Multiplex communications – Pathfinding or routing – Through a circuit switch
Reexamination Certificate
1997-09-16
2001-09-04
Vu, Huy D. (Department: 2664)
Multiplex communications
Pathfinding or routing
Through a circuit switch
C370S399000
Reexamination Certificate
active
06285672
ABSTRACT:
FIELD OF THE INVENTION
The field of the present invention pertains to digital telecommunications. More particularly, the present invention relates to a method and system for increasing data transmission throughput in the public switched telephone network.
BACKGROUND OF THE INVENTION
Digital telecommunications across great distances has become a commonly used and widely adopted method of sharing information and ideas. The widespread adoption of facsimile technology, voice and video teleconferencing, and electronic mail has resulted in the formation of new paradigms of business organization. With the modern telecommunications technology, proximity of location is not nearly as relevant as connectivity bandwidth. Working professionals, governments, businesses, and other segments of the economy are reorganizing themselves around the widespread adoption of point to point telecommunications devices.
Essentially, digital telecommunications have proven to be a far more powerful and compelling method of communication than previous, conventional methods. Communication and exchanges between one user and another distant user occur virtually in real time. For example, a fax machine can communicate information and pictures nearly instantaneously, as opposed to conventional mail. Electronic mail allows one user to send numerous document files and large image files to the distant user immediately. The rise of this new paradigm has led to a huge increase in the demand for point to point digital telecommunications.
The problem, however, is that the public switched telephone network through which the majority of this information passes was not designed to handle high bandwidth digital data. The public switched telephone network (PSTN) was primarily designed for point to point voice communication. However, designers of data modems have attempted to make the best use of this situation, but even so, the vast majority of point to point digital communications across the PSTN proceeds at speeds much slower than the theoretical 64 Kbps limit of the PSTN when using digital switching of PCM channels. Users are demanding faster, reliable 64 Kbps connections.
Prior Art
FIG. 1
shows a diagram of a typical communications channel
100
in the PSTN. The communications channel
100
includes a user modem
101
sending transmit data and downloading receive data to and from a user on the left of
FIG. 1
(not shown). User modem
101
is coupled to a 2-4 wire converter
102
, and in turn, via a local loop, is coupled to a second 2-4 wire converter
103
. In communications channel
100
, the two wire line connecting 2-4 wire converter
102
and 2-4 wire converter
103
is referred to as the local loop. The 2-4 wire converter
103
is coupled to a PCM (pulse code modulation) codec
104
. The 2-4 wire converter
103
and PCM codec
104
are often integrated into one piece of equipment referred to as a subscriber line interface card. PCM codec
104
is coupled to a digital switched network
105
portion of the PSTN, and in turn, is coupled to a PCM codec
106
. PCM codec
106
is coupled to a 2-4 wire converter
107
, then via another local loop to a 2-4 wire converter
108
, and subsequently to a remote user modem
109
. Remote user modem
109
couples received data and transmit data to and from a user on the right of
FIG. 1
(not shown).
The signals from the user on the left of
FIG. 1
to PCM codec
104
are analog signals. The signals from the user on the right of
FIG. 1
to PCM codec
106
are also analog signals. However, the signals that are transmitted and switched across the digital switched network
105
are digital. The analog signals are converted into corresponding digital signals by the digital to analog converters (DACs) and the analog to digital converters (ADCs) of PCM codec
104
and PCM codec
106
. In the case of voice communications, the analog signal is a voice waveform and its corresponding digital signal in the PSTN is a sampled representation of the voice waveform, after it is subjected to A (or &mgr;) law compression. In the case of conventional data communication, the analog signal is an analog waveform onto which is modulated the digital data. The corresponding digital signal within the PSTN is a sampled representation of this analog waveform (also after A or &mgr; law compression).
The 2-4 wire converter
103
and the PCM codec
104
comprise the major portions of a subscriber line interface card. Similarly, 2-4 wire converter
107
and the PCM codec
106
also comprise a subscriber line interface card. The subscriber line interface card (SLIC) functions by converting the user's analog signal into a digital representation of the sampled analog signal after A or &mgr; law compression. Once converted, the digital signal is transmitted and switched through the digital switched network
105
of the PSTN. At the destination, the digital signal is coupled to another SLIC where it is converted from digital form back into analog form.
If the analog signal is a voice signal, the analog signal is used to drive the speaker of a telephone, recreating the user's voice. If the analog signal is a data signal (e.g., a V.21 modem signal), the digital data is extracted from the analog signal for use by the receiving user's computer system by user modems
101
and
109
. Communications channel
100
functions adequately when used to transmit and receive analog information (e.g., voices). However, communications channel
100
proves inefficient when utilized to transmit high bit rate data streams. The present state of the art allows a maximum data rate of 40-50 Kbps for K56-type modems (server to client direction only). The limitation for the latter being caused mainly by the A or &mgr; law expansion process at the local exchange.
One of the major bottlenecks to high speed data transmission through a communications channel is the presence of prior art SLICs. For example, one of the primary challenges in designing 56 Kbps PCM (pulse code modulation) type modems is accounting for the detrimental effects of the DACs and ADCs contained within the SLICs.
Prior Art
FIG. 2
shows a typical prior art SLIC
200
. SLIC
200
includes 2-4 wire converter
103
and PCM codec
104
. PCM codec
104
includes an 8 bit A law ADC
201
and an 8 bit A law DAC
202
. The 8 bit A law ADC
201
transmits information upstream to the digital switched network as a 64 Kbps digital data stream. The 8 bit A law DAC
202
receives downstream information as a 64 Kbps data stream. While SLIC
200
includes A law DAC
202
and A law ADC
201
, those skilled in the art understand that alternatively, SLIC
200
could include a &mgr; law DAC and &mgr; law ADC instead.
As is well known in the art, in the downstream direction (e.g., from an internet service provider to a user), 8 bit A law DAC
202
restricts the number of signaling levels from a theoretical maximum of 256 voltage levels. The number of levels actually available are much lower. Many of the signaling levels around the origin are spaced too closely together and are unusable due to DC offset voltage levels and other signal characteristics of the 8 bit A law ADC
202
. This forces the user modem
101
to use larger amplitude levels with a corresponding increase in the transmit signal power, as the bit rates increase. Thus, one of the factors limiting the maximum bit rate is the maximum allowable power levels on the local exchange lines. Those desiring more detailed information regarding A law and &mgr; law codec standards are directed to “ITU-T G.711, PULSE CODE MODULATION (PCM) OF VOICE FREQUENCIES, International Telecommunications Union” which is incorporated herein as background material.
One solution presently available to the user is to convert to ISDN (integrated services digital network). However, this requires an expensive hardware change to the SLIC
200
(e.g., the removal and replacement with a new SLIC) and software and other hardware changes to PSTN such that the user's local loop is no longer viewed by the PSTN as a regular analog line. Conseque
Beadle Burk William
McVerry Francis
Harper Kevin C.
Rockwell Semiconductor Systems Inc.
Snell & Wilmer
Vu Huy D.
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