Pulse or digital communications – Transceivers – Modems
Reexamination Certificate
1999-08-10
2001-05-08
Vo, Don (Department: 2631)
Pulse or digital communications
Transceivers
Modems
C379S093090, C379S093080
Reexamination Certificate
active
06229846
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
Not Applicable
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not Applicable
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to modem communications. More particularly, the invention pertains to a system and method for supporting high speed, bi-directional analog modem communications at a data rate of substantially 64 Kbps. Still more particularly, the invention concerns a line card providing an interface between an analog modem and a digital transmission network, such as the digital portion of the Public Switched Telephone Network (PSTN).
2. Description of the Prior Art
Considerable attention has been directed toward increasing the data transfer rates of analog modems communicating over conventional local loop POTS (Plain Old Telephone Service) lines. In accordance with the V.90 standard currently in use, the maximum data rate for analog modems operating in the downlink direction (server to client) is 56 Kbps. In the up-link direction (client to server), the current maximum data rate is 33.6 Kbps.
Several factors limit analog modem technology to these speeds. First, filtering implemented at the line card interface between the local loop and the digital portion of the PSTN restricts available bandwidth to approximately 3000 Hz. Second, FCC regulations impose a maximum power limit of −9 dBm for analog signals carried on POTS lines terminating at RJ-11 jacks. This power limit reduces signal to noise ratios and also impairs the detection and processing of high frequency signal components, which are attenuated by filter-induced roll-off. Third, the analog-to-digital conversion and i-law or A-law companding performed by the line card CODEC (enCODer/DECoder), which is intended to reduce quantization noise, limits the CODEC's ability to use more than seven bits of each eight bit PCM codeword for the digital-to-analog conversion performed in the downlink direction. Conventional companding, moreover, has failed to increase the quantization error signal-to-noise floor above approximately 38-42 dB.
Accordingly, there is a need in the modem communication art for increased analog modem speed, in both the up-link and downlink directions. What is required is a system and method that improves signal processing efficiency at the interface between an analog modem and a digital transmission network to which it is connected in order to provide high speed, bi-directionally symmetrical modem communication. In a digital telephone system implementing 64 Kbps PCM codeword transmission, an improved line card at the local switch is indicated, such that bi-directional 64 Kbps analog modem communication may be facilitated on the local loop.
SUMMARY OF THE INVENTION
A system and method in accordance with the invention provide support for increased communication speed between data communication devices connected to a digital communication network, including analog modems communicating with the digital communication network via analog signaling pathways that may also support voice traffic. Following the initiation of network communication between two such data communication devices, determination is made whether the communication devices include at least one analog modem and another data communication device capable of communicating at the increased data communication speed. If so, adjustment is made, as necessary, to the filtering present on the analog signaling pathway between the digital communication network and the analog modem in order to provide increased analog signaling bandwidth.
Analog signals sent by the transmit side of the analog modem are converted to digital form by an Analog-to-Digital Converter (ADC) and compressed by a compression circuit for transmission as digital codewords over the digital communication network. The compression profile is preferably linear so that the digital codewords correspond to equally spaced voltage levels throughout the range of voltages sampled by the ADC. Alternatively, a nonlinear compression profile can be implemented in which fewer digital codewords are used to represent lower voltage levels than for higher voltage levels. In the receive direction, digital data signals carried across the digital communication network are expanded by an expansion circuit in inverse proportion to the aforementioned compression profile and converted to analog form by a Digital-to-Analog Converter (DAC) for transmission across the analog signaling pathway.
In preferred aspects of the invention, the digital communication network carries PCM codewords at a rate of 64 Kbps (per the DSO standard), and the analog modem transmits and receives Pulse Amplitude Modulated (PAM) analog signals in synchronization therewith. The determination as to whether high speed data communication is possible preferably includes a first determination of whether the communication devices include an analog modem and another data communication device, and a second determination of whether the analog modem and data communication device are capable of communicating at the increased data communication speed. The first determination may include detecting a modem tone being exchanged between the communication devices. The modem tone could be a tone for disabling echo cancellation in the analog pathway or any other suitable signal. The second determination may include detecting a signal sent by one or both communication devices indicating that 64 Kbps communication has been negotiated. Alternatively, the second determination could include detecting a signal sent by one of the communication devices that is indicative of the other communication device being unable to support the increased data communication speed (e.g., it is not accepting a 64 Kbps communication request). The filter adjustment preferably includes disabling analog filtering used during voice communications over the analog signaling pathway, or at least disabling any analog filtering on the analog signaling pathway that prevents an analog signaling bandwidth of at least approximately 4000 Hz. The encoding of analog signals to digital form and the subsequent compression thereof preferably includes sampling the analog signals using, by way of example only, a twelve to sixteen bit encoding scheme followed by compressing the samplings into eight bit PCM codewords according to the selected compression profile. The expanding and subsequent decoding of digital signals to analog form preferably includes converting the eight bit PCM codewords carried on the digital network into, by way of example only, twelve to sixteen bit codewords in inverse proportion to the compression profile, and converting these expanded codewords into discrete PAM voltage levels that substantially correspond to the PCM codeword quantization levels.
In a most preferred embodiment, the invention is implemented using a line card interface between the analog signaling pathway(s) and the digital communication network.
REFERENCES:
patent: 5365577 (1994-11-01), Davis et al.
patent: 5801695 (1998-09-01), Townshend
Lassig Mark Alan
Posthuma Carl Robert
Duft Walter W.
Lucent Technologies - Inc.
Phu Phuong
Vo Don
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