IP feeding of DVB-T2 transmitters - Teamcast

Rennes, December, 23 st – 2011Feeding DVB-T2 transmitterswith redundant IP networksIn the connected world around us, IP technology has definitely established itselfas part of our daily lives. Originally regarded as an IT-only transport technologysuitable for WEB data and email traffic, IP has quickly become the dominantstandard for all types of communications in the field of personal communicationsor professional data services.The field of digital terrestrial television is no exception and broadcasters are nowconsidering IP technology to implement their real-time stream transport. Withthis comes a constant search for service convergence, management flexibilityand of course network CAPEX and OPEX costs reduction.In the context of early DVB-T2 network roll-outs, such as in Finland,broadcasters have expressed demand to feed terrestrial transmission sites via IPright from the beginning.The objective of this document is to provide some key elements to explain howIP technology is used to convey time critical MPEG-TS based data stream. Atypical use-case of DVB-T2 is illustrated. Then we explain the way TEAMCASThas implemented IP interfaces onto its modulators providing the latest state-ofthe-artredundancy switching mechanism, so called IP CleverSwitch ® .TEAMCAST is capitalizing on more than 15 years of experience in implementingterrestrial Digital TV solutions. Through this document, TEAMCAST is pleased toshare its own market and technology vision built on more than three years ofsuccessful pioneering experience in the domain of DVB-T2.One of the main sources of inspiration for this document is the DVB-T2commercial network roll-out in Finland, where TEAMCAST is involved and washonored at the IBC 2011 Innovation Awards 2011.

1.1 IntroductionFor broadcasters and network operators worldwide, distribution network costsrepresent a burdensome business expense. During the last decade, IP technologyhas demonstrated its capability to transport video stream services with a highreliability of (QoS) quality of service performance. Consequently, TEAMCAST isparticipating in this progressive shift of existing broadcast network architecturetoward IP based solution.This transition to IP technology has progressively affected already completedDigital TV Network architecture, starting with contribution, multiplexing and nowthe transmitter sites. With the introduction of the second generation of terrestrialdigital TV (DVB-T2) and it’s successful adoption worldwide (8 countries havedeployed DVB-T2 commercial services and more than 40 countries have adoptedthis standard worldwide since its introduction in 2009), feeding transmitter siteswith IP is a core requirement in the market place world-wide.For years, TEAMCAST has been the preferred choice of many transmittermanufacturers to provide best of class digital modulators supporting ASI inputswith redundancy capability. To meet the new increase demand for IP transport,TEAMCAST introduced the MT2 for both DVB-T and DVB-T2 standards. This newmodulation platform integrates two separate Gigabit stream interfaces with avery innovative and unique user configurable switching mechanism based on IPrelated decision criteria.1.2 Typical DVB-T2 network architectureA typical DVB-T2 network architecture is illustrated in the figure below. It is verysimilar to any Terrestrial Digital TV network, where two networks are used: thecontribution network to collect and to transport the different services contenttoward the central head-end, and the distribution network which is utilized todeliver the multi-program stream to the different transmitter sites.Figure 1: DVB-T2 network architecture

In DVB-T2, the T2-MI stream interface has been introduced between the DVB-T2Gateway and the DVB-T2 modulators, thus integrated within the DVB-T2transmitters. This interface defined in ETSI TS 102 773, is used to operate a(SFN) Single Frequency Network and also deliver Multi-PLPs (different data pipesfor different types of service usages) layers.The T2-MI standard defines two different ways to transport the T2-MI packets:over a classical ASI interface and over IP network using RTP over UDPencapsulation.Figure 2: T2-MI protocol stackConcerning the DVB-T2 transmission parameter set-up, there are a large numberof possible combinations, depending on the targeted receiving conditions and onthe service coverage characterization. Early adopters like ARQIVA in UK, DNA inFinland, have selected DVB-T2 transmission mode as detailed in the followingtable. This mode permits to transmit a total of 36.14 Mbps of useful data rate.DVB-T2 ParameterValueFFT32kGuard Interval 1/128Pilot patternPP7LDPC Code Rate 3/5Modulation256-QAMTotal Bit rate36.14 MbpsTable 8: Typical DVB-T2 data bit rate

1.3 IP technology for video networksIP video distribution is now becoming the preferred technology by serviceproviders and network operators constantly searching for CAPEX/OPEX costoptimization, flexibility and reliability.But, IP protocol invented in the 70s was not initially designed for transmittingvideo streams with their high level of (QoS) quality of service required. Indeed IPprotocol, qualified as a “best effort” connectionless transmission mean, isnatively suitable for non-critical data exchanges such as IP networks sufferingfrom unpredictable network delays, packet drops and congestion.Consequently, the challenge of using an IP network to transport a video streamrequired the introducing of several mechanisms to compensate for the native IPtechnology short comings.1.3.1 RTP over UDP rather TCP protocolRTP (Real Time Transport Protocol) defines a standard packet format fordelivering audio and video over the internet. It is defined in RFC 1889. It wasdeveloped by the Audio Video Transport Working group and was first published in1996.In comparison to TCP (Transmission Control Protocol) which favors data integrityrather than delivery speed, RTP favors rapid delivery and has mechanisms tocompensate for any minor loss of data integrity. RTP compensates for jitter anddetects out-of-sequence data arrival, both of which are common during IPnetwork transmission and therefore is suitable for time critical data transmissionsuch as audio and video.RTP itself does not guarantee real-time delivery of data, but it does providemechanisms such as Time Stamping for sending “time sensitive” data (such asvideo/audio streaming) to ease process synchronization.Figure 3: RTP header detailsEncapsulated over UDP, RTP permits transport of data onto a unidirectional linkwhich is typically well adapted to contribution or distribution networks within theDTV architecture. Note that RTP is required when FEC (Forward Error Correction)is employed.

1.3.2 MPEG2-TS over IPTwo methods are currently utilized for the transport of MPEG-2 TS over IP:► The first one is quite simple as it consists simply in carrying a number ofTS packets into the payload of the UDP datagram. Ethernet basednetworks have a Maximum Transmission Unit (MTU) of 1500 bytes, so thiscorresponds to 1500/188≈7 packets.► The second method specified by the IETF the DVB-IPI group uses RTP tocarry MPEG-2 TS packets. In this case, the RTP payload again carries anintegral number of TS packets, 7 for Ethernet based networks.Interestingly, DVB specifically warns users not to make use of the directUDP transport method, even though the RTP method has an extra level ofpacketization with some redundant information.Figure 4: MPEG-TS over RTP/UDP/IP encapsulation details1.3.3 Coping with packet loss with FECAs mentioned earlier, IP transmission is natively a best effort technique with thepossibility to retransmit some received data when used in a connected mode (i.e.with TCP).However, when temporal sensitive data such as video are transmitted onunidirectional networks (using in this case UDP) packet loss issue becomes verycritical.To meet this challenge, the Pro-MPEG forum Group published some guidelines forthe transmission of professional MPEG2-TS packet over IP networks,standardized today as SMPTE-2022.By adding checksums per column and/or per row in a matrix of transmitted RTPmedia packets, lost packets may be recovered by the receiver to a certainextent. On the other hand, the FEC checksums will add significant overhead tothe bit stream and therefore must be carefully adjusted to achieve the besttrade-off between QoS and data rate usage efficiency.

Figure 5: Pro-Mpeg Forum FEC operationWhile the ProMPEG FEC can correct some transmission errors in a large amount,especially when the Row + Column FEC are used, in practice the Column onlyFEC mode is usually used for professional video transmission. Indeed, theColumn FEC (also called 1D FEC) authorizes to correct a minimum of packetburst errors and random errors for an acceptable ratio of data overhead.1D FEC is perfectly suited for IP networks that usually offer an error rate farbelow 1%.The table below highlights a practical example of using a FEC to transmit a T2-MIstream over IP Network.MPEG-TS Useful Bit rate 36.14 Mbps Overhead ratioT2-MI/MPEG-TS 37 Mbps +2.3%RTP/UDP/IP/Ethernet 38,5 Mbps +4%FEC TypeColumn FEC onlyFEC 5/25FEC data rate 1,55 Mbps +4%Total IP Bit rate 40 Mbps +10.6%Table 8: Typical DVB-T2 data bit rateA total overhead of 10.6% has been measured (for both encapsulation andFEC), with the capability to correct between 0.5% and 0.9% of transmissionerrors. In this particular configuration, a processing delay of around 20ms isrequired for FEC decoding at the reception site.

1.3.4 Increase QoS performance on IP network with MPLSTo meet the increased demand for conveyingprofessional services over IP, network operatorshad to significantly improve their IP network QoS inorder to compete with existing ATM based services.This improvement consisted of reviewing “fromscratch” the overall policy of service deliverymanagement, in particular, it needed to be able tohandle different class of services (CoS), verysimilar to the ATM world. Implementing differentclass of services has been possible thanks to several new technologicalinnovations, one of them being MPLS network architecture (Multi-Protocol LabelSwitching).Multi-Protocol Label Switching is a technology that builds on “Layer 3” or routinglayerIP capabilities to simplify and improve the exchange of IP packets. In MPLSnetworks, MPLS-enabled routers use IP routing protocols to exchangeinformation with each other. The challenge for Telco’s is to be able to meet therequired SLAs (Service Level Agreement) for premium video services and toachieve this, IP/MPLS network is a key technology. Telco’s MPLS networks mustprovide:► Extremely low jitter, for example as less than 10 milliseconds► Very low delay (typically less than 80 milliseconds)► Extremely low (ideally zero) packet loss, since even a single droppedpacket can have a major effect on video quality

1.4 IPNetworks to feed DVB-T2transmittersThere are of course a large number of possible networkarchitectures forfeeding digital TV transmitters via IP, andeach broadcaster has his own network structure. Onepossible set-up is illustrated here.Locatedat the head-end, two DVB-T2 gateways are used in parallel in a (1+1)redundancy mode. The gatewaysgenerate the T2-MI packets which areencapsulated onto IP (UDP/RTP/IP)with possibility of adding some FEC.Figure 6: Example of IP network to feed DVB-T2 transmittersThe IP transportt network topology can be very simple, in the case of a privateand dedicated transmission link oreven more complex, such as sharedMPLSbackbone.The two T2-MI sources are generally mapped onto two different Multicastaddresses and cross the IP networkin two separate and redundant paths.At thetransmitter sites, the network operator can provide two physical interfaces, onefor the primary and another for the secondarystreams.Those two IP links are then connected to the two separate Gigabit ports ofone modulator, or two modulators incase of a 1+1 transmitter set-up.1.5 TEAMCASTImplementationon MT2TEAMCAST 2 nd generationn of DVB-T/T2 modulator, referenced as MT2, supportstwo separate Gigabit Ethernet input ports. The TEAMCASTMT2 hasbeencarefully designed to anticipate thedemandd of feeding Digital TV transmissionsites with redundant IP transport streams.The MT2 product supports all the required IP standard mechanisms likeRTP/UDP/IP encapsulation, Unicast and Multicast addressing modes, virtual LANtaggingg and the FEC decoding as defined in SMPTE-2022 standard.

However, there is one key innovation offered by TEAMCAST, it is theimplementation of new input switching process (so called IP CleverSwitch ® )totally dedicated and perfectly-suited for IP stream switching.Indeed, designing an efficient IP stream switching mechanism requires differentmethods compared to classical MPEG-TS/ASI process, based on ASI packetsynchronization and packet error detection.The method implemented by TEAMCAST is based on the RTP packet detection.The principle is quite simple and is illustrated below.Figure 7: Redundant IP switching managementIt is based on detecting any loss of an RTP packet. After a user configurable “RTPPacket Time Out”, then the secondary Gigabit input port is selected. Theswitching back to primary is also configurable from a timer.This mechanism, quite innovative for the terrestrial transmission domain,authorizes management of DVB-T2 Gateway issues at the IP level (interruptionof the source Multicast stream for instance) as well as any possible issuesconcerning the IP routing.Used in combination with the existing MPEG-TS packet switching mechanism, theCleverswitch ® IP process provides broadcasters with the maximum flexibility andefficiency to manage DVB-T/T2 transmitter redundancy.

Coming next: IP based distribution network.There is no doubt that today we are facing a new and exciting way to feedterrestrial transmitting sites via IP networks.Although IP technology has been initially designed for “best effort” services, therelease of complementary protocols such as MPLS, SMPTE-2022 FEC, etc. makethe transport of “mission critical” real-time media data such as MPEG-TS over IPnetwork possible.In practice, the QoS (Quality of Service) performances proposed by the Telco’s,authorizes to run the MPEG-TS over IP encapsulation and transmission withoutany FEC, or with a very limited protection (in the range of 4% to 5%).Depending on Broadcaster’s use cases, an IP network can be utilized as a sparenetwork with a classical primary ASI link or a main distribution network. In thiscase, full IP native redundancy might be required and Digital transmitters are fedwith two separate and redundant IP streaming interfaces.To meet this demand, TEAMCAST highlights the MT2 product, its latestgeneration of DVB-T/T2 modulators, that features natively two separate Gigabitinput interfaces managed by a very innovative and “IP oriented” switchingmechanism (so called CleverSwitch ® ).Author: E. PINSON, Business Unit Manager – TEAMCAST

Company information:TEAMCAST is a renowned and highly active member of the TV Broadcastingecosystem worldwide, with innovative technology offerings based on a solidexpertise in both Digital TV and Mobile TV transmission. TEAMCAST is a companywith strong technical credentials who supplies product technologies toinfrastructure equipment providers for Digital television broadcast networks.TEAMCAST’s strategy consists in developing Terrestrial DTV and Mobile TVsolutions as soon as new standards and new technologies are released.To achieve a very short time to market with high quality products, it is essentialto gain technical knowledge and expertise by taking part in the definition phaseof new standards, as well as contributing to the work of the different task forcesthat aim at finalizing and qualifying the technologies. In the specific frameworkof DVB-T2, TEAMCAST has been involved right from the beginning as an activemember of DVB. Its active participation within several projects such as CelticProject B21C, SME42 project ,and Celtic Project Engines has been a keyopportunity to build a concrete experience concerning this emerging technology.Furthermore, TEAMCAST has developed a close relationship with early DVB-T2adopters such as ARQIVA in UK, TERACOM in Sweden and DNA in Finland, havingdelivered DVB-T2 professional products and solutions.More information is available at www.TEAMCAST.comPress contact: Alain UNTERSEEalain.untersee@TEAMCAST.comTEAMCASTCentre Espace PerformanceF35769 Saint Grégoire cedexFranceTél. : +33 (0)2 23 25 26 80Fax : +33 (0)2 23 25 26 85TEAMCAST Inc.The Burnham Center, Suite1865111 West Washington StreetChicago, IL 60602 - USATel: +1 312 263 0033Fax: +1 312 263 1133www.TEAMCAST.com