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Path: utzoo!watmath!clyde!cbosgd!gatech!amdcad!phil
From: phil@amdcad.UUCP (Phil Ngai)
Newsgroups: net.lan,net.dcom
Subject: Re: Optical Network Links
Message-ID: <11771@amdcad.UUCP>
Date: Fri, 23-May-86 03:38:20 EDT
Article-I.D.: amdcad.11771
Posted: Fri May 23 03:38:20 1986
Date-Received: Sat, 24-May-86 22:15:03 EDT
References: <661@nrcvax.UUCP>
Reply-To: phil@amdcad.UUCP (Phil Ngai)
Organization: AMD, Sunnyvale, California
Lines: 37
Keywords: Links laser
Xref: watmath net.lan:1504 net.dcom:1934

In article <661@nrcvax.UUCP> geoffb@nrcvax.UUCP (G. Geoffrey Baehr) writes:
>	Device: Optical Link, IR Laser/Fresnel Lens Assy
>		Range 1km
>		Bandwidth 100mhz (Also can be cranked up)
>		Aiming - Telescopic Sight
>		Cost ~ $7000 per side
>		Accepts TTL signals. Currently is repeating T1 signals. 
>		Ethernet transceiver being adapted.

I'm not sure of what you mean by "Ethernet transceiver being adapted"
but if it's what I'm afraid of (extending the distance between a
controller and a transceiver from 50 meters to 1000 meters) I would
like to go through a simple calculation. Assume light travels at 1
nS/foot and there are 3 feet per meter. Then light takes 3000 nS to
travel 1000 meters, 6000 nS to make the round trip. Ethernet heartbeat
is supposed to be sent after the end of the transmission. If the
transceiver returned heartbeat immediately after it perceived the end
of transmission, the controller would not see heartbeat until 6 uS
after it ended its transmission. Unless you "adapted" the transceiver,
it would actually delay before asserting heartbeat.

This is well outside the spec for both Ethernet 2 and IEEE 802.3.

You should also be sure the optical link does not add a lot of timing
jitter. Ethernet can not accept very much before it starts having
trouble decoding the signal. We're talking budgets on the order of
1 nS for the transceiver cable. (from memory, I could be off by
a couple of nS)

Finally, what about radiological safety hazards? Does this device
require eye protection?
-- 
 Vote Yes on Proposition 51!

 Phil Ngai +1 408 749 5720
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