES inputs, interoperability between the two formats may be achieved at the PES level.

MPEG Transport StreamsA transport stream consists of a sequence of fixed sized transport packet of188 B. Each packet comprises 184 B of

payload and a 4 B header. One of the items in this 4 B header is the 13 bitPacket Identifier (PID) which plays a keyrole in the operation of the Transport Stream.The format of the transport stream is described using the figure below (a later section describes the detailed formatof the TS packet header). This figure shows two elementary streams sent in the same MPEG-2 transport multiplex.Each packet is associated with a PES through the setting of the PID value in the packet header (the values of 64 and51 in the figure). The audio packets have been assigned PID 64, and the video packets PID 51 (these are arbitrary,

but different values). As is usual, there are more video than audio packets, but you may also note that the two typesof packets are not evenly spaced in time. The MPEG-TS is not a time division multiplex, packets with any PID may

be inserted into the TS at any time by the TS multiplexer. If no packets are available at the multiplexer, it inserts null

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packets (denoted by a PID value of 0x1FFF) to retain the specified TS bit rate. The multiplexer also does notsynchronize the two PESs, indeed the encoding and decoding delay for each PES may (and usually is different). Aseparate process is therefore required to synchronize the two streams (see below).

Single Program Transport Stream (Audio and Video PES).

Transmission of the MPEG-TS

Although the MPEG TS may be directly used over a wide variety of media (as in DVB), it may also be used over acommunication network. It is designed to be robust with short frames, each one being protected by a strong errorcorrection mechanism. It is constructed to match the characteristics of the generic radio or cable channel and expectsan uncorrectedBit Error Rate (BER) of better than 10-10. (The different variants of DVB each have their own outercoding and modulation methods designed for the particular environment.)The MPEG-2 Transport Stream is so called, to signify that it is the input to the Transport Layerin the ISO OpenSystem Interconnection (OSI) seven-layer network reference model. It is not in itself a transport layer protocol andno mechanism is provided to ensure the reliable delivery of the transported data. MPEG-2 relies on underlyinglayers for such services. MPEG-2 transport requires the underlying layer to identify the transport packets, and toindicate in the transport packet header, when a transport packet has been erroneously transmitted.When the MPEG-TS is used over a lower layer network protocol, the lower layer must identify the start of eachtransport packets, and indicate in the transport packet header, when a transport packet has been erroneouslyreceived. The MPEG TS packet size also corresponds to eightAsynchronous Transfer Mode (ATM) cells, assuming8 B overhead (associated with theATM Adaptation Layer (AAL)).

Single and Multiple Program Transport Streams

A TS may correspond to a single TV programme, or multimedia stream (e.g. with two a video PES and an audioPES). This type of TS is normally called a Single Programme Transport Stream (SPTS).An SPTS contains all the information requires to reproduce the encoded TV channel or multimedia stream. It maycontain only an audio and video PESs, but in practice there will be other types of PES as well. Each PES shares acommon timebase. Although some equipments output use SPTS, this is not the normal form transmitted over aDVB link.In most cases one or more SPTS streams are combined to form a Multiple Programme Transport Stream (MPTS).This larger aggregate also contains all the control information (Program Specific Information (PSI)) required to co-ordinate the DVB system, and any other data which is to be sent.

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Streams supported by the MPTS

Most transport streams consist of a number of related elementary streams (e.g. the video and audio of a TVprogramme). The decoding of the elementary streams may need to be co-ordinated (synchronized) to ensure that theaudio playback is in synchronism with the corresponding video frames. Each stream may be tightly synchronized(usually necessary for digital TV programs, or for digital radio programs), or not synchronized (in the case of

programs offering downloading of software or games, as an example). To help synchronization time stamps may be(optionally) sent in the transport stream.There are two types of time stamps:

y The first type is usually called a reference time stamp. This time stamp is the indication of the current time.Reference time stamps are to be found in the PES syntax (ESCR), in the program syntax (SCR), and in thetransport packet adaptionProgram Clock Reference (PCR) field.

y The second type of time stamp is called Decoding Time Stamp (DTS) or Presentation Time Stamp (PTS).These time stamps are inserted close to the material to which they refer (normally in the PES packetheader). They indicate the exact moment where a video frame or an audio frame has to be decoded or

presented to the user respectively. These rely on reference time stamps for operation.

Signalling Tables

For a user to receive a particular transport stream, the user must first determine the PID being used and then filterpackets that have a matching PID value. To help the user identify which PID corresponds to which programme, aspecial set of streams, known as Signalling Tables, are transmitted with a description of each program carried withinthe MPEG-2 Transport Stream. Signalling tables are sent separately to PES, and are not synchronized with theelementary streams (i.e. they are an independent control channel).

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DVB Signalling Tables and Transport Layer PIDs

The tables (called Program Specific Information (PSI) in MPEG-2) consist of a description of the elementarystreams which need to be combined to build programmes, and a description of the programmes. Each PSI table iscarried in a sequence of PSI Sections, which may be of variable length (but are usually small, c.f. PES packets).Each section is protected by a CRC(checksum) to verify the integrity of the table being carried. The length of asection allows a decoder to identify the next section in a packet. A PSI section may also be used for downloadingdata to a remote site. Tables are sent periodically by including them in the transmitted transport multiplex.

MPEG-2 Signalling Tables

PAT - Program Association Table (lists the PIDs of tables describing each programme). The PAT is sent with thewell-known PID value of 0x000.

CAT - Conditional Access Table (defines type of scrambling used and PID values of transport streams that containthe conditional access management and entitlement information (EMM)). The PAT is sent with the well-known PIDvalue of 0x001.

PMT - Program Map Table (defines the set of PIDs associated with a programme, e.g. audio, video,)

NIT - Network Information Table (PID=10, contains details of the bearer network used to transmit the MPEGmultiplex, including the carrier frequency)DSM-CC - Digital Storage Media Command and Control (messages to the receivers)

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Programme Service Information (SI) provided by MPEG-2 and used by DVB

To identify the required PID to demultiplex a particular PES, the user searches for a description in a particular table,theProgram Association Table (PAT). This lists all programmes in the multiplex. Each programme is associatedwith a set of PIDs (one for each PES) which correspond to a Programme Map Table (PMT) carried as a separate PSIsection. There is one PMT per programme. DVB also adds a number of additional tables including those shown

below.

DVB Signalling Tables

In addition to the PSI carried in each multiplex (MPTS), a service also carries information relating to the service as awhole. Since a service may use a number of MPTS to send all the required programs. Information is provided in thePSI tables defined by DVB. Each PSI table refers to the MPTS in which it is carried and any other MPTSs that carryother TS that are offered as a part of the same service.

BAT- Bouquet Association Table (groups services into logical groups)SDT- Service Description Table (describes the name and other details of services)

TDT - Time and Date Table (PID=14, provides present time and date)RST - Running Status Table (PID=13, provides status of a programmed transmission, allows for automatic eventswitching)EIT - Event Information Table (PID=12, provides details of a programmed transmission)

Service Information (SI) provided by DVB

Most viewers have little knowledge of the operation of these tables and interact with the decoder through a graphicalor textual programme guide.

Format of a Transport Stream Packet

Each MPEG-2 TS packet carries 184 B of payload data prefixed by a 4 B (32 bit) header.

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The header has the following fields:

y The header starts with a well-known Synchronization Byte (8 bits). This has the bit pattern 0x47 (01000111).

y A set of three flag bits are used to indicate how the payload should be processed.

1. The first flag indicates a transport error.2. The second flag indicates the start of a payload (payload_unit_start_indicator)3. The third flag indicates transport priority bit.

y The flags are followed by a 13 bitPacket Identifier (PID).This is used to uniquely identify the stream towhich the packet belongs (e.g. PES packets corresponding to an ES) generated by the multiplexer. The PIDallows the receiver to differentiate the stream to which each received packet belongs. Some PID values are

predefined and are used to indicate various streams of control information. A packet with an unknown PID,or one with a PID that is not required by the receiver is silently discarded. The particular PID value of0x1FFF is reserved to indicate that the packet is a null packet (and is to be ignored by the receiver).

y The two scrambling control bits are used by conditional access procedures to encrypted the payload ofsome TS packets.

y Two adaption field control bits which may take four values:1. 01 no adaptation field, payload only2. 10 adaptation field only, no payload3. 11 adaptation field followed by payload4. 00 - RESERVED for future use

y Finally there is a half byte Continuity Counter(4 bits)

Two options are possible for inserting PES data into the TS packet payload:1. The simplest option, from both the encoder and receiver viewpoints, is to send only one PES (or a part of

single PES) in a TS packet. This allows the TS packet header to indicate the start of the PES, but since aPES packet may have an arbitrary length, also requires the remainder of the TS packet to be padded,ensuring correct alignment of the next PES to the start of a TS packet. In MPEG-2 the padding value is thehexadecimal byte 0xFF.

2. In general a given PES packet spans several TS packets so that the majority of TS packets containcontinuation data in their payloads. When a PES packet is starting, however, the

payload_unit_start_indicator bit is set to 1 which means the first byte of the TS payload contains the first

byte of the PES packet header. Only one PES packet can start in any single TS packet. The TS header alsocontains the PID so that the receiver can accept or reject PES packets at a high level without burdening thereceiver with too much processing. This has an impact on short PES packets

MPEG PES mapping onto the MPEG-2 TS

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Option Transport Packet Adaption Field

The presence of an adaptation field is indicated by the adaption field control bits in a transport stream packet. Ifpresent, the adaption field directly follows the 4 B packet header, before any user payload data. It may contain avariety of data used for timing and control.

One important item in most adaption packets is theProgram Clock Reference (PCR) field.Another important item issplice_countdown field. This field is used to indicate the end of a series of ES accessunits. It allows the MPEG-2 TS multiplexer to determine appropriate places in a stream were the video may bespliced to another video source without introducing undesirable disruption to the video replayed by the receiver.Since MPEG-2 video uses inter-frame coding a seamless switchover between sources can only occur on an I-frame

boundary (indicated by a splice count of 0). This feature may, for instance be used to insert a news flash in ascheduled TV transmission.One other bit of interest here is the transport_private_data_flagthat is set to 1 when the adaptation field contains

private data bytes. Another is the transport_private_data_length fieldthat specifies how many private data byteswill follow the field. Private data is not allowed to increase the adaptation field beyond the TS payload size of 184

bytes.

DVB Satellite

DVB transmission via satellite (often known as DVB-S) defines a series of options for sending MPEG-TS packetsover satellite links. The DVB-S standard requires the 188 B (scrambled) transport packets to be protected by 16

bytes of Reed Solomon (RS) coding.

MPEG Transport Service Encoding Specified by DVB-S

The resultant bit stream is then interleaved and convolutional coding is applied. The level of coding may be selectedby the service provider (from 1/2 to 7/8 depending on the intended application and available bandwidth). The digitalbit stream is then modulated using Quadrature Phase Shift Keying (QPSK). A typical satellite channel has a 36 MHzbandwidth, which may support transmission at up to 35-40 Mbps .

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Digital Storage Media Command and Control (DSM-CC)

DSM-CC is a toolkit for developing control channels associated with MPEG-1 and MPEG-2 streams. It uses aclient/server model connected via an underlying network (carried via the MPEG-2 multiplex or independently ifneeded). DSM-CC may be used for controlling the video reception, providing features normally found onVideoCassette Recorders (VCR) (fast-forward, rewind, pause, etc). It may also be used for a wide variety of other

purposes including packet data

DVB: Digital Video Broadcasting

Digital Video Broadcasting (DVB) is a transmission scheme based on the MPEG-2 video compression /transmission scheme and utilizing the standard MPEG-2 Transmission scheme. It is however much more than asimple replacement for existing analogue television transmission. In the first case, DVB provides superior picture

quality with the opportunity to view pictures in standard format or wide screen (16:9) format, along with mono,stereo or surround sound. It also allows a range of new features and services including subtitling, multiple audiotracks, interactive content, multimedia content - where, for instance, programmes may be linked to World WideWeb material.DVB is a European initiative. Equipment conforming to DVB standard is now in use on six continents and is DVBrapidly becoming the worldwide standard for digital TV.At the time DVB was being developed in Europe, a parallel programme of standards and equipment developmentwas also going-on in the U.S.A. by the Advanced Television Systems Committee (ATSC). ATSC is slightlydifferent to DVB (obviously the U.S. was not too worried about this - for years they have lived with an insipid TVstandard called NTSC, which has lead to slightly different TV market). Among other things ATSC adopts a differentaudio coding standard, and Vestigial Side Band (VSB) modulation. The U.S. has adopted a system based on ATSC,called Digital TV (DTV). During standardization this evolved into a hot debate between the PC-based manufacturers(favoring a non-interlaced display) and the TV manufacturers (favoring an interlaced format). There is much incommon between the US and European standards and inter-operation between some DVB and DTV equipment has

been demonstrated.

Flexibility - a key design goal of DVB

The DVB system is based on a generic transport system that does not impose any restriction on the type of materialbeing sent. Transmission techniques have been defined for video, audio, Electronic Programme Guides (EPG)(multi-media magazine-style), Electronic Service Guide (ESG) teletext-style programme guides which are processedand displayed using the receiver's menus), pay-per-view TV, data carousels (resembling traditional teletext). Thereis even provision for the transmission of Internet Protocol packets over the same system.Using DVB, a single 38 Mbps Satellite DVB transponder (DVB-S) may be used to provide one of a variety ofservices:

y

4-8 Standard TV channels (depending on programme style and quality)y 2 High Definition TV (HDTV) channels

y 150 Radio programmes

y 550 ISDN-style data channels at 64 kbps

y A variety of other high and low rate data servicesAlternatively, any combination of services may be simultaneously carried up to the maximum transponder capacity.

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Open Design

The MPEG-2 system is designed to be open - allowing receivers to implement as much or as little functionality asthey need. All parts of the standard allow for manufacturers to add sophistication and additional features todiscriminate their products, while providing the basic functions required for interoperability.

Summary of DVB Features

1. MPEG-2 standard for video & audio2. Fixed rate simplex transmission3. Extends MPEG-2 transport facilities:

i. Programme Guides (both teletext style and magazine style formats)ii. Specifications for Conditional Access (CA)iii. Optional return channel for interactive servicesiv. Various types of packet

DVB Standards and related documents are published by the European Telecommunications Standards Institute(ETSI). These include a large number of standards and technical notes to complement the MPEG-2 standardsdefined by the ISO.

DVB Transmission

DVB builds upon MPEG-2 and uses MPEG-2 Transmission. It also defines additional private sections and providinga definition of the physical medium (modulation, coding, etc) needed to carry the MPEG-2 Transport Streams.

Typical Streams carried in a DVB Terrestrial Transport Multiplex

Each MPEG-2 MCPC multiplex carries a number of streams which in combination deliver the required services.Here is a sample breakdown of the various MPEG-2 streams being used to provide a terrestrial 24 MBPS TVmultiplex:

Stream bit rate (kbps)

SI 300PSI 546

Digital Teletext 754

Total per Mux 1600Sample per-multiplex overheads

Streambit rate(kbps)

TV Video * 5000

Stereo Audio 270

SubTitles 50

Conditional Access 600

Total Programme 5920

Bit rate per programme(* Video at 4-6 Mbps depending on content, Conditional Access may not be required)

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This allows a standard 18 MHz channel to carry 5 TV channels or 4 higher quality channels without conditionalaccess - or 3 high quality channels with conditional access. The remaining bandwidth may be used for other servicessuch as Electronic Programme Guides (EPGs), Audio Descriptions for the visually impaired (~ 70 kbps), Signing forthe deaf (~ 400 kbps using a separate window), house pages, digital data, software downloads, etc.

DVB Bearer Networks

The DVB standards allow a DVB signal to be carried over a range of bearer networks. Various standards haveevolved which define transmission over particular types of link:

y DVB-S (Satellite) ETS 300 421 (Digital Satellite Transmission Systems)

y DVB-T (Terrestrial) ETS 300 744 ( Digital Terrestrial Transmission Systems)

y Interfaces to Plesiochronous Digital Hierarchy (PDH) networks (prETS 300 813).

y Interfaces to Synchronous Digital Hierarchy (SDH) networks (prETS 300 814).

y Interfaces to Asynchronous Transfer Mode (ATM) networks (prETS 300 815).

y Interfaces for CATV/SMATV Headends and similar Professional Equipment (EN50083-9)

Digital Satellite TV (DVB-S)Satellite transmission has lead the way in delivering digital TV to viewers. A typical satellite channel has 36 MHz

bandwidth, which may support transmission at up to 35-40 Mbps (assuming delivery to a 3.5m receiving antenna)using Quadrature Phase Shift Keying (QPSK) modulation.The video, audio, control data and user data are all formed into fixed sized MPEG-2 transport packets. The MPEGTS packets are grouped into 8-packet frames (1503 B). The frames do not have any additional control information,

but to enable the receiver to find the start of each frame, the TS-header byte is inverted (0xB8) in the first TS packetof each frame. The frames are then passed through a convolutional (organ-pipe) interleaver to ensure the datafollows an approximately random pattern, assuring frequency dispersion of the modulated signal. At the start of eachframe, the scrambler is re-initialized.16 bytes of Reed Solomon (RS) coding are added to each 188 byte transport packet to provide Forward ErrorCorrection (FEC) using a RS (204,188,8) code. For the satellite transmission, the resultant bit stream is theninterleaved and convolutional coding is applied. The level of coding ranges from 1/2 to 7/8 depending on the

intended application and available bandwidth. The digital bit stream is then modulated using QPSK modulation.The complete coding process may be summarized by:

y Inverting every 8th Synchronization byte

y Scrambling the contents of each packet

y Reed-Solomon (RS) coding at 8% overhead

y Interleaved convolutional coding (the level of coding ranges from 1/2 to 7/8 depending on the intendedapplication)

y The resulting bit stream is modulated using Quadrature Phase Shift Keying (QPSK).

Digital Terrestrial TV Network (DVB-T)

Legislation passed in 1996 has opened the way to allow new companies to enter the terrestrial TV market, while

protecting the interest of the main players. This lead to the granting of licenses to various companies during 1997,and the planned rollout of the first direct-to-home digital terrestrial services during November 1998.A standard TV channel is 8 MHz wide, accommodating the TV signal in Phase Alternate Line (PAL) format and thesound subcarrier. The same 8 MHz of bandwidth may be used to provide a 24 Mbps digital transmission path usingCoded Orthogonal Frequency Division Multiplexing (COFDM) modulation. This may support up to 6 digital TVchannels.The information is transmitted in the following manner:

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y COFDM or Quadrature Phase Modulation (QPSK) (COFDM uses either 1705 carriers (usuallyknown as '2k'), or 6817 carriers ('8k'))

y Reed-Solomon (RS) coding at 8% overhead

y Interleaved convolutional coding (the level of coding depends on the intended application)

Some countries have chosen to introduce a digital TV service which is based on more robust modulation, allowingmobile reception of the signal, or to pilot high definition TV - in both cases the bandwidth (number of channels) hasbeen traded.

DVB Receivers & Transmission

DVB Receivers

DVB transmissions may be received via a variety of equipment, but will typically be received by one of thefollowing:

y A Set-Top Box (STB), costing $200-$600, and connected via the SCART or standard TV antennaconnection to a TV set. More sophisticated STBs contain more than one DVB decoder, the secondarydecoder(s) providing a bit stream to an internal hard disk where selected programme content can be storedin digital format. Such a STB can be preprogrammed to record programmes (as in a Video Cassette

Recorder (VCR)), but may also be used to implement pause and rewind functions (where users are given

y Playback control of the content, allowing them to defer viewing when, for instance, the telephone rings).

y An in-built decoder forming a part of a digital TV set or a digital VideoCassette Recorder (VCR).

y A receiver located centrally in a house which feeds digital TVs, VCRs, Camcorders, etc with signals viadigital bus (e.g. the IEEE 1394 FireWire bus).

y A DVB compliant PC receiver card which displays the various styles of content (TV, Audio, and Data) onthe PC screen.

y A DVBMultimedia Home Platform (MHP).

The next generation of digital recorders will provide much greater functionality than existing VCRs. By combininginformation held transmitted as MPEG meta-data prior to each programme, the digital recorder may itself determinewhether the broadcast content should be recorded. This decision could for instance be done by matching the meta-data with a user-specified customer profile stored in the digital recorder. This profile also allows a future ElectronicProgramme Guide (EPG) to generate customized schedules.

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Multimedia Home Platform (MHP)

The Multimedia Home Platform (MHP) is being defined to enable DVB receivers to be constructed in a commonand provide common features and interfaces. DVB MHP uses the JAVA programming language (developed by SUNMicrosystems) to access an Applications Program Interface (API) which gives access to DVB services. UsingJAVA code, manufacturers can add additional capability to discriminate their products in the market place, as wellas implementing industrial standard EPGs and user interfaces. Three application profiles have been defined:

y Enhanced Broadcast

y Interactive Television

y Internet AccessThe first MHP standards were defined in ETSI TS 101 812 (2000).

Wide-Screen Format

It has been claimed that the human eyesight is naturally more wide than high and that by increasing the width of aTV picture, one can gain substantial improvement in the viewer experience. Certainly this format allows much morecontrol by the viewer of the scene being presented - with the opportunity to focus on specific interests rather thanfollowing only the intended story line. The UK has piloted a wide-screen service using "PALPlus" in 16:9 format.After much market survey, most people seem to acknowledge the advantages of wide-screen TV (as opposed to thetraditional 4:3 screen format). The 16:9 format has been adopted as the standard forHigh Definition (HD) TV.Attempting to view wide screen TV on a standard TV presents some problems - one either misses the left and right

parts of the picture, or one sees the entire picture through a "letter box" which places broad black stripes across thetop and bottom of the picture. Viewer surveys have shown that there has been considerable resistance to adopting a16:9 "letter box" format for 4:3 TVs. It seems that the best way forward for traditional TVs is to adopt a small letter

box, 14:3 which looses some information at each side, and introduces a small black band at the top and bottom ofthe picture. The plan is therefore to rollout TV in 16:9 format (much TV is already produced in this format), but to"guard" the TV picture, ensuring that all essential material is captured in the central 14:9 portion of the display.

Programme Guides

The purpose of a programme guide is to assist the viewer in selection of the programmes to be viewed. The guidewill present a selection of information about the current TV programmes being shown, and the coming programmes.The programme guide is not bound to the channels being viewed, and it is feasible to see programme informationfrom channels which are not currently transmitting. The programme guide will carry all programme information,including regional variations, which it may not be possible to receive outside a specific area, within a commonMPEG2 multiplex stream.Two types of guides have been suggested: one simpler, allowing more flexibility, and resembling the TV listingsfound in newspapers; the other more sophisticated resembling a web-based TV magazine.

Event Schedule Guide (ESG)

The simplest type of guide resembles that provided by the analogue teletext system. This type of guide maybe constructed by the receiver based on the Service Information (SI) contained in the MPEG2 multiplexstream. Transmitting the information in this way allows the receiver manufacturers and viewers todetermine the presentation format - level of detail, preferred presentation style, channels of interest, type of

programme desired, etc. A bit rate of about 50 kbps is required a weeks programme (assuming 5 channelsof traditional TV).

Electronic Programme Guide (EPG)

An EPG presents an additional service to the user in the form of a multimedia magazine-style channelguide. This type of guide requires support in the manufacturers set top box for a consistent interface toallow the view of video stills, movie clips, buttons, layout, etc. It is expected that the look and feel for this

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type of guide will dictate by the service provider. EPGs have some considerable advantage whenconsidering other services which may be provided: Home shopping, remote banking, etc. There is also a

bigger opportunity for advertising. The cost is in the standardization of the STB interface and the extrabandwidth consumed by the guide.

DSM-CC for Software Download

Most decoders will have software-based user interfaces implemented using a microprocessor. In many cases themicroprocessor will also perform configuration and other housekeeping tasks associated with decoding the variousMPEG2 streams. It has been suggested that the DSM-CC service could be used to reload and update this softwareover the MPEG2 broadcast link. This would use a DSM-CC object carousel to download each type of software(presently each STB will have its own architecture, and therefore will require a specific software image). To ensurethat the software is always available it has been suggested that it should be encrypted with a key, which isindependent of the Conditional Access (CA) system, and the RSA key has been suggested. The details ofdownloading - and how to recover from faults - who is controls the software in the viewers STB, etc are still issuesfor debate.

Data Transmission using MPEG-2 and DVB

Introduction

The growing use of multimedia-capable personnel computers to access the Internet and in particular, the use ofWorld Wide Web, has resulted in a growing demand for Internet bandwidth. The emphasis has moved from basicInternet access to the expectation that connectivity may be provided whatever the location. This presents freshchallenges to the networking community, particularly as users become familiar with the benefits of high-speedconnectivity.Along with an increased use of the Internet, there has also been a revolution in television transmission, with theemergence of Digital Video Broadcast (DVB) .The same system may support a high speed Internet, and is beingsupported on a number of DVB satellite systems. A high speed (6-34 MBPS) simplex data transmission system may

be built using a digital Low Noise Block (LNB) and standard TV antenna connected via an L-band co-axial cable toa satellite data receiver card in a PC (or LAN adapter box). In many cases, a return link may be established using theavailable (standard dial-up modem) terrestrial infrastructure, providing the full-duplex communication required forthe Internet service. Low-cost satellite return channels are also available. The overall system may provide low cost,high bandwidth, Internet access to any location within the downlink coverage of a DVB satellite service.

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Forward Data Transmission

Typical configuration for providing Direct to Home (DTH) Internet delivery using DVB

Data is already being sent over DVB networks using the MPEG-2 transport stream. A variety of proprietaryencoding schemes are being used. Data transmission may be simplex or full duplex (using an interaction channel forthe return) and may be unicast (point-to-point), multicast (one to many) or broadcast (all receivers receiving theassigned PID).

A number of manufacturers supply DVB-S (satellite) receiver cards with data capability and data gateways to allowpacketizing the data to be sent. Some suppliers include:

y Media 4 Inc

y PACE/TFC (see also ASTRA-Net)

y SAGEM

y NDS

y Phillips

y COMSTREAM (MediaCast)

y Adaptec

y Sony (Mediacaster)

In an effort to standardize these services, the DVB specification suggests data may be sent using one of five profiles:

1. Data Piping - where discrete pieces of data are delivered using containers to the destination. There is notiming relationship between the data and other PES packets.

2. Data Streaming - where the data takes the form of a continuous stream which may be Asynchronous (i.e.without timing, as for Internet packet data) Synchronous (i.e. tied to a fixed rate transmission clock, as foremulation of a synchronous communication link) or Synchronized (i.e. tied via time stamps to the decoderclock and hence to other PES packets, as for the display of TV captions). The data is carried in a PES.

3. Multi-Protocol Encapsulation (MPE) - the technique is based on DSM-CC and is intended for providingLAN emulation to exchange packet data

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4. Data Carousels - a scheme for assembling data sets into a buffer, which is played-out cyclic manner(periodic transmission). The data sets may be of any format or type. One example use is to provide the datafor Electronic Program Guides (EPGs). The data are sent using fixed sized DSM-CC sections.

5. Object Carousels - object carousels resemble data carousels, but primarily intended for data broadcastservices. The data sets are defined by the DVB Network Independent Protocol specification. They may beused, for example, to download data to a set top box decoder.

For Internet data transmission, the recommended procedure is to use MPE. Backwards compatibility with proprietary data transmission schemes using piping / streaming is provided by assigning a registered ServiceInformation (SI) code to each format of data. Only the SI codes which are recognized by a receiver / decoder areprocessed, enabling continued processing of the proprietary encoding by receivers with the appropriate hardware.

Digital Storage Media Command and Control

DSM-CC is a toolkit for developing control channels associated with MPEG-1 and MPEG-2 streams. It is defined in part 6 of the MPEG-2 standard (Extensions for DSM-CC) and uses a client/server model connected via anunderlying network (carried via the MPEG-2 multiplex or independently if needed).DSM-CC may be used for controlling the video reception, providing features normally found on videocassetteRecorders (VCR) (fast-forward, rewind, pause, etc). It may also be used for a wide variety of other purposesincluding packet data transport. It is defined by a series of weighty standards, principally MPEG-2 ISO/IEC 13818-6(part 6 of the MPEG-2 standard).DSM-CC may work in conjunction with next generation packet networks, working alongside such Internet protocolsas RSVP, RTSP, RTP and SCP.

Digital Storage Media Command and Control (DSM-CC) Packet Download

Compared to other download protocols, DSM-CC download is designed for lightweight and fast operation in orderto meet the needs of devices that contain limited memory. The DSM-CC download operates over heterogeneousconnections and is applied to a number of network models one of which is the broadcast model with no upstreamchannel. The mechanisms used in download are:

y Sliding window

y No ACK for use in broadcasting

y Maps to MPEG-2 Transport Stream for hardware multiplexing

DSM-CC Multi Protocol Encapsulation

Using the multi-protocol encapsulation, each frame of data is encapsulated in an Ethernet-style section.

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Return Channel Systems

Various types of Return Channel systems have been defined by DVB including:

y

RVB-RC Return Channel Systemy DVB-RCS Return Channel System for Satellite

y DVB-RCT Return Channel System for Terrestrial TV

y DVB-RCC Return Channel System for Cable TV

Return Data Transmission (Interaction Channel) via Satellite

The DVB-S standards provide transmission of data from a satellite hub station up-link to receivers (typically PCs oradoption boxes). The advantages of using DVB include the availability of low cost (mass produced) systemcomponents and the ability to integrate transmission with digital TV distribution (sharing the transmission costs and

providing a future integration path for TV and Internet services). Using these components an Internet service may beeasily offered. Simplex data transfer (using UDP) requires no further components at the receiver, but to provide fullduplex communication (as required for TCP) requires the addition of a return channel (sometimes known as an

"interaction channel").The complete system consists of a packet data processor at the hub site (which typically formats the IP packets usingMPE) and a decoder card at the PC. A client sends requests for data transfer (and later acknowledgments as thesession progresses) through the terrestrial network, while the server transfers the data to the client through the higherspeed satellite link. The client network software is configured to redirect the packets destined to the hub site to a"virtual connection" formed by an IP tunnel which sends packets from the client back to the hub site using the dial-up modem connection. Once the return packets reach the hub site, they may be either forwarded to a server, orrouted to the Internet (typically using a high-speed fibre connection to a terrestrial Internet Service Provider).

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Each user (i.e. DVB/MPEG-2 receiver) is allocated a MAC address (from for example form a subscriber database)to match the IP addresses of the equipment at the remote site. The MAC address is used to uniquely identify the userequipment.

Protocol architecture for (a) Outbound link to client and (b) return link from client

Each frame of data is encapsulated by adding a section header, a Medium Access Control (MAC) address (arrangedto ease processing by receivers with limited capability) and an optional header with Logical Link Control (LLC) /Sub Network Access Protocol (SNAP). The data is protected by a CRC-32 checksum. The entire block of data isknown as a section. The section length is adjusted by adding padding bytes to ensure it may segmented into anintegral number of 188B MPEG-2 transport packets. The transport packets are assigned a PID, based on the routinginformation at the hub site. A set of users may be allocated the same PID - forming a Virtual Private Network(VPN), or may alternatively one PID may be allocated to each user. The first packet (with the start of the section)has a flag bit set to indicate that it contains the start of the section.The packets in the DSM-CC section may be scrambled using conditional access control which scramble the MAC

address (preventing other users from knowing the traffic to each MAC address) and/or the packet data. Encryption iscontrolled by flag bits in the DSM-CC encapsulation header.Normally packets are sent Unicast (i.e. point-to-point), in which case, only one receiver will forward the data, theother receivers in the network will receive, but discard the data (since either the MAC address and/or PID will notcorrespond to their internal filters). Multicast transmission is possible, using a multicast address. No provision is

provided for group management (this is assumed to be provided by some other means, e.g. using a terrestrial returnchannel).

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Although a satellite-based system is capable of providing a high bandwidth (simplex) path from a satellite serviceprovider to a receiver, the connectivity from a user to the service provider is usually provided using a low speedterrestrial link. This results in a network connection in which the capacity to a remote server differs from that from

the server. Such an asymmetry in the network paths may be suited to user needs, since most Internet connectionsreceive much more data than they need to send. However, a high degree of asymmetry in the forward and return

paths introduces a potential bottleneck in performance. There are also other important considerations in providing aTCP Internet service via satellite that is discussed below.

MPEG-2 Encoders and Decoders

The secret of MPEG-2 is the flexibility offered to the manufacturers of the equipment, while adhering to the coreMPEG-2 standard. Encoders and decoders come at a range of prices with a range of capabilities. Not all decodersare equal. Manufacturers decide how much functionality to include in their decoders. They may, for instance,support electronic programme guides, interpretation of optional MPEG data, and support for a range of audioformats. Even a stream of encoded MPEG-2 video may be handled at various levels of sophistication (advancedcodecs provide features for instance which may mask the effect of loss of all or part of a frame of video).In many ways, the encoder is the key item. There are many options when encoding a video picture into an MPEG-2

bit stream. The ability to maintain picture quality with rapidly moving scenes and complex pictures will dependupon the algorithms and processing power of the encoder. For real-time transmission, such as encoding the output ofa video camera, this may necessitate expensive equipment. For non-real-time, converting stored analogue / digitalclips into an MPEG-2 video clip library, less expensive (but slower) equipment may be used.

MPEG-2 Decoders

The various types of MPEG-2 decoder may be broadly classified as shown below:

Software MPEG-2 Decoders

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Software-based MPEG-2 players, which usually require the support of specialized instruction sets to provide the bit manipulations required for video rendering. Such support is provided in most modernprocessors (e.g. MMX, viz). Such players are well suited to delivery from local high speed media (e.g. toreplay video recorded onDigital Versatile Disk(DVD)), but are generally CPU intensive, and allow little

extra processing with current CPUs for other tasks. Display is to a window on the computer display .

PC-Based MPEG-2 Accelerators

PC-based MPEG-2 accelerators are available for personal computers and/or workstations. Theaccelerator provides a range of functions to ease the processing of the MPEG-2 video decompression,

but still requires CPU intervention to decode the MPEG-2 stream. The primary application ofaccelerators is the replay of recorded MPEG-2 video from a local disk. Mediamatics have specified a"standard" interface to such decoders.

Computer MPEG-2 Decoders

MPEG-2 decoders relieve the computer CPU of nearly all processing. They may play via an externalmonitor, a video overlay on the existing computer monitor, or by direct writing of the video bit map tothe display memory. Decoders may receive data from a Network Interface Card(NIC) or a local disk.The host computer is free to do other tasks - although disk access speed may often be a limiting factoron overall system performance.

Network Computers / Thin Clients

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An alternate design is to embedded the MPEG-2 decoder in an equipment which was designed tosupport MPEG-2 decoding together with other basic functions (e.g. web browsing, ftp, telnet,

programme guides). Such equipment may be based around a virtual machine (e.g. a JAVA virtualmachine) and be reprogrammable by the user and/or network applications. A high quality displaywould normally be connected via a S-video or SCART connector. By reducing the functionality ofsuch a design, manufacturers may eliminate interface and configuration options - substantiallyreducing the overall cost of ownership (i.e. little additional software required, easy installation, simpleuser interface) compared to a common PC. The key emphasis will be on ease of use, with the intentionof reaching the many people who do not need (or will not use) a common PC.

The MPEG-2 Set-Top Box (STB)

Most people use TV sets in a different way to computers and locate them in different rooms and havedifferent expectations, particularly in the lifetime of equipment (10 years?), the initial cost, the running

cost (zero for hardware, small charge for programming), and user interface (remote control). There istherefore a need for equipment that allows connection of standard (PAL/NTSC) TVs to a digital videonetwork. Such equipment will take the form of a set top box which replacing or sitting alongsideexisting satellite / cable receivers. It is likely that it will support DVB (and/or ATSC) and provide aninteraction channel (which may be optionally used).

MPEG-2 Consumer Equipment

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Future TVs and video equipment may support in-built decoders, reducing the cost of the set-top boxand allowing further integration of functions. An alternate scenario is also possible, in which the set-top box and a network computer/thin client are brought together into a single piece of equipment witha range of interfaces to various types of network (DVB/ ATSC) and packet/cell data networks). Suchequipment may be viewed more as a utility interface (in the same way as an electricity/gas/watermeter) and be hidden away out of sight. The household equipments (TV, VCR, Camcorder, PCs)would be connected back using a common high-speed communications link (e.g. IEEE 1394FireWire).

Delivery of MPEG-2

MPEG-2 data may be delivered to the decoder in a variety of ways. These may be categorized in one of two groups,depending upon whether the receiver buffers a substantial volume of data prior to playing.

Streaming applications

These applications have only a small buffer at the receiver and therefore require the jitter introduced duringtransmission to not exceed the maximum delay, which the buffer can accommodate. These applications usually haveinsufficient space for retransmission and require data to be sent as a continuous stream (either to a group ofsynchronized receivers or to individual receivers:

y Replay of "live" network media from a streaming video server (e.g. Windows Media Server)

y Replay over a public/private broadcast network (e.g. DVB)

Buffered Applications

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These applications have a buffer at the receiver. The applications may buffer all or part of the video clip beforeplaying. They are therefore less sensitive to jitter introduced during transmission, and may be tolerant of a smalllevel of loss - the protocol used may permit some form of retransmission:

y Replay from local storage media (e.g. DVD)

y Replay from network-downloaded content from a file/web server (e.g. using http)

y Replay using a purpose-designed video-clip based transfer protocol (e.g. TimeLine)

IssuesThere are two important issues to be considered when thinking about MPEG-2 delivery. The first issue arisesdirectly from the bit rate needed to support MPEG-2 applications - rates typically of 3-6 Mbps per programme. Ifeach user is selecting their own material, this may multiply the bandwidth needed by the number of users. This

places exacting demands on the network and the server, and also on the client hosts used by the users (which neednormally handle only one programme but are usually of a lower specification than the server). The other issue is thesubject ofQuality of Service (QOS). Different applications and delivery mechanisms will have different needs interms of maximum tolerable jitter, and maximum tolerable loss rate.

Programming APIs for MPEG-2 Computer CardsApplications may be written to use MPEG-2 on a range of platforms using suchApplication Programming

Interfaces (APIs) as QuickTime (Mac, eta) and others. APIs are also available for the PC platform and may becategorized as:

1. Proprietary APIs developed by individual MPEG-2 decoder card suppliers2. Microsoft MCI, defined as a 16-bit interface providing support for overlay cards3. Microsoft Active Movie-14. Microsoft Direct Show(Active Movie-2) part of the Direct-X API allowing bit-mapped access to the

display and filters to be defined to process the video.At the time of writing, many developers are committed to producing Direct-Show APIs, but few are able to supplythis.

Broadband Multimedia Satellite SystemsWhat is Broadband Multimedia?

The term Broadbandliterally means large capacity, but it means much more than that in the communicationsindustry. The most spoken about broadband systems were those designed based on theAsynchronous Transfer Mode

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(ATM), or cell-based switching. These systems were designed to support multiple services and have becomesynonymous with multimedia capability.

Multimedia is a term that may be applied to digital media-rich content, digital platforms, and networks. MultimediaNetworks enable a number of services to be sent simultaneously over the same network. Examples include supportfor different classes of Internet customers (Home, Business) and high availability video on Demand, Voice over IP,Videoconference, etc. In the case of ATM, multimedia is linked to the ability to control the Quality of Service (QoS)of each class of service, and in some cases to specify individual Service Level Agreements. More recently, the fullrange of multimedia services have been demonstrated over next generation IP networks, leading to questioning as tothe merits of introducing a unifiedATM network.Types of multimedia service:

y TV - well suited to satellite (likely to remain the killer-app for at least the next few years)

y Video on Demand - Ability to download hours of TV programmes.

y Electronic Programme Guide, EPG - now widely used by many digital TV users

y E-commerce services (shopping, gaming, etc)

y Internet proxied services (selected web, email)

y Internet access - offered only by some current systems

y

Games - surprising take-up by some usersMost modern broadband systems also permit some level ofInteractivity. This is true of most current content: TV,Radio, etc, but especially true of the new digital services, where users often wish to sort, manipulate or participate

based on the received content. Although it is possible to conceive a one way Internet service, one-way routedInternet (e.g. supplying supplementary capacity to network service providers requiring bandwidth exceeding their

purchased terrestrial capacity), most services will require some form of return channel to allow two way packetflow.

Multi-Platform delivery is also a key concept for many people thinking of Broadband Multimedia. This is the abilityto deliver content (digital video, web pages, etc) to a range of network devices - TV sets, personal PCs, Multimediadevices, web-enabled telephones (e.g. WAP) and wireless devices. To date the very different user interfaces, displaycapabilities and human factors considerations has made multi-platform delivery difficult. Many proposed networksinstead will restructure information for each type of display. Some suggest this is a short-term solution, and futuregenerations will have to converge on a common technology (presumably Internet-based for data applications).There are five key players in a typical broadband multimedia system:

y Content Owners / Providers

y Middleware developers (platform, user interface, portals, etc)

y Service Providers (customer management, proxies, supporting services, lease networking resources fromNetwork Operators)

y Network Operators (infrastructure & equipment to connect the various players)

y Customers (people / companies willing to pay Service Providers to have the content delivered to them)

Broadband Multimedia Content - Is Content King?

The key issue in bringing broadband multimedia networks to reality was once thought of as Content. Finding theright content was seen as a key part of planning a broadband system. Early content (well-suited) news, sports,weather, financial data - mainly text based. One way to generate additional content for a new format is to usecontent gateways (which rewrite content across HTML, for OpenTV, for WAP, etc), although such gateways mayseem attractive, they are unlikely to provide effective long-term use for access to rapidly evolving Internet content.Convergence is the term given to the perception that many platforms now have (or soon will have) commoncapabilities. Although specific platforms are optimized for specific types of content some capabilities are now

becoming common. The TV set can receive and send email (as can a wireless personal computer, mobile phone,etc), networked PCs can receive broadcast TV, digital music can be downloaded from any digital appliance, anddigital camera pictures uploaded. A multiservice network shows a similar convergence in network capability.

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One advantage of convergence is the ability to access the same information from a TV set as from a PC. Particularlyin Europe, many households do not have their own Internet PC (estimates place the proportion of Europeanhouseholds with a PC to be 50% less than in the USA). In contrast to what is seen as the complexity of a PC, most

potential broadband customers perceive the TV as a less difficult to use device. Never the less to evolve the set-top-box will need to acquire much more sophistication, and may never prove the ideal device for personal access. Anadvantage of such systems is that the service provider can leverage their existing (often-loyal) customer based forsubscriber TV.There is growing perception that content alone is not the solution. There is a need for a sound commercial model forthe new service. Traditionally few Internet users have been willing to pay more than modest charges for their dataservices. There are also legal issues on the distribution (and redistribution) of content, both from the viewpoint ofnational laws and the ownership of copyright / loyalties.

DVB Satellite Return Channel (DVB-RCS)The DVB Return Channel System via Satellite (DVB-RCS) was specified by an ad-hoc ETSI technical groupfounded in 1999. This tracked developments by key satellite operators and followed a number of pilot projectsorganized by the European Space Agency (ESA). The DVB-RCS system is specified in ETSI EN 301 790. Thisspecifies a satellite terminal (sometimes known as a Satellite Interactive Terminal (SIT) or Return Channel SatelliteTerminal (RCST)) supporting a two-way DVB satellite system. The use of standard system components provides asimple approach and should reduce time to market. (There is also a related guideline document for use of EN 301790.)The satellite user terminal receives a standard DVB-S transmission generated by a satellite hub station. Packet datamay be sent over this forward link in the usual way (e.g. MPE, data streaming, etc).In addition, DVB-RCS provides transmit capability from the user site via the same antenna. The transmit capabilityuses a Multi-Frequency Time Division Multiple Access (MF-TDMA) access scheme to share the capacity availablefor transmission by the user terminal. The return channel is coded using rate 1/2 convolutional FEC and ReedSolomon coding. The standard is designed to be frequency independent - it does not specify the frequency band (or

bands) to be used - thereby allowing a wide variety of systems to be constructed. The data to be transported may beencapsulated inAsynchronous Transfer Mode (ATM) cells, usingATM Adaptation Layer 5 (AAL-5), or use a nativeIP encapsulation over MPEG-2 transport. It also includes a number of security mechanisms.

One arrangement of the User Terminal Protocol Stack

DVB-RCS terminals require a two-way feed arrangement / antenna system, able to transmit and receive in theappropriate satellite frequency bands. These are typically connected via a cable (or group of cables) to an indoorunit. This unit could be a Set Top Box (STB) with a network interface, could be integrated in a PC peripheral (e.g. aUSB or FireWire device), or may be integrated in a PC expansion card. A key goal of DVB equipment suppliers isto reduce equipment costs. Since cost is likely to be (at least initially) dependent on the terminal transmit capability(that is the rated transmit power), a number of different classes of terminal are envisaged, supporting a range oftransmit bit rates.

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Terminal Operation

AReturn Channel Satellite Terminal (RCST), once power on, will start to receive general network information fromthe DVB-RCSNetwork Control Center (NCC). The NCC provides monitoring & control functions and generates thecontrol and timing messages required for operation of the satellite network. All messages from the NCC are sent

using the MPEG-2 TS using private data sections (DVB SI tables). These are transmitted over the forward link.Actually the DVB-RCS specification calls for two forward links - one for interaction control, and another for datatransmission. Both links can be provided using the same DVB-S transport multiplex.The term forward link refers to the link from the hub station which is received by the user terminal. DVB-RCSallows this communication to use the same transmission path as used for data (that is the DVB-S receive path), or analternate interaction path. Conversely the return linkis the link from the user terminal to the hub station using theDVB interaction channel. The control messages received over the forward link also provide the Network Clock

Reference (NCR). The NCR contains a 27 MHz clock reference and reception of the NCR is used by user terminalsto adjust the transmit frequency of each user terminal to ensure a common reference for the MF-TDMAtransmissions.All transmissions by a user terminal are controlled by the NCC. Before a terminal can send data, it must first join thenetwork by communicating (logon) with theNCC describing its configuration. The logon message is sent using afrequency channel also specified in the control messages. This channel is shared between user terminals wishing to

join the network using the slotted ALOHA access protocol.

Simplified Transponder Usage by a Group of DVB-RCS User Terminals

After receiving a logon message from a valid terminal, the NCC returns a series of tables including the Terminal Burst Time Plan (TBTP) for the user terminal. The MF-TDMA burst time plan (TBTP) allows the terminal tocommunicate at specific time intervals using specific assigned carrier frequencies at an assigned transmit power.The terminal transmits a group of ATM cells (or MPEG-TS packets). This block of information may be encoded inone of several ways (using convolutional coding, RS/convolutional coding or Turbo-coding). The block is prefixed

by a preamble (and optional control data) and followed by a postamble (to flush the convolutional encoder). Thecomplete burst is sent using QPSK modulation. Before each terminal can use its allocated capacity, it must firstachieve physical layer synchronization (of time, power, and frequency), a process completed with the assistance ofspecial synchronization messages sent over the satellite channel.A terminal normally logs off the system when it has completed its communication. Alternately, if there is a need, the

NCC may force a terminal to log off.One of the strengths of the system is the extreme flexibility this provides to configuring individual transmissioncapabilities. Some also criticize this as a weakness: The current standard allows a range of implementations, andtherefore does not promote equipment interoperability between different systems.

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Broadband Interactive Service (BBI)BBI is an example of a satellite system built according to the DVB-RCS standard. The system has been designed forSociete Europeene des Satellites (SES), the company operating the ASTRA Satellite network. BBI uses ASTRAsexisting Ku-Band system (14 GHz) for transmission from the hub station to the SIT, and Ka-Band capacity (29.5-30.0 GHz) for the return channel from the SIT to the hub station using a 20 MHz MF-TDMA channel.The BBI system supports three classes of SIT, each with similar receive capabilities, but different transmitcapabilities:

SIT II SIT III SIT IIIAntenna 0.75 m 0.95 m 1.2 mSIT Transmit EIRP (estimated) 40 dBW 45 dBW 50 dBW

Return Transmission Speed 144 kbps384kbps

2 Mbps

Forward Link Speed (DVB-S) 38 Mbps 38 Mbps 38 Mbps

TCP over Satellite

Introduction

There is a popular misunderstanding concerning the performance of TCP (used by most Internet applications) oversatellite. This is fuelled by various misconceptions that have become widespread in recent years. Themisunderstanding stem from three areas:

(i) Many people have experience with implementations that are now out of date. Research paperspublished in the later 1980's and early 1990's identified problems with TCP, most of that have nowbeen resolved.

(ii) Many reported experiments have used misconfigured TCP protocol stacks. Until recently fewprotocol stacks have been configured suitably for satellite operation.

(iii) Unfortunately, many researchers still fail to understand the way in which TCP actually operates.This has lead to a lot of bad advice. The IETF has recently published two documents to try andaddress these issues.

The result of all this, is that many people are left with fear, uncertainty and doubt concerning whether TCP will

actually operate over a satellite link. In fact, this doubt is mostly unfounded, as many satellite service users cantestify. TCP was designed to work over satellites. The original goal of the research project that created TCP was tolink a pilot satellite network (SATNET) with the terrestrial Internet (ARPANET). Although there is no practicallimit to TCP's throughput (the maximum theoretical throughput is 1.5 Gbps - faster than any satellite link), there anumber of significant issues which do directly impact the performance which real users are likely to achieve. Tounderstand things more clearly, it is first necessary to examine how TCP has evolved.

SPECIFIC ISSUES THAT IMPACT THE SATELLITE SERVICE

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The main distinguishable characteristics of a satellite link compared to a terrestrial network which effect theperformance of TCP are:Effect of Satellite Link Errors(ii) Satellite Propagation Delay(iii) Bandwidth and Path Asymmetry(iv) Channel Access and Network Interactions

Internet protocols are not optimized for satellite conditions, and consequently, the throughput over satellite networksis restricted to only a fraction of the available bandwidth, data networking over satellites is faced with overcomingthe large latency and high bit error rates typical of satellite communications, as well as the asymmetric bandwidthdesign of most satellite networks. The effectiveness of TCP is optimized for short hops over low-loss cable or fiber.Satellite conditions adversely affect a number of elements of the TCP architecture, including its congestionavoidance algorithms, data acknowledgment mechanisms, and window size limitations, which combine to severelyconstrict the data throughput rate that can be achieved over satellite links.

Congestion Avoidance:In order to avoid the possibility of congestive network meltdown, TCP assumes that alldata loss is caused by congestion and responds by reducing the transmission rate. However, over satellite links, TCPmisinterprets the long round trip time and bit errors as congestion and responds inappropriately. Similarly, the TCP"Slow Start" algorithm, which over the terrestrial infrastructure prevents new connections from flooding an alreadycongested network, over satellites forces an excessively long ramp-up for each new connection. While thesecongestion avoidance mechanisms are vital in routed environments, they are ill suited to single-path satellite links.

Data Acknowledgements:The simple, heuristic data acknowledgment scheme used by TCP does not adapt well tolong latency or highly asymmetric bandwidth conditions. To provide reliable data transmission, the TCP receiverconstantly sends acknowledgments for the data received back to the sender. The sender does not assume any data islost or corrupted until a multiple of the round trip time has passed without receiving an acknowledgment. If a packetis lost or corrupted, TCP retransmits all of the data starting from the first missing packet. This algorithm does notrespond well over satellite networks where the round trip time is long and error rates are high. Further, this constantstream of acknowledgments wastes precious back channel bandwidth. If the back channel bandwidth is small, the

return of the acknowledgments to the sender can become the system bottleneck.

Window Size: TCP utilizes a sliding window mechanism to limit the amount of data in flight. When the windowbecomes full, the sender stops transmitting until it receives new acknowledgments. Over satellite networks, whereacknowledgments are slow to return, the TCP window size generally sets a hard limit on the maximum throughputrate. The minimum window size needed to fully utilize an error-free link, known as the "bandwidth-delay product,"is 100 KB for a T1 satellite link and 675 KB for a 10 Mbps link. Bit errors increase the required window sizefurther. However most implementations of TCP are limited to a maximum window size of 64 KB and manycommon operating systems use a default window size of only 8 KB, imposing

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