Streaming Video on IP Networks
Contents
1. 90 KHz and 27 MHz As soon as the 27 MHz counter reaches the value of 300 it is reset to 0 and the 90 KHz part is incremented by one The reason for having the PCR field split into two parts is due to the fact that MPEG 1 was only using a 90 KHz timebase The timabase extensions fields are displayed in figure 2 7 90 KHz 27 MHz DN 33 bits 9 bits Figure 2 7 PCR fields in transport packet adaptation field The PCR field contains the necessary information required to synchronize the de coder to the encoder clock The decoder needs an internal clock that is locked to the encoder clock in order to utilize the decode time stamp and presentation time stamps contained within the PES packets To make sure that the decoder clock is synchronized with the encoder the PCR signal is transmitted periodically typically every 0 1 seconds Transport stream header Syncbyte Error Start Adapt Adapt Priority SCR quan Cont field Adapt flid Disc Random Elem Opt Extended header PCR OPCR Splice Transport Adaptation countdown private data extension Program clock reference Figure 2 8 The fields contained in the transport stream header The decoder uses a voltage controlled oscillator VCXO to generate a 27 MHz clock When a PCR is received it is compared to the local counter that is driven by the VCXO The difference in time is used to correct the frequency of the VCXO to en sure that the 27 MHz clock is locked to2. LL 6 3 The Setup 11 3 1 Installation O 11 3 1 1 The operating system 11 3 1 2 Streaming software 0 222200000 12 3 1 3 Hardware 0 0 000000 00 0 ov vr 14 3 1 4 Satellite setup 2 20 0 2 0000000 a 16 3 1 5 Configuration files sosse rss ss ess 16 3 1 6 Terrestrial setup 22 2 2 20 000000 ss ss 17 3 1 7 Live capture rr rr rr leen 19 4 Analysis 20 4 1 Preliminaries 0 0 0 0 00 00 eee oo vr 20 4 2 Signalstudy 0 sens 20 4 3 Demonstration 2 2 2 2 2 42 2 24252 2 2 5 22 44 Theories Res 23 5 Conclusion 26 References 27 A dvbrc file 29 B Transport stream error log 30 C SocketReader 31 Chapter 1 Introduction My main objective at Ericsson was to create a system that could stream broadcast quality video over an Ethernet network A commercial video server system was al ready online when I arrived at Ericsson The current video solution is based on a Tandberg TT7116 IP Streamer The streamer receives its video feed from several TT1220 professional IRD units which are basically satellite receivers Each unit is tuned to a specific channel The digital video feed is passed from the TT1220 units to the T T T115 streamer where the signal is converted into a format suitable for Ethernet traffic The sys tem communicates with the outside world through a web server built up of two custom designed Si
3. MPEG standard They also have software at their disposal which is capable of analyzing transport streams The analysis showed that there were discontinuities in the transport stream at three second intervals The error log can be found in the appendix section of this report The discontinuity is due to the fact that the continuity counter fields are nonse quential This may be caused by missing packets or by the software if it changes the values of these fields According to the technicians at ESDG this may very well be 24 the reason why the set top box goes black after approximately four minutes MPEG 2 carried at the 188 byte Transport Stream packet level is in essence un protected and transportation needs to take place across almost error free links If the IP transport network cannot guarantee very low levels of error rates then for IP multicast some form of forward error correction scheme should be used Bit errors in an IP layer will be reflected in whole frames being dropped at the MAC layer due to checksum failures Hence the protection scheme should not work on the bit level as is the case for satellite links for example A more efficient method is to add a Forward Error Correction FEC at the IP frame level The simplest technique is to XOR together the IP frames and then regularly approximately every 20th frame send the XOR sum on a separate UDP port The receiver will be able to detect if a packet has been lost by insp
4. and then later on to transport stream packets The different layers are created with different objectives During PES packetizing information such as scrambling and copyright protection are added to the stream The information added in the transport stream layer is used to transport and deliver the data The following figure shows the relationship between elementary streams and trans port streams Variable size SS _ PES header Picture data TS header Data from PES packet TS header Data from PES packet TS header Mixed data hex FF fills out the Fixed size remaining space Figure 2 5 Relationship between the packetized elementary stream and the trans port stream The entire contents of the PES packet is divided into smaller pieces and inserted into the payload regions of the TS packets Byte stuffing is used if a PES packet does not fit into a discrete number of TS packets The hexadecimal value FF is inserted into the TS packet until its payload is full This is displayed in the third TS packet in figure 2 5 Transport streams can be built up in different ways Single program transport streams SPTS are built up of different PES streams that all share a common time base Common time base means that the programs are related to one another such as in the case of a movie with several different audio tracks The multi program transport stream MPTS is a multiplex of several different transport streams The MPEG 2 tr
5. had a proper satellite dvbrc file I managed to stream a satellite chan nel to the set top box The box could indeed display the channel contents for a short while but the stream was not perfect since the image disappeared after ap proximately four minutes The result was the same with other channels so divine interference was ruled out as a possibility 3 1 6 Terrestrial setup The second part of configuring the computer consisted of setting up a receiver for terrestrial digital video broadcast Terrestrial signals are emitted from TV towers They differ from satellite signals in that they carry additional information needed for error correction Terrestrial DVB signals are prone to interference such as ghost images and electrical noise caused by high rise buildings heavy power usage and interfering radio signals The Linux DVB drivers work with both satellite and terrestrial broadcasts Much like its celestial counterpart the terrestrial receiver refused to work initially The drivers were not the cause of the malfunction so the problem had to lie elsewhere In order to see how sensitive the reception of the Nova card was a test antenna made from an old coat hanger was used 17 Figure 3 6 The coat hanger antenna As a reference I used a Nokia Mediamaster terrestrial digital video receiver as ref erence to position the antenna to get an optimal signal The signal proved to be quite weak due to the fact that t
6. high bandwidth and small transmission delays This is why this problem area is of interest for a com munication equipment company such as Ericsson The object of this report is to document the work I have done on IP television at Ericsson Strommande video over IP Examensarbete Sammanfattning Strommande video har blivit popul rt tack vare ett kande intresse for str m mande media pa internet Trots att str mmande video inte ar n gon ny f reteelse ar det f rst nu som det finns m jlighet att distribuera videosignaler med hog uppl s ning i realtid ver IP Anv ndning av fiberoptik erbjuder mycket hog bandbredd Sollentuna Energi ar ett av flera f retag som for tillf llet erbjuder TV abonnemang ver IP till de kunder som har fiberuppkopplingar i hemmet Abonnenterna kan valja mellan ett stort antal kanaler och kommunicerar med videoservrar genom set top boxar som kopplas till TVn Str mning av MPEG 2 video st ller mycket h ga krav pa natverkens bandbredd och kvalit och ar darfor av intresse for foretag som Ericsson Syftet med den har rapporten ar att dokumentera det arbete om IPTV som jag har utfort pa Ericsson Contents 1 Introduction 1 2 Technical Description 2 2 1 Unicast broadcast and multicast 2 22 MPEG 0 002000000 2 ss srt 3 2 2 1 Multiplexing 222 eee eee eee eee eee se ss 4 2 2 2 Packets 0 0 0 0 20 00 0000 a 5 2 2 3 MPEG Streams oaa 5 2 2 4 Transport Streams LL LL
7. was discovered that the Linux DVB drivers were the cause of the malfunction Several weeks had gone by during the debugging process but once the drivers were working I could promptly tune the NOVA s card to any given frequency I was able to tune the card to different frequencies and zap between channels 3 1 5 Configuration files Once the drivers were up and running VLS had to be configured VLS uses a file called dvbrc to tune itself to different channels This is true for both terrestrial and satellite DVB There is no practical scan program available to search for channels over any given frequency band Ironically there exists a small application supplied with the Linux drivers called scan The scan program is part of the libdvb package No documentation is pro vided for scan It was a challenge to figure out how to use it but by going through the program s source code I could determine how to get it started Scan is a com mand line program that requires a dvbrc as an argument Figure 3 5 shows an example of how to use scan The example assumes that a valid dvbrc file lies in the root directory Enter the directory entitled libdvb xxx Start scan by running the following command libdvb 1 5 2 scan root dvbrc gt temp file temp file will now contain the information contained in the dvbrc file along with all the programs available at the given frequencies A Figure 3 5 Scan
8. 2 9 The PSI tables are used by the MPEG 2 demultiplexer The Program Map Table PMT contains the PID s of the packets that belong to a specific program These PID s identify the transport packets carrying the audio and video packets of a specific program These PES streams are extracted from the transport stream and routed on the decoder The PID linked to the first row in the PAT points to the Network Information lable The main role of the NIT is to deliver information about the delivery net work The information contained in the NIT is not defined by the MPEG 2 standard The Conditional Access table contains information about the encryption methods used for each individual program The PID used to identify this table always has the value 1 The CAT contains the PID s of certain packages that carry information about the scrambling systems 10 Chapter 3 The Setup 3 1 Installation The idea at Ericsson was that I build a video server system based on standard PC equipment The original mockup consisted of a PC running on Linux with a PCI card capable of receiving digital satellite transmission The computer I was supplied with was a low end Pentium II running at 300 MHz The processing power was deemed sufficient since the machine would not be transcoding any part of the MPEG 2 trans port stream My objective was to configure the system to receive broadcast digital TV and stream the corresponding MPEG 2 transport stream through a networ
9. MPEG 2 errors Instead I will list a number of theories to why the MPEG 2 stream causes the set top box to go down after only a few minutes The Program Map Table defines the packets carrying PCR information for a given program If the PMT should fail to point out the correct PID packets then the decoder will lose sync over time resulting in screen freezing or image blanking Another possible cause of error is PCR jitter The use of LAN networks to trans mit resynchronization timestamps introduces certain problems Transmission delays are not always constant within a network If the PCR packets are delivered with varying delays the decoder clock could be affected The decoder system uses a phased locked loop PPL an analog system used to average out disruptions in the PCR flow Should however the delays vary too much then the PPL might not be able to compensate for the variations as shown in figure 4 3 Encoder time delayT 1 T 1 A T 1 B T 1 C T 14D T Decoder time Figure 4 3 PCR transmission The most straightforward method for measuring the network jitter as it affects the MPEG 2 Transport Stream is to use two Transport Stream analyzers one on each end of the network as shown in figure 4 4 23 Video server Video client MPEG 2 transport stream MPEG 2 transport stream Network interface i Kiste Network interface Figure 4 4 Setup for PCR jitter measurement Most Transport Stream analyzers available t
10. and protocol de velopment Ethereal is a network protocol analyzer or packet sniffer that lets you capture and interactively browse the contents of network frames It runs on all popular computing platforms including Unix Linux and Windows The plan was to make two simultaneous dumps of the packets sent out by both systems I tuned both my PC video server and the Tandberg to the same satellite channel I chose a test card channel with a static test pattern for sake of simplicity To my mild disappointment the sniffer was not able to make simultaneous dumps from the two systems This was no cause for concern since the feed from the channel was static All I had to do was take two successive dumps and save them on file for later comparison The configuration is displayed in figure 4 1 20 Tandberg IP streamer PC with 2 network interface cards PC video server Tandberg IP video streamer 1 configured PC video server 1 PC equipped with two network interface cards Ethereal downloadable from www ethereal com or trough the apt Ethereal can easily be downloaded and installed under Linux from www ethereal com or through the apt Ethereal documentation and user manual can be found on www ethereal com docs Figure 4 1 Sniffer setup My intent was to pinpoint crucial differences between the packet streams Every packet seemed different it was found that the Tandberg systems inserts additional information int
11. annels are independent of one another The various video and audio streams of the different channels are multiplexed together before being broadcasted to the consumer Several different broadcasting stations send their feeds to a common uplink sta LC Figure 2 2 Multiplexing requires compression on each input tion There the signals are multiplexed before being transmitted to a satellite for redistribution The multiplexing stage could be a function of the underlying deliv ery network The main drawback of such a system is that the transported data is highly dependent of the distribution network In other words a multiplex based on 1 a certain network topology may not necessarily be compliant with another network topology The MPEG 2 standard is independent of the network s physical implementation The standard is well suited for all types of networks both those that introduce noise and those that are error free The data in the MPEG 2 standard is self consistent in that all required information required to decode the audio and video is packaged within the bit stream 2 2 2 Packets The MPEG 2 standard is based on the usage of packets Packets are data structures commonly used within the networking domain Packets consist of a header and a payload both being variable in size The header contains the necessary information needed to decode the data payload the size of the header depends mainly on which scenario the data pa
12. ansport stream packet is of constant size of 188 bytes The packet consists of a 4 bytes information header an optional header extension called the adaptation field and the data payload that is built up of PES packets The trans port packet header is normally kept quite small in order to increase the data payload carried within the packets The overall size of the packet is always unchanged T 188 bytes SC h JI Figure 2 6 The total size of a transport stream packet is always 188 bytes despite the fact that the header can be variable in length The first byte of the Transport Stream packet header is a synchronization byte of value 0x47 The remaining 3 bytes of the header are used to identity the payload of the packet and provide additional decoding information The most important information in the header is the packet identifier PID The PID information is used to identify the transport packets containing information relevant to a single PES stream When a demultiplexer is instructed to listen to a certain PID it extracts the packets containing the right PID number and discards the rest The adaptation field is an optional extension of the transport packet header The information contained within the adaptation field is used for clock recovery The most important field is the program clock reference PCR The PCR field is a 42 bit counter which is split up into two parts The two parts represents two counters running at
13. at this was not an issue 4 3 Demonstration Towards the end of my period at Ericsson a demonstration system was mounted The demonstration consisted of streaming High Definition TV from my video server through a network controlled by a COPS protocol The COPS protocol regulates quality of service issues such as bandwidth Since the set top boxes were not able to process the high bit rate HDTV transport stream we used a powerful PC as client The client PC was equipped with a Pentium IV processor clocked at 2 0 GHz The PC was hooked up to a high definition projector This is displayed in figure 4 2 Figure 4 2 The demonstration system We noticed a remarkable difference in required processing power between the video server and the client The processing power of the Pentium II running at 300 MHz was sufficient to stream video but the Pentium IV 2 0 GHz client PC was barely able process the tremendous amount of data The client regularly dropped frames to stay in sync with the signal Decoding video requires much more processing power than streaming the same signal onto the network I expected the video to freeze or go blank after a few minutes but the video feed continued working for several hours When left streaming the system stayed up for 22 a continuous period of 12 hours This proves that the MPEG 2 transport stream is not entirely corrupt 4 4 Theories Due to lack of time I was not able to determine the nature of the
14. computer running VLC as a client Both computers were connected through a switch that supported multicast The MPEG 2 signal streamed perfectly for several minutes The required configuration is displayed in figure 3 2 12 Server PC Client PC 2 PC s equipped with network interface cards 1 switch with multicast support VideoLAN Client downloadable from www videolan org Both PC s must have the VideoLAN Client software installed on them The software can be downloaded from www videolan org or through the Debian package manager The installation process is straightforward Additional information regarding installation and user manual for VideoLAN Client is available from www videolan org The file that is streamed must be supported by VideoLAN Client VideoLAN supports various video file formats but it is recommended to use an MPEG 2 file Figure 3 2 PC to PC streaming setup The next step was to stream an MPEG 2 signal to a set top box connected to a TV Ericsson has a standardized set top box that has a built in Ethernet network adapter It is used in conjunction with the Tandberg video streamer In order to send a correct transport stream I was provided with a CD ROM containing an MPEG 2 compliant transport stream file I chose to use the Video LAN Server software VLS instead of the VLC VLS is a scaled down version of the VLC which requires less resources on the PC Saving resources was essential due
15. directly attached router IGMP provides the means for a host to inform its attached router that an application running on the host wants to join a specific multicast group Multicast messages usually employ the User Datagram Protocol UDP UDP pro vides no reliability it sends datagrams but there is no guarantee that they ever reach their destination The difference between multicasting and broadcasting is that broadcasting sends the packets to every machine on the network Multicasting on the other hand only sends out packets to clients belonging to the multicast group This technique is mainly used to reduce the amount of traffic on a network The output from the video server can either be sent as a unicast or a multicast stream Multicast addresses are in the range 224 0 0 0 through 239 255 255 255 The range 224 0 0 0 to 224 0 0 255 are reserverd and should not be used Multicast routers should not forward any multicast datagram with destination addresses in this range The set of hosts listening to a certain IP multicast address is called a host group Hosts can dynamically join and leave a group The number of hosts in a group is unrestricted IANA Internet Protocol v4 Multicast Address Assignments E D A 4 a Joining b Receiving Figure 2 1 Joining and receiving from a multicast group Joining a multicast group can be as simple as starting an application A
16. download new kernels from http www kernel org Once the files are downloaded they need to be unpacked and compiled A good guide for this can be found on http www linux org docs Idp howto Kernel HOWT O Before compilation the kernel must be configured to enable loadable module support i2c and videodev This is done by running make config make xconfig or make menuconfig The Linux DVB drivers can be downloaded from either of the two links Installation instructions can be found in the README and INSTALLATION files contained in the Linux DVB driver tar gz file Figure 3 4 Debian kernel setup 15 3 1 4 Satellite setup The satellite dish on the Ericsson rooftop in Telefonplan Stockholm was a profes sional system Originally it was thought that the NOVA s PCI card was incompat ible with the Low Noise Block downcoverters LNB microwave heads LNB heads are the components inside the satellite dish that receive the satellite signals The LNBs in the satellite dish were locked to specific frequencies Normally LNB heads can be tuned to different frequencies using a satellite receiver Since this was not the case it was believed that the tuning software used with the satellite dish was trying to unsuccessfully tune the LNB heads In order to rule out that possibility I debugged and traced the tuning program s source code In the end I could find no reason for program or hardware conflict It was only later that it
17. e being converted to a transport stream the output of an MPEG 2 encoder is called the elementary stream An elementary stream is an endless stream of audio and video information Storage and distribution system prefer discrete blocks of information so the elementary stream is transformed into a packetized elementary streams PES A PES packet begins with a header containing a unique packet start code its value is 0x000001 as seen in figure 2 4 It is followed by a code that iden tifies the type of the PES stream PES packet lt LLL Start code stream ID PTS DTS Data 0x000001 Figure 2 4 Example of a simple PES packet The amount of information contained within the header is variable The PES packet header is variable in length Certain packet headers contain in formation that is not constantly transmitted The most important of the optional flag are the presentation time stamps PTS and the decode time stamps DTS The presentation time stamps inform the decoder when the actual information should be displayed while the decode time stamp is present if the decoding time of the packet differs from the actual presentation time The DTS is always preceded by a PTS The PES is primarily a logical construct and is not intended to be used for in terchange transport or interoperability The PES is primarily a conversion point between program streams and transport streams The elementary stream is first segmented into PES packets
18. ecting the sequence number in the RTP header The lost packet can be recreated by XORing all the received packets excluding the one lost plus the FEC packet RFC2733 describes a method for this 29 Chapter 5 Conclusion It was determined that a low cost alternative to the Tandberg streamer might be possible Although a perfect replacement machine was not achieved I was able to configure a system capable of demonstrating the potential of a PC based video server Much more analysis work must be done in order to obtain a system capable of delivering a perfectly compliant MPEG 2 stream Various problems such as network jitter and bit errors are probable causes to the incompliance of the MPEG 2 transport stream Network jitter can be studied with the use of network analyzers by doing two point measurements If jitter is present its negative effects can be minimized by using shorter packets with the expense of additional packet overhead The recurring transport stream discontinuity errors are a cause for concern They can be caused by the satellite card the Linux DVB drivers or even the VLS stream ing software Analysis programs will most likely be of great use when trying to pinpoint the exact cause of the transport stream anomaly The fact that a demonstration system streaming HDTV managed to stay up for several hours proves that the generated MPEG 2 transport stream is not entirely corrupt The PC decoding the HDTV
19. he building we were in was covered with aluminum siding The aluminum sheets effectively shielded us from TV signals The Nokia set top box proved to be quite tolerant of weak signals and managed to produce a flickering image on the TV screen The image was often distorted by pixelation due to discontinuities in the MPEG 2 transport stream caused by weak TV signals The result was that the Nova card definetely needed a strong signal Once the antenna was positioned I knew that I should be able to get at least some read out from the terrestrial DVB card I tested different channels and different dvbre settings but none worked By chance I managed to spot a small commen tary line in one of the several header files used by the Linux DVB driver files It stated that in order to make the terrestrial card run under Linux a small dll file should be copied from the Hauppauge Windows driver CD and placed in the Linux system This little suggestion did the trick and the terrestrial card awoke By configuring the terrestrial dvbrc file using the scan program I got VLS to work A faint and completely pixelated image was displayed by the set top box for a few seconds The required configuration can be found in figure 3 7 In order to extract the necessary file one must first install the NOVA t card and its drivers on a PC running Windows When the card is installed and configured in Windows the file ttlcdacc dll can be copied from C Pr
20. k 3 1 1 The operating system My first task was to install a Linux operating system on my PC The Linux oper ating system is available in several different versions called distributions Ericsson uses Debian Linux distribution This is why I opted to install this distribution on my PC The Debian installation process turned out to be quite a laborious task Very little information is provided regarding the install process of the base system The installation program requires a great deal of information about the hardware and network configuration The first installation attempts failed due to the fact that the graphics card in my PC conflicted with the Debian distribution Unable to overcome the problem I attempted to try another Linux distribution The Red Hat distribution installation process worked flawlessly and installed itself on my computer The victory was short lived since issues regarding software installation quickly arose Since Linux bases itself on open source software most programs are downloadable in source code for mat only Because of this dependency issues can take place when attempting to compile programs Some programs require additional source code not available with the base installation These dependency problems often cascade source code files often depend on other source code files The Debian distribution has software that handles these issues It is called the APT manager 11 Because of this I attempted to
21. licon Graphics workstations optimized for I O operations The entire setup is very expensive and not suitable for testing and proof of concept trials Therefore Ericsson wanted me to come up with a low cost alternative that could replace the original setup I was instructed to use low end personal computers and use freely distributable software All software that I would potentially write would be open sourced and available to download for free from the Internet Chapter 2 Technical Description 2 1 Unicast broadcast and multicast The information in section 2 1 refers to 1 and 2 Unicast RFC2073 is communication that takes place over a network between a single sender and a single receiver The data stream can be sent using a crossover cable or can pass through a switch before being delivered to the receiver Multicast RFC2373 is based on the concept of a group An arbitrary group of receivers expresses an interest in receiving a particular data stream This group does not have any physical or geographical boundaries the hosts can be located anywhere on the Internet Hosts that are interested in receiving data flowing to a particular group must join the group using IGMP RFC2236 Hosts must be a member of the group to receive the data stream IGMP stands for Internet Group Management Protocol As the name states it is a protocol used for group manage ment on the Internet The IGMP protocol operates between a host and its
22. n 1992 The standard was designed to fit the bandwidth of early CD ROM s providing quality comparable to that of VHS cassettes The strength of MPEG 1 lies in its powerful ability to provide relatively high quality video images while using a high compression ratio The MPEG 2 standard was established in 1994 and was designed to provide to better image quality at higher bit rates MPEG 2 is capable of delivering true broadcast quality video and is primarily used by broadcasting companies that pro vide their content through satellite and cable networks It is backwards compatible in that any MPEG 2 decoder can play MPEG 1 content MPEG 2 has also become widely spread since it is the coding scheme used by DVD s The High Definition TV standard is based on MPEG 2 MPEG 4 is based on the successful MPEG 1 and MPEG 2 standards and was orig inally intended for usage with very low bit rates The standard was initiated in 1995 and finalized towards the end of 1998 What makes MPEG 4 different from MPEG 2 is its ability to integrate content from different media sources It can add such elements as recorded entities 2D images and moving 3D computer models 2 2 1 Multiplexing The MPEG 2 standard is based on the different applications that video and audio data are used in The standard also takes into consideration the technology used to deliver the data For example in TV broadcasting several different channels are sent at once to the consumer The ch
23. nced configuration in Linux DVB drivers needed to be downloaded and installed on the system in order for the cards to work in Linux A web site called LinuxTV provided numerous links to various Linux DVB drivers The drivers proved to be stubborn and would not compile or install themselves on the system I had previously received help from a person who had altered certain Linux system variables By mistake he had changed these values to point to old header files used by the Linux kernel The faulty system variables were discovered by chance The drivers worked per fectly once the system variables were reconfigured to point to the correct header files Before the error was corrected it was thought that the underlying hardware such as the satellite equipment was at fault Luckily this was not the case In order to prepare the system for the Linux DVB drivers certain steps have to be taken these are displayed in figure 3 4 Debian Linux Kernel Source files version 2 4 18 or greater downloadable from www kernel org or through the apt Linux DVB drivers downloadable from http www metzlerbros org dvb or from http linuxtv org or through the apt In order for the Linux drivers to work the Debian Linux environment must be configured e Source files for kernel 2 4 18 must be downloaded to the PC e The kernel has to be compiled and configured with Joadable module support and i2c and videodev activated You can
24. o the transport stream before it sends it to the Ethernet network A byte per byte comparison of the signals would therefore be difficult Instead I wrote a small JAVA application that opened a multicast socket to the video server and proceeded to record a raw transport stream without any network protocol overhead The source code for the JAVA application can be found in ap pendix C The recording would enable me to search through the transport stream data to find any errors Unfortunately the specifics of the MPEG 2 standard are very complicated and a manual parse of the data would require much knowledge of the form of the transport stream There exists software capable of analyzing and determining the compliance of an MPEG 2 transport stream such as the Manzanita MP2TSA Unfortunately no such software was at my disposal but some findings will be reported in section 4 4 below 21 There were concerns that the packets might be larger than maximum allowable Ethernet frame size of 1500 bytes Measurements made with Ethereal showed that the packets transmitted trough the network had a total size of 1358 bytes When packets are larger than the maximum frame size they are divided into series of smaller packets that fit into Ethernet frames These smaller frames are sent out on the network as bursts of packets This sudden amount of traffic on the network can cause congestion and packet jitter The measurements proved however th
25. oday will measure the PCR errors di rectly in nanoseconds usually as a histogram By comparing the PCR jitter at the output of the network with the PCR jitter measured at the input of the network this error can be used as a direct measure of the maximum cell delay variation experienced by the transport stream as it propagates through the network Jitter performance over IP based networks should improve if the streaming protocol uses shorter packets at the expense of additional packet overhead The set top boxes contain memory chips that buffer the incoming video stream If the memory buffer fills up for some reason it might not be able to continue de coding the MPEG 2 signal properly The PC system used during demonstration stayed up for a period of 12 hours perhaps due to the fact that it was more resilient to incompliant MPEG 2 bit streams The 256 MB of RAM were also probably more than sufficient when used as buffer memory It is unclear at this time how the Linux drivers handle the PCR packets it might relocate them erroneously in the transport stream The source code for the drivers should be looked at in more detail in order to rule this possibility out As I was writing this report I was supplied with more information regarding errors contained within the transport stream A 100 MB file was analyzed by a company called ESDG which is based in Link ping The company specializes in MPEG and have a great deal of knowledge of the
26. ogram Files Hauppauge WinTV NOVA This file should be renamed tda1004x mc and placed in the letc dvb directory Figure 3 7 NOVA t setup 18 3 1 7 Live capture The idea of using a software MPEG 2 capture card to stream video from a video camera was abandoned quite early since the equipment I had at my disposal was not capable of real time MPEG 2 encoding According to various MPEG 2 encod ing message boards the minimum requirements for real time software encoding of MPEG 2 is a Pentium 4 running at a speed of at least 2 4 GHz with at least 256 MB RAM I had a Pentium II running at 0 3 GHz with 128 MB RAM 19 Chapter 4 Analysis 4 1 Preliminaries During test periods the set top box could receive and display MPEG 2 transport streams for a period varying between 4 and 7 minutes The up time depended on the set top box used According to various experts at Ericsson such as Mikhail Soloviev certain boxes were crankier than others Quite simply some boxes proved to be more tolerant to lawed MPEG 2 transport streams 4 2 Signal study In order to determine the cause of the erroneous signal I set out to compare the MPEG 2 transport stream generated on my PC with the correct MP EG 2 transport stream generated on the Tandberg IP streamer I placed two network cards in a second PC and installed an Ethernet network sniffer program called Ethereal Ethereal is a program used for troubleshooting analysis software
27. pplica tions that receive multicast transmissions send requests to join multicast groups this is demonstrated in a in figure 2 1 The host sends a membership report mes sage to its nearest router as well as to all hosts on the attached interface Each membership report contains the multicast address of a single group that the host has joined Once a member has joined a multicast group it will receive all multicast messages sent to the group as can be seen in b in figure 2 1 In order for multicast to work over multiple networks routers and switches need to support the IGMP protocol Multicast routers send queries at regular intervals to see if there are any multicast members left on any of its interfaces Using queries multicast routers keep tables of which of their interfaces have one or more hosts in a multicast group 2 2 MPEG The information in section 2 2 refers to 3 4 5 6 MPEG stands for Moving Pictures Experts Group The Moving Picture Experts Group is a working group of ISO IEC in charge of the development of standards for coded representation of digital audio and video MPEG is the name given to standards used for coding audio and video content in a compressed digital format There exists a wide variety of MPEG formats such as MPEG 1 MPEG 2 MPEG 4 and MPEG 7 MPEG uses lossy compression in that information is lost when the video and audio data is compressed The first MPEG standard MPEG 1 was established i
28. qi ST KTHS S VETENSKAP J9 OCH KONST 26 SS E KTH Numerical Analysis and Computer Science Streaming Video on IP Networks Stefan Freyhult TRITA NA E04024 Numerisk analys och datalogi Department of Numerical Analysis KTH and Computer Science 100 44 Stockholm Royal Institute of Technology SE 100 44 Stockholm Sweden Streaming Video on IP Networks Stefan Freyhult TRITA NA E04024 Master s Thesis in Computer Science 20 credits at the School of Computer Science and Engineering Royal Institute of Technology year 2004 Supervisor at Nada was Henrik Eriksson Examiner was Stefan Arnborg Abstract With the increasing demand of streaming multimedia applications over the Inter net video streaming is gaining more and more popularity Though the concept of streaming video is not new it is but recently that networks offer the possibility of high quality real time video over IP The use of fiber optics has practically elimi nated the issue of limited bandwidth Various companies such as Sollentuna Energi currently offer IP TV subscriptions to homeowners who have fiber optic links into their homes Subscribers can choose from a wide variety of channels The users communicate with video servers through set top boxes Transmission of MPEG 2 encoded video is one of the most demanding applications in terms of network re sources and Quality of Service QoS requirements It demands very
29. reinstall the Debian package on my PC Through hours of Google searching I had managed to pinpoint the reason why the first in stallations had failed The Matrox graphics card installed in my computer did not support frame buffering This caused Debian to display No screens found on the screen during start up By editing a configuration file the problem was solved The required configuration is displayed in figure 3 1 Various problems can arise during the Debian installation process Certain graphics cards are not 10096 compatible with the distribution notibly certain Matrox cards In order to work around the incompatibilty issue it is imperative that one does not select frame buffering option when asked during the installation process Should frame buffer misstakenly be selected it can be unselected after the installation by running dpkg reconfigure xserver xfree 86 and not selecting frame buffering Figure 3 1 Debian Linux Setup 3 1 2 Streaming software Through the Debian APT manager I downloaded a program package known as the Video LAN Client VLC The VLC package is software written by a group of com puter students at the Ecole Polytechnique in Paris The software can be used for streaming video over a network and for viewing various video formats My first attempts to stream video over our local network proved to be success ful By using VLC as a server I managed to send a MPEG 2 file from my computer to a nearby
30. rver is used for streaming to set top boxes It proved to work better than the VideoLAN Client VideoLAN Server is available through the apt or from www videolan org VideoLAN Server is available in source code only if downloaded from the videolan homepage Complete VideoLAN Server installation instructions and user manual are available on http www videolan org doc The Apache web server does not require any special configuration Apache documentation can be found on www apache org It is recommended to include the Apache web server during the Debian Linux installation process The html interface is used as a menu and allows users to select a desired channel on the TV screen It is essentially a html document that contains special HTML tags Tag information and set top box setup instructions are included in the MediaBase Video Server amp Ericsson Set Top Box document written by Ulf Collovin It is available from Ericsson i Figure 3 3 PC to set top box streaming setup 3 1 3 Hardware Once the software proved to work correctly I proceeded to work on the reception cards I was fitted with two reception cards from Hauppauge one for satellite Digital Video Broadcasting DVB called NOVA s and another for terrestrial DVB called 14 NOVA t Both models were PCI cards None of the cards contained any MPEG 2 decoding hardware The boxes containing the cards stated that they were Plug amp Play models in Windows but needed adva
31. signal had more processing power and more buffer memory than the Ericsson set top boxes I used This might the reason it could withstand the discontinuities in the transport stream and decode the stream for more than 14 hours 26 References 1 Francois Fluckiger Understanding networked multimedia Prentice Hall 1995 2 Atul Puri and Tsuhan Chen Multimedia systems standards and networks Marcel Dekker 2000 3 Martin J Riley and Iain E G Richardson Digital video communications Artech House Publishers 1997 4 Tektronix A guide to MPEG fundamentals and protocol analysis 5 John Watkinson The art of digital video Focal Press 3 edition 2000 6 John Watkinson MPEG 2 Focal Press 2000 27 28 Appendix A dvbrc file The following pages contain an example configuration for both terrestrial and satel lite digital video broadcasting 29 Appendix B Transport stream error log The following pages contain the error log that was created while analyzing a trans port stream generated by the video server 30 Appendix C SocketReader Source code for SocketReader java 31
32. the PCR The MPEG 2 standard requires the PCR packets to arrive in at most 0 1 sec intervals to maintain synchronization Control and management information is contained within the transport stream Some of this information is used to group audio and video streams together The MPEG 2 standard defines a program as a number of elementary streams that share a common time base This information is periodically sent within the transport stream All required information concerning audio video and data streams are grouped together in Program Specific Information tables PSI PSI tables are struc tures that are linked together The four basic PSI tables adhere to a certain hierar chy The four tables defined by the MPEG 2 standard are the following e PAT Program Access Table e PMT Program Map Table e NIT Network Information Table e CAT Conditional Access Table The starting block is the Program Access Table which is always contained in trans port stream packets with the reserved PID value of 0 It provides the initial in formation on the programs contained in the transport stream The PAT contains entries for each program number and their corresponding PID value The packets corresponding to the PID values are in turn tables that contain program specific information These tables are called program map tables PAT PIDO Suem4 D 41 PMT 2 PID 50 Sueams D 60 PMT 1 PID 35 Figure
33. to the nature of the task The set top box communicates with the network through an html interface The html interface consists of a web page containing a list of available programs in ad dition to several special tags essential to the set top box The special tags contain information such as PID numbers for the audio and video streams By configuring an Apache web server on my PC I provided the set top box with the necessary communication to view the test transport stream By clicking on one of the links on the web page the set top box tuned itself to the parameters specified within the tags The VLS bases its streaming on a configuration file called vls cfg This file can be found at the end of this report The configuration file controls such parameters 13 as the multicast address the multicast port and the type of file being streamed Once configured VLS managed to stream the Ericsson promo transport stream con tained on the CD ROM The required configuration is displayed in figure 3 3 Server PC Switch Set top box and TV set 1 PC equipped with a network interface card 1 switch with multicast support 1 Ericsson set top box and a TV set VideoLAN Server downloadable from www videolan org Apache Web Server can be selected during Linux installation or installed through the apt MPEG 2 compliant Transport Stream Ericsson promo Transport Stream is available from Orjan Sahlin The VideoLAN Se
34. usage example 16 A dvbrc file contains amongst other things frequencies symbol rates and FEC codes In other words the initial dvbrc file must contain most of the information one would hope to obtain using a scan program A compliant and working dvbrc file can be found in Appendix A at the end of this report VLS expects the dvbrc file to lie in the root directory The only lines in the dvbrc file that need to be modified for additional channel reception are the TRANSPON DER rows The TRANSPONDER rows are required to have unique ID numbers the first TRANSPONDER row should have ID 001 the next ID 002 and so on It is of extreme importance that the TRANSPONDER rows refer to the same SAT ID number throughout the dvbrc file The values in the TRANSPONDER rows that should be modified for each transponder are FREQ POL SRATE and FEC All these values can be found on http www lyngsat com By selecting the desired satellite one can identify the four values in that satellite s frequency table FREQ is labelled Freq POL is labelled Tp SRATE is labelled SR and FEC is labelled FEC in the lyngsat tables Once I provided a correct dvbrc file to the scan application it managed to find the channels available at the frequencies I had added to the file The output of the scan program was a file almost identical to the input dvbrc file the difference being that it also contained the channels available at each frequency Now that I
35. yload is used in This can be seen in figure 2 3 Packet E M M Figure 2 3 Packets consists of a header and a payload Depending on the underlying transportation medium variable or fixed packet length is used This is due to the fact that certain problems arise when packets are sent along error prone communication systems Bit errors can damage packets seriously which can have grave consequences on the transmitted information Variable length packets are commonly used if the medium guarantees error free transmission If on the other hand packets are subject to various kinds of faults during transport it is useful to have packets of fixed sizes that are also relatively short 5mall package size means that little data is lost if a packet is corrupted 2 2 3 MPEG Streams In order to cope with various conditions the MPEG 2 standard defines two basic tools that support different delivery systems These are known as theprogram stream and the transport stream The program stream focuses on short distance delivery systems such as hard drives CD ROM s DVD s and other local media storage sys tems These environments have a very low error rate which makes it possible to use long data structures that are variable in length The transport stream is used over error prone systems such as local area networks The transport stream packets are short and fixed in si
36. ze in order to minimize consequences of packet loss and to be able to cope with network congestion My work done at Ericsson consisted of arranging a system that would deliver MPEG 2 data over an Ethernet network This text will focus on the transport stream since it is the technology used in environments that can cause bit errors 2 2 4 Transport Streams The transport stream offers the possibility of multiplexing several audio and video streams together It can also take an existing MPEG 2 multiplex and extract a single program or a collection of programs Remultiplexing a transport stream is also possible remultiplexing is the process of taking existing transport streams ex tracting some of their programs and rearranging them into a new transport stream One of the most important objectives of the transport stream is maintaining the synchronization between the audio and video during the multiplexing process This is made possible by adding information in the form of time stamps to the transport stream Control and management information are also embedded into the transport stream Unfortunately there is no error recovery system built into the transport stream The control and management information can merely inform the decoder that er rors have made their way into the system Error recovery can be handled by external systems such as network protocols by using information provided by the MPEG 2 transport stream Befor
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