Pulse or digital communications – Transceivers – Modems
Reexamination Certificate
2000-09-08
2004-01-13
Fan, Chieh M. (Department: 2634)
Pulse or digital communications
Transceivers
Modems
C375S260000, C375S285000, C375S346000, C370S201000, C379S406010, C379S417000, C455S295000, C455S501000
Reexamination Certificate
active
06678316
ABSTRACT:
I. BACKGROUND OF THE INVENTION
1. Technical Field
This invention relates to the field of providing high speed digital data services to a digital service subscriber and, more particularly, to so-called asymmetric digital subscriber line (ADSL) services and to a method of reducing near-end crosstalk in discrete multi-tone (DMT) modems located at a central office and at the subscriber's premises for providing ADSL services.
2. Description of the Related Arts
In the field of cable television, cable modem technology is emerging which provides increased bandwidth services to the home. Cable television equipment manufacturers are promoting the upgrading of cable television distribution plant to comprise so-called hybrid optical fiber and coaxial cable (hybrid fiber/coax) facilities. It is anticipated in the cable field that bandwidths to and from the subscriber can be increased so that bidirectional voice and data services may be provided in addition to traditional cable television programming. In the related field of telecommunications, there exists a considerable amount of embedded distribution plant comprising high capacity twisted wire pair cable. Historically, each cable pair was specially loaded with inductance coils at periodic intervals along the path from a serving central office to the subscriber's premises to improve voice telephony performance. The inductance loading countered the effects of the high capacity cable and provided a flat bandwidth on each twisted cable pair at voice frequencies. On the other hand, frequencies higher than voice bandwidth were intentionally attenuated to such a degree that the twisted wire cable pair was unusable for other than a voice channel. When the loading is removed, the twisted cable pair bandwidth improves and becomes more comparable to that of coaxial cable.
An emerging technology in the telecommunications arts that competes with cable modem technology is so-called asymmetric digital subscriber line (ADSL) technology. Referring to
FIG. 1
taken from American National Standards Institute standards document T1.413-1995, there is shown a public switched telecommunication network (PSTN)
105
and a digital network (for example, a frame relay, asynchronous transfer mode (ATM), Internet or other digital network)
110
at the left. At the right is the subscriber's premises. The digital network
110
is coupled via a logical interface V to an ADSL transceiver unit (ATU) at the serving central office (C). Also at a serving central office are located a splitter
120
for splitting the telecommunications services from the digital services, typically based on frequency. For example, a voice channel may still be preserved at from 0-4000 Hz. The splitter function may be integrated into ATU-C
115
(and at the remote subscriber site, into ATU-R
135
). Interface U-C represents the subscriber loop (twisted pair) interface at the central office C and interface U-R represents the subscriber loop interface at the remote subscriber terminal end of the twisted wire cable pair or other facility
125
. Facility
125
may comprise, for example, a twisted wire pair or a hybrid optical fiber/twisted wire pair facility or other wired or wireless facility having comparable or greater bandwidth. Service module (SM)
150
or
155
at the remote location may comprise an intelligent telecommunications terminal, a personal computer, a television terminal, an energy management system, a security system or other service module known in the art. Plain old telephone service is (POTS) module
145
represents a traditional telecommunications terminal such as a facsimile terminal, voice bandwidth modem or standard telephone. Facility Cl distribution
140
within the subscriber premises may comprise, for example, a bus such as a home bus or a star network or other configuration. By bus as used herein is intended a communications link that may be wired or wireless connecting a plurality of devices together. The bus may be arranged so that there is contention for access to the bus according to priorities or be provided sufficient capacity to alleviate the likelihood of contention. Interface T represents the interface between a service module (SM) and/or a bus/star
140
to other service modules (SM's).
Referring to
FIGS. 2A and 2B
, there are shown respectively an ATU-C transmitter whose reference diagram is taken from A.N.S.I. T1.413-1995 and an ATU-C receiver whose reference diagram is derived therefrom. In
FIG. 2A
, there is shown an expanded functional block diagram of the transmitter portion of ATU-C
115
of
FIG. 1. A
multiplexer/sync control unit
200
provides the interface to the digital network
110
. Various high speed data rate links AS
0
, AS
1
, AS
2
and AS
3
at multiples of 1.536 Mbits/sec are provided toward digital network
110
. In particular, each AS link represents an independent downstream simplex (unidirectional downstream) bearer of data traffic. Lower speed data services are also shown and represented by LSO (16 or 64 kbits/sec), LS
1
(160 kbits/sec) and LS
2
(384 or 576 kbits/sec). Each LS link may represent a duplex bearer (bidirectional) carrying both downstream and upstream traffic or, in the alternative, a unidirectional simplex bearer.
CRC
205
and CRC
210
represent cyclic redundancy check in each direction of transmission. Scrambler and forward error correction
215
,
220
represent scrambling and forward error correction, for example, using Reed-Solomon error correction coding, in each direction of transmission. Interleaver
225
provides a data interleaving function as is further described in A.N.S.I. T1.413-1995, incorporated by reference as necessary. Tone ordering function
230
provides tone selection and control functions as are also described by A.N.S.I. T1.413-1995. Constellation encoder (if used) and gain scaling functions are represented by block
235
. The inverse discrete Fourier transform function applied for data modulation is represented by block
240
. Two data directions are shown coupling IDFT
240
and output parallel to serial buffer
245
where a cyclic prefix is added to each data frame. Finally, a digital to analog converter and analog signal processing function are represented by block
250
which interfaces the subscriber facility
125
.
Referring to
FIG. 2B
, the ADSL receiver at the central office is shown. The horizontal arrows are reversed in direction from FIG.
2
A. Data demultiplexer
255
interfaces the digital network
110
. Descrambler
265
,
258
, deinterleaver
270
, decoder
280
, DFT
285
, input serial to parallel buffer
290
and analog to digital converter
295
represent the significant changes in function between ATU-C transmitter and receiver.
Referring to
FIG. 3
, at a subscriber terminal, the transmitter (ATU-R) is similarly configured as ATU-C but it is assumed that channels operate at LS
0
, LS
1
or LS
2
toward the subscriber's equipment. Cyclic redundancy checks
305
/
310
are provided for each direction of transmission to/from subscriber equipment
375
. Scrambler and forward error correction circuits
315
and
320
, for example, using Reed-Solomon error correction coding, are provided for especially secure data transmission. An interleaver
325
is provided in one transmit path. Tone ordering circuitry
330
is necessary for generating and ordering the discrete multi tones of the discret multi-tone (DMT) modem. The constellation encoder and gain scaler
335
may or may not provide a form of trellis data encoding and gain scaling for controlling the tone ordering. IDFT block
340
performs an inverse discrete Fourier transform for modulating the digital data. The output parallel to serial buffer
345
is provided for providing parallel to serial conversion to a digital to analog converter and analog signal processing interface
250
which interfaces the subscriber loop
125
.
The analog signal framing (
FIG. 4
) used in ADSL technology is obtained by passing quadrature amplitude modulation (QAM) samples through a D/A converter
250
or
350
. These s
Helms Howard David
Miller II Robert Raymond
Shively Richard Robert
AT&T Corp.
Fan Chieh M.
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