Randy, and All,
I would like to re-open discussions on last-mile outside plant technologies with some questions that have been perplexing me for quite some time. From my discussions with some heavy hitters in the field, I've found that I am not the only one unsure, I may add. The issues have to do with EMI/RFI, coupling effects and some of the Voodoo rules of the past that are in some cases anachronisms, and in other cases... I'm not quite sure. The issues center on the ability of existing telco plant to support T1/E1 and ADSL/VDSL signals in the distribution plant (Tom's SAC/SAI to the home/business), without causing detrimental cross-talk to adjacent facilities due to coupling. In part, I focus on the ADSL Forum's views of these issues, and seek to get clarifications and opinions on their relevance. Note: I haven't any axe to grind with the ADSL Forum. They've simply provided me with a convenient prop for presenting my questions. And in what follows, I am not necessarily promoting or advocating T1 to the home. Rather, I seek answers to the technical issues that I cite below.
I can recall all kinds of spectrum charts going back to the Sixties and Seventies when good ol' Mom told us that you couldn't place a 56 kbps line in the same cable as a T1, and that it 'had to' use shielded cable; T1s "always" required shielded cable and couldn't be placed in 50-pair bundles of UTP that were being used for voice or low speed data; IBM 25-pair EIA cable couldn't be longer than 50 feet; you cant do this and you cant do that.
Today we know that many of these rules were overkill. While some of these were verifiable facts based in state-of-the-art scientific application, there were also many instances where they were scare tactics and overkill, akin to the "network protection device" kludges that were required for customer owned CPE shortly after Carterfone. From these legacy facts and notions (be they factual or fictional) the ADSL Forum has promoted and adopted some positions that ADSL is good for you, and one of the reasons is that T1/E1 is NOT good for you... or at least it is not good for your neighbor if you are using it. From the ADSL Forum, I quote from their "General Introduction to Copper Access Technologies:" <bear with me>
"Until recently, T1 and E1 circuits were implemented over copper wire by using crude transceivers with a self-clocking Alternate Mark Inversion (AMI) protocol. AMI requires repeaters 3000 feet from the central office and every 6000 feet thereafter, and takes 1.5 MHz of bandwidth..."
(FAC: this is not correct - A T1 requires 772 KHz of bw),
"....with a signal peak at 750 kHz (U.S. systems). To a transmission purist, this is profligate and ugly, but it has worked for many years and hundreds of thousands of lines (T1 and E1) exist in the world today."
"Telephone companies originally used T1/E1 circuits for transmission between offices in the core switching network. Over time they tariffed T1/E1 services and offered them for private networks, connecting PBXs and T1 multiplexors together over the Wide Area Network (WAN). Today T1/E1 circuits can be used for many other applications, such as connecting Internet routers together, bringing traffic from a cellular antenna site to a central office, or connecting multimedia servers into a central office. An increasingly
important application is in the so-called feeder plant, the section of a telephone network radiating from a central office to remote access nodes that in turn service premises over individual copper lines. T1/E1 circuits feed Digital Loop Carrier (DLC) systems that concentrate 24 or 30 voice lines over two twisted pair lines from a central office, thereby saving copper lines and reducing the distance between an access point and the final subscriber."
""""""Note, however, that T1/E1 is not a very suitable service for connecting to individual residences. First of all, AMI is so demanding of bandwidth, and corrupts cable spectrum so much, that telephone companies cannot put more than one circuit in a single 50 pair cable, and must put none in any adjacent cables. Offering such a system to residences would be equivalent to pulling new wire to most of them. Secondly, until recently no application going to the home demanded such a data rate. Thirdly, even now, as data rate requirements accelerate with the hope of movies and high speed data for everyone, the demands are highly asymmetric -- bundles downstream to the subscriber, and very little upstream in return -- and many situations will require rates above T1 or E1. In general, high speed data rate services to the home will be carried by ADSL or VDSL (or similar types of modems over CATV lines). """"""
I am not comfortable with this for several reasons: (1) ADSL data rates, and hence the underlying carrier frequency components, are now pushing above those used for T1 lines, and despite the differences in signalling and coding techniques, if the above hypothesis were true, then we could be looking at plenty of problems from ADSL and especially VDSL down the road, as well; (2) DSP techniques today have come a long way in mitigating spurious emissions and harmonic content; (3) their rationale for asymmetry escapes me, or at least makes me uncomfortable, because I think that they are leveraging one fact to foster a defense (or offense?) for their own cause. On this, the Forum goes on to state:
""As its name implies, ADSL transmits an asymmetric data stream, with much more going downstream to the subscriber and much less coming back. The reason for this has less to do with transmission technology than with the cable plant itself.""
[[[At that point I get really uncomfortable...]]]
""Twisted pair telephone wires are bundled together in large cables. Fifty pair to a cable is a typical configuration towards the subscriber, but cables coming out of a central office may have hundreds or even thousands of pairs bundled together. An individual line from a CO to a subscriber is spliced together from many cable sections as they fan out from the central office (Bellcore claims that the average U.S. subscriber line has twenty-two splices). Alexander Bell invented twisted pair wiring to minimize the interference of signals from one cable to another caused by radiation or capacitive coupling, but the process is not perfect. Signals do couple, and couple more so as frequencies and the length of line increase. It turns out that if you try to send symmetric signals in many pairs within a cable, you significantly limit the data rate and length of line you can attain. "
Is this a convenience or a fact? Or both? The idea here is, they would have you believe, that in the proximity of the CO, cable bundles are densely stacked, and the likelihood of coupling is greater there than out in the boondocks, and hence, at this point the weaker signals would be prone to severe coupling, whereas in the field (at the home) there is less aggregate noise due to coupling effects, and therefore, acceptable. Or something along those lines.
In contrast to the conclusions that you can draw from the foregoing, I recently read that NEC is teaming with Amati to deliver a 60 Mbps VDSL product that will directionally scale on demand - or on a programmable basis, depending on the user's requirements. I.e., 45 up and 15 down, or 30 up and 30 down, or 51 up and 9 down, etc. How are these chips going to pass muster if what the Forum states about coupling at the higher frequencies is, in fact, relevant and true?
I know that outside plant cabling and splicing techniques have vastly improved since the days of pulp insulation and lead sheathing, and I suspect that these improvements have not been taken into consideration in these analyses, or they have been ignored. If nothing else, the balance of the outside UTP pair is greatly improved over what it was twenty or thirty years ago. It's been a while since I picked up a slide rule <grin> and applied calculus to the parameters affecting transmission over twisted pairs.
So, would you or someone here give me a hand in making sense out of these assertions and allegations, and help me sort fact from fiction? What's the real story with regard to the effects of coupling? And what are the new rules, if high-speed ADSL and VDSL are to flourish.
Also, what is the breakdown of twisted pair to untwisted (or parallel pair) cable (the drop cable) in the last 200 feet to the house, and what is the impact of NOT having continuous twists to the end point of the loop when DSL (or any high-speed approach) is employed? Is this an issue that the FTTC and ADSL camps are concerned about?
Thanks in advance, Frank |
