Pulse or digital communications – Equalizers – Automatic
Reexamination Certificate
1997-02-20
2001-05-15
Bocure, Tesfaldet (Department: 2731)
Pulse or digital communications
Equalizers
Automatic
C375S242000
Reexamination Certificate
active
06233275
ABSTRACT:
BACKGROUND OF THE INVENTION
The field of the present invention pertains generally to data communications equipment, and more particularly to a device for transmitting digital data over a telephone connection.
Data communication plays an important role in many aspects of today's society. Banking transactions, facsimiles, computer networks, remote database access, credit-card validation, and a plethora of other applications all rely on the ability to quickly move digital information from one point to another. The speed of this transmission directly impacts the quality of these services and, in many cases, applications are infeasible without a certain critical underlying capacity.
At the lowest levels, most of this digital data traffic is carried over the telephone system. Computers, facsimile machines, and other devices frequently communicate with each other via regular telephone connections or dedicated lines which share many of the same characteristics. In either case the data must first be converted into a form compatible with a telephone system designed primarily for voice transmission. At the receiving end the telephone signal must be converted back into a data stream. Both tasks are usually accomplished by modems.
A modem performs two tasks corresponding to the needs above: modulation, which converts a data stream into an audio signal that can be carried by the telephone system, and demodulation, which takes the audio signal and reconstructs the data stream. A pair of modems, one at each end of a connection, allows bidirectional communication between the two points. The constraints on the audio signal create the limitations on the speed at which data can be transferred using modems. These constraints include a limited bandwidth and degradation of data by noise and crosstalk. The telephone system typically can carry only signals that range in frequency between 300 Hz and 3,400 Hz. Signals outside this range are sharply attenuated. This range was built into the design of the telephone system since it covers a significant portion of by the human voice spectrum. However, the bandwidth of a channel is one factor that determines the maximum attainable data rate. With all other factors constant, the data rate is directly proportional to the bandwidth.
Another factor is the distortion of the audio signal or any other signal that the communications endpoints cannot control. This includes electrical pickup of other signals being carried by the telephone system (crosstalk), electrical noise, and noise introduced by conversion of the signal from one form to another. The last type will be expanded upon in later discussion.
For general utility, modems are designed to be operable over most telephone connections. Thus, they must be designed for worst-case scenarios, which include bandwidth limitations and significant noise that cannot be removed. Even so, substantial progress has been made on modem design in the past several years. Devices capable of operating at speeds up to 28,800 bits per second are now commonly available. See International Telecommunication Union, Telecommunication Standardization Sector (ITU-T), Recommendation V.34, Geneva, Switzerland (1994) which is hereby incorporated herein by reference. However, theoretical arguments based on the channel bandwidth and noise levels show that the maximum possible speed has nearly been obtained and further significant increases are highly unlikely with the given constraints. This is discussed in C. E. Shannon, “
A Mathematical Theory of Communication
,” Bell System Technical Journal, 27:379-423,623-656 (1948) which is hereby incorporated herein by reference.
Unfortunately, although speeds approaching 30,000 bits per second (or 3,600 bytes per second) make many data communications applications feasible, conventional modem transmission is still not fast enough for all uses. At these speeds, transmission of text is fast, and low-quality audio, such as digitized speech, is acceptable. However, facsimile or still-image transmission is slow, while high-quality audio is limited and full-motion video has not been satisfactorily achieved. In short, what is needed is greater data transmission capability. This is a prerequisite for the new applications and is a necessity for maximizing the performance of many existing applications.
Of course the telephone companies, cable-television providers, and others are not ignorant of these increasing data transmission needs. One approach to providing higher speed data connections to businesses and residences is to provide end-to-end digital connectivity, eliminating the need for additional modems. One offering of such a service is the Integrated Services Digital Network (ISDN). See: International Telecommunication Union, Telecommunication Standardization Sector (ITU-T), “
Integrated Services Digital Networks
(
ISDNs
),” Recommendation I.120, Geneva, Switzerland (1993), and John Landwehr, “
The Golden Splice: Beginning a Global Digital Phone Network
,” Northwestern University (1992) each of which is incorporated herein by reference. ISDN replaces the existing analog local loop with a 160,000 bit/second digital connection. Since the bulk of long-distance and inter-office traffic is already carried digitally, this digital local loop can be used for end-to-end digital voice, computer data or any other type of information transfer. However, to achieve these data transmission rates on the local loop, special equipment must be installed at both ends of the line. Indeed, the entire telephone network is currently undergoing a transformation from a voice transmission network to a general data transmission service, with voice just being one particular form of data.
Once installed, each basic ISDN link will offer two data channels capable of 64,000 bits/second, a control channel with a capacity of 16,000 bits/second, reduced call connection time, and other benefits. At these rates, facsimile and still image transmission will be nearly instantaneous, high-quality audio will be feasible, and remote computer connections will benefit from a fivefold speed increase. Some progress toward full-motion video may also be achieved.
The down side of ISDN is its availability, or lack thereof To use ISDN, the user's central office must be upgraded to provide this service, the user must replace its on-premises equipment (such as telephones) with their digital equivalents, and each individual line interface at the central office must be modified to carry the digital data stream. This last step, the conversion to a digital link of the millions of analog connections between every telephone and the central office, is formidable. The magnitude of this task dictates that the deployment of ISDN will be slow and coverage will be sporadic for some time to come. Rural and sparsely populated areas may never enjoy these services.
Another existing infrastructure potentially capable of providing high-speed data communications services is the cable television system. Unlike the telephone system, which connects to users via low-bandwidth, twisted-pair wiring, the cable system provides high-bandwidth connectivity to a large fraction of residences. Unused capacities on this wiring could provide data rates of tens, or even hundreds, of millions of bit per second. This would be more than adequate for all of the services envisioned above including full-motion digital video. However, the cable system suffers from a severe problem - - - its network architecture. The telephone system provides point-to-point connectivity. That is, each user has full use of the entire capacity of that user's connection - - - it is not shared with others and does not directly suffer due to usage by others. The cable system on the other hand, provides broadcast connections. The entire capacity is shared by all users since the same signals appear at each user's connection. Thus, although the total capacity is high, it is divided by the number of users requiring service. This architecture works well when all users require the same data, such as for cabl
Bocure Tesfaldet
McDonnell & Boehnen Hulbert & Berghoff
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