Xref: utzoo comp.arch:17317 sci.electronics:13044 sci.physics:13717 comp.lsi:1095 Path: utzoo!utgpu!news-server.csri.toronto.edu!rutgers!cs.utexas.edu!wuarchive!decwrl!elroy.jpl.nasa.gov!ames!aplvax!ames!ucsd!swrinde!zaphod.mps.ohio-state.edu!brutus.cs.uiuc.edu!ux1.cso.uiuc.edu!ux1.cso.uiuc.edu!aglew From: charlie@sunoptics.caltech.edu (Charles Stirk) Newsgroups: comp.arch,sci.electronics,sci.physics,comp.lsi Subject: Re: Electro-optic bus Message-ID: <9007232305.AA11439@sunoptics-gw.caltech.edu> Date: 23 Jul 90 23:05:22 GMT References: Sender: usenet@ux1.cso.uiuc.edu (News) Distribution: comp Organization: University of Illinois, Computer Systems Group Lines: 68 In-Reply-To: aglew@oberon.crhc.uiuc.edu's message of 23 Jul 90 20:39:37 Return-Path: charlie@sunoptics.caltech.edu Return-Path: X-Ph: V3.2@ux1.cso.uiuc.edu To: andy-glew@uiuc.edu Dear Andy, Sorry I don't know how to post to usenet, so if you could put this there for me I would appreciate it. The latency of an optical interconnect, like an electronic one, depends as much on the circuit as on the individual device characteristics. Optoelectronic devices are composed of diodes, transistors, resistors inductors and capacitors. Only with very exotic devices should the device characteristics be very much different than those found in electronics. Since the maturity of the optoelectronic devices lags behind those of electronics in most cases, due to a smaller and younger market, it is difficult but not impossible to make a case for optical interconnects in computer architecture purely on device characteristics. The lower noise and higher bandwidth that is an optical circuit advantage in telecommunications has also been useful in some computer architecture applications like buses and peripheral connections. The advantages come from the passive part of the circuit, the optical waveguide. The advantages gained by better circuit layout with optics can be significant and are easy to demonstrate in principle. For instance, in vlsi complexity theory the minimum amount of chip area it takes to layout the shuffle exchange graph, which is useful in parallel processors, is at least (N/logN)^2, where N is the number of nodes in the graph. My collegues and I demonstrated an optical shuffle exchange network that uses only N units of area. Area costs money in vlsi yield models and small area circuits can also mean high speed. Similar arguments can be made for other graphs like the hypercube. Practical demonstrations of these advantages, however, are difficult due to the technological immaturity alluded to above. That doesn't mean demonstrations haven't been attempted, they just don't irrefutably show a significant practical advantage. A good example of this is public relations extravaganza coming out of Bell Labs in the last year on optical computing. In addition, the amount of freedom available in choosing values for the parameters for the devices and circuits in a comparison between optics and electronics is wide enough so that almost any conclusion can be drawn, both for and against optics. It is an active area of research, nonetheless. For an introduction to the field of optical interconnects, I recommend the papers by J. W. Goodman of Stanford and his associates published in Proc. IEEE, Applied Optics and Optics Communications since 1984. Sincerely, Charlie Stirk charlie@sunoptics.caltech.edu -- Andy Glew, andy-glew@uiuc.edu Propaganda: UIUC runs the "ph" nameserver in conjunction with email. You can reach me at many reasonable combinations of my name and nicknames, including: andrew-forsyth-glew@uiuc.edu andy-glew@uiuc.edu sticky-glue@uiuc.edu and a few others. "ph" is a very nice thing which more USEnet sites should use. There is an info-ph mailing list, or, contact Steve-Dorner@uiuc.edu. UIUC has ph wired into email and whois.