Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!uflorida!gatech!udel!burdvax!gvlv2!kleonard
From: kleonard@gvlv2.GVL.Unisys.COM (Ken Leonard)
Newsgroups: comp.arch
Subject: Re: One aspect of bandwidth (backplane bus)
Keywords: Backplane
Message-ID: <206@gvlv2.GVL.Unisys.COM>
Date: 21 Apr 89 13:13:44 GMT
References: <407@bnr-fos.UUCP[ <17500@obiwan.mips.COM[ <607@geovision.UUCP> <710@ultb.UUCP>
Distribution: usa
Organization: Unisys Great Valley Laboratory
Lines: 32

* [stuff deleted] 
* Is anyone out there actually doing work with fiber optic busses? 
--well, our so-called VHSLAN (now building prototype) can be run can be run 
as a "bus" (sort of ring-like) as well as a "net" (sort of crossbar-like.) 
* I remember reading that the bandwidth of current fiber optic 
--true, but watch out for what "bandwidth" really means 
* cables is in the 3500 _Gigahertz_ range in the IR band alone. 
--let's say that the channel bandpass is 3500 gigabits max with present 
technology but not forget that by the time we're almost economically reasonable 
and have gotten the error rate really under control, we're talking about 
2000 to 2400 gigabaud at approx 1/2 bit information per baud (yes, I typed 
it right, about 2 line state times per INFORMATION bit) 
* This would seem to be adequate for the next generation of 
* computers (but probably not the generation after that....), esp 
--well, it just about fits the current generation top-end systems with 
one fiber-wavelength-channel per data channel 
* considering that fiber optic cables were meant for multiplexing. 
--OUCH--that word again--_multiplexing_--multiple frequencies/wavelengths 
per fiber is NOT MULTIPLEXING--unless you regard the common case of umpteen 
zillion radio and TV stations sharing the same luminiferous aether to be 
also a case of multiplexing 
--so-called frequency- or wavelength-division multiplexing is NOT AT ALL 
SIMILAR TO time-division or contention or code-division multiplexing 
and is almost an entirely separate discussion 
--FDM/WDM is really PROPAGATION MEDIUM SHARING 
--TDM/CM/CDM etc is SIGNAL CHANNEL SHARING, at best, and is often (usually 
at the present?) more like DATA CHANNEL SHARING 
* Each board could have it's own frequency range, allowing 
* multiple boards to talk at the same time. Any comments? 
--and let's not forget ((decision) engine sharing?) and (switch space sharing?) 
and (information sharing?) and (?) 
--ain't this fun?