1. Introduction
The terrestrial ATSC 3.0 broadcasting system is anext-generation broadcasting standard that efficiently combines IP-based communication techniques with features of broadcasting services which delivering high-quality data to alarge number of users in real time to provide flexible service configuration and personalized services to viewers. By thesereasons, the ATSC 3.0 broadcasting network should use IP based MMT (MPEG Media Transport) or ROUTE (Real Time Object Delivery over Unidirectional Transport) format in order to provide converged broadcasting services using the HTTP/TCP/IP protocol when transmitting the data through network[1,2,3,4]. In case IP based data transmission technology is used, it may inevitably cause a packet loss during transmission depending on the network environment. In order to cope with a broadcasting accident caused by transmissionerror or a malfunction of a broadcasting apparatus, the Intelligent IP switch shall be required to accurately detect errors and switchover broadcast streams with recoverable IP packetwhen various types of errors occur.
In this paper, we propose a design and implementation method of an intelligent IP switch, which is suitable for broadcast stream and reliable in comparison with the simple IP Switch technology used in existing network environment. The proposed Intelligent IP Switch consists of IP Stream Analysis Module, ALP Stream Analysis Module, STL Stream Analysis Module and SMPTE 2022-1 based FECEncoding/Decoding Module. When the input stream is damaged, the seamless switching module performs a s witching to a multiplexer that outputs a normal stream and can restorethe stream transmitted through the gateway, thereby providing a normally restored stream output.
In this paper, related works concerned with the propsed intelligent IP switch for ATSC 3.0 broadcasting system are explained in Section 2. The design and implementaton method of intelligent IP switch with packet FEC is described in Section 3 and 4, respectively. To verifiy the validity of the propsoed intelligent IP switch, experimental conditions and results are suggested in Section 4. And finally this paper is concluded in Section 5.
2. Related Works
2.1 Characteristics of Terrestrial ATSC 3.0 Broadcasting System
ATSC 3.0 is a suite of voluntary technical Standards and Recommended Practices that is fundamentally different frompredecessor ATSC systems. With higher capacity to deliver Ultra High-Definition services, robust reception on a widerange of devices, improved efficiency, IP transport, advancedemergency alerting, personalization features and interactive capability, the ATSC 3.0 Standard provides much morecapability than previous generations of terrestrial broadcasting. The ATSC 3.0 System is designed with a layered architecturedue to the many advantages of such a system, particularlypertaining to upgradability and extensibility[5].
The ATSC 3.0 broadcasting system improves the frequency efficiency and stability of transmitting UHD broadcasting service which can be provided at any time, any where without changing the frequency, by changing the MFN (Multiple Frequency Network) transmission methodapplied to the ATSC 1.0 standard, which is a domestic broadcasting standard, to the single frequency network (SFN) transmission method. Fig. 1 shows the MFN structure of redundant ATSC 3.0 broadcasting system.
(Figure. 1) MFN Structure of redundant ATSC 3.0broadcasting system
2.2 Layer Interfaces of ATSC 3.0 Broadcasting System
As the fisrt step of receiving ATSC 3.0 broadcasting signal, in general, a receiver should access to a low levelsignaling stream with pre-defined IP address and port number to receive Low Level Signaling (LLS) data. The receiver canobtain information for desired broadcasting signal through Service List Table (SLT) located in the received LLS data[5].
The ATSC 3.0 broadcasting system supports the interface between the IP layer and the physical layer through the ATSC Link-layer Protocol (ALP), which can support the convergence service of the broadcasting network and the communication network. It is required to process the STL(Studio to Transmitter Link) packet for stable transmission between the studio and the transmitter by receiving the ALP packet from the ATSC 3.0 gateway[6].
ALP is a protocol used in the link layer that serves as an interface for data transmission between the network layerand the physical layer, and ALP also provides dataencapsulation, compression and signaling functions at the linklayer. Fig. 2 shows the structure of ALP component andinterface.
Through ALP, it is possible to encapsulate all types of packets mainly, ALP encapsulates IP packets, MPEG-2 TSpackets, and link layer signaling packets. An encapsulated ALP packet enables a single processing on one packet formatin the physical layer, regardless of the type of packettransmitted from the network layer. It also compresses theduplicated header information of the input data beforeproceeding encapsulation step for transmission efficiency.
(Figure. 2) Structure of ALP component and interface.
3. Design of Intelligent IP Switch with Packet FEC
A switch technology, which has been applied to ATSC 1.0broadcasting system, is incompatible with ATSC 3.0broadcasting system which supporting IP-based stream transmission against MPEG-2 TS-based technology. Since ATSC 3.0 introduces a single frequency network and uses an IP-based STL-TP transmission technology, an additional error correction function shall be required unlike the ATSC 1.0broadcasting system [1].
Table 1 shows the supported specifications comparisons between the existing ATSC 1.0 switch and the ATSC 3.0 intelligent IP switch designed in this paper. Fig. 3 shows the module configuration of the Intelligent IP Switch that reflects the characteristics shown in Table 1.
(Table 1) Supported specifications comparisons between the existing ATSC 1.0 s witchand the ATSC 3.0 intelligent IP switch
(Figure 3) Module Configuration of Intelligent IPSwitch
In using Intelligent IP switch, the ALP (ATSC 3.0 Link-Layer Protocol) stream analysis module and the STL(Studio-to-Transmitter Link) stream analysis module are required to detect an error of a packet stream incoming through the network.
The IP stream analysis module can analyze and verify the SLT (Service List Table) data and the system time data, which are bootstrap information for describing information on the signaling transmission protocol. The ALP stream analysis module can analyze the ALP-TP, ALP packet, and IP packet. The STL stream analysis module can analyze the preamble data, time management data, and baseband packet data. It is confirmed through the respective analysis modules that the stream input is normally performed.
If the input stream is corrupted, the seamless s witching module can perform a switchover to the multiplexer thatoutputs the stable stream. After performing the switchover, itadds the FEC packet through the FEC encoding generator so that recovery is possible in case of possible damage of the data transmitted through the gateway. In addition, through the FEC decoder, a decoding algorithm can be applied to anerroneous input stream to output a normally reconstructed stream. At this time, the FEC encoding and decoding algorithm conforms to the SMPTE 2022-1 standard. The process of analyzing the input stream and applying FECencoding after switching is shown in Fig. 4.
Fig. 5 shows the proposed overall block diagram of ATSC3.0 Tx system with Intelligent IP switch which has kinds of packet analyzers, seamless switching module and packet FEC generator based on SMPTE 2022-1[7].
(Figure 4) Process Flow of Intelligent IP Switch
(Figure 5) Overall Block Diagram of ATSC 3.0 Tx System with Intelligent IP Switch
4. Implementation of Intelligent IP Switch with Packet FEC
SMPTE 2022-1 scheme has the advantage of burst error correction capacity in comparison with RFC2733[2]. Forrecovering the lost packet this standard operates XOR which has the ability to recover any one lost packet. Moreover, for the case of more than one packet has lost, two dimensional scheme can be used. Since two or more lost packets cannot be recovered with one dimensional scheme. Example of dual FEC mode structure is shown in Fig. 6 as below.
(Figure 6) Example of Dual FEC Mode Structure
To form two dimensional scheme, this standard designateL for the number of columns and D for the number of rows. The flow of FEC Encoding algorithm is shown in Fig. 7.
First, depending on the conditions set by the user, generate a matrix structed by Row(L) and Column(D). Next, after the data which will be sent is sorted in the form of FIFO, it has to be inserted in the FEC Matrix which is generated in the previous step. If FEC Matrix row field is filled completely, FEC encoder creates Row FEC Data by operating XOR . And the column FEC data also generated in the same way. In FEC data generating step, the additional FEC Header and the RTP Header of original stream has to beinserted before the FEC payload. These procedures should be performed before the data stream is emitted to the STL transmitter.
(Figure 7) Flowchart of FEC Encoding Procedure
Since the proposed FEC encoding algorithm applied twodimensional scheme, the first FEC stream has to be sent toport that is 2 greater than destination UDP number and thesecond FEC stream has to be sent to port that is 4 greater.
Before the data emitted to transmitter, it has to be decoded after transmitted through gateway in order to verify the integrity of the encoded data stream. The decoding algorithm’s flow shows in Fig. 8. Each Row(L) and Column (D) FEC data is received from the UDP port number which is 2 and 4 greater than input stream’s UDP portnumber. After receiving FEC data, by analyzing FEC Headerextract Row(L) and Column(D) FEC data.
(Figure 8) Flowchart of FEC Decoding Procedure
With these extracted data, generate the appropriate structure of matrix and insert original stream and FEC data to it. If the matrix has loss data decode the FEC data and recover RTPHeader with FEC Header. Next, by operating XOR of FEC payload data, recover the RTP payload of original stream and be emitted the recovered data to stream analyzer.
As described in Section 2 above, the ALP / IP packet can be extracted from the ATSC 3.0 STL-TP stream. The extracted main parameters are Preamble and Time & Management. In addition, the validity of L1 signal can be analyzed through data analysis and also BBP (Base-Band Packet) can be extracted, whose the validity is analyzed. Fig. 9 shows UI screen shot of packet analyzer of the proposed intelligent IP switch.
(Figure. 9) UI Screen Shot of Packet Analyzer
To measure the delay time and the switching time, thenetwork switch was installed to analyze the signal output from the DUT and the signal output from the gateway as shown in Fig. 10(a). The delay time test was proceed as follows. The signal generator make an event packet every 200 ms with the original video. The gateway could receive the original video and event packet from the transmissionsignal generator and outputs the same two videos. The one was transmitted to the DUT and the other was transmitted to the network switch at the same time. The DUT could receive the video from the Gateway and then transmit the one to the RF Out Exciter and the other to the Network Switch. Thenetwork switch could receive each video from the gate way and the DUT, respectively. And then two video could be sent to the packet analyzer which can compare the differences ofarrival time of the two video packets.
(Figure. 10) (a) Experimental Configuration and (b) Main UI of the Proposed Intelligent IP Switch
For the switching time test, an event packet at the moment the DUT changes the image source from Primary to Secondary could be generated. The switching time could be measured by adding the time of arrival of the first packet to Secondary after the occurrence of the corresponding event.
The delay average time and the switching average time were measured at 1.37ms and 823.8ms, respectively, by repeating 10 times under the conditions described above.
The main UI supporting the user condition setting of the proposed Intelligent IP Switch is shown in Fig. 10(b). As shown in Fig. 10(b), the switch condition can be set on the main UI screen and the status of input stream and output stream can be monitored as well.
5. Conclusions
In this paper, we proposed the design and implementation method of an intelligent IP switch, which is suitable for broadcast stream and reliable in comparison with the simple IP Switch technology used in existing network environment. The proposed Intelligent IP Switch consists of IP Stream Analysis Module, ALP Stream Analysis Module, STL Stream Analysis Module and SMPTE 2022-1 based FECEncoding/Decoding Module. When the input stream is damaged, the seamless switching module performs as witching to a multiplexer that outputs a normal stream and can restore the stream transmitted through the gate way, thereby providing a normally restored stream output.
The proposed intelligent IP switch is expected to be applicable to the areas that require redundancy between broadcasting systems and transmission signal duplication as well.
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