One of the most frequently asked questions about delivering online
video is "What's the difference between streaming video and downloading
video?" Let's face it, as a user clicking a video link on a web page,
you often won't know which method you're using, unless you poke around
a little. But streaming and downloading are distinct methods of
delivery, each with its own benefits and limitations. We'll take a look
at the difference between the two methods, and make some suggestions
about when you should choose one or the other for your projects.
Delivering From A Web Server:
Delivering your video file using a web server is sometimes referred to
as "progressive download" or "http streaming". In fact, it's not really
streaming at all, but a very simple bulk download of the video file to
the viewer's computer. Let's say you have a video file encoded at
200kbps. You place that file on your Web server, and put a link to the
file on your web page.
The web server does not know or care that it's a 200kbps video file. It
simply pushes the data out to the client as fast as it can. It may
appear to be streaming since playback can begin almost immediately. The
"progressive download" feature in most media players allows them to
begin playing the file as soon as enough data has been downloaded. Of
course, you can't fast-forward to the end of the file until the whole
file arrives from the server.
If the actual network bandwidth is smaller than the 200kbps that the
file is encoded at, then you may have to wait a while before you can
begin playing it. But even on a 56kbps connection, the video will look
great “ you're essentially trading waiting time for video quality.
The temporary file is saved to the user&qts computer, so they can re-play
it if they want to without having to download it again.
Web servers use HTTP (Hypertext Transport Protocol) to transfer files
over the network. One of the features of HTTP is that it operates on
top of TCP (Transport Control Protocol), which controls the actual
transport of data packets over the network. TCP is optimized for
guaranteed delivery of data, regardless of its format or size. For
example, if your browser or media player realizes that it&qts missing a
data packet from the server, it will request a resend of that packet.
Resend requests take time, take up more bandwidth, and can increase the
load on the server. And if the network connection is sketchy, you could
begin to use more bandwidth for resends than you&qtre using for the video
itself! TCP is not designed for efficient real time delivery or careful
bandwidth control, but for accurate and reliable delivery of every bit.
Delivering From a Streaming Server:
A streaming media server is a specialized piece of software that
accepts requests for video files, knows about the format, bandwidth,
and structure of those files, and in many cases, pays attention to the
performance of the player that's receiving the video. Streaming servers
deliver just the amount of data necessary to play the video, at
precisely the rate needed to play it.
Unlike the web server, which simply starts dumping as much video data
onto the network as it can, the streaming server opens a conversation
with the media player. There are two sides to this conversation“ one
to transfer the video and one for control messages between the player
and the server. Because they continue to exchange these control
messages with the player, streaming servers can adjust to changing
network conditions as the video plays, improving the viewing
experience. The control messages also include user actions like play,
pause, stop, and seeking to a particular part of the file. Since the
server sends video data only as it's needed and at just the rate it's
needed, it also allows you to have precise control over the number of
streams you serve and the maximum bandwidth you consume.
If you've got a 56kbps connection to the network, you won't be able to
receive that 200kbps video. You'll have to settle for a lower-quality
version that's encoded for 56kbps connections. But streaming delivery
of video data does have some advantages:
- You can skip ahead in a video, or begin playback at a point somewhere
in the middle. This is a convenience to users, but also a boon to you
as a provider. It enables interactive applications like video search
and personalized playlists.
- It lets you monitor exactly what people are watching and for how long
they are watching it.
- It makes more efficient use of bandwidth since only the part of the
file that's watched gets transferred.
- The video file is not stored on the viewer&qts computer. The video data
is played and then discarded by the media player, so you maintain more
control over your content. In a pinch, streaming servers can use HTTP and TCP to deliver video
streams, but by default they use protocols more suited to streaming,
such as RTSP (Real Time Streaming Protocol) and UDP (User Datagram
Protocol). RTSP provides built-in support for the control messages and
other features of streaming servers. UDP is a lightweight protocol that
saves bandwidth by introducing less overhead than other protocols. It's
more concerned with continuous delivery than with being 100% accurate “ a feature that makes it well-suited to real time operations like
streaming. Unlike TCP, it doesn't request resends of missing packets.
With UDP, if a packet gets dropped on the way from the server to the
player, the server just keeps sending data. The idea behind UDP is that
it&qts better to have a momentary glitch in the audio or video than to
stop everything and wait for the missing data to arrive.
Finally, a streaming server is necessary to deliver live webcasts and
to use multicast. For networks that support it, multicast allows more
than one client to tune in to a single stream, saving bandwidth at
every part of the delivery chain.
Conclusions:
All of this adds up to a few simple rules of thumb for when to use
streaming and when to use http downloading to deliver your video. The
main reason for downloading video from your Web server is that it's
simple and you can do it with infrastructure you already have. It's
most useful when your videos are short, when you're more interested in
delivering high-bitrate encodings than in delivering in real time, or
when you want your viewers to be able to keep a copy of the video on
their own computers.
Streaming is the better solution when your clips are more than a few
minutes long, when you want to enable interactive applications like
video search or linking deep into a file, or you want to collect
statistics on what's actually being watched. Streaming is the way to go
when you want to control the impact of video on your network, or when
you need to support large numbers of viewers. And of course, it's the
only way to do live Webcasts and multicasting.
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