Xref: utzoo comp.arch:17333 sci.electronics:13065 sci.physics:13731 comp.lsi:1098 Path: utzoo!utgpu!news-server.csri.toronto.edu!rutgers!uwm.edu!zaphod.mps.ohio-state.edu!brutus.cs.uiuc.edu!ux1.cso.uiuc.edu!ux1.cso.uiuc.edu!aglew From: charlie@sunoptics-gw.caltech.edu (Charles Stirk) Newsgroups: comp.arch,sci.electronics,sci.physics,comp.lsi Subject: Re: Electro-optic bus Message-ID: Date: 24 Jul 90 21:31:20 GMT References: Sender: usenet@ux1.cso.uiuc.edu (News) Distribution: comp Organization: University of Illinois, Computer Systems Group Lines: 73 In-Reply-To: aglew@oberon.crhc.uiuc.edu's message of 23 Jul 90 20:39:37 Inductance can be very important in packaging. The reason for this is common ground bounce, where large current transients through power and ground pins due to large numbers of devices switching rapidly and the inductance on the pins of the package cause the power rail to drop in voltage and the ground rail to come up. There are ways to reduce this effect. The first is large numbers of power and ground pins for high-speed and high-current circuits. The second is using large decoupling capacitors in the package to keep the power and ground levels relatively constant. The third method is to design circuits that don't have current transients through the pins. Another, though much less severe inductance problem is the crosstalk noise due to induced currents in pins and wires. Designers get around this by running a ground pins between the signal pins. I did not see the statement that optical fibers are limited to about 1Gbit/sec, so I'm not sure if the author meant present state of technology or fundamental limits. Optical fibers have supported bandwidths far in excess of 1 GHz, and the fundamental limit is ridiculously high. The main limit on increasing bandwidth at a fixed bit error rate is the power budget between the transmitter and receiver and channel crosstalk noise. Using wavelength division multiplexing of many channels in one fiber, extremely high-bandwidths have been demonstrated. In practice, one is limited more by the source and detector characteristics than the characteristics of the fiber. The time it takes to do electrical->optic->electrical conversion is limited by the acceptable bit error rate, the optical power of the transmitter, the sensitivity of the detector and amplifier, the noise sources, and the time of flight. All of these factors must be considered when determining the latency of a link. Increasing the first two and decreasing the second two will decrease the latency of the link. The time of flight is significant because light travels at about a foot per nanosecond. A paper that discusses several of these issues and their effect on bandwidth is by Anil Jain in Optical Engineering a couple of years ago. A note of caution when designing optical interconnects into computer architecture. Present architectures have been designed with the the known characteristics of electrical interconnects in mind. Optical circuits, however, have a markedly differents set of characteristics. While some are improvements over those in electronics like bandwidth, distance and noise, and can be used to incrementally upgrade the present architectures, the cost may not be worth it. Many optical devices are presently much more bulky, expensive and unreliable than their electronic counterparts. They will probably only be commercially feasible in high-end systems, where their added cost can be justified, until these factors change. Optical interconnects can have many disadvantages if not designed correctly. A true revolution in computer architecture could occur, however, when systems are designed from scratch to take advantage of the many useful characteristics of optical interconnects. 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. UIUC has ph wired into email and whois (-h garcon.cso.uiuc.edu). The nameserver and full documentation are available for anonymous ftp from uxc.cso.uiuc.edu, in the net/qi subdirectory.