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Sample essay topic, essay writing: Overview Of Video On Demand Systems - 2071 words
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.. he public. ( This paper would probably be caughtby such a publisher.) This auditing of compressed video information is not asstraight forward. A particular video stream can flow through an MPEG-2 encoderwithout incident while a second stream will bog-down the system (possiblyinducing errors). Rapidly changing backgrounds , like sports coverage can causeproblems.
The MPEG-2 standard is complex and requires more than just an astutesystems engineer to ensure that equipment designers of the encoders have notinterpreted the MPEG standard differently (from the decoder designers). HewlettPackard suggests that the industry needs to consider testability as a primaryrequirement of VOD systems. One way to resolve encoding concerns could be tocreate standardized test that carefully verify the implementation of the MPEGstandard. Bit error rate testers can test transport layers, traditional dataanalysis tools can also be used to build new test tools for MPEG. It should beno surprise that testability is the last area of standardization for the VODmarketplace.Summary Preparing video information for VOD archiving has reached a point thatdevelopers are able to concentrate on accelerating the compression phase. Thecompression techniques are relatively well documented
The industry is nowaddressing how to implement them faster; HW vs. SW, Digitizing Cameras vs. DSPcards. Most experts agree that even though today's workstations have theprocessing power to perform the MPEG compression it is usually more efficient toperform as much processing in HW (like dedicated video cards) as possible. Thisis not always the case in Multimedia applications where the end product (do toBW limitations) is not really Broadcast Quality . Quality of Imagery the userexpects is also a major consideration in selecting a content preparation element.If the user cannot take advantage of a hi-resolution 2k X 2k image; or if theBW of the distribution network is limited; then a hi-resolution MPEG-2 CODECmight not be justified. If the CODEC implements the 'Spatial scalabilty'capability of MPEG-2 then the encoder provides the video in a two part format.This lets low-resolution decoders extract the video signal and with additionalprocessing in more capable decoders, a high resolution picture can be provided.Video ServerRequirements Once the content is uploaded to the video server in the contentpreparation phase, and registered appropriately in the database, it becomesavailable for the end user.
In order for this data to be available and viewableby the end user the server should have at least a Raid 5 SCSI controller, 4GBHard Drives with 7200 RPM, and a high speed network interface. The servershould support MPEG-2 compression at 4.0 Mpbs to deliver approximately 28 hoursor 96 Hours of MPEG-1 compression of 30-fps, 640-by-480 pixel video on demandwhich equates to a minimum of 50 GB of Hard disk space. The server shouldemploy RAM in order to buffer the data being received from the disk drive toensure a smoother transfer of the video to the end user. A minimum of 256MB isrecommended. The server should be able to handle MPEG-2 and MPEG-1 in NTSC, PALor SECAM video formats and be able to meet broadcast and cable requirements foron-air program applications and video caching.Compression Method *Storage Required in Mb per 30 Second video clipStorage Required in Mb per 60 Second video clipTotal Capacity 52GB HDD HoldsMPEG-1 @ 1.2 Mbps367296.3 HoursMPEG-2 @ 4 Mbps12024028.8 Hours* Assumming the standard compression ratio per method type.Limitations There are several major limitations that must be addressed in order tounderstand why the above requirements are imposed. 1) Storage--There appears to currently be a storage limitation on videoservers because of retrieval and transmission time associated with video.Multiple servers will be needed to store and retrieve from large archives ofvideo information. These servers should be distributed remotely to maximizelocal retrieval and viewing while minimizing WAN traffic.
2) Data stream--in order to view video information with a minimum oflatency and without jitter the data stream needs to be constant anduninterrupted (with the exception of some buffering as necessary). There areseveral forms of buffering: a) Media stream storage on hard disk. b) cached at the transmit buffer c) network transit latency and buffers may be viewed as anotherbuffer. d) the receive end may buffer a sufficient amount of the mediastream to maintain a continuous stream for displayand suitable synchronization with the transmit end. 3) Concurrent users--The video server should be limited to 100concurrent users in order to ensure that each user is able to access therequested data as expeditiously as possible.
4) Network bandwidth size--The network needs to directly proportionalto the number of simultaneous video streams. The bandwidth of the system iseffectively limited by the bandwidth / transmission capabilities originating atthe server. 5) Latency--Although hard to determine, there should be no more than 2seconds for a video file retrieved locally and no more than 10 seconds for avideo file retrieved over the WAN from a remote site. 6) ODBMSProducts Several products that are currently being marketed as video servers are: 1) The Network Connection, M2V Video Server: a) 120 simultaneous 1.2 Mbps MPEG-1 video streams b) 112GB, RAID 5 storage. c) In excess of 200 Hours MPEG-1, and 60 Hours MPEG-2.
d) Supports JPEG, M-JPEG, DVI, AVS, AVI, Wavelte, Indeo andother video formats. e) Supports Ethernet, Token Ring, FDDI and ATM. 2) Micropolic Corp, AV Server: a) 16 Mpeg-2 Video Decoder Boards with 4 Channels per card is64 channels at 6Mbps per channel. b) 252GB, Raid storage. c) In excess of 120 hours MPEG-2 d) Supports only MPEG-2 3) Sun Microsystems, Media Center 1000E Video Server: a) 63GB, RAID4 storage.
b) In excess of 32 Hours MPEG-2, and 81 Hours MPEG-1 c) Supports MPEG-1 and MPEG-2 d) Supports ATM and Fast-EthernetDistribution Network: Video on Demand (VOD) requires predictability and continuity of trafficflow to ensure real-time flow of information. MPEG and MPEG-2 (as describedabove) require an effective BW of 1.5 - 4 Mbits/sec. Multiplying this 'mediastream' BW requirement by the number of clients will give a rough estimate ofthe effective distribution networks bandwidth. The Common Imagery Ground/SurfaceSystem (CIGSS) 1 Handbook suggests the following steps to size and specify theLAN technology use for Image dissemination systems: 1. Approximate the system usage profile by estimating the amounts ofimage, video and text handling that will be required. 2. Convert the amount of images, video and text to be processed intoaverage effective data rates. Raw data transferred directly toan archive ( our video server) and near real- time processed imagery should be estimated separately.
The bandwidth requirementscan be combined later if needed. 3. Adjust calculated rate for growth. The growth factor should be atleast 50%. 4. Add a fraction (about .3 to .4) of the peak capacity to the growthadjusted rate for interprocessor communications.Updating heritage networks to this new BW requirement can incur substantialcosts.
The cost of implementing a hi-speed network varies depending on thenetwork architecture.LAN Types Several LAN architectures are being used in 'trial' VOD systems. ATM,FDDI token ring and even variations of the Ethernet standard can provide therequired 10-100Mb/sec BW. A version of Ethernet called switched Ethernet can provide up to 10Mbpsto all clients. Since this is a switched architecture the full 10 Mbps can beavailable to each client. This architecture provides the quickest most costeffective method of upgrading legacy systems since it does not require upgradeof existing 10baseT wiring.
A voice grade Ethernet 100VG-AnyLAN can also beimplemented in a VOD system. This architecture however will require somecable upgrades from CAT 3 to CAT 5. Ethernet 100VG is expected to 'top-out' at100Mbps, no further upgrades are foreseen. Token ring networks have been implemented in a few VOD trail systems.FDDI can be setup to provide 100Mbps and because of the Token-ring architecture,the network can specify BW for each client. A simulated system, described inthe Sept '95 edition of Multimedia Systems would be capable of handling 60simultaneous MPEG-1 video streams.
The video server (486DX) not the 100-Mbit/sec token ring limited the system size. This is of course a small system,and due to the 'shared' nature of a token ring FDDI architecture , it shouldnot be implemented for larger (1000+) systems. ATM provides the highest BW and probably the most expensive networksolution. ATM provides the proper class of service for video on demandapplications. ATM connections running at OC3 rates (155Mbps) are currentlypriced at approx. $300-$500.
ATM is not a 'shared' topology. BW is notdependent on the number of users. In fact, as the number of users on an ATM netis increased, the effective BW of the ATM network increases. ATM can havehundreds of services operating simultaneously; voice, video, LAN and ISDN.These services can all be guaranteed, and assured that they won't interfere witheach other. The LAN marketplace is currently providing 155Mbps products.
Someof the ATM forum leaders (such as FORE systems) are also providing 622Mbps(OC12) network interface cards (NICs). The problem is that ATM is a relativelynew protocol. Several companies have come together to form the ATM Forum, tohelp standardize the architecture. For most network application software thecell-based ATM layer is not an appropriate interface. The ATM adaption layer(AAL) was designed to bridge the gap between the ATM layer and the applicationrequirements. The Forum's efforts have been very successful at the lower ATMadaptive layers but some interoperability issues still exist. The American ATMForum has standardized on ATM AAL 5 to map MPEG-2 for transport.
While theEuropean ETSI has chosen AAL2. These inconsistencies effect the transport ofmultimedia only through ATM LANS.Protocols There are several transport protocols that can be implemented for audio-video applications; TCP, UDP, SONET, TCP/IP Resource Reservation Protocol (RCVP)and IPX/SPX. Do to the effective data rate necessary to support VOD, protocolsthat minimize client/server interaction are preferable, except in cases where anover-abundance of network bandwidth exists. In ATM nets supporting mostly non-VOD applications retransmission of lost packets or corrupt packets will not bepossible. For example, if cells are lost the Fore Systems AVA Real-time DisplaySW uses pixel tiles from a previous frame. In a typical VOD system , withouterror correction, QOS is directly proportional to network/LAN BER (Bit ErrorRate). VOD systems which provide error correction as part of network protocolhave to be designed to allow for the latency created by their error correctingprotocols. (DSS currently implements interleaving, Reed Soloman and viterbidecoding) QOS trade-offs can be quantified and analyzed (see ' QOS control inGRAMS for ATM LAN', IEEE Journal of Selected Areas in Communications, by JosephHui) Networking, DBMS and server companies have been adopting upper layerprotocols to VOD processes. Oracle Media Net utilizes a 'sliding window'protocol.
Sliding Window protocol is a well established methodology forensuring transmission over lossy data links. Medianet monitors the responsebetween client and server, lengthens the response checking time to the point oferror and then backs off. (This process theoretically diminishes disruptivelatencies ) . Novell developed the Novell Embedded System Technology (NEST) andNetware to run over IPX/SPX protocols. The Novell implementation providesprioritization for video users. Flow control from the client to the server doesnot yet exist.
(Interoperability, 10/95).WAN Types Distributing VOD information outside the LAN requires either a very highbandwidth WAN with guaranteed availability, or substantial buffering and latencyallowances at the client in order to ensure and maintain a constand display ofdata. When many people think of information distribution over a WAN, sourced bymany different servers, to many isolated users; the Internet naturally comes tomind. The Internet was used by the National Information Infrastructure (NII)workshop as a model for the delivery of video services. This commercialorganization conference in addition to supporting HDTV and DSS , is interestedin providing VOD services to 'all Americans'. The Internet was seen as a goodfirst attempt for distributing information.
The Internet is inexpensive,requires no gatekeepers, provides search utilities and has several proven HumanMachine Interfaces (HMIs). Unfortunately the Internet is also bandwidth limited,provides insufficient: traffic control, security, directories and no guaranteeddelivery functions. The Internet may not be the solution to the VODdistribution problem, but it will expedite the development of an openarchitecture commercial VOD WAN. Commercial enterprises have been considering hybrid fiber/coaxial cableas one possible solution. This implementation also referred to as 'fiber to thecurb' requires a partial upgrade to existing telephone distributioninfrastructures. Signals are transmitted over fiber to a neighborhooddistribution (Gateway) point. The signals are then either converted to RF andsent to the User (home) via coax, or converted to a lower data rate networkInterface and sent onto the home. The RF implementation requires the 'Set-TopBox' for decoding the RF , The latter could be a PC implementation.
ISDN-Bthe broadband version of ISDN will probably evolve as the leading WAN technology.Narrowband ISDN is already an excepted method of providing the higher serialdata rates necessary for minimal quality multimedia applications, liketeleconferencing. True motion picture quality VOD implementations will requirethe Mbps data rates that should be provided by ISDN-B. The DOD has also been interested in the distribution of video andimagery across WANs. The Defense Airborne Reconnaissance Office (DARO) hasdeveloped the Common Imagery.
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