As you may have read in several previous articles, the Anchor
Tag is very versatile. It can be used to link to a new page, open a
mail program or execute a Script. In fact, the anchor tag can be
used to link to any file on your web site. I have posted links to
the latest version of IMS Web Dwarf and to Keith Parkinson's
picture of the Dwarf iss47fig1.htm
But so what? Links are used everywhere!
Yes they are but if you look at these two you will see that the
first is a link to an executable ".exe" file and the second is a link
to a JPEG ".jpg" file. You will also notice that the link to the
".exe" file opens a dialog to start downloading the file to your
computer. The second link displays the JPEG file in your
browser.
Why do they do that? Neither file is a HTML document. Even
the Executable file is not executed but downloaded.
The reason is based in the World Wide Web itself and how web
content is delivered to your computer from a server. Many
people tend to confuse the Internet with the World Wide Web.
The Internet has been around for over 25 years as a means to
connect computer systems together. Initially, the majority of
information on the Internet was transferred as files and later
through newsgroups and e-mail.
It was not until the invention of the World Wide Web by Tim
Berners-Lee in 1990, only 10 years ago, that Web Pages and
Web Sites became possible. The WWW provides a protocol by
which information can be located, transferred and processed.
HTTP allows HTML web pages to be sent so that a Browser can
receive and interpret the data. But the WWW is not restricted to
HTML.
A question I often receive asks how to add multi media files to a
web site. In many instances this is very simple. A multi-media
file can be transferred in exactly the same way as a HTML,
JPEG or GIF file. Simply create a HTTP link to the file and
away you go. Try this 315 kb avi file: slogo.avi
Did it work? For most of you it probably did but for a few of you
it may not. The reason is that the WWW provides the framework
by which files are transferred and processed but it does not
provide the application to actually process the file. I am sure that
most of you will understand that you need a Browser in order to
view Web Pages. HTTP provides the mechanism to transfer the
HTML file but it is the Browser that will interpret it. The same is
true of the link to the above "avi".
Browsers include a lot of applications that many of us take for
granted. It is Obvious that the browser will interpret the HTML
and display the text. It will also display GIF, JPEG and many
other image file formats. Audio files however, are not officially
supported by all Browsers. Internet Explorer will recognize and
process Wave and Midi files through the BGSOUND tag but
Netscape will not. In order for Netscape to play audio it must use
a 3rd party Plug-In or a Helper application. (A Plug-In works
inside the Browser while a Helper Application is completely
independent of the Browser). In either case, it is not Netscape
that plays the audio but the 3rd party application.
When a multi-media file can be loaded and executed without any
other information being provided, it is usually enough to simply
provide a hyper-link to it as I did above. When you wish to
provide more control over the multi media application, you will
need to use a specialize tag such as BGSOUND, EMBED or
OBJECT. These tags allow you to pass parameters to the Plug-
In that can control many of its functions and operations. I will
talk about these tags in more detail next week.
The bottom line here is that in order to play a Multi Media file,
the viewer will need to have an appropriate application installed
on their computer. Just because an AVI, MPEG, Flash or other
file works on your computer don't assume that it will work for
all your visitors. The general rule is if you are going to use
Multi-Media files, you should also provide a link to a
downloadable player and you should not automatically play the
multi media file unless you have warned your visitor and given
them a chance to either download the player or leave. Since
Multi-Media files tend to be very large, you should also warn
your visitors of the size of the files before they are downloaded.
So you have set up your multi media files, provided links and
been courteous enough to inform your visitors of everything they
need to know. Is that it? Not quite.
You cannot take if for granted that any multi media file type can
be put on your server even if you do provide a player. The
reason is that the Server and your Browser may not know what
to do with the file.
Why not? If you think about it, it will become apparent. When
you link to a HTML document the server knows that you want it
transmitted to your Browser. If you access a CGI script however,
the server will not transmit the file but will execute the script.
When the browser does start to receive a stream of date from the
server how does it know what to do with it? It may receive a
stream of Image data followed by some HTML, a bit of
Javascript etc.
Your Browser will know how to process the data because the
Server will add a MIME type to each block it sends to it. MIME
is an acronym for Multipurpose Internet Mail Extension and is
used to enable the Browser to identify the data it is receiving so
that it can launch the appropriate program to process it.
For the system to work, it is consequently necessary for both the
server and the visitors browser to understand the Mime type.
Unfortunately this presents a problem since you as the Web Page
author, probably do not have access to either your ISP's server or
your visitors Browser.
Most ISP's I have dealt with are receptive to adding a new
MIME type to their servers provided it is legitimate and does not
conflict with any other MIME types. Setting a MIME type up for
your visitors is more problematic especially for Helper Apps
which are not pre configured in most Browsers.
For most software companies distributing Multi-Media
applications over the Internet, this is a real challenge. Most web
surfers are very reluctant to download and install additional
applications on their computer unless they have a real desire to
do so. Even well established applications such as Real Audio
and Flash are not installed on a majority of end users computers.
As a Web Page author it is important to know which multi media
additions to your Web Page will be generally supported and
which ones may cause problems for many of your visitors. Does
that nifty sound track or streaming video really add a lot to your
web site or does it just server to drive away a lot of your
visitors?
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