larry@kitty.UUCP (04/19/87)
In a recent article Greg Earle (earle@jplpub1.JPL.NASA.GOV) writes: > I live in the 213 Area code in L.A. I recently moved, and want to get two > lines for my new abode, one of which I will use exclusively for a modem > line. When I talked to Pacific Bell I was told I could (for a nice high > fee, of course) get a `Data Access Line' which would (presumably) run from > the local switching office to my home; a higher grade line would replace the > normal voice grade phone line. I was told that this was recommended for > anyone doing data transmissions of 2400 baud or higher. I almost bit; but > then I thought, what about the rest of the way? I would be calling JPL in > Pasadena 99% of the time, which is in Area code 818, prefix 354. Since I'm > not a TELECOM expert, I just surmised that the calls I would make would go > from my home, over my `good' data line, to the local switching office; then > to whatever the local switching office for Pasadena is, and then over a > (presumably) standard voice grade line to my other modem. > > My question for you experts is (a) is this something like the real path that > the call will take (3 hops; home <=> switching office <-> s.office#2 <-> work) > and (b) if this is so, then is there any point in getting a higher grade line > for one's home, when one has no control over the line quality for the other > 2/3 of the connection ?!? Let's break up this discussion into two areas: (1) quality of central office subscriber lines (i.e., between your home/office and the telephone company central office); and (2) quality of lines between telephone company central offices. I'll answer (2) first, because it is the easier answer. In general, the quality of an interoffice trunk (i.e., a line connecting two telephone company central offices) is FAR superior to the quality of any subscriber line. In keeping with the DDD network operating goals and an overall transmission design plan called VNL (Via Net Loss), the transmission loss on most interoffice trunks originating at End Offices (Class 5) trunks is carefully kept below 4.0 dB. Interoffice trunk transmission loss on Toll Center (Class 4) and up to Regional Center (Class 1) switching offices is carefully kept below 2.6 dB, or even below 1.4 dB, depending upon the path. Such interoffice trunk design is generally done so precisely that loss is kept within +/- 0.1 dB of the design goal on any interoffice trunk of a given path to assure a uniform transmission quality. In addition, such interoffice trunks are generally equalized to have a reasonably flat transmission characteristic between 300 and 3,000 Hz. Furthermore since most interoffice trunks originating in End Offices (except in high-density urban areas where adjacent central offices are close together) are four-wire (i.e., separate receive and transmission paths - one for each direction) and are terminated in a precision hybrid-network, the transmission quality will be far superior to anything which could ever exist on a two-wire subscriber loop. All interoffice trunks of Toll Center and up origin are four-wire. Noise level, ERL (Echo Return Loss), and other parameters which affect the quality of transmission are also kept within precise design goals on interoffice trunks. The BOC's and larger independent operating telephone companies check the transmission quality of interoffice trunks on a regular basis, often using automatic test apparatus such as ATMS (Automatic Transmission Measuring System), CAROT (Centralized Automatic Reporting of Trunks), TFMS (Trunk and Facility Maintenance System), etc. Trunks which fail to pass these automatic tests are disabled until repair is effected. So the point is: under virtually all circumstances, you should have little concern about the transmission quality of interoffice trunks, as compared to your own subscriber loop. (IMPORTANT NOTE: The above applies to what is traditionally known as the DDD network; some of this standardization has gone to hell with the advent of Alternate Long Distance carrier. The above information should still be safely applicable if your call is intra-LATA in length, is inter-LATA but served by the same operating telephone company at both ends, or is routed through AT&T. This is NOT a "plug" for AT&T; it's just a simple fact of life since AT&T still runs all the major toll switching centers in the U.S.) Now we'll get back to the first topic, which is the local subscriber loop. Subscriber loops are generally designed based upon only two parameters: (1) DC resistance and (2) transmission loss at 1.0 KHz. Since most central office apparatus has subscriber loop resistance limits between 1,200 and 1,500 ohms, resistance of a subscriber loop is controlled to be within this range by selecting cable layout with sections that have large-enough wire gauges (the SMALLER gauge sections are generally CLOSEST to the central office). If the resistance limit still cannot be economically met with wire gauge selection alone, then a signaling range extender (loop extender) will be connected to the line; this device is always located in the central office. Under this condition, the subscriber loop resistance may be >> 1,000 ohms, but the loop extender has the sensitivity to support such a higher resistance. In simple terms, the transmission loss of a subscriber loop is directly proportional to its DC resistance - so a long loop will also have a large transmission loss. Invariably, subscriber loops greater than 10 kft (kilofeet) in length will have loading coils installed every X-kft (there are different loading schemes which use different spacing between loading coils); these loading coils add inductance which compensates for the attenuation of the loop that results from distributed capacitance. The worse case loss that any reasonable telephone company would impose on a subscriber loop is about 9 dB. New York Telephone, as an example, tries to keep loop loss to no more than 6 dB - but not every loop is that lucky. :-) If a maximum subscriber loop transmission loss goal of 6 to 9 dB cannot be met through loading and cable routing, then a voice-frequency repeater is installed on the subscriber line; this repeater is almost always installed in the central office on the required lines. Sometimes a combination loop extender-repeater is used, but in many cases there will be two discrete devices in the central office. Under most circumstances, subscriber loop loss is measured at only 1.0 kHz. However, a frequency-vs-attenuation plot of a subscriber loop can look like a roller coaster! Since the human ear is rather forgiving, for voice applications most telephone companies care little about the frequency-vs-attenuation curve on a POTS (Plain 'Ole Telephone Service) subscriber loop. However, MODEMS can care about this curve! Subscriber loops which run through mixed gauges of loaded cable, and/or run through voice-frequency repeaters (especially of the older E6 variety) can have some pretty ugly frequency-vs-attenuation curves. The only way to flatten the curve (and thereby make the line more attractive to data) is by means of an equalizer, a better quality repeater (hybrid rather than a negative impedance type like the E6), along with more careful design engineering of the particular loop. This equalizer, repeater change, and additional engineering is not necessary for most voice applications - so it isn't done. However, an equalizer and additional engineering DOES result in a superior subscriber loop for data purposes. So, telephone companies generally charge more money for a better subscriber loop design for data applications. If you are making serious use of data transmission at > 2,400 bps, the comparatively small additional monthly and installation charge is well worth it to get a better subscriber loop. I don't make a habit of defending telephone companies, but I must say that I feel such an additional charge is reasonable. They do have to install additional equipment (under most circumstances), and certainly do have to perform specific engineering on the design of the subscriber loop. As far as noise level on subscriber loops is concerned, this is generally caused by wet terminals and splices and is really a repair problem. There is little that can be done to reduce noise level on a subscriber loop other than to track down wet or poor splices. However, for the price of a better quality loop, one generally gets a quantitative noise measurement with some attempt at repair if the noise level is beyond normal limits. Now, to sum up and answer the $ 64 question: In my opinion, for data transmission > 2,400 bps on LONGER SUBSCRIBER LOOPS (say > 2 miles from the central office), an additional charge for a better quality loop (i.e., flatter frequency response and lower attenuation) IS a worthwhile expense. At least, with a known good loop of known characteristics, one can look elsewhere should data errors become a problem. <> Larry Lippman @ Recognition Research Corp., Clarence, New York <> UUCP: {allegra|ames|boulder|decvax|rocksanne|watmath}!sunybcs!kitty!larry <> VOICE: 716/688-1231 {hplabs|ihnp4|mtune|seismo|utzoo}!/ <> FAX: 716/741-9635 {G1,G2,G3 modes} "Have you hugged your cat today?"
ciaraldi@rochester.UUCP (04/28/87)
In article <KPETERSEN.12295906723.BABYL@SIMTEL20.ARPA> larry@kitty.UUCP writes: >In a recent article Greg Earle (earle@jplpub1.JPL.NASA.GOV) writes: >> if this is so, then is there any point in getting a higher grade line >> for one's home, when one has no control over the line quality for the other >> 2/3 of the connection ?!? > > Let's break up this discussion into two areas: (1) quality of central >office subscriber lines (i.e., between your home/office and the telephone >company central office); and (2) quality of lines between telephone company >central offices. > > I'll answer (2) first, because it is the easier answer. In general, >the quality of an interoffice trunk (i.e., a line connecting two telephone >company central offices) is FAR superior to the quality of any subscriber >line. > (IMPORTANT NOTE: The above applies to what is traditionally known >as the DDD network; some of this standardization has gone to hell with the >advent of Alternate Long Distance carrier. The above information should >still be safely applicable if your call is intra-LATA in length, is >inter-LATA but served by the same operating telephone company at both ends, >or is routed through AT&T. This is NOT a "plug" for AT&T; it's just a >simple fact of life since AT&T still runs all the major toll switching >centers in the U.S.) This is from a long and very informative article, showing much more familiarity with the characteristics of phone systems than I have. Nevertheless, I have a comment, based on my personal experience. My house, and the University of Rochester, are both served by Rochester Telephone, at one time the largest independent telephone company in the world (i.e. not part of AT&T or a government). I live about 10 miles from the University, but both sites are well within the local free calling area. For many years I have been able to communicate at 1200 bps with almost no noise, using even rather inexpensive modems, between my house and most parts of Rochester and the surrounding suburbs. I can call bulletin boards, Rochester Institute of Technology, and individuals with no problem. But when I call the U of R, or someone in that neighborhood calls a BBS near me, the error rate goes way up. Sometimes you can't even hold the carrier. I have a regular residential line, but the University and the BBS's have conditioned lines. I finally found someone at Rochester Telephone who knew what I was talking about, and explained it to him. He said that the two central offices involved happened to be connected by some of the oldest trunk lines in the city, over 30 years old. Until they were replaced, which would not be for several years, the only thing I could try would be calling again and hoping for a different (better) trunk. Recently, Roch Tel has become more concerned about data comm users, so I expect things will improve, but I just wanted to point out that there is only so much you can do with your own line. Mike Ciaraldi ARPA: ciaraldi@cs.rochester.edu uucp: seismo!rochester!ciaraldi
larry@kitty.UUCP (Larry Lippman) (04/28/87)
In article <27354@rochester.ARPA>, ciaraldi@rochester.ARPA (Mike Ciaraldi) writes: > >> if this is so, then is there any point in getting a higher grade line > >> for one's home, when one has no control over the line quality for the other > >> 2/3 of the connection ?!? > > > > I'll answer (2) first, because it is the easier answer. In general, > >the quality of an interoffice trunk (i.e., a line connecting two telephone > >company central offices) is FAR superior to the quality of any subscriber > >line. > > This is from a long and very informative article, showing much more > familiarity with the characteristics of phone systems than I have. Thank you! > Nevertheless, I have a comment, based on my personal experience. > > My house, and the University of Rochester, are both served by > Rochester Telephone, at one time the largest independent telephone > company in the world (i.e. not part of AT&T or a government). > I live about 10 miles from the University, but both sites are well within > the local free calling area. > > For many years I have been able to communicate at 1200 bps > with almost no noise, using even rather inexpensive modems, > between my house and most parts of Rochester and the surrounding > suburbs. I can call bulletin boards, Rochester Institute of > Technology, and individuals with no problem. > But when I call the U of R, or someone in that neighborhood calls > a BBS near me, the error rate goes way up. Sometimes you can't even > hold the carrier. I have a regular residential line, but the > University and the BBS's have conditioned lines. > > I finally found someone at Rochester Telephone who knew what I > was talking about, and explained it to him. He said that the > two central offices involved happened to be connected by some of > the oldest trunk lines in the city, over 30 years old. > Until they were replaced, which would not be for several years, > the only thing I could try would be calling again and hoping > for a different (better) trunk. Sorry about the long quote, but I felt it necessary in order for my reply to be meaningful... I have had the "pleasure" [read "misfortune" :-( ] of dealing with Rochester Telephone in the past. Most of the problems I have encountered in the past dealt with their outlying central offices, such as Avon. These outlying CO's were mostly Stromberg-Carlson XY electromechanical switches, and were real trash for dial-up data transmission. The interoffice trunks feeding these outlying CO's also left something to be desired. However, within the population center of Rochester, the CO's have been mostly Western Electric #5 Crossbar, #1 ESS, and now #5 ESS. It is my understanding that Rochester Telephone is the largest non-Bell System customer of Western Electric (now AT&T Network Systems). I believe that Rochester Telephone also has Northern Telcom DMS-100 electronic switches, but I am not certain. It is my understanding that the central office quality and the interoffice trunk quality within the population center of Rochester has been rather decent. I presume this area to encompass your example area. While I am not in a very good position to dispute a Rochester Telephone engineer, I nevertheless feel your problem is NOT interoffice trunks. I am more inclined to believe that you are being victimized by faulty or maladjusted T-carrier and/or a digital central office which is not handling data very well. A T-carrier problem IS an interoffice trunk problem, but not in the same vein as my original article or the statement of the Rochester Telephone engineer. I don't want to get off on a tangent in this article, but suffice it to say that problems in sampling clock synchronization in digital switching offices can wreck incredible havoc with data. It is my understanding that some DMS-100 offices have exhibited this problem more severely that the AT&T digital offices. <> Larry Lippman @ Recognition Research Corp., Clarence, New York <> UUCP: {allegra|ames|boulder|decvax|rocksanne|watmath}!sunybcs!kitty!larry <> VOICE: 716/688-1231 {hplabs|ihnp4|mtune|seismo|utzoo}!/ <> FAX: 716/741-9635 {G1,G2,G3 modes} "Have you hugged your cat today?"
gary@percival.UUCP (04/30/87)
Expires: References: <KPETERSEN.12295906723.BABYL@SIMTEL20.ARPA> <27354@rochester.ARPA> <1731@kitty.UUCP> Sender: Reply-To: gary@percival.UUCP (Gary Wells) Followup-To: Distribution: Organization: Percy's UNIX, Portland, OR. Keywords: line quality, switches I can't ( and wouldn't ) speak for your local telco's, but here in Pacific Northwest Bell-land we have DMS-100 & #5 ESS digital CO's; we have #1A & #4 ESS switchers; we have LOTS of digital carriers ( both cable & microwave ); & we have fiber optic ( also digital ). It is part of my job to travel all around the state of Oregon & parts of SW Washington & modem back to Portland and Seattle. For every 8 hours of data transmission ( at 1200 ), I will see maybe 1 or 2 seconds of noise ( garbaged data ). While it is true that T-Cxr's that lose sync will do terrible things to data, at least around here, the connection will be gone long before the a data user will see garbled data. The channel banks have detection circuitry, & kill the system if sync is lost. They start ringing alarms if the sync starts to slip, which happens long before a loss. The point is, the PSTN ( Public Switched Telephone Network, aka DDD ) is _designed_ to be transparent to the users. If your telco's are doing their job,what type of switcher, transmission facility, or routing should not be visible to you, or effect you in any manner. If you _can_ tell, you should be yelling at your Utility Commissioner(s). -- -------------------------------------------------------------------------------- Wudda yeah mean, I'm gonna get in trouble? I AM in trouble! ...tektronix!reed!percival!gary