Asymmetric modem communications system and method

Details Asymmetric modem communications system and method Asymmetric modem communications system and method

1999-03-22

2002-07-16

Ngo, Ricky (Department: 2664)

Multiplex communications

Data flow congestion prevention or control

Flow control of data transmission through a network

C370S276000, C370S410000, C370S493000, C375S222000

Reexamination Certificate

active

06421320

ABSTRACT:

FIELD OF THE INVENTION
The present invention generally relates to data communications and, more particularly, to an asymmetric modem communications system and method for achieving high speed data transfers through a telephone network that includes both digital and analog communications mediums.
BACKGROUND OF THE INVENTION
A telephone network is often used as an interface between a digital modem and an analog modem. Generally, a digital modem is a device that communicates digital data by using digital signals that replicate analog waveforms. An analog modem is a device that communicates digital data by encoding the data on analog waveforms.
FIG. 1
shows a typical telephone network
99
for interconnecting a digital modem
101
and an analog modem
102
. The digital modem
101
is usually interconnected with a digital network
113
via digital connections
112
a,
112
b.
For instance, the digital modem
101
may be interconnected to a digital network
113
in the form of a public switch telephone network (PSTN) via a Local Exchange Carrier (LEC) subscriber loop. The digital network
113
may comprise, among other things, a T1 carrier system, a basic rate or primary rate Integrated Services Digital Network (ISDN), a fiber optic cable network, a coaxial cable network, a satellite network, or even a wireless digital communications network. Communications over the digital network
113
are conducted in accordance with a pulse code modulation (PCM) scheme. Channel capacity through these digital facilities is typically between 56 and 64 kilobits per second (kb/s). Coding of the signals is also employed so that compression and a constant signal/distortion performance over a wide dynamic range is achieved for optimal transmission of voice signals. A commonly used coding technique is a nonlinear mu-law coding.
The digital network
113
is in turn interconnected with another LEC subscriber loop that includes a coder/decoder (codec)
106
. The codec
106
is interconnected with the digital network
113
via digital connections
114
a,
114
b.
The codec
106
is often situated at a telephone company office or along a street near the analog modem subscriber in an SLC device. The codec
106
provides an interface between the digital network
113
and an analog telephone connection
118
, sometimes referred to as a copper loop. For communications in the direction from the digital network
113
to the analog modem
102
, the codec
106
includes a mu-to-linear-analog converter
109
, which includes digital-to-analog (DAC) conversion functionality. The converter
109
converts nonlinear mu-law levels to a linear analog signal. For communications in the direction from the analog modem
102
to the digital network
113
, the codec
106
includes a linear-analog-to-mu converter
107
, which includes analog-to-digital (ADC) conversion functionality. The converter
107
converts the linear analog signal to nonlinear mu-law levels.
A hybrid
103
is in communication with the DAC and ADC via respective LPFs
111
,
105
. The hybrid
103
serves to separate the bidirectional analog signals from the analog telephone connection
118
into unidirectional transmit and receive analog signals sent to and received from the ADC
107
and the DAC
109
respectively.
Furthermore, the analog modem
102
is connected to the analog telephone connection
118
and communicates analog signals therewith. Thus, communications occur between the digital modem
101
and the analog modem
102
by way of the digital network
113
and the codec
106
.
Researchers have been attempting to increase the speed at which data can be transferred through the telephone network between the digital and analog modems
101
,
102
. U.S. Pat. No. 5,394,437 to E. Ayanoglu et al. describes a high speed analog modem
102
that is synchronized to the DAC and ADC clocks of the codec
106
. Further, a pulse level modulation scheme is utilized to communicate data along the telephone connection
118
. With pulse level modulation, a plurality of voltage levels are communicated along the analog telephone connection
118
. This system permits data transfer rates above 40 kb/s.
Although the aforementioned system is meritorious to an extent in terms of increasing data transfer rates, it suffers from various undesirable problems and disadvantages.
A primary disadvantage of the Ayanoglu system involves echo problems. Generally, there is sensitivity to quantized echoes because detection occurs at the codec quantizer, and there is an inability to provide echo cancellation prior to detection. More specifically, echo cancellation at the analog modem
102
is not a problem given its exceptional linearity. However, the echo at the codec is a major problem due to the mu-law coding and limited hybrid quality. On a poor subscriber loop, the receive signal is attenuated. The echo is increased due to the impedance mismatch. In fact, the echo level can exceed the receive signal level. Accordingly, both the analog modem
102
and the digital modem
101
will attempt to utilize all PCM levels. When the digital modem
101
echo results from one of the upper compander levels and the analog modem
102
has transmitted on one of the lower levels, then the echo will control the channel bank encoder step size. In this case, it is difficult to resolve the symbols from the analog modem
102
.
Another disadvantage of the Ayanoglu system is that it requires complex timing synchronization with the codec.
Hence, there exists a need in the industry for systems and methods for increasing the speed of data transfers through a telephone network
99
, which comprises both a digital and analog communications mediums, between a digital modem
101
and an analog modem
102
.
SUMMARY OF THE INVENTION
The invention provides for an asymmetric modem communications system and method for achieving high speed data transfers through a telephone network that includes both digital and analog communications mediums. In general, the system includes means for concurrently communicating first and second signals, respectively, in opposite directions along the connection between the communications devices and means for modulating the first and second signals with different modulation techniques. The communications occur in full duplex manner.
In a possible implementation, a digital modem is interfaced to a digital network. The digital network is connected with a coder/decoder (codec). The codec is interfaced with a two-wire analog telephone connection, sometimes referred to as a copper loop. Finally, the telephone connection is interfaced with an analog modem.
Both the digital and analog modems have a transmitter and a receiver. The digital modem has a transmitter that pulse modulates digital data in the sense that it generates and transmits pulse levels and a receiver that receives and demodulates signals in accordance with the standard V.34 communications protocol. Generally, the V.34 protocol employs a form of quadrature amplitude modulation/demodulation. The analog modem has a transmitter that transmits and modulates signals in accordance with the V.34 communications protocol and a receiver that demodulates the pulse levels into digital data.
Communications over the digital network are conducted in accordance with pulse code modulation (PCM). Moreover, communications over the analog connection occur via encoding of digital data on analog waveforms.
With the foregoing configuration, asymmetric data communications are realized. Specifically, the analog modem communicates to the digital modem using the V.34 communications protocol at a data rate of between 33,600 b/s and 2400 b/s, inclusive, while the digital modem communicates to the analog modem at a data rate of between 64,000 b/s and 2400 b/s, inclusive.
Worth noting is that the invention can also be broadly viewed as providing a method for bidirectionally communicating information between first and second communications devices along a connection. The method can be summarized as follows: concurrently communicating first and second signals in opp

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