Path: utzoo!telecom-request Date: 24 Jun 91 17:01:08 GMT From: "Fred R. Goldstein" Newsgroups: comp.dcom.telecom Subject: Re: What The Heck is "Fiber Optic Quality" Anyway? Message-ID: Organization: Digital Equipment Corp., Littleton MA USA Sender: Telecom@eecs.nwu.edu Approved: Telecom@eecs.nwu.edu X-Submissions-To: telecom@eecs.nwu.edu X-Administrivia-To: telecom-request@eecs.nwu.edu X-Telecom-Digest: Volume 11, Issue 489, Message 7 of 12 Lines: 48 In article , riacs!rutgers!ttidca.tti. com!jackson@decwrl.dec.com (Dick Jackson) writes... > In other words fiber is no better than other MODERN digital facilities. Yes and no. For ordinary voice users, that's probably true: A copper or radio digital transmission facility is usually tagged as in trouble when its bit error rate is anywhere near the level needed to make PCM audio audibly degraded. And that's not common. But it can occur: Radio links (including satellites) are subject to rain fade. That raises the raw BER a bit. Minor fade, however, is usually compensated for by trellis coding, a form of forward error correction. Severe fade can lead to real errors, though. Microwaves are not happy when it rains _very_ hard. System designers generally account for some probability of this, but you can never say never. Copper links (like T1) are subject to electrical impulse noise. If lightning strikes nearby, a pole line can pick up a hit. Again this is unusual, but it can occur. It also matters how marginal the link is in the first place. Motors and other such devices can impair some marginal copper links. The nature of these bursts varies with the cause and with the type of transmission system. Fiber optic is usually transmitted in raw NRZ format, perhaps scrambled, where each bit is represented by an on or off. (It is not group coded like most radio and some copper systems, where a transmission symbol represents several bits.) It is immune to electrical noise or rain fade. (Backhoe fade, of course, is harder to avoid :-(. ) When it works well, the BER is often on the order of 10^-12, versus 10^-8 or so on many copper systems. That's the difference between very good and very, very, very good. Because it's not group coded, most errors only affect a single bit at a time, rather than a cluster of bits as might be hit on a modem or digital radio link. It's gaussian noise, and the S/N-ratio to BER curve can be taken from a simple graph in Mischa Schwartz's textbook on noise, whose title escapes me now (I've got the new edition at home). This all can be significant when designing error detecting and correcting techniques. What works best for a _pure fiber optic_ network may not be ideal for a mixed-media digital network. Fred R. Goldstein Digital Equipment Corp., Littleton MA goldstein@delni.enet.dec.com voice: +1 508 952 3274 Do you think anyone else on the planet would share my opinions, let alone a multi-billion dollar corporation?