Pulse or digital communications – Transmitters – Antinoise or distortion
Reexamination Certificate
1996-11-15
2001-02-20
Bocure, Tesfaldet (Department: 2731)
Pulse or digital communications
Transmitters
Antinoise or distortion
Reexamination Certificate
active
06192087
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
Not Applicable
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not Applicable
REFERENCE TO A “MICROFICHE APPENDIX”
Not Applicable
BACKGROUND OF THE INVENTION
The present invention relates to transmission of digital information over analog medium connected to a digital network and more particularly to the PCM modems (“Pulse Code Modulation”).
The world based on the Internet has seen tremendous growth in recent months. As more users begin browsing and downloading information from the World Wide Web, there has been a great desire to be able to increase the data rate. The desire is even greater for users accessing the Internet through an Internet service provider (ISP), since most of the users are linked up to the Net through a personal computer and a modem. Conventional analog modems, such as the V.34 modems, however, view the public switched telephone network (“PSTN”) as an analog channel, even though the signals are digitized for communications throughout most of the network. As such, various effects and impairments due to quantization impose a limitation on the data rate of the channel to about 35 Kbps. This limit has been commonly known as Shannon's Limit. (See Shannon, C. E. and W. Weaver,
The Mathematical Theory of Communication,
University of Illinois Press, 1949).
There has been a recent development of a high-speed communications technology based on the PCM modems, where data rate of at least 56 Kbps is said to be actually attainable. The PCM modem technology is based on the simple realization that the public switched telephone networks (PSTN) is increasingly a digital network and not an analog network. Also, more and more central site modems are connected to the PSTN through digital connections, i.e. T1 in the U.S. and E1 in Europe, without utilizing a CODEC (coder/decoder). The conventional modem, however, still interprets this digital stream as the representation of the modem's analog signal. With the PCM modems, a much higher data rate can be achieved without the complicated task of re-wiring the user's site or modifying the telephone network.
Note that by “central site modems,” it is referred to those modems installed at an ISP, or at a corporation to allow many simultaneous connections for remote LAN access. Also note that a CODEC is a device which sits between the digital portion of the network and the analog local loop for converting between analog and digital.
The recent 56 Kbps technology seeks to address an impaired section of the communications path of the PSTN digital network, where the impairment is due to the hybrid and the copper wire connection between the telephone central office and the user's home, usually referred to as the analog local loop.
Since recently, much has been described about the PCM modems and how they can and should facilitate downstream data communication at a much higher rate than present paradigm. For example, the PCM modem has been the subject of a recent Telecommunications Industry Association (TIA) Technical Committee TR-30 Standards meeting on Oct. 16-17, 1996. The submitted technical contributions include Mr. Guozhu Long's
DC Suppresser for
56
K Modems,
Mr. David C. Rife's 56
Kbps Channels,
Mr. Veda Krishnan's
V.pcm Modem Standard,
Mrs. Vedat Eyuboglu's
PCM Modems: A Technical Overview,
Mr. Richard Stuart's
Proposal for a High Speed Network Access Modem,
and Mr. Vladimir Parizhsky's
U.S. Robotics' x
2
Technology: Technical Brief.
These contributions are hereby incorporated by reference.
Also, there have been recent publications on the overall data communication system based on the PCM modem. The first one is a 1995 presentation disclosed by Pierre A. Humblet and Markos G. Troulis at Institute Eurecom, entitled
The Information Driveway,
1995, which purports to explain the basic concepts on the high speed modem. The second one is a PCT Patent Publication, dated Jun. 13, 1996, International Publication Number WO/9618261, by Brent Townshend, which discloses a High Speed Communications Systems for Analog Subscriber Connections. This Publication, on pages 17-19, discloses an overall high speed system based on the PCM modems, which also implements DC null elimination on the transmitter side. These are also hereby incorporated by reference, since they provide a fair reference to the basics of the high speed PCM modems and their environment.
Additionally, U.S. patent issued to Ender Ayanoglu of AT&T, U.S. Pat. No. 5,528,625 dated Jun. 18, 1996, entitled High Speed Quantization-Level-Sampling Modem with Equalization Arrangement, discloses a QLS modem for high-speed data communication. Another U.S. patent also issued to Ender Ayanoglu of AT&T, U.S. Pat. No. 5,394,437, dated Feb. 28, 1995, entitled High-Speed Modem Synchronized to a Remote CODEC, discloses a high-speed modem for data transmission over an analog medium in tandem with a digital network. These references are also hereby incorporated by reference.
FIG. 1
depicts a conceptual diagram of the high-speed communication path using the PCM modem technology. An ISP, or “central site”,
100
is digitally connected to a telephone network
130
through its transmitter
110
and receiver
120
. The network
130
is connected to a local loop
150
through a central office line card
140
. The line card typically has a Pulse Code Modulation (“PCM”) CODEC implemented within. The local loop
150
is connected to the user's PC at the user's site through the user's modem
160
. As can be appreciated by those skilled in the art, the connection between the ISP modem's transmitter
110
to the telephone network
130
is a digital connection with a typical data rate of about 64 Kbps. Since the parameters of the telephone network
130
and line card
140
are dictated and set by telephone company's specification and operation, the central site transmitter
110
will need to transmit the digital data in certain way to fully exploit its digital connection to the network. However, dealing with the central site transmitter in this new paradigm has its obstacles.
In this type of data communication systems, such as the transmitter
110
in the central site, the transmit signal points are determined by physical constraints, and cannot be made part of the overall transmitter design. An example is when a signal is transmitted from within the digital part of the telephone network to the residential customer. That system will be the focus of the following description, although the methods described hereinafter will in many cases be applicable to other systems.
The transmitter
110
sends 64 Kbps of data into the network
130
, which eventually gets translated to an analog signal in a digital-to-analog converter in a central office line-card CODEC
140
. To send the maximum of 64 Kbps of data, all transmitted bits must be determined by the incoming data. Any modification of the outgoing sequence of bits resulting in correlation between the bits represents a redundancy in the signal, and will thus result in a lower data rate. Therefore, if the analog signal is to be controlled in any way, the data rate must be lowered. The main task confronting the designers, then, is to perform the desired control with the least amount of redundancy added.
As will be described in the present application, methods and apparatus for controlling the spectrum of the line codec's output signal are disclosed. In particular, methods of minimizing the energy of the signal at unwanted frequencies are described. These methods can be readily extended to other parts of the spectrum by those skilled in the art based on the teaching of the present invention.
Impairments Presented by the Communication Channel
There are other difficulties from the channel which affect the implementation of the PCM modem and the information transmission. The communication channel from the line card PCM CODEC to the user's modem can be characterized by 4 primary functional units: the codec's anti-
Olafsson Sverrir
Zhang Xuming
Zhou Zhenyu
Bocure Tesfaldet
Conexant Systems Inc.
Snell & Wilmer L.L.P.
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