Multiplex communications – Duplex – Communication over free space
Reexamination Certificate
1997-09-08
2001-12-25
Chin, Wellington (Department: 2664)
Multiplex communications
Duplex
Communication over free space
C370S286000
Reexamination Certificate
active
06333920
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to telephone transmission and, more particularly, to the efficient transmission of high-speed digital signals between a telephone central office and the customer premises.
2. Description of the Related Art
Perhaps the most flexible and least expensive approach for transmitting data over telephone lines is to use the existing voiceband telephone channels normally used to carry speech. The channel between the transmission endpoints may be either a switched network connection which may be established by the user at one endpoint by simply dialing the telephone number of the other endpoint, or it may be a permanent, private line connection which is set up for the user by the telephone company. In either case, once the connection has been established, data from the user's data communication processing equipment is input to a voiceband modem which generates an output analog line signal having a frequency spectrum which matches the passband of the voiceband telephone channel. At the receiving end, a matching modem recovers the data from the received line signal and passes it to the user's equipment at that end.
For a given level of noise and distortion, the rate at which data can be communicated over a channel is limited by its bandwidth and noise, including distortion, within the channel. The bandwidth of the typical voiceband telephone channel is about 4 kHz. For typical levels of noise, this limits the transmission rate over such channels to a theoretical maximum to about 20 to 30 kilobits per second (kb/s). For many applications, such as database input or retrieval or other applications typically involving a human being at one end of the transaction, these data rates are wholly satisfactory.
For many other applications, however, such as computer to computer file transfers, videotext, transmission of digitized speech or video and the like, voiceband telephone data transmission is unacceptably slow. Advantageously, most of the transmission facilities interconnecting telephone switching offices communicate their information in the form of multiplexed, high speed digital bit streams. These facilities can be configured to provide not only the standard 4 kHz voiceband channels, but also wideband channels capable of carrying customer data at, for example, the so-called DS-1 rate of 1.544 megabits per second (Mb/s) and higher.
The challenge, however, is to get the customer's high-speed data to the central office, and high speed data from the central office to the customer. In the future, this may be accomplished by linking customer premises with central offices through optical fiber. However, it could be well into the twenty first century before such a system is put in place. For the immediate future, the existing telephone local distribution system-comprising copper wire pairs will continue to be the physical medium for delivering high-speed data to customer premises.
Telephone engineers have been successful in providing transmission schemes that allow for high-speed data transmission from customer locations to the central office. In the mid-1970's, for example, AT&T introduced a digital data communications network, the Digital Data System (DDS), in which data at rates up to 56 kb/s was transmitted from the customer locations to the central office using a four-wire local circuit, that is, two two-wire pairs. The essence of the transmission scheme was to use bipolar baseband transmission in combination with, inter alia, fixed equalization to compensate for linear distortion and thereby provide a channel with flat loss up to frequencies sufficient to transmit at the required bit rate. This scheme allowed for transmission over distances of almost eight miles at the 56 kb/s rate and even greater distance at lower rates without the use of repeaters, thereby providing high-speed customer-premises-to-central-office transmission over the “local loop” for a significant base of customers. See, for example, E. C. Bender et al., “Digital Data System: Local Distribution System”, The Bell System Technical Journal, Vol. 54, No. 5, May-June 1975, which is incorporated herein in its entirety.
Subsequently, a 1.544 Mb/s speed was added to DDS, and data transmission at that rate was thereafter provided in other digital data transmission offerings. This transmission rate was achieved by using the technology developed for the so-called T1 carrier system which had to that point been principally used to interconnect telephone central offices. Here again, the transmission scheme involved a four-wire circuit and a bipolar transmission format. Indeed, the design of DDS was based on the previously existing T1 technology. At the 1.544 Mb/s rate, however, compensation for channel distortion and noise required equalization and regeneration of the line signal typically at every 3000 ft (6 kft).
The above approaches are certainly technically sound and are used quite extensively. However, not only is transmission based on T1 technology relatively expensive to provide and maintain see for example, Method and Apparatus for Wideband Transmission of Digital Signals Between, For Example, a Telephone Central Office and Customer locations, U.S. Pat. No. 4,924,492 issued to Gitlin et al., which is hereby incorporated by reference, more demanding applications for the telephone network have arisen. Although video on demand services, for example, can be accommodated within the framework of Asymmetric Digital Subscriber Line services (ADSL), at a data rate of only 1.544 Mb/s see for example, “PSTN Architecture For Video On Demand Services”, U.S. Pat. No. 5,247,347 issued to Litteral et al., which is hereby incorporated by reference, even higher data rates will be required for some applications. Very high data rate subscriber line (VDSL) systems will address the requirements of these applications such as the delivery of high definition television. Further, the trend has been that data rates considered high today are considered low several years later.
As noted above, at some point a high bandwidth medium, such as optical fiber, may very well provide a communications path from every telephone operating company to every customer location, thus allowing high speed data communications through the telephone network. In the interim, however, VDSL systems may employ a mix of technologies to establish high speed communications to every customer location. A two wire pair can support data rates up to 51.84 Mb/s, 25.92 Mb/s, or 12.96 Mb/s for respective distances of 1 kft, 3 kft, and 4.5 kft. Because the ubiquitous two wire pairs currently provide connection from most customer locations to the PSTN, it would be advantageous to capitalize on this enormous installed base. That is, rather than incurring the expense and inconvenience associated with replacing all two wire pairs with optical fiber, telephone companies could employ optical fiber to distribute data to a point within the range of twisted pairs for a desired data rate. From this point, a distribution cable containing several twisted pairs could connect to individual premises within the neighborhood. In some cases, the entire neighborhood may be within the desired two-wire range of a central office, for example, within 3 kft where 25.9 Mb/s services are desired. In those cases, the distribution cable could run directly from the central office to customer locations. For those situations where the customer locations are outside the desired two-wire range, an optical fiber could connect the central office to an optical network unit which would provide an interface between the optical fiber and one or more two-wire distribution cables.
Generally, it would be desirable to support both symmetric and asymmetric services within such a neighborhood and, therefore, within a single distribution cable. Asymmetric services would accommodate such applications as video on demand, in which an upstream channel, that is the channel from the customer locations to the telephone network, requir
Nguyen Mai-Huong
Werner Jean-Jacques
Chin Wellington
Lucent Technologies - Inc.
Nguyen Steven
Priest & Goldstein PLLC
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