Electrical computers and digital processing systems: multicomput – Computer-to-computer protocol implementing – Computer-to-computer data transfer regulating
Reexamination Certificate
1998-01-22
2001-12-11
Harrell, Robert (Department: 2152)
Electrical computers and digital processing systems: multicomput
Computer-to-computer protocol implementing
Computer-to-computer data transfer regulating
Reexamination Certificate
active
06330613
ABSTRACT:
COPYRIGHT NOTICE
Contained herein is material which is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction of the patent disclosure by any person as it appears in the Patent and Trademark Office patent files or records, but otherwise reserves all rights to the copyright whatsoever.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is related to the field of high speed modems for use in telecommunications. More specifically, the present invention is related to a modem comprised entirely of digital components. No analog to digital (A/D) or digital to analog (D/A) converters are utilized by the modem of the present invention when modulating or demodulating data transmitted to or received from a digital transmission medium, thereby increasing performance while decreasing cost of the modem. Additionally, a DSP-based embodiment of the modem of the present invention may be dynamically configured by downloading a software image.
2. Description of the Related Art
With respect to
FIG. 1A
, a traditional telecommunications path is illustrated over which a computer such as Personal Computer (PC)
100
communicates with another computer such as PC
160
. The communication utilizes the Public Switched Telephone Network (PSTN) as may be necessary with communication between computers that are separated by a distance greater than can be accommodated by present Local Area Network (LAN) technology. The computers may communicate with each other as part of a client-server network, or to exchange electronic mail, or the like.
PC
100
is shown connected to a MOdulator/DEModulator (modem)
110
over line
170
. Line
170
is typically an asynchronous link, e.g., an RS-232 line that transmits and receives data as a digital signal
160
as indicated in FIG.
1
A. Modem
110
converts the digital signal
160
originating from PC
100
to an analog signal
161
for transmission over analog line
171
, commonly referred to as a “local loop”. In this particular example, modem
110
is external to PC
100
. However, it is understood that modem
110
may be internal to PC
100
, in which case, line
170
is unnecessary. In either case, modem
110
operates in the same manner. Likewise, PC
160
, line
175
and modem
150
function in the same manner as described for PC
100
, line
170
and modem
110
, to provide analog signal
163
for transmission over line
174
, the analog local loop with respect to PC
160
.
Analog Transmission over the Local Loop
A computer, just like a telephone, is commonly connected to a Central Office Exchange (CO) over a “local loop”. This local loop is commonly a single pair of wires that transmits and receives voice or data transmissions in the form of an analog signal. For example, PC
100
transmits a digital signal
160
containing data to modem
110
, which converts the digital signal to an analog signal
161
for transmission over analog line
171
. Line
171
represents a local loop between PC
100
/modem
110
and CO
120
.
While the local loop today carries voice and data transmissions, it originally carried only voice transmissions. Voice signals are, of course, characterized by an analog, not digital, waveform of varying amplitude and frequency. Voice frequency bandwidth, i.e., voiceband, is approximately 4000 Hz, but the frequency band actually passed in the telephone network is in the 300 Hz to 3000 Hz range, which is adequate for the human ear to recognize basic qualities associated with speech such as voice recognition, intonation, emotion, etc. This frequency range was chosen as a compromise-increasing the frequency range results in better voice quality, but at a greater expense. Hence, the local loop was initially designed to transmit analog signals of limited bandwidth.
Virtually every home or small business initially transmitted voice information as an analog signal over a local loop comprised of a twisted wire pair to a local telephone company's CO. Presently, particularly in metropolitan areas, the pair of wires comprising the local loop is being replaced by a transmission medium, e.g., wire, fiber optic or radio, capable of transmitting voice and data as a digital signal. However, the local loop portion of the worldwide telecommunications infrastructure remains overwhelmingly analog. Thus, there is an ongoing need for modems to convert from digital to analog signals when transmitting data over the local loop or to convert from analog to digital signals when receiving data over a local loop.
While the local loop remains mostly analog, the switching systems in the central office and transmission lines in the PSTN are commonly digital. Thus, a brief review of CO systems and digital transmission systems utilized in the PSTN, such as T
1
, is useful for an understanding of the present invention.
Digital Transmission over the PSTN
A Central Office (CO) is a switching system operated by a local phone company for connecting, maintaining and releasing a local call between two devices, such as telephones, facsimile machines, or modems, within the geographical area managed by the local office. A toll, or long distance connection between devices in different cities or states, however, requires utilization of the Public Switched Telephone Network (PSTN). For example, given a scenario in which telephones are located in different cities or states managed by separate COs, the PSTN provides an interconnection between the COs so that the telephones can establish a toll call for voice transmission between them. While a local loop transmits a single local call at a time, the PSTN multiplexes multiple toll calls at the same time over the same transmission medium utilizing, for example, a time division multiplexing (TDM) technique, to improve the efficiency of the PSTN.
Additionally, digital transmission techniques are often utilized to further enhance the performance and efficiency of the PSTN. As a result, analog signals transmitted by a telephone over the local loop to a local central office are generally converted to digital signals and coded, for example, using Pulse Code Modulation (PCM) techniques, at the local central office before being transmitted over the PSTN. The digital signals may then be decoded and converted back to analog signals at a receiving central office to recover the voice transmission before being transmitted over another analog local loop to a receiving telephone. Thus, a central office utilizes a COder/DECoder (CODEC) to convert analog signals received from an attached local loop to an encoded digital data stream for transmission over the PSTN.
Likewise, a CODEC in the receiving CO decodes the encoded digital data stream received from the PSTN into analog signals for transmission over the local loop. As will be discussed below, if the local loop to which a receiving telephone is connected is capable of carrying digital transmissions, the digital signals need not be converted back to analog signals at the receiving central office, but may be transmitted as digital signals until converted to analog signals at the receiving telephone.
Analog to Digital/Digital to Analog Conversion
Given that the local loop portion of the telephone network is generally analog and the PSTN portion is generally digital, the need for analog to digital and digital to analog signal conversion at the CO is readily apparent. For example, with respect to
FIG. 1A
, digital signals
160
representing data transmitted from PC
100
must be converted to analog signals
161
at modem
110
. At CO
120
, the analog signals are converted to digital signals
162
for transmission over PSTN
130
. Again at CO
140
, the digital signals are converted to analog signals
163
and transmitted to modem
150
, where the analog signals are converted yet again to digital signals
164
for transmission to PC
160
.
Recall that human speech produces an analog waveform of varying amplitude within a frequency range of 0 to 4000 Hz. To convert an analog signal with sufficient accuracy to a digital signal for transmissio
Livshitz Michael
Vlajnic Milan
Blakely , Sokoloff, Taylor & Zafman LLP
Harrell Robert
Nortel Networks Limited
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