In both cases the ad servers are linked to the streaming servers to generate the correct URLs. The ad servers often use solicited cookies to track users and ensure that targeted advertisements are used where possible. This means that a webcast, although streamed to a wide geographic area, can carry advertising that is local or relevant to the viewer. Playlists If you are streaming regular webcasts you will need a means of playing out clips to a prepared schedule. Just like a television station, clips can be put in playlists and streamed on cue. This means that corporate presentations and distance 224 The Technology of Video and Audio Streaming Disk storage array Load balance Streaming servers Stats & logging router control LAN Internet Figure 11.10 On-demand server farm.
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client playlist cannot be changed once received, whereas the server-side allows dynamic changes to the playlist; for example, targeted ads. Windows Media use active server pages to generate dynamic playlists. RealServer can use a proprietary ad tag in the SMIL file ( RealAdInsert/ ) to dynamically assign URLs to the advertisement clip. Since the advertisements may be changing hour to hour, this avoids the necessity to keep updating the SMIL file. Stream serving 223 LAN remote clients East coast office LAN remote clients South coast plant LAN 500 clients West coast HQ media server VPN VPN single stream single stream encoder live presentation transmitter router media server media server Media players Figure 11.9 Splitting a live stream at remote sites.
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RealNetworks supports a form of multicasting with a control back-channel. This allows full statistics to be gathered from the clients, but has the advantage of multicasting the media data. It is best suited to small audiences; a very large multicast would have problems with the server capacity required to handle the unicast control traffic. On-demand serving On-demand serving is more like running a web server. The viewers choose their own content, and then a fast disk storage array delivers the content, as required, to the streaming servers. Each client has a unicast connection with the server, so the more viewers, the higher the server loading. A popular site will use many servers in parallel. The Internet traffic loading can be balanced across all the servers. The server hardware does not need many facilities: a fast CPU, plenty of RAM, and at least two network-interface cards (NICs). If the server has very high loadings or there are network problems, you will need access by a separate network port for control messages, so always install at least two NICs. The system will be more reliable if the load is spread over several small servers, rather than one large multiple processor server. This also gives redundancy against hardware failure. Inserting advertisements If you are running a commercial site you will want to add advertisements to the content. They can be the same banner ads used on web pages. The alternative is to insert clips into the stream just like a regular television spot. This is called interstitial advertising. The simplest way to place video advertising around an on-demand clip is to top-and-tail the content with preroll (gateway) and postroll ads (bumper) using an SMIL file (Synchronized Multimedia Integration Language) to play the clips serially. SMIL has a time container that can be programmed with a fixed sequence of media elements. The sequence command is used to place ads before and after clips. The player will run the playlist of content and advertisements as programmed. The viewer cannot step through the playlist manually to jump over the ads. An associated SMIL element, parallel, commands the group within the time container to run together. This ensures that the following clip is correctly prerolled to avoid any glitches as the playlist is running. Windows Media Services offers two ways to deliver advertising and other interstitial material: with either a client playlist or the server-side playlist. The 222 The Technology of Video and Audio Streaming
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Multicasting If you are webcasting live to a large audience, a regular unicast establishes a separate network connection to each client. This uses considerable network resources, even though each client is watching exactly the same material. Multicasting offers a solution by serving a single stream. This is then routed to all the clients. So the network routers are doing the distribution rather than the streaming servers. The drawback with multicasting is that you need to have control over the routers. First they have to be multicast enabled, which is not necessarily the case. There are different algorithms for setting up multicast routes: dense and sparse. If you are streaming over a corporate network within a single domain, multicasting has much to offer. It is when you want to multicast to the public Internet that problems can arise. The network of peer-to-peer connections that link a server to a client may not be multicast enabled, or the routers may not be set up to multicast across domains. If you are multicasting you cannot use automatic rate changing. The server transmits a single stream and is not aware of the clients, so it cannot negotiate to serve at a certain rate. To get around this you may have to transmit three or four different rate streams from different server ports. The player connects to an appropriate port for the bandwidth available at their local end. Multicast network If you are setting up a multicast to several sites, and want to use Virtual Private Networks (VPN) for the intermediate connections across the corporate WAN, note that IPSec does not support multicasting. The way around this is to unicast to a splitter server at the remote site, then multicast locally. You will need to make sure that clients can see only one multicast server. Since the same IP address is used for the multicast, the client potentially could receive several duplicate packets. The player will not decode the stream correctly in these circumstances. Announcing a multicast is different from retrieving on-demand content. With on-demand, the browser requests the media file. When the file is retrieved, the header carries information necessary for the player configuration. Once you join a multicast, there is no header. Instead, a small message file gives the browser/ player the necessary information (like the port number to use). This message can use Session Description Protocol (SDP RFC.2327); Microsoft uses the media station NSC file. The media station is analogous to a television station, so the station represents the channel and not the media streamed (programs transmitted) over the channel. The NSC configuration file will set up the player correctly to receive a multicast. The ASX file that announces a multicast points the player to the NSC file. Stream serving 221
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If only a few clients were watching at the remote sites, the streams could be delivered directly from the server at headquarters. But each client will be using bandwidth on the trunk networks, all carrying the same data. If a large number of clients at each site are watching the webcast, the server will be unable to handle the load. The alternative is to use local servers at the remote sites to receive a single stream, and then locally distribute it. This saves bandwidth on the long-distance circuits and reduces the load on the originating server. You may be thinking that this is a really important broadcast, what happens if the link goes down? Companies like RealNetworks have come up with a number of solutions. RealNetwork s Helix Universal Server includes the facility to set up backup links. You set up redundant transmitters, usually two servers in parallel. If the link from the designated transmitter breaks, the receiving server automatically falls back to the next available transmitting server. The clients will see the webcast has stopped; if they refresh the link, the stream will continue using the backup path. 220 The Technology of Video and Audio Streaming Disk storage array Live encoder Streaming server Stats & logging router live content pre-recorded content control LAN Server-side playlist Internet Figure 11.8 Live and simulated-live webcasting.
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serve to a large number of clients, extra servers have to be used. For ondemand serving, they can be added in parallel, but for live streams a different architecture is used to save network resources. A relay server splits the live streams downstream, so what starts as a single live stream from the encoder fans out like the branches of a tree. Take the example of a CEO s webcast from the headquarters on the west coast to two other remote sites on the south and east coasts. Stream serving 219 web page web server pointer link to metafile HYPERLINK starts player or plugin player requests media clip SUCCESS! http://address/filename.ram pointer metafile or redirector rtsp rtp http http streaming server plug -in plug -in plug -in clip 1 2 3 4 5 6 rtsp://address/clipname.rm Figure 11.7 Linking to streaming media.
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Announcing the content The player usually locates streaming media by a hyperlink embedded in a web page. This link contains not only the URL for the content, but also the instructions to start the media player. Web links The usual way to announce streaming media files is by a hyperlink in a web page. The link points to a small file on the web server. Windows Media calls this the stream redirector, or ASX. Real uses the RealAudio Metafile or Ram file. Once the browser receives this file, using the MIME type, the metafile is passed to a streaming plug-in. The metafile has the full address and filename for the streaming content. The media player than transmits a request to the specified streaming server for the media file. This may use MMS or RTSP for communication rather than the HTTP used with the web server. If all goes well the correct clip is streamed to the player success. The metafiles can list several files to be played in sequence. SMIL If you are streaming rich media, then a number of different clips and images have to be synchronized for correct playback at the client. One way to do this is to use the Synchronized Multimedia Integration Language (SMIL) to write a control file. SMIL is supported by the QuickTime, Real, and Windows Media architectures. Webcasting Webcasting can be live, prerecorded, or a mix of both. A webcast at its simplest can be just a single clip. If you want to play out a sequence of clips you can set up a playlist. Even if you are streaming a presentation, you may want an introductory promotional film, and possibly some follow-up material after the main presentation. The playlist controls the server to play the relevant clips at the specified time. Splitting and relays A streaming server can handle several hundred simultaneous clients. The only real way to establish how many clients would be by conducting load tests. To 218 The Technology of Video and Audio Streaming
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MPEG-4 and scalable streams The MPEG team has proposed a different way to cope with variable network bandwidth. The server transmits a basic low-resolution stream. Additional helper streams can carry more detail. If the bandwidth is available then these extra streams allow a better quality picture to be assembled by the player. MPEG-4 also supports scalable encoding. This means that a basic player may decode only part of the stream to create the video, albeit at a lower quality than a more complex player, which can decode and display all the stream information. Loading content Whether you are using a managed service or doing your own serving, the first step is to deliver your content to the streaming servers. The encoding probably takes place near the video editing facility or, for a live webcast, at the venue. The servers have to be located close to an Internet backbone, unless you are streaming only over a local area. So in all probability the encoder and server are separated geographically. The simplest way to deliver the content is via a file transfer, using FTP. Some encoding systems have the ability to transfer a file automatically, immediately after the encoding has finished. Live streaming The file can, of course, be sent on a CD-ROM. If the content is a live broadcast, then neither of these methods is suitable; it has to be streamed. This is covered in the chapter on live webcasts. The media encoder typically connects to the server using TCP for a bidirectional control link and a unidirectional media stream, using UDP. It is very important that the circuit used for this connection has more than sufficient bandwidth and a high QoS; that means low packet loss and timing jitter. Any data loss or corruption will be visible by all receivers of the webcast. This generally means using an uncontended circuit like T-1/E-1, rather than a domestic circuit like ADSL or a cable modem. Stream serving 217 control (TCP) origin server to players media encoder media data (UDP) Figure 11.6 Encoder connections. 1
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RealNetworks SureStream SureStream allows different encoding rates to be combined into a single media file. The streaming server will choose the appropriate data rate for the prevailing conditions by negotiating with the player. The lowest rate, called a duress stream, is a backstop, and will be streamed if congestion is very bad. When you are calculating the server disk space, a SureStream file takes the space of the sum of all the components. Because only the Helix Server can extract the correct component, a web server will serve the file in its entirety, including all the different rates. Windows Media Intelligent Streaming This is a similar feature, allowing multiple constant bit rate streams to be encoded and wrapped in a single file. The streaming server will then stream the best rate for the current network conditions. Windows Media Encoder comes with predefined multiple bit rate profiles, but the profiles also can be customized to suit your special requirements. If you want to multicast a file that has been coded at multiple bit rates, only the highest rate will be transmitted. Drawbacks If you want to serve at different rates, then better results can be obtained by encoding a small picture at low frame rates for low bit rate streams, and a larger picture for streams at higher rates. So you may want a 480 360 pixel frame at 30 fps for a 1 Mbits/s stream and 160 120 at 6 fps for an ISDN link. Multiple bit rate encoding has to use the same picture size, so that the player can switch seamlessly between the different rates. The automatic rate-shifters have their uses, but are not a complete answer to serving the same content over very different networks. The big advantage is that the process of changing bit rate is invisible to the user. QuickTime and alternate movies This is a less sophisticated method of offering different bit rates. You can encode a movie as several separate movies encoded at different bit rates, and maybe with different language audio tracks. These are the alternates: The player follows a pointer to the master movie, which then references the alternate movies. The player negotiates with the server to request the correct alternate file for the player settings. 216 The Technology of Video and Audio Streaming
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Adapting to network congestion Both RealNetworks and Windows Media offer a way of changing the bit rate of a stream as network congestion varies. To get the best viewing experience we want to stream at the highest rate possible. But if the network slows down, rather than attempting to continue with a high bit rate, it makes sense to throttle back the bit rate. If the congestion eases then the bit rate can revert to a higher level. That way the viewer is not subject to stalling streams, just a graceful degradation in quality. These technologies work only with unicasting. The Real-Time Protocol maintains the correct delivery rate over UDP/IP (or TCP/IP if bandwidth permits). The RTSP framework supports the client interaction with the stream, the VCR-like controls Play, Pause, and so on. The streaming server application can use RTCP reports from the player to measure network congestion and switch stream rates for multiple bit rate media files. The player can report lost and delayed packets, and the reception of out-ofsequence packets. Stream serving 215 lowbandwidth highbandwidth select encoded bandwidth 40k SETUP PLAY PAUSE TEARDOWN time reference packetizer media data player commands player reports RTCP RTSP media RTP file parse Media files 100k 500k Figure 11.5 Streaming control.
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