Method and apparatus for telephone network impairment...

Details Method and apparatus for telephone network impairment... Method and apparatus for telephone network impairment...

1999-04-22

2003-02-25

Pham, Chi (Department: 2631)

Textiles: cloth finishing

Beating

Beetling

C375S222000, C379S092040

Reexamination Certificate

active

06523233

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to signal processing, and more particularly it relates to the resolution of errors caused by analog and digital impairment of telephone networks during the signal transmission between high speed modems.
BACKGROUND OF THE INVENTION
Telephone networks are generally used for voice as well as data communications with reference to data communications, a plurality of high speed modems (one of which is typically remotely disposed at the premises of a customer) are coupled together via a telephone network. Each modem is typically connected to the telephone network by a local exchange carrier (LEC) subscriber loop.
Generally, telephone networks include both analog and digital components. The digital components may include a T1 digital link, an integrated service digital network (ISDN), a fiber optic cable network, a coaxial cable network, a satellite network, and/or a wireless digital communications network. Further, the LEC subscriber loop may comprise either an analog or a digital communications path.
During a data communication session between remotely disposed modems, one modem (the transmitting modem) initiates a data transmission and another modem (the receiving modem) receives the transmission. Usually an A/D converter is required for converting analog signals from an analog portion of the network into digital signals for subsequent transmission over the digital portion of the network. Similarly, a D/A converter is required for converting digital signals carried on the digital portion of the network into analog signals for subsequent transmission over the analog portions of the network.
In North America and Japan, data communications over the telephone networks are conducted in accordance with pulse code modulation (PCM) “&mgr;-law” encoding and decoding techniques. In Europe, communications are conducted in accordance with “A-law” encoding and decoding technique. In accordance with those well-known PCM &mgr;-law or A-law techniques, signals are transmitted in the form of PCM signals. PCM signals consist, in general, of a series of binary code words in which each word represents an instantaneous value of a periodically sampled and quantized analog signal. In normal usage, these code words are transmitted in the form of a serial bit stream to a receiving subscriber loop where they are decoded into a reconstructed version of the original analog signal.
When a PCM code goes through a telephone network, the PCM code values can be changed due to digital switching in the telephone networks. These changes introduce an error element known as “impairment” which can have both digital and analog components. The combined analog and digital components are known collectively as “network impairment.” The network impairment causes PCM modems to mis-code signals and interferes with the performance of the data communication.
Attempts have been made to compensate for network impairment. These techniques include the use of encoders/decoders which hold digital impairment tables depicting preselected analog levels corresponding to the digital code words in accordance with the &mgr;-law or A-law standards.
FIG. 4
illustrates a prior-art telephone network configuration
10
implementing one of these prior-art techniques. In
FIG. 4
, two remotely disposed (client site) modems
101
and
115
are coupled via a typical telephone network. In the outgoing (transmitting) direction, modem
101
transmits a signal to a first hybrid
103
. The hybrid
103
separates a bidirectional analog signal received from the modem
101
into a unidirectional transmit signal. This unidirectional transmit signal is then converted to a digital signal at A/D converter
105
. The digital signal is then encoded into a PCM signal at encoder
107
, which routes the encoded signal to a digital network
113
. After traversing the digital network
113
, the encoded signal is decoded at decoder
109
and is then converted back (regenerated) to an analog signal at D/A converter
111
. The analog signal is then output to a second hybrid
103
A which separates the bidirectional signal received from the D/A converter
111
into a unidirectional signal and delivers the unidirectional signal to the second modem
115
.
In the reverse incoming (receiving) direction, the signal follows an essentially identical path in the reverse direction via the second hybrid
103
A, A/D converter
105
A, encoder
107
A, digital network
113
, decoder
109
A, D/A converter
111
A, and the first hybrid
103
.
In the configuration of
FIG. 4
, a typical signal is encoded/decoded many times while being transmitted through the telephone network
10
. The constant encoding/decoding and conversion between digital and analog signals produces an inherent quantization error in the transmitted signal. This error arises from the fact that the amplitude of the regenerated analog signal does not exactly match the analog signal level of the original signal.
FIG. 5
illustrates an alternative telephone network configuration
20
in which the A/D or D/A conversion and the need for encoding/decoding between digital and analog signals is reduced. This is a typical configuration of a communications network wherein a central site modem
201
(a network modem) downloads information to a client modem
210
(for example, an internet application where a network modem downloads large blocks of information to client site modem
210
). In the configuration of
FIG. 5
, central site mode
201
is coupled directly to the digital network
213
. The quantization error is reduced because there is no conversion or encoding/decoding between the modem
201
and the digital network
213
. The connection between the digital network
213
and the client site modem
210
is essentially identical to the same connection in FIG.
4
.
U.S. Pat. No. 5,724,393 (the '393 patent), assigned to the assignee hereof, discloses a method of reducing the quantization error resulting from the encoding/decoding in transmissions between modems. As explained in the '393 patent, the quantization error can be reduced by scaling down the amplitude levels of signals generated by the transmitting modem by a predetermined reduction factor and then performing an inversion mapping in which the amplitude level of the factored, &mgr;-law encoded signals is scaled up by a predetermined inversion factor.
Typical prior art systems for reducing telephone network impairment rely on the use of predefined impairment tables associated with the most commonly used telephone switches. The need to use large predefined telephone network impairment tables reduces the flexibility of these prior art systems and increases the cost. In addition, this solution can be difficult to deploy in a long distance telephone network which combines different types of switches (e.g., mixed A-law and &mgr;-law standards) because the encoders and decoders used in the system will vary system-wide depending on which standard is in use.
Thus, there exists a need for a general, low cost method for detecting and compensating telephone network impairment which may result in a constant signal performance over a wide dynamic range (with minimum quantization error) and which may be applicable to a variety of telephone networks including mixed &mgr;-law and A-law networks.
SUMMARY OF THE INVENTION
A method and apparatus for automatically detecting and compensating for telephone network impairment in a telephone network is disclosed. The inventive method eliminates the need for predefined digital impairment tables and can be applied to any telephone network including those which utilize different types of switches and &mgr;-law/A-law mixed switches. The inventive method also eliminates the quantization error typically present in the transmission of signals due to the use of encoders/decoders. It is a low cost solution and does not require any pre-knowledge about the telephone network and may be used in telephone network deploying switches which are currently under development or often upgraded.
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