Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!watmath!clyde!cbatt!ihnp4!inuxc!pur-ee!uiucdcs!uiucdcsb!jabusch
From: jabusch@uiucdcsb.UUCP
Newsgroups: comp.dcom.lans
Subject: Re: What can you do with an existing fi
Message-ID: <167800002@uiucdcsb>
Date: Wed, 28-Jan-87 20:45:00 EST
Article-I.D.: uiucdcsb.167800002
Posted: Wed Jan 28 20:45:00 1987
Date-Received: Fri, 30-Jan-87 02:37:56 EST
References: <5082@reed.UUCP>
Lines: 49
Nf-ID: #R:reed.UUCP:5082:uiucdcsb:167800002:000:2965
Nf-From: uiucdcsb.cs.uiuc.edu!jabusch    Jan 28 19:45:00 1987


	The problem with most commercial fiber systems is that they are
equivalent to baseband coaxial systems.  That is, they transmit at a given
light frequency (actually a range) and everyone on the fiber uses that 
range.  In order to get multiple stations to transmit near-simultaneously,
each station is in effect running TDM (Time Division Multiplexing).  In 
other words, CSMA/CD, in effect.  More normal systems that provide video,
such as CATV, run broadband networks, each station using a 6MHz range for
the transmission of video.  This allows continuous broadcast of video in
each of many frequency ranges, with receivers tuned to catch the channel
desired.
	In order to support video on fiber, now, you would need buffering
at the receiver to store the incoming data, which would then have to be 
transmitted at higher frequency rates than typically possible from most
video equipment.  The buffering is necessary because the receiver would 
continue to re-create the analog signal from its digital source (fiber)
at the 6MHz rate to provide decent video.  Since the channel is shared 
between multiple transmitters (TDM), the basic data transfer rate would
have to be fast enough to guarantee that no matter how many transmitters
are on a given fiber, enough packets would arrive at the receiver in a
given time frame (forgive me for not doing the appropriate math here) to
keep the receiver buffer from emptying.
	There are a lot of factors to consider here.  One possible aid is
to buffer at the transmit end, so that packets would always be ready to send
when the channel is open for transmission.  This implies that the receiver
is always behind by some "lag" time which is proportionally (or perhaps
exponentially) related to the number of stations on the network.
	Add to this problem the fact that there just aren't too many A-D
(Analog to Digital) Converters out there than can keep up with the 6MHz
bandwidth needed for the video broadcast.  There are people working at
several fiber vendors out there now, trying to solve these problems. 
One proven method for turning a fiber into a broadband system is to
transmit at different freqencies of light.  This is expensive to do, but
technology evolves and costs decrease.  I know of at least one lab that
has done this (right here, the Information Engineering Laboratory, 
University of Illinois, Urbana, Illinois).  It was referred to at the 
time as "Spectrum Sampling".  There is a Master's thesis on it, written 
by Apostolos Dollas, who can now receive e-mail as:
	dollas%mcnc.csnet@RELAY.CS.NET.
There is, of course, much that I haven't even begun to discuss here, and
which I will receive many flames for, if all continues with the SOP
on this net.
	I hope that I've helped a little.

John Jabusch

p.s.	don't bother to send me e-mail.  I am between hosts, and even
though I can post notes from this site, I can't receive mail here.  I will
have another mailing address by the end of the week.