Digital Video Broadcast Return Channel via Satellite (DVB-RCS) Protocol Analyzer

Transcription

1 Digital Video Broadcast Return Channel via Satellite (DVB-RCS) Protocol Analyzer Mr.Mohanchur Sarkar, Dr.K.K.Shukla, Dr.K.S.Dasgupta Department of Computer Engineering, Institute Of Technology, BHU, Varanasi Advanced Digital Communication Technology Group, Space Applications Centre (ISRO), Ahmedabad Abstract--- Digital Video Broadcast Return Channel via Satellite (DVB-RCS) is the well accepted international open standard for two-way satellite interactive network. The intent of the open standard is to accelerate economies of scale thereby generating lower-cost solutions and opening the market in a shorter timeframe than could be possible with competing proprietary solutions. The work on DVB-RCS is initiated at Space Applications Centre (ISRO), Ahmedabad as a part of the R&D activity for the development of a two-way satellite network, which will support e-learning program in India. The primary objective of this work is to design and develop an analyzer that can analyze DVB-RCS compliant data stream. This work deals in capturing the DVB-RCS compliant data, detect the Service Information (SI) Tables, and analyze the contents of those tables. This analyzer also analyzes the IP Encapsulated data and PCR data received from the DVB-RCS Forward Link stream. The basic motivation in developing the Protocol Analyzer is to create a test environment to support research and development program in the field of Satellite based Networks. The Analyzer will act as a very important tool for developers of DVB-RCS HUB, network operators using this technology and for any R&D on this technology. This work is a part of the ongoing research on performance enhancement algorithms for Satellite based Networks. I. INTRODUCTION Digital Video Broadcast-Return Channel via Satellite (DVB- RCS) is an European standard formalized by the European Telecommunication Standard Institute (ETSI) and is envisaged to become the de-facto standard for Satellite Interactive Networks in the future the way GSM has established itself in mobile communication. This standard is typically targeting at broadband access network requirements and also offering a solution that is totally based on satellite communication. In the present information centric society, there is a growing need for wideband networks capable of supporting bandwidth hungry applications; applications requiring broadband connectivity. Besides these, there is a need for robust access scheme in order to provide reliable interconnection between user s terminal and the core of the network backbone. The key element to define the system characteristics of a satellite network is the access scheme. It has to care for the assignment of satellite bandwidth to the individual station as well as the ports of the station. In the past, access consisted typically of analogue links equipped with modems, dial-in connections or digital leased lines. But these conventional access techniques are no longer adequate for broadband multimedia network. In this context, DVB- RCS is a valuable alternate for overcoming the last mile issues. ISRO proposes to develop a Satellite Based Two Way Communication Network for its Multimedia Based e- Learning Program. This network will be used to provide education to all levels starting from primary education in rural areas to sophisticated specialized education for high-end professionals pursuing professional courses. Education being a basic need of the country, satellite based network has a tremendous growth potential with a long-term significance in terms of technology and other economic issues. Such a network can be developed using conventional VSAT terminals. But all the VSAT manufacturers use their proprietary protocol so both the terminals and HUB don t have the necessary flexibility and interoperability. This leads to a dependency on a particular vendor, which is not desirable for a network of long-term importance. So in this scenario the use of an open standard, which has, international acceptance becomes very much obvious. Keeping in view of these advantages of inter operability and growth potential it is decided to use the DVB RCS standard for our proposed EDUSAT network. The primary objective of this work is to design and develop an analyzer that can analyze DVB-RCS compliant data. This work deals in analyzing the DVB-RCS compliant data, detect the Service Information (SI) Tables, and analyze the contents of those tables. The software also analyzes the IP Encapsulated data in the DVB-RCS Forward Link stream. The Analyzer will act as a very important tool for developers of DVB-RCS HUB, network operators using this technology and for any R&D on this technology. The DVB-RCS compatible data can be received from a FILE containing prerecorded data or from a network where, the data comes as a streaming data or can be received from Satellite. The software has the capability to generate reports of all the individual SI tables analyzed or to display the important fields of the Tables with the help of a GUI. II. SYSTEM OVERVIEW This section gives an overview of the system architecture of DVB-RCS based broadband network. DVB-RCS as

2 mentioned earlier is a standard developed for broadband network services which uses space based communication system. The basic building blocks of this network are - Ground station Hub, one or more satellite in the forward link, a Satellite Interactive Terminal (SIT) at user s location, and a satellite in the return link. Forward traffic to the individual user is multiplexed into a conventional DVB/MPEG-2 broadcast stream at a centralized broadcast Hub and relayed to the SIT. The SIT acts as a router/multiplexer for different traffic sources (voice, data, fax and audio). The individual components are routed via Hub to their final destination. The SIT employs a scheduled MF-TDMA scheme to access the network and participate in bi-directional communication. MF- TDMA allows a group of SITs to communicate with a hub using a set of carrier frequencies, each of which is divided into time slots. The forward link data format supports up to 45 Mbps whereas a MF-TDMA scheme allows up to 2 Mbps in the return channel. DVB-RCS being an open standard provides seamless inter network with other networks and supports data transfer from Satellite Interactive Terminals (SITs) to the Hub station using Industry standard Internet Protocol (IP) and Asynchronous Transfer Mode (ATM). A. Actual System Configuration This is the configuration of the total working system, which will be eventually used in Space Application Centre (ISRO), Ahmedabad. Fig1 gives the overall system architecture of the DVB RCS Protocol Analyzer. The main activity of the system is to receive the DVB-RCS compliant data, transmitted from a DVB-RCS HUB and analyze the contents of the data. The L- band signal is feed to the input of a COTS available Integrated Receiver and Decoder (IRD) manufactured by Tandberg Incorporates. The IRD receives the data and decodes it and outputs the MPEG2TS data through an ASI (Asynchronous Serial Interface). The PC on which the Analyzer software runs is equipped with a commercially available ASI card manufactured by Computer Module Incorporates, USA. ASI is the latest interface for high serial rate data transfer from 1Mbps to 240Mbps. The ASI card has three inputs one for receive, one for transmit and one to take external 27 MHz clock. The external clock input taking highly stable 27 MHz (better than 10ppm accuracy) reference in needed for a proper analysis of the PCR data received from the forward stream and an analysis for the jitter associated with the data. The 27Mhz signal is available from the GPS based NTP (Network Time Protocol) Server which maintains a common timing for all the units used in the system like Burst Mode Demodulator and Analyzer software ASI card reception timing. The DVB-RCS Protocol Analyzer Software runs on a Linux platform and the task of the software can be broadly functionally divided into the following modules i. MPEG2TS Decoding and de multiplexing: This is the first step of the decoding process as the data in DVB-RCS Forward Link is based on MPEG2TS following DVB-S specification. After the successful decoding and de-multiplexing the data depending on the PID value contained in the header is put into the respective buffers. PID is a 13 bit value sent in TS Header to identify different DVB Streams. ii. The forward link of the DVB-RCS protocol broadly contains three types of data like IP Encapsulated data, Service Information Table data and PCR data. So respective decoders are used for the decoding of these information. iii. IP Encapsulation and Decoding: Decoding of the IP Encapsulated Data iv. Service Information Table decoding: Decoding of the service information tables v. PCR decoding: Decoding of the PCR values B. Present System Configuration Due to the cost implication and logistic support needed to use a live Satellite channel, the DVB-RCS Protocol Analyzer is developed with the capability to capture data from prerecorded file and capture data from LAN. In the later case the satellite channel is simulated on LAN where one PC streams prerecorded DVB-RCS data. This data is received by the Analyzer through LAN interface instead of using an ASI card as will be in the actual case of working with live Satellite Interactive Network. III. SOFTWARE ARCHITECTURE The DVB-RCS Service Information tables can be broadly divided into two categories static tables and dynamic tables. The static tables generally don t change unless there is a change in the configuration of the network. The dynamic tables change with time and the HUB has to generate those taking data from the return channel. A brief introduction about the main decoders comprising the Analyzer software is given below. A. NIT/PAT/PMT/PMT_RMT/RMT Decoder This decodes the static tables of the Forward Link, like Network Information table, Program Map Table, Program Map Table for the DVB-RCS Tables and the RCS Map table. NIT and RMT combined give the network parameters and tuning parameters for the terminal like the frequency in which the forward link is transmitted, symbol rate, coding rate etc.. After analyzing these tables the analyzer or terminal understands the way in which all the SI tables are organized in the Forward Link. The contents of these tables generally don t change unless there is a change in the network configuration, which can be displayed using the GUI of the DVB-RCS Protocol Analyzer Software. B. SCT/FCT/TCT/SPT Decoder This decodes the Superframe Composition Table, Frame Composition Table, Timeslot Composition Table and Satellite Position Table. These four tables combined give the fequency plan of the return link to the terminal. The FCT, TCT, SPT are static tables and contents generally don t change.

3 . From Satellite Antenna L-Band TANDBERG Integrated Receiver & Decoder ASI Input DVB-RCS Protocol Analyzer PC 1. SI Table Decoding 2. IP Decoding 3. PCR Decoding 4. Report Generation 5. GUI Output ASI Output GNU GTK Fedora Linux Core4 Multi carrier Burst Mode Demodulator GPS Integrated NTP Server Fig 1. Analyzer System Configuration with Data from Satellite The SCT is a dynamic table and contains all information about a superframe like when the superfarme starts, how long it lasts and the sequence number of that superframe. It contains the superframe_start_time_base field, which signifies the start of a superframe in terms of PCR ticks, duration field to specify the duration in terms of PCR ticks and superframe counter to signify the superframe number. The Analyzer displays these values accordingly in the GUI. Each new superframe is updated with the new value of PCR and count. The TIM decoder decodes the Terminal Information Message Table. It can be a unicast TIM that is generated for a specific SIT or broadcast TIM that conveys such network conditions, which affect all terminals. The TIM contains the descriptors like Logon_initialize_descriptors, ACQ assign descriptor, SYNC assign descriptor needed to handle logon of terminals to the network and inform specific information to SITs. The TIM decoder is mainly used to analyze the logon of terminals to the network. The CMT decoder decodes the Correction Message Table, which contains the timing, frequency and power corrections of all terminals in ACQ or SYNC phase. The CMT decoder is needed to analyze the coarse synchronization and fine synchronization and synchronization maintenance phase of the HUB Management. The TBTP decoder decodes the Terminal Burst Time Plan for all the terminals logged on to the network. This decoder is used to analyze the traffic assignment of terminals, after it has passed the ACQ and SYNC phase. The MMT decoder decodes the Multicast Map Table, which signify the PIDs in which the multicast data is to be transmitted in the forward link. The MPEG2 TS Decoder is the first part of the decoding process. The data received is either from a File containing prerecorded MPEG2 TS data or from LAN when satellite link is simulated in LAN or directly from Satellite where, the Tandberg IRD provides the MPEG2TS data in an ASI interface. The decoder takes as input MPEG2 TS data and demultiplexes it into IP Encapsulated data, DVB-RCS SI Tables and NCR data and keeps them into their respective buffers. The MPEG2 TS data are constant length 188 bytes, which contain a 4-byte header and 184-byte payload. The decoder

4 strips out the header and checks whether the first byte of the TS data is 0x47, which signifies the start of a MPEG2TS data. Then it analyzes the fields and finds out the PID associated with the TS packet. Depending on the value of the PID the decoder puts the payload to either SI Table buffers or IP Encapsulated data buffer or PCR data buffer. C. IP De-capsulation and Decoding The IP Encapsulation into DVB is done using the Multiprotocol Encapsulation as specified [2]. The software module for IP Decoder, which is an important function of the Protocol Analyzer, performs the following major functions 1) The IP encapsulated data are generally more than 188 bytes length so they are distributed in a number of MPEG2 TS packets or the IP Encapsulated data are fragmented during transmission. 2) The job of the decoder is to re-assemble the IP encapsulated fragments of data received from the different packets and find the start of a new IP payload. 3) The start of a new IP payload is associated with a DSMCC header, which for IP data has a table index of 0x3e as per the standards of IP Encapsulation (ETSI TR101202). The DSMCC header is 12 bytes long and contains along other fields the length of the DSMCC payload. The DSMCC header is analyzed and from the length the number of TS packets containing the IP data is calculated and reassembly done. 4) Once the total IP data is received it is passed to the IP decoder module. The IP decoder module analyzes all the fields of the IP header and UDP header if UDP is carried in IP and stored in respective objects and even in file if requested. IV. IMPLEMENTATION APPROACH The software is implemented in GNU C++ on Fedora LINUX Platform. The GUI of the software is developed using GTK on a Pentium PC. An Object Oriented Design approach is adopted in designing the software. All the Service Information tables, Descriptors, IP Encapsulated data and PCR are coded as separate objects, with associated routines for decoding them as member functions. The general SI Header objects and DSMCC Sections [1] objects are inherited by SI Table and IP Encapsulation objects. The Object Oriented programming concept of Abstraction, Multi-Level- Inheritance and Dynamic Polymorphism are extensively used in the software which gives the software its needed flexibility, reusability and ease in testing. A. Software Features Generally when a DVB-RCS Hub development starts people code the different SI Tables, IP Encapsulated Data and PCR in files. Then there is a need for individual verification of the different SI Tables to be conforming to the standard or not. The capture from file option provides lot of flexibility for the basic protocol verification and highly needed in the initial phase of DVB-RCS Hub development or R & D on this technology. After the initial basic protocol verification of the different tables is done there is a need to verify the DVB-RCS compliant data, as it will come in a real satellite network. As there are a lot of dynamic tables the behavior of the Hub has to be analyzed in a real time scenario. This leads to the Capture from Network option where before up-linking to the satellite the Data is streamed on LAN with the Satellite system characteristics and verified by the Analyzer. After the data is verified by the Analyzer in the Network mode the confidence of the system to work over a satellite link becomes quite high and the data generated can be up-linked to satellite. This is the mode in which it is actually intended to be used. In this case the system will connected with antenna with additional hardware necessary to receive data from Satellite. This mode of the analyzer will be used after the initial phase for HUB development is over and can be used even for network monitoring activities to monitor the condition of active DVB-RCS based Satellite networks. B. Multithreaded Architecture The software is multithreaded using POSIX compliant threads. There are separate threads for capturing the data either from the File or Network interface. The display of the values in the GUI of the dynamic SI Tables like SCT, CMT, TBTP, and TIM, PCR as well as the IP de-capsulated data is achieved by separate threads. The monitoring of the number of packets received, packets in CRC error and BER are calculated using separate threads so the capturing and decoding module of the software can work in parallel. V. TEST AND EXPERIMENTS CONDUCTED This section discusses the testing procedure and results. The system is tested taking prerecorded data from file as well as receiving streaming data from the LAN. A. Offline Testing The software is tested in the offline mode using a binary file, which contains prerecorded DVB-RCS compliant data. This data is verified to be DVB-RCS compliant by verifying it through a standard commercial DVB-RCS analyzer. The reports generated by the Verisat Analyzer are stored. A brief introduction about the Verisat DVB-RCS Forward Link Analyzer tool is given below. The Verisat Analyzer being a commercial standard analyzer, which is certified by the European Space Agency (ESA) to comply the Sat Labs recommendation, forms as a base for the development of the analyzer. The DVB-RCS compliant Protocol Analyzer, which is developed in this project is tested in the offline mode taking

5 data from prerecorded file and the Reports are generated for all the Service Information Tables, IP encapsulated data and PCR with Statistics information and Error analysis. The reports generated are compared with the output of the Verisat Analyzer and found to be exactly similar. This verifies the working of the developed analyzer. B. On Line Testing The Satellite Channel is simulated in a LAN environment. There are twp PC in the test setup one simulating the DVB- RCS Hub by streaming the DVB-RCS compatible prerecorded data on the LAN. The Analyzer software running on the other PC receives the data from LAN and does the MPEG2 decoding and de-capsulation. After the processing of data is complete by the Analyzer the analyzed data is displayed in a GUI as shown in the screen shots below. The SI table, IP data and PCR can be analyzed in run time. The main screen of the software displays are number of packets captured the number of packets in error and number of packets received error free. It also calculates the Bit Error Rate as errors are intentionally introduced in the data so as to simulate the Satellite Channel Characteristics. The generated data in the GUI is compared with the output of Verisat Forward Link Analyzer [16] and is found to be same. This test confirms the capability of the developed Analyzer and the test result is shown below as screen shots.. Fig 2. GUI of the DVB-RCS Analyzer displaying NIT VI. CONCLUSION AND FUTURE WORK The system will be very useful to DVB-RCS Hub developers and act as an efficient tool to understand the technology and work on it. The following extensions are possible on this software as future work. The objects used in this software can act as a library for other programs to be developed where a DVB-RCS Protocol is used. Any software, which needs the functionality of MPEG2 decoding and de-capsulation can take help of this software for development. Recently multimedia streaming is using the IP encapsulation standard (ETSI TR101202) so decoders both software and hardware generally need the functionality of MPEG2 decoding and IP De-capsulation. This software for the purpose of analysis has this feature so part of the code can become a major part of Software and Hardware based IP De-capsulator. Research and analysis on Satellite Network often need data to be analyzed and certain feature of a network to be studied in detail. In that case separate software patches may be developed suitable to the problem to be handled and in those cases this software will provide a basic framework on top of which other more meaningful applications and tools can be developed This work is a part of on going research on Satellite Network protocols and this software will provide the necessary test, evaluation and analysis environment. ACKNOWLEDGEMENT Authors thank all members of Department of Computer Engineering, I.T. BHU, Mr. V.S.Palsule Head, Mr. N.G.Vasantha Kumar, Mr.Vishal Agarwal, Mr. Manish Kumar Soni and all staff members of Advanced Communication Technology Division (ACTD), SAC/ISRO for their cooperation and technical support. REFERENCES [1] ETSI EN : "Digital Video Broadcasting (DVB); Interaction channel for satellite distribution systems" [2] ETSI TR : "Digital Video Broadcasting (DVB); Implementation guidelines for Data Broadcasting" [3] Novra S Receiver Instructions in [4] Series 2000/3000 IDU User Manual from EMS Satellite Networks,Canada, [5] User Manual DM240 Digital Modulator from Radyne ComStream [6]References of Tym Server 2000 from Symmetricom, [7] Multi-Channel Multi-Frequency Burst Mode Demodulator Version 1.1 Specification. Doc:BMD-ABSP-SPE-0004 from Alcatel Bell Space N.V [8] Project Report on GSAT-3 (EDUSAT) Applications Programme Macch 2003 Space Applications Centre [9] SatLabs System Recommendations January 2004 [10] ISO/IEC (1996): "Information technology - Generic coding of moving pictures and associated audio information; Part 1: Systems" [11] ETSI EN : "Digital Video Broadcasting (DVB); Framing structure, channel coding and modulation for 11/12 GHz satellite services"[12] ETSI ETS : "Digital Video Broadcasting (DVB); Network-independent protocols for DVB interactive services" [13] ETSI EN : "Digital Video Broadcasting (DVB) Specification for Service Information (SI) in DVB systems" [14] ETSI EN : "Digital Video Broadcasting (DVB); DVB specification for data broadcasting" [15] A proposal On Study, Simulation, & PROFF OF CONCEPT OF DVB RCS [16] User Manuals on FRLA at [17] Project Report and Software Manual of DVB-RCS Protocol Analyzer Software submitted in IT.BHU.