Methods and apparatus for terminating a line and supporting...

Telephonic communications – Telephone line or system combined with diverse electrical... – Having transmission of a digital message signal over a...

Reexamination Certificate

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C379S093050, C379S093080, C379S093240, C379S399010, C379S412000, C379S414000, C379S406010, C379S406020

Reexamination Certificate

active

06393110

ABSTRACT:

§1. BACKGROUND OF THE INVENTION
§1.1 Field of the Invention
The present invention concerns methods and apparatus for terminating a line and for supporting the asymmetric digital subscriber line (or “ADSL”) protocol. More specifically, the present invention concerns a digital access arrangement for performing line isolation, transmit/receive signal separation, and/or echo cancellation functions.
§1.2 Related Art
As set forth in detail below, the ADSL protocol is believed to be an important technique for data communications, particularly for data communications needed to provide video-on-demand services.
§1.2.1 Unmet Demand for Telecommuting and Video-On-Demand Services
In the near future, it is believed that applications for providing video-on-demand and for facilitating telecommuting will be developed in order to meet perceived demand for such services. First, regarding video-on-demand, current techniques for delivering video entertainment, such as movies, to consumers include television broadcasts, cable services, and video tape and disk rentals. Though television broadcasts and cable services deliver video entertainment to customers' homes, such services are limited in that customers must view such entertainment at a time dictated by a fixed schedule—not necessarily when they want to view such entertainment. Further, VCR type functions such as pause, stop, fast forward, and rewind, are not available.
Although video recorders permit customers to record video programs for later viewing at a convenient time, customers are often put off by the task of setting such video recorders. On the other hand, video tape and disk rentals permit customers to watch a particular movie or program at a time convenient. However, such movies or video programs are not delivered to their home—the customer must pick up and drop off the video tapes or disks. Thus, a service that provides video entertainment to customers (i) at their residences and (ii) when they want it would serve a strong, yet unmet, demand.
§1.2.2 Transmission Facilities to Customer Premises and Their Limitations
Video data can be transmitted to a customer's premises by a physical transmission medium, such as co-axial or fiber optic cable for example, or by a wireless method, such as television broadcast or satellite transmission for example. The limitations of each of these broad categories is discussed below.
The problem with using physical transmission medium is, to the extent not already facilitated by existing plant (e.g., telephone lines, co-axial cable, etc.), such physical transmission medium must be provided to the customer's premises. Installing new transmission medium to the premises of many customers (also referred to as “last mile” transmission) entails tremendous costs. Thus, to the extent that existing infrastructure, and in particular, existing physical transmission medium, exists, such existing infrastructure should be exploited.
Presently, the most prevalent physical transmission media entering customer's premises are (a) twisted pair copper wire (also referred to as “twisted pair”) and (b) coaxial and/or hybrid fiber-coaxial (or “HFC”) cable. Twisted pair has been used, traditionally, for voice telephone services, and more recently, for data communication by means of modems. Some believe that the use of twisted pair for video-on-demand and telecommuting applications has advantages over the use of coaxial cable or hybrid fiber-coaxial cable for the reasons discussed below. In the following, ADSL is a communications protocol supported over twisted pair. Cable modems are used to communicate data over coaxial or hybrid fiber-coaxial cable.
Installed infrastructure presents the largest advantage of ADSL over cable modems. In 1998, the global ratio of telephone lines to HFC lines is about 400 million to 6 million, or about 60 to 1. Aggressive upgrades from coaxial cable to HFC cable over the next five. (5) to six (6) years will not improve the ratio to better than 10 to 1. Even in the United States, the ratio of telephone lines to HFC lines is on the order of 20 to 1. Based on estimates of the International Telecommunications Union (or “ITU”), in 1995, 700 million telephone lines existed, about 500 million of which served residences and the balance serving businesses and pay telephones.
Regarding line quality, cable modems have the advantage that they do not depend on coaxial cable distance. This is because amplifiers in the cable network boost signal power sufficiently. Unfortunately, however, most existing CATC systems are not HFC, but rather, tree and branch networks of coaxial cable. These networks use one-way amplifiers that preclude upstream data flow. Since 1993, many CATV lines have been installed with two-way amplifiers creating an upstream data path from 5 to 45 MHz. However, the sheer size of these networks (sometimes, as many as 10,000 customers may be served from a single headend) and the noise and channel problems with so many subscribers attached to a common line, make high speed upstream channels unattainable after a few subscribers have joined the line. A unidirectional coaxial CATV network may be upgraded to a bi-directional one by physically replacing amplifiers, at a cost of around $25 per home passed. Upgrading from coaxial to HFC cable requires more work and costs.
On the other hand, ADSL modem speeds will depend on line distance. The longer telephone lines found today may support speeds no greater than 1.5 Mbps. However, the average telephone line will support speeds up to 6 Mbps. Variable rate ADSL modems will be able to adapt their rate to line length.
Finally, telephone networks have historically been more reliable and stable than coaxial cable networks. Thus, some believe that ADSL, rather than cable modems, will be the preferred means of providing video-on-demand services.
Although wireless video transmission methods, such as satellite for example, do not suffer from the “last mile” problem of wiring to each customer's premises, the transmission is one way—from the service provider to the customer. There is no backchannel communications path from the customer's premises to the service provider. User commands are to a tuner at the customer's premises, not to the service provider. Thus, video-on-demand and VCR type functions are not supported by satellite television systems.
§1.2.3 Overview of ADSL
Having described the relative advantages of ADSL for providing video-on-demand services for example, a technical description of ADSL, known to those skilled in the art, is provided in §1.2.3.1 below for the reader's convenience.
§1.2.3.1 Technical Description of ADSL
§1.2.3.1.1 Data Rates
Recall that video-on-demand is believed to be a significant, yet un-served, market. Present video and audio data compression standards, such as the MPEG-2 (i.e., motion pictures expert group) standard, permit full motion video to be represented by a data stream having a rate of about three (3) Mbps. Providing back channel commands, such as program selection and menu navigation, VCR type commands, etc., obviously requires much less bandwidth. ADSL provides a downstream (i.e., from a service provider to a customer) data rate of 0.5 to 8 MBPS, an upstream (i.e., a back channel from the customer to the service provider) data rate of 64 to 640 KBPS, and traditional telephone service (also referred to as “POTS” or “plain old telephone service).
As a practical matter, the length of the local loop, that is the length of the twisted pair copper wire from a customer premises to a central office or other network node, as well as the gauge of the copper wire used in the twisted pair, limits the downstream data rate of ADSL. For 24 gauge (i.e., 5 mm cross section) wire, the downstream data rate limits are as follows:
Up to 18,000 feet
1.544 MBPS (T1)
Up to 16,000 feet
2.048 MBPS (E1)
Up to 12,000 feet
6.312 MBPS (DS2)
Up to 9,000 feet
8.448 MBPS.
Downstream data rates for 26 gauge (i.e., 4 mm cross section) wire are slightly less.
As discussed above, variable rate ADSL modems will be ab

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