Source: http://www.google.com/patents/US6393110?dq=6125447
Timestamp: 2017-02-24 02:34:46
Document Index: 652446743

Matched Legal Cases: ['§1', '§1', '§1', '§1', '§1', '§1', '§1', '§1', '§1', '§1', '§1', '§1', '§4', '§4', '§4', '§1', '§1', '§4', '§4', '§4', '§4', '§4', '§4']

Patent US6393110 - Methods and apparatus for terminating a line and supporting the asymmetric ... - Google PatentsSearch Images Maps Play YouTube News Gmail Drive More »Sign inPatentsA digital access arrangement for (i) isolating downstream components from twisted pair copper wire and (ii) separating upstream and downstream communications channels. The line isolation is performed with relatively small, lightweight components, such as an optical isolation unit for example, and can...http://www.google.com/patents/US6393110?utm_source=gb-gplus-sharePatent US6393110 - Methods and apparatus for terminating a line and supporting the asymmetric digital subscriber line protocolAdvanced Patent SearchTry the new Google Patents, with machine-classified Google Scholar results, and Japanese and South Korean patents.Publication numberUS6393110 B1Publication typeGrantApplication numberUS 09/163,661Publication dateMay 21, 2002Filing dateSep 30, 1998Priority dateSep 30, 1998Fee statusLapsedPublication number09163661, 163661, US 6393110 B1, US 6393110B1, US-B1-6393110, US6393110 B1, US6393110B1InventorsTim Urry PriceOriginal Assignee3Com CorporationExport CitationBiBTeX, EndNote, RefManPatent Citations (10), Non-Patent Citations (13), Referenced by (50), Classifications (14), Legal Events (10) External Links: USPTO, USPTO Assignment, EspacenetMethods and apparatus for terminating a line and supporting the asymmetric digital subscriber line protocol
US 6393110 B1Abstract
A digital access arrangement for (i) isolating downstream components from twisted pair copper wire and (ii) separating upstream and downstream communications channels. The line isolation is performed with relatively small, lightweight components, such as an optical isolation unit for example, and can be operated with signals modulated at relatively high frequencies and having relatively high data rates and amplitudes.
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.
§1.2.1 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
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.
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).
§1.2.3.1.2 Modulation Frequencies
The frequency band of the telephone network was historically limited, by low pass filters at the fringes of the network, from 0 to 3.3 KHz. ADSL requires that these filters be removed. Frequency division multiplexing (or “FDM”) is then used to separate the frequency band of the twisted pair into a band for downstream data, a band for upstream data, and a band for POTS. As shown in FIG. 1A, a POTS channel is provided from 0 to 4 KHz, while a POTS guardband extends to 25 KHz. The upstream data channel is provided from about 25 KHz to about 100 KHz. Finally, the downstream data channel is provided from about 120 KHz to about 1.142 MHz. Also, as shown in FIG. 1A, the amplitude of the upstream and downstream signals is +20 dBm. The downstream data channel may be time division multiplexed (TDM) into one or more high and low speed channels. As shown in FIG. 1B, if echo cancellation is provided, the upstream data channel can extend to about 138 KHz such that it partially overlaps the frequency band of the downstream data channel.
§1.2.3.1.3 Modulation Techniques
Two alternative, and incompatible, modulation techniques have been competing; namely, carrierless amplitude phase (or “CAP”) modulation and discrete multitone (or “DMT”) modulation. Each is briefly discussed below.
CAP modulation is a variant of quadrature amplitude modulation (or “QAM”) which is the scheme used in most voice grade modems following the V.32 standard. Basically, CAP modulation uses both multilevel amplitude modulation and phase modulation and initially transmits a “carrierless” signal. The CAP modulation system may use a number of different error correction schemes such as: (i) Reed Solomon forward error correction (or “FEC”) in the downstream direction for improving reliability in the event of impulse noise; (ii) an interleaving technique to reduce block error; (iii) Trellis encoding, typically provided in the upstream direction, for minimizing cross-talk, background, and white noise. Since each of these error correction techniques are known to those skilled in the art and not particularly relevant to the present invention, they are not described in further detail. It suffices to note that the forward error correction schemes are important in time sensitive (real time) data applications, such as video-on-demand for example.
DMT modulation divides the bandwidth of the copper line twisted pair into 256, 4 KHz wide, sub-channels, referred to as “bins”. These bins are creating by using the fast Fourier transform (or “FFT”) at the receiver and inverse fast Fourier transform (or “IFFT”) at the transmitter. DMT allocates data to the bins based on the noise then being experienced in each bin. The digital signals may be encoded with error-correcting codes, for dealing with occasional bursts of impulse noise, similar to those employed on compact disks.
§1.2.4 Exemplary Environment in Which the Present Invention May Operate
FIG. 2 is block diagram which depicts an environment 200 in which the present invention may operate. This environment is similar to the ADSL Forum System Reference Model, set forth in Annex A of Technical Report TR-007, entitled “Interfaces and System Configurations for ADSL: Customer Premises” (March 1988). As shown, at a high level, a customer premises 220 is coupled with an access node (such as a central office of a local telephone service provider) 210 via a local loop 230 comprising a twisted pair copper wire. Recall that the data rates which can be supported over the local loop 230 will be limited by the gauge and length of the local loop 230.
A voice network 250, such as the public switched telephone network (or “PSTN”) for example, may connected, via one or more trunks 219, with a trunk (concentrated) side input of the voice traffic switch 218. A data network 240, which may also be the PSTN for example, may be connected, via one or more lines 217, with a concentrated side input of the data traffic switch or router 216.
The customer premises 220 may include POTS equipment 224 (such as a telephone and/or a modem for example) and a terminal 228 (such as a personal computer and/or a set top box for example). Like the splitter 212 at the access node 210, a voice/data splitter 222 separates the POTS frequency band from the data frequency bands. Once again, referring back to FIGS. 1A and 1B, the splitter 222 may include a low pass filter for passing frequencies from 0 to 25 KHz to the POTS equipment 224. The splitter 222 may also include a high pass filter for passing frequencies above 25 KHz to an ADSL transmission unit (which may also be referred to as a “terminal adapter”) 226. The ADSL transmission unit 226 may be coupled with the terminal 228. Although not shown in FIG. 2, the functions of the voice data splitter 222 may be incorporated into, or provided downstream from, the digital access arrangement 260.
The ADSL transmission unit 226 at the customer premises 220 may include a digital access arrangement (or “DAA”) 260, a digital signal processor (or “DSP”) 270, and a controller 280. The present invention concerns the digital access arrangement 260. The digital access arrangement 260 basically functions to: (i) isolate the downstream components from the twisted pair copper wire 230; and (ii) separate the upstream and downstream communications channels, for example, by hybrid splitting, transmit/receive signal filtering, and/or echo cancellation. The digital signal processor 270 basically functions to: (i) convert digital signals to analog signals (or “DAC”) and analog signals to digital signals (or “ADC”); (ii) modulate outgoing (i.e., upstream) signals and demodulate incoming (i.e., downstream) signals in accordance with either the CAP or DMT modulation methods; and/or (iii) cancel echo. Finally, the controller 280 basically functions to throttle outgoing (i.e., upstream) data rates to 64 Kbps to 640 Kbps and to buffer the incoming (i.e., downstream) data stream so that it may be provided at a data rate compatible with the terminal 228.
§1.2.5.1 Challenges to the Digital Access Arrangement
§1.2.5.1.1 Line Isoloation
The twisted pair copper wire 230 may be terminated at the digital access arrangement 260. The twisted pair copper wire 230 includes wires known as a “tip” wire and a “ring” wire. To protect (for example, in the event of lightening striking the local loop or a power line crossing the local loop) a customer and equipment at their premises, the digital access arrangement 260 isolates the twisted pair copper wire 230 from downstream equipment. Traditionally, transformers were used to inductively isolate the tip and ring lines from downstream equipment, such as telephones for example. As shown in FIGS. 1A and 1B, the POTS band is at a lower frequency (i.e., 0 to 4 KHz) and POTS uses a lower power than the upstream and downstream data in ADSL. At the higher frequencies and power used for upstream and downstream data in ADSL, the magnetic cores of transformers must be designed large enough so that they do not saturate or operate in a non-linear region.
Optical components are relatively small and light components and have been used by modems for line isolation. More specifically, a photo-transmitter (e.g., a light emitting diode, photo diode, photo transistor, etc.) provides a line signal to an adjacent photo-receptor. The receptor generates a voltage based on the state of the adjacent transmitter. Thus, the optical isolators used in modems operate in a “photo-voltaic” mode. In addition, the optical isolators used in modems operate in a binary (i.e., either ON or OFF) mode. Unfortunately, these optical components, operating a photo-voltaic mode, cannot provide line isolation in relatively high data rate applications.
§1.2.5.1.2 Transmit/Receive Signal Separation
Recall that the tip and ring copper lines of the twisted pair 230 can simultaneously carry both upstream data and downstream data (in addition to POTS which may have already been filtered out). Referring back to FIG. 1A, if frequency division multiplexing (or “FDM”) is used, the upstream and downstream data channels can be easily separated by separating, by appropriate filtering for example, the upstream frequency band (e.g., 25 KHz to 100 KHz) from the downstream frequency band (e.g., 120 KHz to 1.142 MHz). Referring back to FIG. 1B, if the frequency bands of the upstream and downstream data channels overlap to some extent, echo cancellation may be used to separate the upstream and downstream data.
The present invention provides a method for transmitting and receiving data over tip and ring lines. Data may be transmitted by (i) buffering data received from an external source to generate buffered data, (ii) modulating the buffered data in accordance with an ADSL modulation technique (such as CAP or DMT) to generate modulated data, (iii) converting the modulated data to an analog signal, (v) amplifying the analog signal, while passing it over an optical isolation boundary, to generate an amplified analog signal, (v) selectively filtering the amplified analog signal to generate an amplified and filtered analog signal, and (vi) applying the amplified and filtered analog signal to the tip and ring lines. Data may be received by (i) taking a signal appearing across the tip and ring lines, (ii) blocking a DC component of the signal, to generate a non-biased signal, (iii) amplifying the non-biased signal to generate an amplified signal, (iv) converting the amplified signal to a digital signal, and (v) demodulating the digital signal in accordance with an ADSL demodulation technique (such as CAP or DMT).
FIG. 1A illustrates the spectrum of frequency bands, and power used in an ADSL service employing frequency division multiplexing.
The present invention concerns a novel digital access arrangement, and more specifically, a digital access arrangement to be used with a line carrying ADSL service. The following description is presented to enable one skilled in the art to make and use the invention, and is provided in the context of particular applications and their requirements. Various modifications to the disclosed embodiments will be apparent to those skilled in the art, and the general principles set forth below may be applied to other embodiments and applications. Thus, the present invention is not intended to be limited to the embodiments shown.
First, the functions to be performed by the present invention will be described in §4.1 below. Then, an exemplary structure for practicing the present invention will be described in §4.2 below. Finally, an example of the operation of the present invention will be described in §4.3 below.
The present invention concerns what is commonly referred to as a digital access arrangement. As discussed in §1.2.4 above, digital access arrangements basically function to: (i) isolate the downstream components from the twisted pair copper wire; and (ii) separate the upstream and downstream communications channels, by echo cancellation for example. Such transmit/receive signal separation may be accomplished by providing appropriate filters, such as RC filters for example. Echo cancellation may also be accomplished by providing appropriate filters and/or via known digital signal processing techniques. As discussed in §1.2.5.1.1 above, the line isolation should be performed with relatively small, lightweight component and should be operable with signals modulated at relatively high frequencies and having relatively high data rates and amplitudes. Such isolation is accomplished by providing a transmission stage of a digital access arrangement with an optical isolation unit (which may be operated in a photo conductive mode) and by providing a reception stage of the digital access arrangement with isolation capacitors.
§4.2 Exemplary Structure of the Present Invention
FIG. 4 is a schematic diagram of a digital access arrangement (or “DAA”) circuit 400 which may be used to practice the present invention. The line isolation boundary is depicted with dashed line 401.
The components are arranged as shown in FIG. 4. Thus, a detailed description of the individual connections is not necessary and is not presented here. Further, the values of the components are depicted in FIG. 4. The circuit of FIG. 4 may use a known power supply, such as the one described in the design application note, “LAN Power Supply Generates Isolated 9V”, downloaded from http://www.maxim-ic.com/AppNotes on Sep. 21, 1998 (from Maxim Integrated Products of Sunnyvale, Calif.).
§4.3 Example of Operation of Disclosed Embodiment
An example of the operation of the exemplary digital access arrangement 400 will now be described with reference to FIGS. 3 through 6. First, an example of transmitting data will be described in §4.3.1 below with reference to FIGS. 3 through 5. Then an example of receiving data will be described in §4.3.2 below with reference to FIGS. 3, 4, and 6.
§4.3.1 Example of Transmitting Data
An example of transmitting data will now be described. Referring to FIG. 3, the terminal 228 has some data to send upstream. For example, a customer may have selected a movie to view and entered a personal identification number or an account number. Naturally, other information, such as VCR type control information for example, may be transmitted upstream. This data is buffered in the output buffer 382 of the controller 280. (See, e.g., step 510 of FIG. 5.) Recall from FIGS. 1A and 1B that the upstream data is transmitted at a rate of 64 Kbps to 640 Kbps. Thus, the data rate controller 384 may slow down or speed up the data rate. The transmit data stream is then provided to a digital signal processor 270 which modulates the data in accordance with the carrierless amplitude and phase (or “CAP”) modulation technique or the discrete multitone (or “DMT”) modulation technique. (See, e.g., step 520 of FIG. 5.) This modulation is done numerically, i.e., digitally. The modulated transmit data stream is then converted from a digital signal to an analog signal and provided to the digital access arrangement 260. (See, e.g., step 530 of FIG. 5.)
§4.3.2 Example of Receiving Data
Referring now to FIG. 3, the output at pin 1 of the op amp U3A is provided to an analog to digital converter 372 of the digital signal processor 270. (See, e.g., step 650 of FIG. 6.) Further echo cancellation may be then carried out by echo cancellation facility 374. (See, e.g., optional step 655 of FIG. 6.) Finally, the received signal is demodulated at demodulation facility 376. (See, e.g., step 660 of FIG. 6.) The received signal may be provided to an input buffer 386 which may be emptied at a rate determined by the data rate controller 388 (which, though not shown, may be provided with control signals from the terminal 228). (See, e.g., steps 670 and 680 of FIG. 6.) The buffered received data is then processed at the terminal 228. For example, if the terminal 228 is a set top box, it could decode (or decompress) an MPEG video stream and provide an NTSC (“National Television Standards Committee”) or PAL or S-video or HDTV (“high definition television”) signal to a television monitor.
Thus, the digital access arrangement designed in accordance with the present invention (i) isolates the downstream components from the twisted pair copper wire; and (ii) separates the upstream and downstream communications channels. The line isolation is performed with relatively small, lightweight components and can be operated with signals modulated at relatively high frequencies and having relatively high data rates and amplitudes.
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