Net Data Design, LLC Blog

Mentioned in my previous two posts, Smooth Streaming  is Microsoft’s completion of HTTP-based adaptive streaming, which is a truly a hybrid media delivery method. It may act like streaming, but it is in fact based on HTTP progressive download. The HTTP downloads are performed in a series of small chunks, allowing the media to get easily and cheaply cached along the edge of the network, closer to the end users. The Expression Encoder provides multiple encoded bitrates of the same media source also allows Silverlight clients to seamlessly and dynamically switch between bitrates depending on network conditions and CPU power. The end users experience is one of reliable, consistent playback without stutter, buffering or “last mile” pipe-line clog.

A Microsoft media format and no ASF? That’s right. Why MP4 and not ASF? Well, here are a few reasons:
•    MP4 is a lighter container and uses less overhead.
•    MP4 are easier to “digest” by .NET.
•    MP4 is more widely supported.
•    MP4 was architected with H.264 video codec support, although ASF can be as well, it’s not as native.
•    MP4 was designed to natively support payload fragmentation

There are actually two parts to the Smooth Streaming format: the wire format and the disk file format. In Smooth Streaming, a video is recorded in full length to the disk as a single file (one file per encoded bit rate), but it’s transferred to the client as a series of small file chunks. The wire format defines the structure of the chunks that are sent by IIS to the client, whereas the file format defines the structure of the contiguous file on disk, enabling better file management. Fortunately, the MP4 specification allows MP4 to be internally organized as a series of fragments, which means that in Smooth Streaming the wire format is a direct subset of the file format.

So exactly what are these “fragments”? The basic unit of an MP4 file is called a “box.” These boxes can contain both data and metadata. The MP4 specification allows for various ways to organize data and metadata boxes within a file. In most media scenarios, it’s considered useful to have the metadata written before the data so that a player client application can have more information about the video/audio that it’s about to play before it plays it. However, in live streaming scenarios it’s often not possible to write the metadata up-front about the whole data stream because it’s simply not yet fully known.

The following figure is a high-level overview of what a Smooth Streaming file looks like on the inside:

In a nutshell, the file starts with file-level metadata (‘moov‘) which generically describes the file, but the bulk of the payload is actually contained in the fragment boxes which also carry more accurate fragment-level metadata (‘moof‘) and media data (‘mdat‘). (The diagram only shows 2 fragments, but a typical Smooth Streaming file has a fragment per every 2 seconds of video/audio.) Closing the file is a ‘mfra‘ index box which allows easy and accurate seeking within the file.
When a Silverlight client requests a video time slice from the IIS Smooth Streaming server, the server simply seeks to the approriate starting fragment in the MP4 file and then lifts the fragment out of the file and sends it over the wire to the client. This is why we refer to the fragments as the “wire format.” This technique greatly enhances the efficiency of the IIS server because it requires no remuxing or rewriting overhead.
Here is what an MP4 fragment looks like in more detail:

We say that the Smooth Streaming format is based on the MP4 file format because even though we’re following the ISO specification, we specify our own box organization schema and some custom boxes. In order to differentiate Smooth Streaming files from “vanilla” MP4 files, we use new file extensions: *.ismv (video+audio) and *.isma  (audio only). I keep forgetting to ask the IIS Media team what the acronyms exactly stand for, but my best guess would be “IIS Smooth Streaming Media Video (Audio)”.

Smooth Streaming Media Assets

A typical Smooth Streaming media asset therefore consists of the following files:

• MP4 files containing video/audio
  • *.ismv – contains video and audio, or only video
    • 1 ISMV file per encoded video bitrate
  • *.isma – contains only audio
    • In videos with audio, the audio track can be muxed into an ISMV file instead of a separate ISMA file
• Server manifest file
  • *.ism
  • Describes the relationships between media tracks, bitrates and files on disk
  • Only used by the IIS Smooth Streaming server – not by client
• Client manifest file
  • *.ismc
  • Describes to the client the available streams, codecs used, bitrates encoded, video resolutions, markers, captions, etc.
  • It’s the first file delivered to the client

Both manifest file formats are based on XML. The server manifest file format is based specifically on the SMIL 2.0 XML format specification.

A folder containing a single Smooth Streaming media asset might look something like this:

A folder containing a Smooth Streaming media assets

In this particular case the audio track is contained in the NBA_3000000.ismv file.

Smooth Streaming Manifest Files

The Smooth Streaming Wire/File Format specification defines the manifest XML language as well as the MP4 box structure. Because the manifests are based on XML they are highly extensible. Among the features already included in the current Smooth Streaming format specification is support for:

• VC-1, WMA, H.264 and AAC codecs
• Text streams
• Multi-language audio tracks
• Alternate video and audio tracks (i.e. multiple camera angles, director’s commentary, etc.)
• Multiple hardware profiles (i.e. same bitrates targeted at different playback devices)
• Script commands, markers/chapters, captions
• Client manifest Gzip compression
• URL obfuscation
• Live encoding and streaming

Smooth Streaming Playback: Bringing It All Home

Microsoft’s adaptive streaming prototype (used for NBC Olympics 2008) relied on physically chopping up long video files into small file chunks. In order to retrieve the chunks for the web server, the player client simply needed to download files in a logical sequence: 00001.vid, 00002.vid, 00003.vid, etc.

As I’ve explained in this and previous posts, Smooth Streaming uses a more sophisticated file format and server design. The videos are no longer split up into thousands of file chunks, but are instead “virtually” split up into fragments (typically 1 fragment per video GOP) and stored within a single contiguous MP4 file. This implies two significant changes in server and client design too:

1. The server must be able to translate URL requests into exact byte range offsets within the MP4 file, and
2. The client can request chunks in a more developer-friendly manner, such as by timecode instead of by index number

The first thing a Silverlight client requests from the Smooth Streaming server is the *.ismc client manifest. The manifest tells it which codecs were used  to compress the content (so that the Silverlight runtime can initialize the correct decoder and build the playback pipeline), which bitrates and resolutions are available, and a list of all the available chunks and either their start times or durations.

With IIS7 Smooth Streaming, a client is expected to request fragments in the form of RESTful URLs:

http://video.foo.com/NBA.ism/QualityLevels(400000)/Fragments(video=610275114)

http://video.foo.com/NBA.ism/QualityLevels(64000)/Fragments(audio=631931065)

The values passed in the URL represent encoded bitrate (i.e. 400000) and the fragment start offset (i.e. 610275114) expressed in an agreed-upon time unit (usually 100 ns). These values are known from the client manifest.

Upon receiving a request like this, the IIS7 Smooth Streaming component looks up the quality level (bitrate) in the corresponding *.ism server manifest and maps it to a physical *.ismv or *.isma file on disk. It then goes and reads the appropriate MP4 file, and based on its ‘tfra’ index box figures out which fragment box (‘moof’ + ‘mdat’) corresponds to the requested start time offset. It then extracts the said fragment box and sends it over the wire to the client as a standalone file. This is a particularly important part of the overall design because the sent fragment/file can now be automatically cached further down the network, potentially saving the origin server from sending the same fragment/file again to another client requesting the same RESTful URL.

As you can see, requesting chunks of video/audio from the server is easy. But what about dynamic bitrate switching that makes adaptive streaming so effective? This part of the Smooth Streaming experience is implemented entirely in client-side Silverlight application code – the server plays no part in the bitrate switching process. The client-side code looks at chunk download times, buffer fullness, rendered frame rates, and other factors – and based on them decides when to request higher or lower bitrates from the server. Remember, if during the encoding process we ensure that all bitrates of the same source are perfectly frame aligned (same length GOPs, no dropped frames), then switching between bitrates is completely seamless – and Smooth.

In the last installment of this series I covered the basic tools and theories behind delivering great streaming content to your audience without breaking the bank. Either out of pocket or in bandwidth, so lets talk about Bit Rate Throttling to begin with.

Imagine this – a client connects to your server, clicks on your featured video, watches 5 seconds of it to realize they have no interest in watching further, and move on to the next video.

In those 5 seconds, the server could have sent out 5 minutes worth of the video, and you paid for 5 minutes worth of bandwidth! With the Bit Rate Throtter + media bitrate detection, the server would only end up sending a little over 5 seconds worth, and you would end up paying only for what was used.

Bit Rate Throttling (BRT) is a module plug-in for IIS 7 0r IIS 7.5. It’s an extension of the IIS Media Service. The IIS7 Bit Rate Throttling module was first announced at the National Association of Broadcasters (NAB) show last April as part of the TCO advantage for Silverlight.  Enabling Web masters to throttle the delivery of any file based on the file type. It also has additional functions for digital audio/video files. Throttling can be set at the following levels within Internet Information Services (IIS) Manager: server, site, virtual directory, and file.

BRT contains the following features:

• Fast Start – the ability to send the first part of the media file without rate limiting, to seed the playback buffer in the player and make sure that playback can begin as soon as possible (most players try to pre-buffer a certain amount of the video, often 5 seconds, before starting playback). This also insures that if the connection suffers a hiccup, the playback can continue uninterrupted.
• Disconnect detection - when the client stops watching the video, goes to another page, or closes the video, the BRT detects the connection closure and stops sending the file.
• Built-in support for detecting the playback rate for common media formats, including .asf, .avi, .flv, .m4v, .mov, .mp3, .mp4, .rm, .rmvb, .wma, and .wmv.
• Ability to configure static throttling rates, and media auto-detection rates at any configuration level.

It should also be noted you can configure the BRT module to handle static files as well, such as a large [progressive] .jpg file. So, if you feel you need to trim the bandwidth, Bit Rate Throttling module is a great place to start.

In the next segment I’ll focus on the Smooth Streaming IIS 7+ features and capabilities.

At first glance Silverlight seems like just another platform for creating interactive website, desktop and mobile applications. Much like Adobe’s Flash or AIR. but there is a lot more to it. As I’ve mentioned before, I’m all about giving my clients a product that suites them best even though it may be outside of my “comfort zone.”

I have a client (to be named later) that had a need to deliver high-quality videos without breaking the budget. By budget I mean in terms of bandwidth, encoding, and preparation. We also host this clients website which was recently moved to a new server running Windows 2008 R2, with IIS 7.5. So in looking for a solution I came upon Silverlight, which is a natural fit with IIS 7.5’s  Media Services and Smooth Streaming.

To make this scenario work, you’ll need Microsoft’s Expression Web 3 or Microsoft’s Expression Studio 3. The Expression Encoder 3 is bundled with those packages with sell from (estimated) $150 to $500 dollars. Both packages offer a wide array of tools that will help in your video encoding as well as many other tasks such as website development and desktop applications.

With Expression Web 3 alone, you be able to code in PHP, HTML/XHTML, CSS, JavaScript, ASP.NET, ASP.NET AJAX, all using standards-based code (yes, I checked), however Expression Encoder is the key. It’s not only easy to navigate and use, but allows you to drop in QuickTime, AVI, MPEG, and other formats into the encoder, and utilize the latest quality and speed improvements to the VC-1 or H.264 codecs, choose a Silverlight template (or make your own), and publish directly to a web server using Expression Encoder Publishing Plug-in. It should be noted that the video can also be published locally and viewed.

Smooth Streaming is the production version of a technology first used by Microsoft to deliver on-demand video of the 2008 Summer Olympics for NBCOlympics.com. By dynamically monitoring both local bandwidth and video rendering performance, Smooth Streaming optimizes playback of content by switching video quality in real-time.

Using Smooth Streaming, visitors with high bandwidth connections and modern computers can experience full HD quality streaming, while others with lower bandwidth or older computers receive the appropriate stream for their capabilities. The result is visitors across the board enjoy a compelling, uninterrupted streaming experience, which ultimately lead to extended visitor stays and increased revenue opportunities.

In my next installment of this topic, I’ll cover the nuts and bolts of Bit Rate Throttling and Smooth Streaming.

There are times when a product that you use, like, program for and have become very familiar with begins to lose it’s luster. I’m speaking of a very well known CMS, who will at this point remain nameless. We’ll just refer to it as BrandX for now. That time comes when you have this “feeling” and its confirmed by a long-time good client. It’s at that point you know that ignoring it won’t make it go away.

Over the past year or so I have become more familiar with and almost comfortable with the LAMP stack (Linux, Apache, MySQL and PHP.) I have been a die-hard Microsoft developer and admit that my strength is in the IIS, MSSQL, and .NET stack. All developer have they’re comfort zone, but I digress. Lets talk Joomla!.

Lets start with the test server. It’s VM running Ubuntu 9.10 with the LAMP stack installed. A freshly downloaded copy of Joomla! 1.5 in place and the installation begins. Here is where some other CMS’s can take a lesson (including BrandX)! The installation was very straight forward and took literally a minute to complete! A few minor configuration settings,a database name, user and presto, your in business.

That was the easy part. Now what to do in this new interface? A new world compared to BrandX! As with any “new” product a learning curve is a certainty, but Joomla! is truly a whole new world. I’ll write back with my finding and I look forward tou your comments.