Re: Streaming Video across the IT

In article <1151296088.508442.77200@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>,
<erniemac@xxxxxxxxxxxxx> wrote:

If real-time anything is impossible over the Internet, real-time
anything is impossible anywhere, then. If you are sitting in a
football stadium watching a game, the action you are seeing actually
happened some nanoseconds before you saw it.

(Bandwidth has nothing to do with your argument -- you're talking about
latency. A videotape has a ton of bandwidth, approximately 30 MB per
second of tape, but if I have to get in my car and drive to the store
to get the tape, the latency of videotape is very high.)

There's a multi-second delay on my satellite TV service compared to the
local over-the-air broadcast of the same stations, and another second
or two caused by TiVo buffering, and then another second or so if I'm
watching it on my laptop on the LAN via Slingbox, but I certainly
wouldn't think not to call my TV "real time." It's close enough.

The Internet is certainly capable of a similar "close enough" level of
performance -- i.e., what your vaunted "normal person" would consider
real time -- especially compared to satellite TV broadcasting, since
for most users there won't be any satellite hops in Internet streaming.

It is a little more complex than that. Internet messages are sent in
"packets", not comtinuously as with TV over dedicated telecommunication
lines. SO, no breaks in TV although may be some seconds between sending
and receiving. Not a problem ?

With Internet, packets are sent, not in a contimuous stream and not
necessarily over the same link. This is why on receiving the "packets"
need to "sorted" and joined to give reliable video. Once sorted, the
recieving computer needs to send some confirmation to the sender to
confirm receipt. With Video, some loss of data MAY NOT be importan,
loss of frames allowable. However, with data, the transmission needs to
be 100%- a "checksum" is sent to the sender. If "checksums" don't
agree, the last "packet" is resent. This is called a DUPLEX
communication.

This is why you use UDP if you can -- no checksums, no confirmation,
the receiver just deals with any missing packets, just as a TV receiver
must deal with occasional interference.

Sometimes using UDP isn't practical (firewalls etc.) but when it is, it
works well. When it isn't, you buffer a few seconds and it's close
enough.

This is why videos are often jerky and lack resolution. Even though
some tolerance is allowed with non- impotant video, utilties such as
QT Player, MS Window Media Player, Real Player etc, use buffering to
get a continuous video stream. Great for veiwing Movies and such, but a
disaster in "Video conferencing". Who wants to wait around for up to a
minute while a buffer fillls ?

With sufficient bandwidth and UDP this is not really much of an issue,
as anyone who has ever videoconferenced knows.

--
Jerry Kindall, Seattle, WA <http://www.jerrykindall.com/>

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