larry@uunet.uu.net (Larry Lippman) (10/12/89)
In article <telecom-v09i0436m04@vector.dallas.tx.us> gabe@sirius.ctr. columbia.edu (Gabe Wiener) writes: > I was just thinking about the AT&T Long Lines that have been used in > this country for decades. I'm sure all of these questions have > ridiculously simple answers, but here goes anyway. > 1. Over the _really_ long runs, such as through the Rocky Mountains, > or through the deserts of the southwest, how do they prevent > line resistance from degrading the signal to a point where it would > become undetectable? For the "really long runs" to which you refer, we are dealing with coaxial cable and microwave systems, as opposed to individual pairs per circuit, and/or short-haul carrier such as N-type FDM or T1 PCM. Analog coaxial cable and microwave systems all work on single-sideband suppressed carrier to provide FDM (Frequency Division Multiplex). Analog coaxial cable transmission systems, such as L1, L3, L4 and L5 carrier utilize various types of repeaters which are placed at periodic intervals along the cable route. L1 repeater spacing is 8 miles; L3 is 4 miles; L4 is 2 miles; and L5 is 1 mile. Most repeaters are self-powered using DC which is superimposed over the RF signal on the coaxial cable center conductor. Every 160 miles for L1, L3 and L4 and every 75 miles for L5 are power feed points which are located in small repeater buildings or hardened underground facilities. These power feed points require AC power, but have back-up batteries and generators. Microwave transmission systems are by their very nature limited by the curvature of the earth, and therefore require repeater towers every 20 to 30 miles, with each tower obtaining local power with battery/generator backup. Some examples of long haul microwave systems are the TD-2, TD-3, TH-1 and TH-3. Digital coaxial cable transmission systems, like the T4M, require digital *regenerators* (like a repeater, but not quite the same) every mile, with each regenerator being powered by DC superimposed on the coaxial cable center conductor. Incidentaly, the capacity and specs of the L-type coaxial cable systems, as per their original design are: L1 (obsolete) 600 channels per two coaxial tubes ~2.8 MHz bandwidth L3 1,860 channels per two coaxial tubes ~8.3 MHz bandwidth L4 3,600 channels per two coaxial tubes ~18 MHz bandwidth L5 10,800 channels per two coaxial tubes ~70 MHz bandwidth Most L-type coaxial cable systems use either 12-tube or 20-tube coaxial cables. Accordingly, as an example, an L4 system using a 20-tube cable (with 2 tubes spare) provides a total of 32,400 voice-grade channels per cable. That's a LOT of channels! L5 is three times greater in capacity. The L-type coaxial cable systems are used to provide "hardened" communication routes which are relatively immune to natural and man-made distaster (including nuclear war). As a result, the hardened routes are always underground, but some non-hardened L-type coaxial cable are above ground. Underground routes are also preferred because they result in greater transmission stability since cable temperature changes are minimized; when one is trying to push maximum bandwidth from a coaxial cable and maintain amplitude stability, temperature effects become significant. Digital methods (PCM) are being used to update both coaxial cable and microwave facilities. As an example, the T4M system transmits digital data at a rate of 274 megabits/sec (DS4), and can use the same coaxial cable as in the L5 system; L5 and T4M can furthermore co-exist in the same cable. In the case of the T4M, however, the repeater modules are replaced with regenerator modules for the designated coaxial tubes. It is important to understand that practical digital transmission systems require MUCH MORE BANDWIDTH THAN ANALOG SYSTEMS. While a single T4M channel with two coaxial tubes (one for each direction) provides 4,032 individual voice-grade circuits, the equivalent L5 analog FDM channel provides 10,800 voice grade circuits. Stated another way, the T4M system will handle 168 24-channel D1 channel banks. A new generation of microwave systems have been designed to directly interface with digital carrier. As examples, the 1A Radio Digital System will handle one 1.544 megabit/sec DS1 line per channel; the 3A Radio Digital System will handle one 44.736 megabit/sec DS3 line per channel; and the 18A Digital Radio System will handle one 274 megabit/sec DS4 line per channel. There are also DS4 interfaces for fiber optic systems. It is therefore possible for a signal to leave one customer premises as digital and travel thousands of miles through all manner of wire, coaxial cable, microwave and fiber optic systems and enter a second customer premises while remaining 100% digital. > 2. When one of those lines is damaged out in the middle of nowhere, > and the damage is _inside_ the cable, how do they locate it? There are two general methods of fault location, usually performed sequentially. The first involves supervisory pilot tones to localize repeater and/or gross cable faults. Obviously, if there are say 16 repeaters in a given line segment, and from ONE END of the segment supervisory pilot tones can only be received from repeaters 1 through 11, then the fault is most likely between repeater 11 and repeater 12. A similar feature using audio tones is used to localize faulty regenerators in digital transmission systems. Once an approximate fault location is made as above, entry to the cable is made from a repeater location where impedance measurements can be taken and/or a time domain reflectometer be used to pinpoint the exact fault location, often to within a few feet. > Moreover, how do they splice in a new piece of cable? Very carefully. :-) Critical circuits are generally re-routed using alternate facilities to permit any cable work. Splicing coaxial cable is not trivial, and some time must be allowed following a splice for moisture to be purged from the cable tubes, and for the transmission characteristics of the cable to stabilize before equalization can be performed and the cable placed in service. > In other > words, how do they connect up those hundreds of individual lines? One at a time. :-) As implied above, on long-haul coaxial cable circuits there are comparatively few lines since multiplex is used. A 20-tube coaxial cable has, of course, 20 tubes, plus maybe a dozen or so 19 AWG conventional pairs for order wire, repeater power and test purposes. > It would be like trying to rewire a spinal cord. Nah, it's not that complex once you get into it. > 3. Are the long-lines used today by AT&T digital or analog? Sprint > obviously is touting their fiber-optics, but what is AT&T doing? > Do they still use the analog long-lines that they've been using for > years? Or do they send the signals over them via a digital encoder? I'm afraid that I have been out of the mainstream of the telephone industry far too long to quantitatively comment on the percentage of digital versus analog FDM circuits in use by AT&T. The only thing I can state with certainty is that there is still a SIGNIFICANT number of "long line" circuits which run through analog FDM facilities. One of the reasons why a large number of analog circuits remain is that going digital using the same cable facility results in at LEAST 2-1/2 TIMES >FEWER< voice-grade channels. It is not easy to economically justify converting, say, an existing L5 facility to T4M while at the same time >reducing< its channel capacity by 250% just to go digital for the sake of going digital. Sprint, as an example, has somewhat of an "unfair" advantage over AT&T. Sprint started with ZERO plant investment, and immediately had the freedom to go state-of-the-art digital over the MOST LUCRATIVE high-density circuit routes. AT&T does not have this luxury, and furthermore has a lot of toll plant serving CO's in the middle of nowhere where circuit revenue is far less that circuit installation and maintenance cost. The eventual goal is, of course, for AT&T to have all-digital network using fiber-optic communications, but the reality of the situation is that so much money is invested in existing microwave and coaxial cable plant, including FDM carrier, that it will be a LONG time before the goal of an all-digital transmission network can be realized by AT&T. Don't forget, the AT&T network is larger than Sprint by orders of magnitude! <> Larry Lippman @ Recognition Research Corp. - Uniquex Corp. - Viatran Corp. <> UUCP {allegra|boulder|decvax|rutgers|watmath}!sunybcs!kitty!larry <> TEL 716/688-1231 | 716/773-1700 {hplabs|utzoo|uunet}!/ \uniquex!larry <> FAX 716/741-9635 | 716/773-2488 "Have you hugged your cat today?"