Pulse or digital communications – Transmitters – Antinoise or distortion
Reexamination Certificate
1996-11-26
2001-08-21
Bocure, Tesfaldet (Department: 2731)
Pulse or digital communications
Transmitters
Antinoise or distortion
C375S254000
Reexamination Certificate
active
06278744
ABSTRACT:
FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not Applicable
BACKGROUND OF THE INVENTION
The present invention relates to transmission of digital information over an analog medium connected to a digital network and more particularly to Pulse Code Modulation (PCM) modems.
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 increase the data transmission rate, or simply called data rate. The desire is even greater for users accessing the Internet through an Internet service provider (ISP), since most users are linked up to the “Net” through a personal computer and a modem. Conventional analog modems, such as 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 PSTN. As such, various effects of and impairments due to signal 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 C. E. Shannon and W. Weaver,
The Mathematical Theory of Communication
, University of Illinois Press, 1949).
There has been much recent development of high-speed communications technology based on PCM modems, where data rates of at least 56 Kbps are said to be actually attainable. The PCM modem technology is based on the simple realization that the 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., T1in the U.S. and E1 in Europe, without requiring a CODEC (coder/decoder). A CODEC is a device which connects the digital portion of the network to the analog local loop and converts between analog and digital.
The conventional modem, however, still interprets this digital stream as the representation of the modem's analog signal. With the PCM modems, however, a much higher data rate can be achieved without the complicated task of re-wiring the user's site or modifying the telephone network. It should be recognized that “central site” modems refer to those modems installed at an ISP, or at a corporation, for example, to allow many simultaneous connections for remote local area network (LAN) access.
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 interface 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 PCM modems and how they can and should facilitate downstream data communication at a much higher rate than the 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 Guozhu Long's
DC Suppresser for
56
K Modems
, David C. Rife's 56
Kbps Channels
, Veda Krishnan's
V.pcm Modem Standard
, Vedat Eyuboglu's
PCM Modems: A Technical Overview
, Richard Stuart's
Proposal for a High Speed Network Access Modem
, and Vladimir Parizhsky's
U.S. Robotics' ×
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 PCM modems, which also implements DC null elimination on the transmitter side. These papers provide a fair reference to the basics of the high speed PCM modems and their environment, and are hereby incorporated by reference.
Additionally, U.S. Pat. No. 5,528,625, issued to Ender Ayanoglu of AT&T, 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 typical high-speed communication path using 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 PCM CODEC implemented therein. 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 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 the telephone company's specifications and operation, the central site transmitter
110
needs to transmit the digital data in a particular way to fully exploit its digital connection to the network. However, dealing with the central site transmitter in this new paradigm has some obstacles.
In this type of data communication systems, such as that including 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 is eventually translated to an analog signal in a digital-to-analog converter in the 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 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 designers, then, is to perform the desired control with the least amount of redundancy added.
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 four primary functional units: the CODEC's anti-aliasing filter, the line card circuit, the subscriber loop and the modem's input circuit.
First, the CODEC anti-aliasing filter is mostly flat, and starts to attenuate the signal at about 3.5 kHz, increasing to 15-20 dB at 4 kHz.
Second, the line card circuit is in some ways the most challenging impairment. It is difficult to characterize, since there are many types in use, and it has been designed to transmit a voice signal rather than a data signal. The hybrid transformer circuit includes a null at 0 Hz, and may induce high levels of nonlinear distor
Olafsson Sverrir
Zhang Xuming
Zhou Zhenyu
Bocure Tesfaldet
Conexant Systems Inc.
Snell & Wilmer
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