Pulse or digital communications – Transceivers – Modems
Reexamination Certificate
1999-04-29
2003-02-04
Phu, Phuong (Department: 2631)
Pulse or digital communications
Transceivers
Modems
C375S220000, C375S221000, C375S231000, C375S232000
Reexamination Certificate
active
06516025
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
Not applicable.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not applicable.
BACKGROUND OF THE INVENTION
This invention is in the field of telecommunications, and is more specifically directed to improved upstream communications from a client modem to a telephone network central office over conventional analog telephone lines.
With the recent explosion in Internet usage by both business and consumers, the use of dial-up modem communication between personal computer users and Internet service providers (ISPs) over conventional telephone networks, such as the Public Switched Telephone Network (PSTN) in the United States, has become widespread. Important advances have been made in recent years that have greatly increased the data rate at which communications may be carried out over the existing telephone network, realized to a large degree over twisted-pair wires. These important advances have also greatly reduced the cost of equipment required for such high data rate communication, as well as the cost of providing communications services.
These advances have also improved the capacity of telephone networks to carry higher volumes of voice and data traffic. One such advance is the advent of digital communications over the telephone network among the various “central offices”, generally carried out over fiber-optic cable. One well-known technique used to implement such digital communications utilizes coder/decoder functions (“codecs”) at the central offices, as will now be described.
FIG. 1
illustrates a simple example of communication between a pair of telephones U over a conventional telephone network. In this example, telephones U
1
and U
2
are remote from one another, such that telephone U
1
is associated with central office CO
1
, and telephone U
2
is associated with central office CO
2
. As is well known in the art, the term “central office” refers to a field location of the local telephone company at which switching and other telephone network management functions take place. Communications between telephones U
1
and telephone U
2
, in this conventional example, will be carried out by way of analog connections between telephones U
1
, U
2
and their respective central offices CO
1
, CO
2
; the analog communication system between telephones U and central offices CO are referred to as “subscriber loops”. Codecs
2
1
,
2
2
are provided within central offices CO
1
and CO
2
, respectively, to convert the analog signals received from their respective telephones U
1
, U
2
into digital signals for communication over PSTN
4
, and vice versa. As illustrated in
FIG. 1
, each codec
2
includes analog-to-digital converter (A/D)
6
, for converting analog signals traveling from its associated telephone U into digital signals to be sent over PSTN
4
; conversely, each codec
2
also includes digital-to-analog converter (D/A)
7
, for converting digital signals received from PSTN
4
into analog signals for communication to its associated telephone U. The digital communication over PSTN
4
may be carried out over fiber optic facilities or other high speed communications trunks, according to conventional standards such as DSO, SONET, and the like. As a result of the conversion of the analog voice signals into digital bitstreams, the volume of telephone traffic carried by PSTN
4
can be quite large.
Each of codecs
2
include an associated A/D
6
, as noted above. According to conventional implementations, each A/D
6
converts the incoming analog signal into a stream of digital values by way of sampling. Typically, the sampling rate of A/D
6
is 8 kHz. As is fundamental for A/D conversion, each sample of the analog signal is converted into a digital value that most closely approximates its amplitude. In modern voice telephone networks, the conversion carried out by A/D
6
typically follows a &mgr;-law estimation, rather than a linear mapping of analog to digital values, in order to provide a pleasant voice signal with relatively accurate dynamic range. In any event, error results from the approximation that is necessarily in A/D conversion, amounting to the difference between the true amplitude of the sampled analog signal and its digital approximation; such error is commonly referred to as “quantization error”. The amount of quantization error, or conversely the precision of the conversion, depends directly upon the number of bits used to express the digital value. Current standards for voice communication call for an eight-bit analog-to-digital conversion at the central office, such that each analog sample amplitude is assigned to one of 256 possible values. As such, quantization error may be significant, especially at high data rates.
As noted above, modem communications among computing devices has now become widely popular, especially with the widespread use of the Internet by business and individuals. As is fundamental in the art, a modem (shortened from “modulator/demodulator”) is a device that converts signals to be communicated from the computer over the telephone network from a digital bitstream into an analog signal within the voice band (and vice versa), so that data communications can be readily carried out over the same telephone network used for voice communications.
FIG. 1
illustrates a typical home computer C
1
having a modem M
1
that is connected into the telephone network, for example at the same location at which telephone U
1
is connected. As illustrated in
FIG. 1
, digital communications are of course carried out between computer C
1
and modem M
1
, but communications between modem M
1
and central office CO
1
are analog, just as are the voice communications between telephone U
1
and central office CO
1
. As such, communications from computer C
1
and PSTN
4
begin as digital data, are converted into analog signals by modem M
1
, and are converted back into digital form by A/D
6
1
in central office CO
1
.
In the arrangement of
FIG. 1
, codec
2
1
necessarily inserts error into the communication of digital data from computer C
1
to PSTN
4
. This error is directly due to the quantization error of A/D
6
1
. Because of the approximation carried out by A/D
6
1
, its digital output will not always match the corresponding digital value presented by computer C
1
to modem M
1
for conversion into the analog domain. The presence of this quantization error at central office codecs is currently the limiting factor in the data rates at which modem communications may be carried out over modern telephone networks, especially considering that Federal Communications Commission rules limit the maximum power of telephone communications.
By way of further background, the V.34 standard for modem communications at data rates of up to 33.6 kbps utilizes a precoder in the transmitting modem. The modulation used according to this standard is quadrature amplitude modulation (QAM). This precoder is intended to implement the feedback coefficients obtained in decision feedback equalization for the communications channel, partially compensating for channel distortion; the remainder of the distortion is compensated by a linear equalizer at the receiving modem.
Recently, it has been observed that the bulk of modem communications are telephone companies carry out digital communications between the PSTN and the ISPs, whether or not such communications travel through a central office. An example of this conventional arrangement, for a single remote computer C
1
, is shown in FIG.
2
. In this example, computer C
1
communicates digitally with modem M
1
as before; modem M
1
in turn communicates with central office CO
1
using analog signals. Central office CO
1
includes codec
2
1
as before, with A/D
6
1
, converting the analog signals from modem M
1
into digital signals for communication over PSTN
4
; codec
2
1
also includes D/A
7
1
for converting digital traffic into analog signals, for communication to modem M
1
. As shown in
FIG. 2
, however, ISP
10
communicates digitally with PSTN
4
(perhap
Ali Murtaza
Warke Nirmal C.
Brady III W. James
Hernandez Pedro P.
Phu Phuong
Telecky , Jr. Frederick J.
Texas Instruments Incorporated
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