"Infrastructure for an NTSC/ATSC-Supported Nationwide Broadcasting : Present and Future" David Boroughs PBS National cast Alexandria, VA Abstract This paper reviews the implementation and operation considerations to support a nationwide data broadcasting transmission architecture for non-program related data. It reviews architectures utilized by PBS National cast: VBI, dntsc and migration to DTV. The network management requirements to support the data delivery are also discussed. For over ten years, PBS National cast (NDI) has been embedding data streams into existing PBS analog broadcast signals through the use of the Vertical Blanking Interval (VBI), and has been datacasting a variety of multimedia services. Currently NDI delivers data via 260 of the total 348 TV transmitters in the PBS system. While VBI has provided a viable option in the past and continues to provide a niche for certain applications, the data environment today demands more bandwidth, and savvy users utilize multiple applications requiring varied types of data. The next step for PBS National cast (NDI) is the implementation of the Dotcast dntsc (data in NTSC video) technology. The Dotcast system uses patent-pending technologies that can inject a 4.5 Mbps digitally encoded data stream into an NTSC broadcast television signal without degrading the signal or interfering with regular TV reception. The dntsc system can make valuable use of existing NTSC assets, and provide a smooth transition path to DTV data broadcasting. This paper lays the framework for a data broadcasting architecture where multiple data services are vying for a piece of the transport stream, and local data broadcasting opportunities at the station will be competing with national opportunities for bandwidth. 1
NDI BACKGROUND The notion of data broadcasting has been around for a while, long before the capability became a critical element with the advent of Digital TV. For over ten years, PBS National cast (NDI) has been embedding data streams into existing PBS analog broadcast signals through the use of the Vertical Blanking Interval (VBI), and has been datacasting a variety of multimedia services. PBS National cast is a for-profit subsidiary of PBS and offers real-time, nationwide data broadcasting services through a partnership with participating PBS member stations, reaching up to 99% of television households in the United States. Profits of NDI are shared by the participating stations, thereby helping the stations to achieve their financial goals, through the use of otherwise unused capacity. Currently NDI delivers data via 260 of the total 348 TV transmitters in the PBS system. broadcasting can be either program-related, or non-program related. Each type has its own unique, but related requirements for transmission. NDI primarily provides a network for the nonprogram related data, which does not require a close synchronization with TV programming that may be transmitted at the same time as the VBI data. VBI: THE FIRST DATA BROADCASTING NETWORK TRANSMISSION ARCHITECTURE TO SUPPORT NATIONWIDE REQUIREMENTS The PBS network distributes via satellite to its member stations over a dozen video programs at one time. Each member station can pick and choose from those programs to record for later airplay, or to pass it through in real time. For VBI data delivery, this is the first challenge that must be overcome: how to bridge data from the network feed, and insert it into the local program actually going out over the air, so real-time data delivery can be achieved. The solution for this is to place a databridge at each station inserted between the national and local video feeds. The present NDI VBI data broadcasting architecture for nationwide delivery evolved from a totally analog environment of the early 90 s, where the national feed was analog video and VBI rode video the entire way. But, driven by the evolution of the PBS transmission architecture to digital video feeds, the transition to the hybrid network was necessary. Figure 1 shows the architecture in place today to support VBI data broadcasting. Digital data is delivered to the PBS station over a Motorola (formally General Instrument) DigiCipher II (DC-II) isochronous data stream fed by satellite, and at the PBS station, the data stream is output from a Motorola DSR4000 IRD. At the headend PBS Technical Operations Center (TOC) in Alexandria, VA, data received by the customer is formatted into frame relay packets for a composite 256kbps synchronous data stream, which is input to a DC-II encoder, along with digital video. The composite data stream is then transported via microwave radio to PBS s Satellite Operations Center (SOC) for uplink to the GE-3 satellite. At the station, the databridge receives the data stream, and extracts the data from the isochronous stream, sorting it by frame relay address. A VBI line is associated for each frame relay address, and the data is then inserted into the local program video. At the headend delivery point in Alexandria, the customer (content provider) supplies 2
the data to be transmitted via internet, leased line, or even dial-up. Each frame relay packet has a length equal to a NABTS packet (36 bytes, including synchronization bytes, header overhead, and data payload), which is the VBI packet format primarily used in the network. The databridge basically strips off the frame relay envelope, and inserts a ready-made NABTS packet on the VBI line. The databridge also has the capability to allow a station to locally insert serial data directly on a VBI line. In this case the serial data stream bypasses the frame relay demultiplexing stage. At the receive location, VBI decoder cards are necessary to receive the analog TV signal, look for a specified address in the VBI packet header, and then strip the VBI data. The recovered data is generally sent as an asynchronous serial data stream to an application resident on a PC or settop box. INTERNET, OR LEASED LINE Line 1 Lie 2 INFORMATION PROVIDERS GI DSR-4000 Stream In/VBI out 256K sync bridge Frame Relay 19.2K async channel RS422 In Monitor Module FR Demux/ insert Backplane Bus FR Demux/ insert FR Demux/ insert... Local Station 19.2 Kbps Asynchronous Insertion Serial Inserter Video Keyer DATA DIST AMP DATA PROCESSOR PBS DATA DIST AMP 19.2 kbps Async channel for EPG Frame Relay Mux Network Management Center TOC/SOC 256K ISOC CH DC-II Encoder m/w Link GI DSR-4000 Reporting From Network Monitoring Elements Reporting To Network Management Center Local Video Source PBS STATION Off-Air/Cable Subscriber Premise NTSC TV PC local EPG Processor Local Video + VBI FIGURE 1: VBI Broadcasting Architecture In a video signal, theoretically there are 10 VBI lines available to insert data - lines 10 through 18, and 20. Line 19 is reserved for a Ghost Canceling Reference (GCR) Signal, and Line 21 reserved for Closed Captioned Format (CCF) services. In reality, all of the 10 lines may not be available at a station for national access; some VBI lines may be used for local applications or test signals. Each VBI line can support a raw bit rate of 16 kbps (264 information bits per line (33 Bytes times 60 fields per second), but with the NABTS format, there may be 28 Bytes available for actual data payload. Actual effective throughput may range anywhere from 12 to 16 kbps, depending on the format of the data as it is packetized, which is a function of header information and Forward Error Correction (FEC) used. 3
The capability exists to either treat each VBI line as a separate data service, or multiplex several lines together to increase throughput. However, the net throughput achieved through VBI is slow compared to today s internet delivery capability, but comparative to a dial-up line. Throughput can be increased through multiplexing ( ganging ) of VBI lines together. There has also been some experimentation in utilizing some compression techniques that may provide higher speeds on a single VBI line. If 10 VBI lines were to be available, anywhere from 120 to 170 kbps may possibly be achieved, which is comparable to a dial-up or IN, but not as good as DSL. However, a dial-up return path is still required. Reliability of transmission is traded off with throughput the more FEC and less compression utilized, the more reliable the delivery, but at the expense of throughput. CUSTOMER APPLICATIONS AND REQUIREMENTS The NDI customers are the content aggregators that provide rich content that drive specific applications and deliver a data stream to NDI for network transmission. The users of the data (at receive locations) are typically the customers of the content aggregators. Thus NDI can be viewed as the network carrier. The nature of the VBI transmission sets the stage for the advantages and characteristics of data broadcasting that have become recognized for DTV applications, but at lower data delivery speeds. But VBI delivery does lend itself to a niche in certain content services such as: News, weather, sports Stock quotes Music and video samplers Software updates to home PC users Electronic Guides (EPG) Web page downloads These types of applications do not require massive data pipes, and can fit nicely into data rates that can be supplied by VBI. The nature of VBI receivers has essentially been tuner/decoder cards that plug into a slot in the PC. Several vendors have supplied such cards, including Hauppague, ATI, and Norpak. A specific application takes the received data and formats it into a useable form. EPGs such as Gemstar have evolved from a standalone settop box dedicated to the function, to software inherent in multifunction settop boxes or TV sets. External VBI decoders have also been available, where the TV signal is input and the output is a serial data stream that connects to a COM port on the PC. Basic software exists to allow software downloads or text delivery in the simplest and economical form. Some applications allow top level web pages to be downloaded to a PC, with hot links to more information that can be accessed via the telco connection. This allows a basic level of managed content. STANDARDS TO SUPPORT VBI DATACASTING Figure 2 shows the two VBI standards used in the NDI data broadcasting network that defines the physical layer of data transmission. These standards include EIA-516 for full-rate VBI transmission, and EIA-608, which defines Closed Captioned format (CCF). EIA-608 actually defines Closed Captioning for Line 21, however the same data format can be used on other 4
lines to reliably transmit applications having low speed data requirements, such as an Electronic Guide. EIA-516 EIA-516 defines the NABTS format for a VBI packet, and the format is shown in Figure 2(a). The NABTS packet is a 36-byte packet. The overhead bytes include: The 2-byte clock synchronization and one-byte sync at the beginning of every VBI line used to synchronize the decoding sampling rate and byte timing. The 3-byte packet address hamming encoded with 4 data bits per byte provides up 4096 possible addresses. These addresses are used to distinguish related services originating from the same source. This is necessary for the receiver to determine which packets are related, and part of the same service. The one byte continuity index incremented by one for each subsequent packet of a given packet address. The value of the CI sequences from 0 to 15 and increments by 1 each time a packet is transmitted. This allows the decoder to determine if packets were lost during transmission. A one-byte packet structure field, which contains information about the structure of the remaining portions of the packet. For a raw transmission, 28 bytes remain for user data. The use of FEC at the NABTS level will decrease throughput, and throughput is traded off with the robustness offered by the FEC scheme used. Figure 2(a): EIA-516 VBI Full-Rate Line Structure COLOR BURST CLOCK SYNC (16 bits) P 1 PREFIX P 2 P 3 C I P S OPTIONAL S DATA BLOCK BYTE 5 BYTES 28 BYTES SYNC (8 bits) DATA PACKET DATA LINE TV LINE 0, 1, 2, OR 28 BYTES Figure 2(b): EIA-608 VBI CCF Line Structure CHARACTER ONE CHARACTER TWO COLOR BURST CLOCK RUN-IN START BITS (3 bits) 2 BYTES DATA PACKET DATA LINE TV LINE EIA-608 5
While NABTS is the full rate data format, the Closed Caption Format (CCF) may also be used for services that do not need higher data rates. The TV Electronic Guide (EPG) is a typical application in this category. The data packet is formatted following EIA-608 for Line 21 data services, but is applied on the other VBI lines as desired. The use of CCF provides a greater degree of reliability. The symbol time for a CCF bit is on the order of 2 microseconds, which is approximately 10 times longer than a NABTS bit. Thus there is more time for a decoder to capture the CCF bit under varying conditions. RFC 2728 RFC 2728 describes the transmission of IP over the VBI, and allows further compatibility with the type of internet (IP multicast) delivery schemes that users are now more accustomed. Figure 3 shows the protocol stack. Each layer has no knowledge of the data it encapsulates. IP datagrams are fragmented and spread over multiple VBI packets, utilizing an IP header compression scheme. Application UDP IP SLIP-style encapsulation FEC IP NABTS NTSC/Other Cable, off-air, etc. FIGURE 3: Protocol Stack for IP over VBI CONSIDERATIONS FOR VBI DATACASTING Bandwidth Supporting the media rich applications available over the internet today can be accomplished within certain types of applications. Non-program related data typically includes real-time delivery of information such as stock quotes, news, etc.; streaming media requires as much VBI bandwidth as possible, requiring the ganging of several VBI lines together. 6
Cable Carriage The rights to carriage of certain types of VBI data over cable systems is a gray area, and cable systems in some instances have demanded compensation for carriage. It s the question of who has the rights to the signal and its contents along the transmission path to its destination. VBI Receivers VBI receivers with applications software must also be available at a price point that is not outrageous. This has typically been in the range of well below $200. External devices that plug into the PC have been found to be more acceptable than a PC card that requires a user to pop the hood of his/her PC and install it. That just sets the stage for continuing problems. Today, the thrust of the manufacturers efforts is gearing toward DTV receivers. Transition to Digital Many stations are converting their plant to SMPTE 601 digital in anticipation of simultaneous transmission of their NTSC and DTV signals; the NTSC signal is digitized and included in the digital stream from the source at the studio, over the STL, up to the transmitter. At the NTSC transmitter, the digital video is converted to NTSC for transmission over the analog RF channel. Unfortunately, the full-rate VBI necessary to convey a full data service cannot be encoded efficiently; since the data is changing every second, it cannot be compressed reliably with a reasonable number of bits, without impacting other services, including video. While some equipment may be able to encode the VBI in a 270 Mbps data stream, the VBI reliability falls apart when the composite digital data stream is compressed to a transport stream that is formatted for transport over terrestrial STL facilities, such as digital microwave radio. The only solution found so far is to extend the data stream delivered from the network to the transmitter site, and place the databridge at the transmitter site for VBI insertion at the point where the digital video is converted to NTSC. The search for the better vendor solution continues NETWORK MANAGEMENT REQUIREMENTS OF A DATA BROADCASTING NETWORK To make this an effective service for customers, a given level of performance must be maintained, to allow reliability of delivery, and a consistent service. Just like any other telecommunications network, a level of performance guarantee has to structured; this allows the content providers to determine their application level requirements for FEC, carrousel, and retransmission, and the impact on effective throughput, and design the delivery systems likewise. To accomplish the goal of meeting customer performance expectations, a network management system (NMS) must be in place. The NMS has two primary purposes: (1) to manage a costeffective and efficient process to deliver data throughout the network, and (2) to verify that the network is performing to a predetermined level of performance (network availability) that the customer expects, for the money being paid. Additionally, the system must also be designed to minimize the workload of the TV station engineer, who has a higher priority of maintaining the video signal. The network management requirements discussed in this section will apply to the current as well as future data broadcasting network architectures. The management issues are consistent, regardless of the technology being employed. Network Management Requirements 7
To accomplish the first goal, the NMC should have the following capabilities to monitor and manage the data. REAL-TIME STATUS MONITORING. Report events in real time-when an event occurs that affects the network availability, it must be reported ASAP to minimize the downtime. Figure 1 also shows some critical sampling points that are monitored in a nationwide data distribution network that provide feedback on network operation status. DIAGNOSTIC CAPABILITIES. Diagnostics must be as automatic as possible. This will minimize the time to troubleshoot and isolate problems, and will not require the TV station engineering resources; important if the engineer is working on other issues at the station at the time. To do this the remote station network equipment must have a high degree of "smarts"- to isolate the problem, and send the appropriate alarm informing of the exact problem. CONFIGURATION/ACCOUNTING/BILLING MANAGEMENT. The NMS should have a way to store information on network inventory of equipment and configurations, customer and station contact people, and all the administrative information and provide the information on demand when such information is needed. When required, the system should provide accounting charges, and billing information based on a per packet, data segment, or other measurable unit of usage for each customer using the network. CONFIGURATION CONTROL. Once again, to minimize having to take up a lot of the station engineers time, configuration changes to be made for any existing or new customer should be as automatic as possible at any site, allowing everything to be done remotely from the NMS. Network Availability To accomplish the second goal, to verify the network is at a given level of performance, calculation of network availability is the best measurement tool. Network availability is defined as the time that the network is available to the user, generally falling under 2 criteria: 1) the network allows data to be received at the user destination, and 2) the data that is received meets or exceeds a certain BER threshold. If either of these 2 criteria is not met, the network is down to the customer/user. To determine if network availability is being met, a means to collect statistics on performance needs to be in place. To be able to react in real time, a network monitoring/alarm reporting system must be operational to alert a Network Management Center (NMC). A Daily summary of network availability for each VBI service and the databridge should be provided, taking into account downtimes due to absence of VBI data and critical/majors alarms that would occur to prevent data delivery. These types of reports give the network manager feedback on how the network is operating, and the tools to improve it. Shown in Figure 4 are network availability figures for certain segments of the data distribution path. The data broadcasting network availability should be comparable to the industry for similar service (I.e. streaming media over landlines). Measurement is performed over a given time period, such as month. Typical objectives are in the range of 99%, end to end. 8
The local station has control to a point right outside the transmitting antenna; the user receive location has too many variables to be committed by the broadcaster. A level of service can be assumed in the Grade A contour, for example (based on signal level calculations), but not guaranteed (no control over a nearby building at a receive location that causes multipath). The lowest network availability has been assigned at the broadcast station level to account for nonredundant plant at the station (i.e. varying availability of spare satellite receivers or full-power standby transmitter that automatically switches inline upon the primary transmitter failure). There would also be a demarcation point between the NDI headend and the customer (data content provider) to define the boundaries for troubleshooting responsibility. Methodology for Network Monitoring and Validation The network monitoring system must be capable of global monitoring, as well as local monitoring, as depicted by the necessary sampling points in Figure 1. A global outage is obviously critical from a network availability standpoint, since it is the product of an outage duration, multiplied by the number of stations affected. Fault isolation ability is key this can be accomplished by locating data and signal sampling points along the transmission path, that feed back into a master controller/processor from the process of elimination, the section of plant can be identified where the data signal becomes absent. At the local station a monitor module resides on the databridge to act as the network management agent to collect key information, and report back to the NMC. The NMC must have a way to collect statistics, and generate the results in a user-friendly report that can be provided to customers or to operations center personnel. There are 4 categories: Network Outage times: The time stamps of alarms are used to define network outage and down time. It must have the capability to collect the information on a total network, station specific, or customer specific basis. To gather information for network availability calculations, the NMC should have the capability to get timestamps when a point is "down" and when it returns to an "up" status, so that an elapsed downtime can be calculated. can be considered down either when (1) there is a loss of video that carries the data, or (2) when data is absent from the video signal. 9
Microwave radio Content Provider Internet or Leased Transmission Facilities 99.9% Isochronous Digicipher II Encoder Stream Transmission Interface PBS TOC Network Video Microwave radio 99.99% PBS SOC HPA 99.953% Network Feed from NDI Alexandria IRD 256 kbps PBS Member Station Broadcast Studio Local NTSC Video bridge Local NTSC Audio STL/ TSL 99.963% 99.0% Video/Audio + NTSC Broadcast Transmitter Audio STL/ TSL Video 99.467% FIGURE 4: End-to-End Network Availability Throughput: Capability to collect data for the network capacity planning function to determine if network "bottlenecks" are being approached. This is more critical in applications where multiple customers share a VBI line, and access it on demand. Network design then becomes more akin to the classic traffic engineering that the telephone engineers have to worry about. Typical information to be collected for each VBI service would include: No. of packets sent No. of packets missed/replaced (FEC efficiency) No. of Bytes sent No. of Bytes in error Bit-Error-Rate Average throughput Alarms: Collected and summarized by total network, per station or group of station, or per customer basis. Important to be able to identify trends or occurrences that suggest corrective actions are needed in the network, before a catastrophic event actually occurs. 10
Typical types of alarms may include different categories, denoting the severity of their impact on network performance. Below are typical alarm categories and alarm types. I. Catastrophic Alarms -any event that causes a loss of all data to the local stations II. Critical Alarms -bridge hardware failure that affects service integrity -loss of program video output from the databridge -loss of program video input coming into the databridge -loss of AC input power (assumes monitoring function has a battery backup) -loss of all data from video III. Major Alarms -loss of any VBI line from video -loss of data from any input to the databridge -BER has exceeded bit error threshold BER: Bit-error-rate (BER) statistics, like alarms, are also collected and summarized to be able to identify trends or suggested areas of network performance degradation. At the local station, a monitor module can perform a bit by bit comparison of data that is sent, vs what is received back to determine a true bit-error-rate (BER). The statistic is recorded for each packet, and reported on a periodic basic back to the NMC. BER statistics can also include FEC additional information to determine the amount of FEC correction taking place, to determine signal quality. Often, it is also handy to establish a benchmark VBI data test pattern to test system integrity when a customer s data delivery is in question. A benchmark BERT pattern is also sent out as a baseline data signal to measure its quality compared to operational data. The BERT pattern is a known pattern, such as a 2047 (a known ordered string of 2047 ones and zeros) and can provide a baseline to performance. Implementation of monitors for this type of data can be more efficient and economical that actual monitoring of each customer s data, and provides a reliable and representative snapshot of system operation. A benchmark monitoring location that gives a representative signal for evaluation is typically the studio location of a station, since it is generally 10 to 15 miles away from the transmitter, and can receive a good representative air sample. Other Considerations for Network Operation The broadcaster must also have the capability to monitor the content of the data being broadcast. As stated in the FCC rules, the broadcaster has the responsibility for the content that is delivered over the air to receivers; thus a means to monitor the content or application being broadcast is important. Typically, a decoder and PC are set up at the station running the application, allowing the station personnel to experience the same events and information that a typical consumer would get. If multiple applications are being broadcast, then a PC/decoder for each application may be necessary. Another important aspect of network management and service level agreements is making provisions for disaster recovery. There is a trade-off required for a level of redundancy, given a probability of occurrence, versus the cost of adding the plant. All of this is weighed 11
against the cost of the downtime - the lost revenue opportunity that is realized due to the absence of the data delivery for the time period of being down. dntsc: USE OF NTSC VIDEO TO SUPPORT HIGHER THROUGHPUT SUMMARY OF TECHNOLOGY AND APPLICATIONS While VBI has provided a viable option in the past, the data environment today demands more bandwidth, and savvy users utilize multiple applications requiring varied types of data. The next step for PBS National cast (NDI) is the implementation of the Dotcast dntsc (data in NTSC video) technology. The Dotcast system uses patent-pending technologies that can inject a 4.5 Mbps digitally encoded data stream into an NTSC broadcast television signal without degrading the signal or interfering with regular TV reception. The dntsc system is an RF process, and achieves the 4.5 Mbps data rate by injecting: 3 Mbps into the visual carrier of the signal using Quadrature Phase Shift Keying (QPSK) modulation in conjunction with a proprietary abatement circuit to negate distortion. The dntsc visual data is a data envelope combined additively, in quadrature, with the visual carrier. 1.5 Mbps into the aural carrier, using eight level negative amplitude modulation, applied directly to the FM aural carrier. The system is designed for modularity, and is inserted between the transmitter exciter and final stages. It does not interfere with VBI; thus both services can operate simultaneously. The combined dntsc process adds about 1% power to the overall NTSC signal, which is within the FCC required transmitter power variation of +-5%. Dotcast has identified nine configurations of transmitters for which the dntsc will be compatible and is in the process of developing the system utilizing state-of-the art digital signal processing (DSP) technology. The transmission model for PBS does not rely on cable carriage, and is designed for reliable over-the-air reception utilizing state-of-the art wireless data reception techniques. The bandwidth provided by the Dotcast dntsc system opens up entirely new market opportunities from the limiting throughput that is offered by VBI. It maximizes bandwidth, including fast downloads, immediate access to rich media and electronic commerce options and direct access to applications service providers. It will also enable PBS and its member stations to participate in delivery of the new digital services that provide additional value and information to homes, businesses, government, and schools. Operation Figure 5 shows the signal flow of the dntsc system over the PBS network. Dotcast will provide to NDI a data stream containing the content to be distributed over the data broadcasting network. From NDI/PBS TOC in Alexandria, a high-speed data stream is fed over a DigiCipher II encoding system to the participating stations, to a server which will generally be located at the local PBS station studio. The server is also connected to a local internet connection, and also has a port to allow the local station to utilize some of the capacity to distribute its own information. The output of the server is at 4.5 Mbps which must be fed to the transmitter site, via an STL or other means. At the transmitter, the 4.5 Mbps data stream is input into the transmitter injection equipment, where it is segmented into the separate aural and visual data streams. 12
At the receive location, the dotbox contains a hard drive, which is expected to be up to 100 GB to capture the transmitted information off-air. is distributed to the user s PC depending on predefined preferences and subscribed services. TRANSMISSION REQUIREMENTS TO SUPPORT NATIONWIDE REQUIREMENTS The dntsc system allows the PBS data broadcasting network a graceful way to proceed toward data broadcasting in the DTV domain. The Dotcast 4.5 Mbps data stream with the managed content is packaged to support the MPEG-2 format, and can be transported through the station plant via T1 formatted lines. At the transmitter, the data stream is modulated on to the analog RF via the dntsc encoder. At the receiving end, the consumer uses an off-air antenna connected to a Dotbox that can receive either NTSC or ATSC signals. Thus all the elements are in place for the DTV transition. At the time to convert to DTV, the Dotcast data stream can merely be redirected to a DTV multiplex for inclusion in the 19 Mbps transport stream-accommodated by fixed or opportunistic data transmission schemes. At the consumer end it should remain transparent the off-air antenna is already in place, and the Dotbox will retune to the DTV channel with the data. This is an important attribute in providing a data service transition make it as seamless as possible to the users! The STL between the studio and transmitter provides the biggest area of consideration-how to transport 4.5 Mbps to the transmitter for injection into the RF. The goal is to determine the most cost effective way to add digital capacity: either by upgrading the existing STL, adding a second RF channel, utilization of fiber or leased facilities, or even direct downlink to the transmitter. Each station will be a case-by-case basis. 13
DotNOC Stream Delivered by Public Transmission Facilities Isochronous Stream Transmission Interface Digicipher II Encoder Microwave radio Network Video PBS TOC Internet Connection Receive Location Network Feed from NDI Alexandria Microwave radio PBS SOC Dot Box IRD 2 Mbps PBS Member Station Broadcast Studio Local Monitoring Station Dotcast Server Local NTSC Video/Audio Monitor Return Path 4.5 Mbps STL/ TSL Video/Audio STL/ TSL Audio NTSC Broadcast Transmitter Monitor Return Path 4.5 Mbps Video Aural Transmitter 1.5 Mbps dntsc 8-level Encoder dntsc QPSK Encoder Diplexer Power Amp Up Converter 3 Mbps Exciter/ Modulator Internet Connection FIGURE 5: dntsc Signal Flow Diagram NETWORK MANAGEMENT-MONITORING OPERATION, PERFORMANCE The dntsc system will have a Transmitter-Studio Link (TSL) that allows a return path from the transmitter to the studio, where a local monitoring point will be established at a staffed location, such as Master Control. This will allow continuous monitoring of the critical transmission parameters such as data throughput, injection levels, phase error/delay, as well as provide a means to disable the encoder should the need occur, and detect when the station is off the air. A return path can be established via the internet through the local server at the station. At NDI, the NMC will monitor the transmission path infrastructure (i.e. satellite, video status at stations. Dotcast will maintain a Network Operations Center (NOC) operating 24 hours a day, 7 days a week, that will manage content delivery, as well as the operations of their servers, dntsc encoders, and STL data links. STATUS OF DEVELOPMENT AND DEPLOYMENT The dntsc system is targeted for deployment in the PBS system in 2001. Given the DTV issues at hand, it is expected that the dntsc system will become and remain NDI s primary high speed delivery means for quite a few years! As DTV opportunities become clearer, and agreements are 14
worked with PBS member stations, the Dotcast dntsc can lay the groundwork for the next generation network. DATA BROADCASTING IN THE DTV ENVIRONMENT At this point in time, 26 PBS stations have signed on the air with DTV transmitters, and at least that many more are expected to turn up in 2001. PBS has two services available to its member stations for network feeds: (1) a four channel service which is interrupted by HD content when it is available, and (2) an HD loop which offers a single channel of HD service for demonstration purposes. As DTV gets geared up for service with more local programming and enhanced television experiences, data services may include interactive, program related data, as well as data that is not specifically bound to a TV program event, to deliver data to users as a more efficient means than conventional landlines. Thus, all of the data services will be vying for a piece of the DTV transport stream, not to mention TV and HDTV tradeoffs of bandwidth requirements. In addition local data broadcasting opportunities at the station will be competing with national opportunities for bandwidth. In the DTV environment, the PBS network will continue to distribute via satellite to its member stations, over a dozen digital video programs at one time. Each member station can pick and choose from those programs to record for later airplay, or to pass it through in real time. As with VBI data delivery, the challenge of how to bridge data from the network feed, and insert it into the local program actually going out over the air, for real-time data delivery will be the issue, as well as the investment requirement. In the DTV case, the databridge will be some form of MPEG2 multiplexer, capable of supporting the datastreams described in the ATSC A/90 Broadcasting ation, as well as IP multicast, of which a specification is currently being developed in the ATSC T3/S13 Committee. Non-program related data may include delivery of self-contained web sites, e-commerce, as well as the video and audio streaming media. The streaming media will require the considerations of downtime and real-time data delivery, and quick reaction to outage reports. Not so real time, like static web sites for anytime access could have a latency of even several days. The nature of the data to be delivered will drive the service level agreements that must be supported by a network management and operations system. Figure 6 shows the resource allocation of the PBS DTV feed prior to 2003, and Figure 7 shows the anticipated allocations beginning 2003 and beyond. By 2003, improvements in compression techniques will allow more efficient coding of the video, and provide more room for opportunistic types of data. The efficient utilization of spectrum saving opportunistic data could be a significant consideration as legislators warn that HDTV should continue to be the primary focus of the DTV transition and that alternative uses of the digital spectrum could be deal breakers. The bandwidth available includes a data pipe that will be a fixed, committed rate that can be guaranteed for delivery, and an opportunistic data rate that will vary from second to second, but may provide an effective throughput over time. What is envisioned is to combine the use of fixed data rates plus opportunistic data to achieve an overall data delivery scheme. Of course, to handle wide variations in the opportunistic data delivery will require considerations of buffering in a data receiver to store, hold, and smooth the data to output at a constant leak rate. A goal of NDI is 15
to perform some testing of opportunistic data delivery during various video programs, to develop a profile of throughput efficiency. A couple of alternatives may be available to accomplish the delivery of data from the network headend to the PBS member stations in the DTV data broadcasting environment. The first is to deliver a data channel to each station just as is done with the VBI and Dotcast architectures, and at the station, input the data stream into a local MPEG-2 multiplexer, and integrate into the local station DTV transport stream. The second is to multiplex the data with the PBS network DTV transport stream at the PBS headend (consistent with Figures 6 and 7), extract it out at the station, and then reinsert it into the local DTV stream at the PBS member station. This could become an advantage if at some point, alternate terrestrial delivery of the network DTV transport stream were to supplement or replace the satellite; the data would be part of the transport stream and a cost savings would be realized by not having separate terrestrial facilities for data. NDI will review these options as the infrastructure becomes defined. Over the migration from the VBI, to dntsc and on to DTV data broadcasting, the transmission infrastructure for national delivery is expected to remain somewhat similar in concept get the data at the headend, distribute it to the station edge servers for over-the-air last mile transmission either on-demand or at scheduled times. The network management will be the critical element in providing a level of service that can be competitive with alternative means such as IP multicast over the internet, internet delivery over wireless portable devices, or direct satellite. However, there is still the return channel issue that must be factored into the network architecture. References 1. Development and Performance of the PBS VBI Delivery System. Aderemi A. Adeyeye and Mark S. Richer. 129 th SMPTE Technical Conference paper number 129-56. 2. EIA 516: North American Basic Teletext ation (NABTS). Electronic Industries Association. 1984 3. EIA-608: Recommended Practice for Line 21. Electronic Industries Association. 1993. 4. RFC 2728: The Transmission of IP Over the Vertical Blanking Interval of a Television Signal. The Internet Society Network Working Group. November 1999. 5. 4.5 Mbps Compatibility Transmitted in 6 MHz Analog Television. Walter S. Ciciora. 1998 NCTA Convention, Atlanta, GA. 6. ATSC Standard A/90: Broadcast Standard. Advanced Television Systems Committee. July 2000 16
FIGURE 6: 19.4 Mbps 6 MHz Channel Prior to 2003 3-4 Mbps 3-4 Mbps 3-4 Mbps HDTV 17+ Mbps Fixed 1-2 Mbps NTSC Equivalent 4-5 Mbps 3-4 Mbps Opportunistic Variable Rate Legacy Opportunistic Variable Rate Fixed 1-2 Mbps Non-Prime Time Prime Time FIGURE 7: 19.4Mbps 6 MHz Channel As Of 2003 1-3 Mbps 1-3 Mbps 1-3 Mbps NTSC Equivalent 4-5 Mbps Legacy Opportunistic Variable Rate 1-3 Mbps 1-3 Mbps HD 7-12 Mbps Fixed 1-2 Mbps 1-3 Mbps Opportunistic Variable Rate Fixed 1-2Mbps Non-Prime Time Prime Time 17
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