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Posted by john4321 on 07/24/06 04:57
http://www.shopzero.net/article/videoconferencing/4090.php
You may think you understand video conferencing pretty well until
someone who isn't at all familiar with it approaches you for a simple
definition. When they ask, "What exactly is video conferencing?" you
could suddenly realize you're at a loss for words.
The simplest definition of how video conferencing works is simply by
the integration of video, audio and peripherals to enable two or more
people to communicate simultaneously over some type of
telecommunications lines. In other words, you are transmitting
synchronized images and verbal communications between two or more
locations in lieu of them being in the same room. How video
conferencing works is a little bit harder to explain than answering the
question, "What is video conferencing?"
Millions of people use video conferencing every day around the globe,
but very few people know just how the technical aspects of the process
work. The main ingredients of successful video conferencing are video
cameras, microphones, appropriate computer software and computer
equipment and peripherals that will integrate with the transmission
lines to relay the information.
The analog information recorded by the microphones and cameras is
broken down into discreet units, translating it to ones and zeros. A
Codec encodes the information to a digital signal that can then be
transmitted to a codec at the other end, which will retranslate these
digital signals back into analog video images and audio sounds.
The theory's the same, the transmission has changed
In the earlier days of video conferencing, T1, ATM and ISDN lines were
used almost exclusively but were really only practical for room-based
video conferencing systems. These dedicated lines were expensive and
only large corporations tended to have the facilities and money to
invest in this type of set-up.
As the Internet became more a part of the everyday lives of all
businesses, however, it changed how video conferencing was conducted.
The TCP/IP connections of the Internet are much less expensive and can
carry large quantities of information, including video packets for
conferencing, relatively easily. Because of this, video conferencing
has become much more prevalent in small businesses and in desktop
packages that can be set up with software for computer-to-computer
networking.
Compression makes video transmission practical
The problem that arises when you convert analog to digital for
transmission is the loss of clarity in an image. Analog signals are a
continuous wave of amplitudes and frequencies showing shades and ranges
of color as well as depth and brightness. When you convert to digital,
which is strictly 0's and 1's, you then need to develop a grid to
represent values, intensities and saturations of different color values
so that the image can be interpreted and reformed at the receiving end.
This vast amount of digital information requires huge bandwidth and
means that the time it would take to transmit video images would be
impractical for most applications. That's where compression is crucial.
When determining how video conferencing works, one of the most
important elements is the compression ratio.
The higher the compression ratio, the more quickly the information is
capable of being transmitted. In many cases, however, this also means
some loss in clarity or audio/video quality. For instance, a
compression ratio of 4:1 would be terribly slow but have a fantastic
picture quality. But by the time it was transmitted, everyone at the
other end would probably have left the room for a cup of coffee. Lossy
compression discards unneeded or irrelevant sections of a signal in
order to transmit only the essentials, speeding up the transmission
time significantly but sometimes resulting in loss of quality.
Compression can either be intra-frame or inter-frame for material that
is repetitive or redundant, such as that wall behind the conference
participant. Since the wall remains static and never changes, this
image is redundant and can be eliminated from transmissions to an
extent with proper compression. Intra-frame compression assumes the
redundancy will be present in parts of a frame that are close to each
other. Inter-frame compression assumes that there is redundancy over
time (i.e., like that wall). Either of these can achieve a fairly high
degree of accuracy and reduce the bandwidth needed for transmittal of
signals.
A newer version of compression/decompression is SightSpeed technology,
developed by Cornell University. SightSpeed compresses only images
considered essential and eliminating what is considered 'filler,'
relying on the brain to fill in the decompression at the other end.
Based on an artificial intelligence model, SightSpeed achieves
compression of about 90:1, compared to the typical 15:1 for video
conferencing.
Any video conferencing session you use will provide compression of the
transmission signal. The key is determining the balance between speed
and video picture quality that is right for your needs.
Point to point video conferencing
Point to point video conferencing is just what it sounds like ? a link
between two different points on the planet, or two different video
conferencing terminals. It could be between an office in New York City
and a conference room in Munich. Point to point video conferencing can
easily be initiated by someone on one end contacting the other end as
though making a standard telephone call. There are no special
arrangements to be made other than knowing that the participants will
be there.
Multipoint conferencing is more complex
Multipoint conferencing is more complicated because it has to
coordinate several different locations simultaneously. Since you can't
be in direct contact with several places at once while they are all in
contact with others, you need one source that will tie them all
together. In video conferencing, this is called a multipoint bridge or
multipoint conferencing unit (MCU).
An MCU enables multi-location video conferencing by providing a sort of
"central processing center" for all of the locations through which all
the information flows. The MCU receives all information from the
various locations and then sends it out to each location. In some cases
the MCU is located on a particular PC, and in other cases it is located
on a remote server (the most common structure, particularly for more
powerful MCU networks).
Audio is usually sent and received simultaneously in all locations with
an MCU with no problem because of the relatively small bandwidth needed
for transmittal. It is broadcast in what is called "full duplex" mode,
meaning everyone can talk and hear at the same time with no cutting off
when one person or another speaks.
Video transmission, however, can be broadcast in a number of ways with
an MCU depending upon the quality of the software and the complexity of
the system. Some common types of video transmission for video
conferencing include:
Continuous Presence video conferencing, which allows up to four
conference sites to be seen simultaneously on split screens. This is
usually used if you have a small group or individuals in separate
locations and will primarily be seeing close-up shots.
Universal Control video conferencing is controlled by the initiating
conference site. The primary site determines who sees what at all other
sites.
Voice Activated video conferencing is by far the most common type used
today. The image with these systems shifts to the site that is
currently activating the microphone so that you can always see whoever
is speaking. However, if there is a good deal of background noise
participants should mute their microphones when they aren't talking in
order to avoid the image jumping about needlessly.
Overcoming the language barrier
Obviously, communicating through video conferencing can't be achieved
unless both ends of the conference are "speaking the same language."
That is, whatever is being transmitted electronically will need to be
reassembled properly and heard and seen clearly at the other end. The
Codec system (Coder-Decoder) is useless if both ends aren't using the
same virtual language to interpret the signals.
The International Telecommunications Union (ITU) developed a set of
standards in 1996 dubbed H.323 to outline specific guidelines for Video
Conferencing standards and protocols so that compliance and support
across networks would be easier to achieve and maintain. Since then,
many manufacturers and developers of video conferencing tools have
adopted the H.323 guidelines as their own.
Web conferencing solutions such as Click to Meet, Lotus's SameTime, and
WebEx also offer corporate solutions that are based on Internet video
conferencing. These systems have shared protocols that can be
downloaded and used anywhere at any location for subscribers through
the Internet. These are becoming more popular with companies who like
the convenience and user-friendliness. They will no doubt become more
and more refined over time, vying with and perhaps surpassing the H.323
standards.
Overcoming firewall issues
There are, of course, obstacles to overcome when you take a look at how
video conferencing works. After all, you're sending vast amounts of
translated data either directly or through a gatekeeper system (the
MCU) that is switching and transferring information between a variety
of computers. Just about any business these days has a firewall system
to provide security and protect the system from potential viruses.
Trouble is, many firewalls also block the transmission of data for
video conferencing.
Recent innovations have largely circumvented these problems by
designing firewall solutions that recognize video conferencing
signaling requests and allow the information packets to bypass the
firewall or router without disabling the firewall protection for other
traffic. Even with this, however, there may be occasions when packets
are dropped because of heavy traffic on the system, so investing in a
firewall system that can handle substantial traffic is essential to
quality video conferencing performance.
How video conferencing works will certainly evolve over time and
improve in the coming years, but a basic understanding of what it is
and how it works now will help you make the best choice for you when
you're ready to begin using video conferencing yourself.
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