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To: RoseCampion who wrote (25246)	5/24/2000 1:28:00 AM
From: D. Newberry	Respond to of 54805

Hi Rose, So the author is technically correct about the relative speed of photons vs. electrons, but of course (as everyone else has already pointed out) that's completely irrelevant to the bandwidth issue at hand. I believe that optical fiber comes out far on top because light is at a much higher frequency (=higher Shannon limit); you also can do fun stuff like 192-channel DWDM which would be impossible in a copper wire because fiber's interference and attenuation characteristics permit such things while copper doesn't. Well put Rose, although I must confess, I haven't visited Shannon's theorem for some time and don't recall the specifics of his very important observations. DN

To: RoseCampion who wrote (25246)	5/24/2000 2:12:00 AM
From: Doren	Read Replies (1) \| Respond to of 54805

The reason is simple -- photons travel through glass Although photons do travel through glass faster the real reason glass is better is that light waves can pass through each other unchanged. Think of two beams from flashlights at 90ø angles passing through each other. This means multiple streams of data can pass through a single fibre concurrently without disturbing each other. Can't do that with electrons/magnetism.

To: RoseCampion who wrote (25246)	5/24/2000 11:12:00 AM
From: rushnomore	Read Replies (2) \| Respond to of 54805

OT -- photons vs. electrons The reason is simple -- photons travel through glass much more quickly than electrons travel through wire. Rose wrote: ....electrons themselves pass through a copper wire very, very slowly....It's the electromagnetic wave that travels through the copper wire at near light-speed or a goodly fraction of same.....So the author is technically correct about the relative speed of photons vs. electrons, but of course (as everyone else has already pointed out) that's completely irrelevant to the bandwidth issue at hand. I believe that optical fiber comes out far on top because light is at a much higher frequency (=higher Shannon limit); you also can do fun stuff like 192-channel DWDM which would be impossible in a copper wire because fiber's interference and attenuation characteristics permit such things while copper doesn't. Someone with a real engineering background can probably explain this more accurately. Rose et al: I have a real engineering background, and you have described it pretty accurately. Yes, electrons do travel very slowly, and yes, the data (or any electronic signal) travels at some large fraction of the speed of light because the signal is carried by the electromagnetic wave. Light is just another kind of electromagnetic wave (light can alternatively be represented by photons), but the difference is that the optical fiber has the capability of carrying a much wider bandwidth than copper wires do. With this bandwidth many different light beams, each of a different color (frequency, or wavelength) can travel over the same fiber. And each light beam can be modulated at a very high bit rate.

To: RoseCampion who wrote (25246)	5/24/2000 11:14:00 AM
From: broken_cookie	Respond to of 54805

OT Rose, Very well explained. Electrons travel through wire at a rate of millimeters per second. The wave travels through the conductor at appreciable fraction (1/2 to 2/3 's) the speed of light in a vacuum. Since actual electron speed is miniscule, there is no need to worry about relativistic mass gain. Photonic waves propagate through glass slightly faster than electromagnetic waves propagate through a conductor. But that is not where the benefit comes from. The spectrum available in fiber is 10000 greater than wire allowing transmission of 10000 times more information. So the wider highway analogy is best. In your garden hose analogy, water will begin flowing out of a full hose almost instantly after the tap is turned on. The speed at which this happens is the speed of sound in water. To use another analogy, everyone is familiar with those desk toys where 5 ball bearings are arranged in a cascading pendulum. You lift up and release one bearing and as it strikes the other still bearings, a bearing on the end swings away almost instantly. (Speed of sound in steel). Fiber has a Nyquist/Shannon channel capacity limit of about 50 Tbs (50 terabits a second or 5*10^13 bps). But the bottleneck is the opto to electrical conversion which runs about 1-2 Gbps (10^9 bps). Rich