Mark Miller <miller@dg-rtp.dg.com> (06/16/91)
OK, this one has been bugging me for some time, but what is the highly touted "Fiber Optic Quality"?? Now as I understand it, virtually all long distance calls are sent in a digital format. Last time I checked, digital data didn't really care whether it was sent by copper, microwave or fiber. So if the proceeding assumption is correct, then I should be able to "hear a pin drop" regardless of the transmission media. Now of course I understand the benefits of fiber to the LD carrier, in that they can send many more calls, cheaper. So, is this "fiber optic quality" spiel just some marketing drival intended to mislead the consumer, or did I miss the point somewhere?? Mark T. Miller miller@dg-rtp.dg.com ...uunet!xyzzy!miller
barmar@bloom-beacon.mit.edu> (06/17/91)
In article <telecom11.463.2@eecs.nwu.edu> miller@dg-rtp.dg.com (Mark Miller) writes: > OK, this one has been bugging me for some time, but what is the highly > touted "Fiber Optic Quality"?? > Now as I understand it, virtually all long distance calls are sent in > a digital format. Last time I checked, digital data didn't really > care whether it was sent by copper, microwave or fiber. Of course it does. Digital data can be corrupted by static or crosstalk on the line, just as analog data can. Computer communication generally uses error detection and retransmission on top of the digital medium in order to produce error-free transmission. Audio telephone communication is generally more concerned with transmission speed than fidelity, because listeners are able to deal with minor errors. Higher fidelity lines mean fewer such errors. Barry Margolin, Thinking Machines Corp. barmar@think.com {uunet,harvard}!think!barmar
Jim.Redelfs@uunet.uu.net (Jim Redelfs) (06/19/91)
Mark Miller wrote: > as I understand it, virtually all long distance calls are sent in a > digital format. I believe this is in error. Although AT&T is actively replacing the analog facilities within their VAST system, I'm sure they have a LONG way to go. It wouldn't surprise me to find that only HALF of their traffic makes the long haul digitally -- perhaps even less. > is this "fiber optic quality" spiel just some marketing drival You can bet that it is actively persued my the marketing honchos, but there is certainly a lot of truth to the implication that data transmitted via optical fiber is usually of a higher quality than that which is not. As far as I know, ALL transmissions over fiber ARE digital, whereas NOT all (at LEAST!) transmissions over OTHER type of plant is. Hear a pin drop? So what?!! A fat lot of good that kind of "quality" means when there's a Sports Illustrated "SneakerPhone" on one end and yet another CheapieChirper phone on the other!! JR --- Tabby 2.2 MacNetOmaha(402)289-2899 Multitasking w/MacOrphans (1:285/14)
carroll@cs.washington.edu> (06/21/91)
In article <telecom11.471.2@eecs.nwu.edu> ivgate!macnet!jim.redelfs@ uunet.uu.net writes: > Mark Miller wrote: >> as I understand it, virtually all long distance calls are sent in a >> digital format. > I believe this is in error. Although AT&T is actively replacing the > analog facilities within their VAST system, I'm sure they have a LONG > way to go. This is certainly *not* the case in our corporate network, and I'd guess that AT&T is still some time away from being fully digital, as well. While Barry Margolin made a good point about noisy cable and error rates, I've never *noticed* any impairments on copper-carried digital lines, and I'd assert that you can't actually *hear* the difference. (The only digital call I can remember hearing problems on was overseas, and carried by satellite, I believe; there the error rates *can* be significant.) > As far as I know, ALL transmissions over fiber ARE digital, whereas > NOT all (at LEAST!) transmissions over OTHER type of plant is. Yup. And that's why I'm a Sprint customer (as bad as their customer service is). > Hear a pin drop? So what?!! A fat lot of good that kind of "quality" > means when there's a Sports Illustrated "SneakerPhone" on one end and > yet another CheapieChirper phone on the other!! And *that's* why all the phones at my house are genuine AT&T. Jeff Carroll carroll@ssc-vax.boeing.com
David Nochlin <dhn@ulysses.att.com> (06/21/91)
In response to Jim Redelfs message from June 18, 1991 ... here is the current scoop from the AT&T Fact Book as of April,1991 [available from the AT&T Customer Information Center 1-800-432-6600 as Sel. Code 015-005-001 $0.72]: AT&T Worldwide Intelligent Network Average daily calls handled 120 million --> Digitally transported 100 percent Digital network route miles 72,124 Reach 273 countries/territories Direct dial capability 177 countries Busiest calling day Monday after Thanksgiving At some point within the last year and a bit the network went 100% digital! Cheers, David Nochlin AT&T Bell Laboratories Murray Hill, NJ dhn@ulysses.ATT.COM / or +1 908 582 2663
Bryan Richardson <richard@cs.purdue.edu> (06/21/91)
In article <telecom11.472.9@eecs.nwu.edu> Jeff Carroll<ssc-vax!carroll@ cs.washington.edu> writes: > In article <telecom11.471.2@eecs.nwu.edu> ivgate!macnet!jim.redelfs@ > uunet.uu.net writes: >> Mark Miller wrote: >>> as I understand it, virtually all long distance calls are sent in a >>> digital format. >> I believe this is in error. Although AT&T is actively replacing the >> analog facilities within their VAST system, I'm sure they have a LONG >> way to go. > This is certainly *not* the case in our corporate network, and > I'd guess that AT&T is still some time away from being fully digital, > as well. Every switch in the AT&T network is a digital switch with the exception of a single cross-bar (in North Dakota, I think), and perhaps a few TSPSs which haven't been replaced. If I remember correctly, the transmission facilities within the network are over 99% digital -- fiber, microwave, etc. Thus, it is highly likely that any call placed over the AT&T network is digital from the moment that it enters through the moment that it leaves the network. What often affects sound quality on a call (in addition to the Sports Illustrated phone), is the copper loop from the subscriber to the CO (always analog), the local CO, and the transmission medium from the CO to the AT&T network. Bryan Richardson richard@cs.purdue.edu AT&T Bell Laboratories and, for 1991, Purdue University Disclaimer: Neither AT&T nor Purdue are responsible for my opinions.
jobrien@eecs.nwu.edu> (06/21/91)
In article <telecom11.471.2@eecs.nwu.edu>, ivgate!Jim.Redelfs@ uunet.uu.net (Jim Redelfs) writes: > Hear a pin drop? So what?!! A fat lot of good that kind of "quality" > means when there's a Sports Illustrated "SneakerPhone" on one end and > yet another CheapieChirper phone on the other!! All of which makes me wonder if anyone has done a Consumer Reports- style comparison of phone equipment (phones, answering machines, electrical connections) to compare sound quality?
jackson@decwrl.dec.com> (06/22/91)
In article <telecom11.472.9@eecs.nwu.edu> Jeff Carroll <ssc-vax!carroll@ cs.washington.edu> writes: and > In article <telecom11.471.2@eecs.nwu.edu> ivgate!macnet!jim.redelfs@ > writes: >>> as I understand it, virtually all long distance calls are sent in a >>> digital format. > While Barry Margolin made a good point about noisy cable and > error rates, I've never *noticed* any impairments on copper-carried > digital lines, and I'd assert that you can't actually *hear* the > difference. How about this for a summary of "fiber quality"? If your call is sent over an all digital path AND IF all the facilities are working to spec, you will experience a near perfect performance whether the transmission is on fiber, wire or microwave (digital microwave of course). Older digital systems (e.g. T1) often did not perform to spec all the time and did not have automatic monitoring, therefore they could be left in service in degraded mode. I believe that PART of the development of the phone system to provide the very low error rates characteristic of fiber results from the installation of continuous quality monitoring (such as ESF). In other words fiber is no better than other MODERN digital facilities. Dick Jackson
"Marc T. Kaufman" <kaufman@neon.stanford.edu> (06/22/91)
Bryan Richardson <richard@cs.purdue.edu> writes: > Thus, it is highly likely that any > call placed over the AT&T network is digital from the moment that it > enters through the moment that it leaves the network. An amusing anecdote, which may possibly be untrue today: I wanted to order 48 lines of MegaCom bulk 800 service (2 T1 lines) from AT&T. I was told that the circuits were entirely digital to the toll office, where they were broken down into 48 two-wire analog circuits, then sent downstairs (in the same building) to PacTel, where they were re-multiplexed (on a channel bank *I* had to buy) into digital T1 for shipment to the customer site. I couldn't even get four-wire circuits between the channel banks. Marc Kaufman (kaufman@Neon.stanford.edu)
"Fred R. Goldstein" <goldstein@delni.enet.dec.com> (06/25/91)
In article <telecom11.482.8@eecs.nwu.edu>, 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?