What's the difference between streaming and downloading a video, and how does it impact the limits my ISP imposes?

Let's start with the terminology: what's it mean to "stream" versus "download"?

Download you're probably already familiar with: it's simply a file copy - nothing more, nothing less. When you download a file you're just making a copy of that file which is stored on a server somewhere on your local machine.

A stream is a different concept; there is no file. A stream is simply a that: a stream of data, 1's and 0's, that are being sent from some server to your machine.

A bottle of water might be comparable to a download - you can move it around, you can put it in different places, use it when you like. A hose with water pouring out of it is more comparable to a stream - you have to use the water as it come out of the hose, or it's gone.

The Download Approach: YouTube

YouTube actually uses a download model to provide you with videos.

When you first begin to watch a video, the player begins to download it - it quite literally copies the file from the YouTube servers to your internet browser cache. The "trick", if you want to call it that, is that it begins playing the file before the download is done. That can make it seem like it's streaming, but it's not.

You can even see this in action by watching the YouTube progress bar:

You can see that the video has begun to play, even though it's not fully downloaded. If the player ever catches up to the download progress point, then the playback will pause or stutter - the download's not fast enough to keep up.

If you pause, or if you have a connection that's fast enough, eventually you'll reach this state:

At this point, the file has been fully downloaded.

If you replay the file without leaving the page, the file does not need to be downloaded again. Each replay simply reads the file from the browser cache again to display the video.

If you do something else - say you watch a few other videos - and then come back to this video it may still be in the cache, and thus may not need to be downloaded again.

On the other hand, you might have viewed enough other content that the browser needed to delete older content - such as your video - to make room. As a result, your video might not be in the cache, and thus might need to be downloaded again.

Ultimately, whether or not a video you've previously viewed needs to be downloaded depends on these three things:

• the size of your browser cache

• the amount of browsing you've done since the last time you viewed the video

• whether or not the video was updated on the server (this can also force a download)

Of course you can tell what's happening by watching the YouTube progress bar.

The Streaming Approach: Hulu

All that is complex in comparison to the very simple concept of a streaming video, which gives you no choice. Each time you watch such a video it's streamed directly from the server to the video player without being cached at all. There is no separate "download progress" indicator, since there's no separate download. If you restart the video, it restarts the stream back at the beginning.

I believe videos at Hulu.com are streams.

Because internet connections can be inconsistent, streaming video players often include some kind of buffering, where they actually do receive a certain amount of the stream before they actually begin playing it, or in response to pausing the video they continue to receive it to fill that buffer:

Change your position in a streaming video, and the buffer is invalidated; the stream must restart at the new position you've selected.

Managing your bandwidth and data transfer limits.

Everything you watch, every time you watch it, using a streaming video, counts towards any data transfer limits imposed by your ISP. Watch a 100 megabyte movie 10 times, and you've just eaten up a gigabyte of your allotment.

What counts against your allotment when watching a download-style video depends on whether or not it's already been downloaded and in your browser's cache. If it's already there, subsequent viewings don't count. If it needs to be re-downloaded, then that data transfer does count.

There are tools that in some cases can help, but they may skirt or even cross the line on some legal issues.

For example, there is software that will allow you to capture streaming video into a file that you can then keep and replay from your hard drive as often as you like without needing to re-download. How successful this is really depends on many factors including the speed and capabilities of your PC and the speed of your internet connection. Screen recording tools like Camtasia may work, and there are dedicated utilities such as the various tools from Replay and others.

For YouTube specifically there are many results returned when you search for "download YouTube" that will place the downloaded file not in the browser's cache somewhere, but in a location that you can specify and then keep. Unfortunately, this type of download is not officially sanctioned by YouTube, and thus these tools often break as YouTube makes changes to their technologies.

The bottom line is that video can be large, and regardless of whether or not you actually have the ability to control it, it's important to understand exactly what's being downloaded or streamed, particularly if your ISP is imposing data transfer limits.