Multiplex communications – Channel assignment techniques
Reexamination Certificate
1999-12-01
2004-05-25
Vu, Huy D. (Department: 2665)
Multiplex communications
Channel assignment techniques
C370S464000, C370S480000, C370S482000, C370S484000, C370S487000, C370S490000
Reexamination Certificate
active
06741604
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to high-speed data communication systems, and specifically to Asymmetric Digital Subscriber Line (ADSL) systems.
BACKGROUND OF THE INVENTION
Digital Subscriber Line (DSL) is a modem technology that enables broadband digital data to be transmitted over twisted-pair wire, which is the type of infrastructure that links most home and small business subscribers to their telephone service providers. DSL modems enable users to access digital networks at speeds tens to hundreds of times faster than current analog modems and basic ISDN service. A range of DSL standards have been defined, known generically as “xDSL,” wherein the various standards have different data rates and other associated features but share common principles of operation.
The present patent application is concerned mainly with Asymmetric DSL (ADSL) service, which allows data to be conveyed downstream to the subscriber at a rate of about 8 Mbit/s, and to be returned upstream from the subscriber at about 640 kbit/s. ADSL is based on a discrete multi-tone (DMT) transmission system, in which data are encoded using 256 different “tones,” each corresponding to a frequency band 4.3125 kHz wide. Recommendation G.992.1 (ex-G.dmt) of the International Telecommunication Union (ITU), which is incorporated herein by reference, specifies the physical layer characteristics of the ADSL interface to the subscriber line, including the allocation of the tones in the frequency spectrum to upstream and downstream service.
Annex A of G.992.1 specifies requirements for ADSL operating on a telephone line in the frequency band above that used for “plain old telephone service” (POTS). Upstream ADSL service is allocated tones
6
through
31
(25.875 to 138 kHz). Downstream service may use all of tones
6
through
255
(25.875 up to 1104 kHz), as long as echo cancellation is applied in the upstream service range. Alternatively, downstream service may be limited to tones
32
through
255
if necessary to reduce near-end crosstalk (commonly referred to as “NEXT”) with the tones of the upstream signals. The range below about 25 kHz is left for POTS audio signals.
FIG. 1
is a schematic diagram showing allocation of frequencies when ADSL and ISDN (Integrated Service Digital Network) services are provided over a common subscriber loop, as specified in Annex B of G.992.1. The diagram schematically illustrates a set of spectral masks
20
given in the standard. The shapes of the masks are simplified here for clarity of illustration, the horizontal axis is not drawn to scale, and the vertical scale of power spectral density (PSD) is arbitrary, As shown in the figure, the range below 138 kHz (corresponding to tones
0
-
31
) is unavailable to ADSL, as it must be left clear for ISDN transmission in a lower band
22
. Tones
32
through
63
are allocated to ADSL upstream transmission in a middle band
24
, while tones
32
through
255
are allocated to ADSL downstream transmission in an upstream band
26
. In practical implementations, however, it is frequently necessary to limit the downstream transmission to an upper band
25
, roughly covering the range of tones
64
-
255
, in order to reduce NEXT.
ADSL service providers must be concerned not only with crosstalk between upstream and downstream transmissions on a single subscriber loop, but also with crosstalk between different loops in the same central office. This crosstalk can be particularly severe in the range of tones
32
through
63
, which is allocated to downstream transmission in Annex A and to upstream transmission in Annex B. For this reason, service provider's may find it necessary to use the Annex B ADSL spectrum not only when ADSL is provided over an ISDN line, but also for ADSL over POTS. This solution wastes valuable bandwidth and can severely degrade the ADSL downstream performance.
A range of variations on the standard ADSL spectral profiles have been proposed, in order to increase the upstream and/or downstream data rates while reducing crosstalk interference. For example, U.S. Pat. No. 5,519,731, whose disclosure is incorporated herein by reference, describes a DMT transmission scheme for use in ADSL, which is intended to mitigate T
1
crosstalk noise in the ADSL signals. The scheme uses 512 subchannels, rather than the conventional 256 tones. Some of the added subchannels may be made available for Upstream communications, so as to increase the upstream data rate.
ITU Temporary Document NG-079r1, which is incorporated herein by reference, proposes an optimal asymmetric power spectral density mask for all-digital services, particularly for ADSL. In this proposal, the band of tones
0
-
6
, which is set aside for POTS in the above-mentioned Annex A, is shared between upstream and downstream ADSL transmissions. In other words, both upstream and downstream masks are allowed to extend down to tone
0
in order to increase their data rate. Because NEXT typically increases strongly with frequency, however, the tones in the upper range of the upstream band, between about 90 and 138 kHz (roughly tones
21
-
32
), are masked out of the downstream band.
SUMMARY OF THE INVENTION
It is an object of some aspects of the present invention to provide methods and apparatus for reducing crosstalk in ADSL transmissions.
It is a further object of some aspects of the present invention to provide methods and apparatus that enable improved exploitation of the spectrum available for ADSL, particularly in the presence of other network services, such as ISDN.
In preferred embodiments of the present invention, an improved spectral utilization scheme is introduced for use in ADSL service on a group of subscriber loops, at least some of which are also used for an existing digital network service, such as ISDN. Lower, middle and upper frequency bands are defined for transmission over the subscriber loops. The existing digital network service is provided on the lower band, and the middle and upper bands are used respectively for upstream arid downstream ADSL transmission. This scheme accords with G.992.1 Annex B. For those subscriber loops that do not receive ISDN service, however, the lower band, at least down to the POTS range, is exploited to provide additional transmission bandwidth for both upstream and downstream ADSL transmissions.
Preferred embodiments of the present invention offer a number of advantages over ADSL spectral utilization schemes known in the art. The use of both the lower and middle bands for upstream transmission by non-ISDN subscribers increases the available upstream bandwidth. On the other hand, because there is no downstream transmission in the middle band, crosstalk in this band is substantially eliminated, including both NEXT on individual subscriber loops and inter-subscriber crosstalk between the non-ISDN and Annex B subscriber loops that are served by the same central office. Non-ISDN subscribers thus receive ADSL service with substantially greater bandwidth than can generally be offered in a mixed Annex B and non-ISDN environment that uses methods and apparatus known in the art.
In some preferred embodiments of the present invention, a modem is configurable so as to provide either Annex B service, combining ADSL and ISDN transmission, or ADSL service (with or without POTS) using the novel spectral utilization scheme described hereinabove. This type or modem may then be used for all of the subscribers served by the central office. The choice of Annex B or non-ISDN service is implemented simply by switching the lower frequency band in or out of ADSL use, using either hardware or software switching. Alternatively, different modems may be used for different subscribers, depending on their choice of services.
Although preferred embodiments are described herein with reference to ADSL and ISDN services and certain specific spectral bands used for these services, it will be appreciated that the principles of the present invention may similarly be applied to other types of digital transmissions, and particular to t
Frenkel Liron
Rippin Boaz
Abelman ,Frayne & Schwab
Ryman Daniel
Tioga Technologies Inc.
Vu Huy D.
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