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Non-Tech : Amati investors -- Ignore unavailable to you. Want to Upgrade?

To: Larry Brew who wrote (30458)	2/4/1998 9:16:00 AM
From: polarisnh	Read Replies (2) \| Respond to of 31386

Larry, First of all, I do not profess to be an expert on ADSL or any other WAN technology but I will try to help you out. Pat Mudge, Eric Goethals, or anyone else please feel free to jump in at any time. I agree that copper is a rather crude means for transmission particularly Cat 1 Transmission cable but unfortunately it is one that we will have to live with for years to come. The optimum media type would obviously be a dedicated fiber optic cable. Until all of world has dedicated fiber supporting each residence or commercial location I guess we must live with what we have. Distance is certainly a consideration with most technologies due to inherent transmission delays and they are factored into the limitations of the specifications. One of the limiting factors is associated to the media type. So to answer your question, there are significant losses associated with these `crude' lines. As you may know the bandwidth limitations are not caused from the transmission lines themselves, but in fact are limited by filters at the edge of the voice network, as well as loss associated with splicing. These filters are used primarily for lines that exceed Subscriber loop plant lengths of 18,000 feet and only represent less then 20% of all users in the U.S. That is why xDSL typically utilizes clean lines or lines that aren't conditioned. There are many charts available that outline the distance constraints broken down by data throughput (One-directional), but it is my understanding that a T-1 which supports 1.544 Mbps up to 18,000 feet +/-. Most telephone companies have been working to shrink the average loop length to stretch the capacity of existing central offices. The typical technique involves installation of access nodes remote from central offices, creating so-called Distribution Areas with maximum subscriber loops of 6000 feet from the access node. Remote access nodes are fed by T1/E1 lines (now using HDSL) or fiber. In suburban communities a Distribution Area connects an average of 1500 premises; in urban areas, the figure is double, about 3000 premises. Of course the number of premises served dwindles as service data rates increase. A Fiber to the Curb system (FTTC) offering STS-1 rates may only be within reach of twenty homes in some suburban areas. ADSL transmits an asymmetric data stream, with much more going downstream to the subscriber and much less coming back. Twisted pair wiring was designed to minimize the interference of signals from one cable to another caused by radiation or capacitive coupling, but obviously there is some signal leakage. Signals do couple, and couple more so as frequencies and the length of line increase. Symmetric signals in many pairs within a cable will significantly limit the data rate and length of line you can attain. As ADSL transmits digitally compressed video, among other things, it includes error correction capabilities intended to reduce the effect of impulse noise on video signals. Error correction introduces about 20 msec of delay, which is much too much for LAN and IP-based data communications applications. Therefore ADSL must know what kind of signals it is passing, to know whether to apply error control or not (this problem obtains for any wire-line transmission technology, over twisted pair or coaxial cable). Furthermore, ADSL will be used for circuit switched (what we have today), packet switched (such as an IP router) and, eventually, ATM switched data. ADSL must connect to personal computers and television set top boxes at the same time. Taken together, these application conditions create a complicated protocol and installation environment for ADSL modems, moving these modems well-beyond the functions of simple data transmission and reception. I would say that for the next 15 to 15 years that the xDSL technologies will provide reasonable solutions until everybody has fiber to the residence. I hope that answers your questions. Cheers, Steve