Pulse or digital communications – Transceivers
Reexamination Certificate
2000-10-30
2004-05-04
Tran, Khai (Department: 2631)
Pulse or digital communications
Transceivers
C375S222000, C375S260000, C379S093070
Reexamination Certificate
active
06731678
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed to a system and method for extending the operating range and/or increasing the bandwidth of a communication link. The invention is applicable to the transmission of information over any type of “band limited” media, including metallic twisted wire pairs, coaxial cables, fiber optic cables, and radio frequency (“RF”) paths.
2. Description of Related Art
Over the last several years, the demand for high-speed data communication services has exploded. Most homes and small businesses, for example, have at least one personal computer (“PC”) with a dial-in modem that can be used to connect with an Internet service provider (“ISP”) via the public switched telephone network (“PSTN”). These dial-in modems generally operate at 14.4 kb/s, 28.8 kb/s, 33.6 kb/s, or even 56 kb/s. As such, dial-in Internet connections are capable of providing basic Internet services, including Web access, electronic mail, home banking, electronic commerce and the like.
There are a number of problems, however, associated with the provision of basic Internet services via the PSTN. For example, dial-in modems cannot be used to provide high-speed Internet or other broadband services that require a communication link of 1-2 Mb/s or greater, such as broadcast video, movies-on-demand, distance learning and the like. Also, because Internet data packets are transmitted between the dial-in modems and the ISPs through a voice switch located at the telephone company central office, a significant number of Internet connections (which typically have long holding times) can cause blockage to occur in the switch. Under extreme conditions, essential calls (e.g. 911 calls) may experience dial tone delays. In addition, high volumes of Internet traffic can overload the trunking network that connects the voice switch to the ISPs.
In an effort to overcome the problems associated with dial-in Internet connections, various x-type digital subscriber line (“xDSL”) technologies have been developed, the most prevalent of which is asymmetric digital subscriber line (“ADSL”) technology. Referring to
FIG. 1
, a typical ADSL system includes a plurality of remote ADSL transceiver units (“ATU-R”) a located at various homes and businesses, and a plurality of complimentary central office ADSL transceiver units (“ATU-C”) b located at the telephone company central office. Each ATU-R a communicates with each ATU-C b over a single twisted wire pair c. ATU-R a and ATU-C b can typically support data rates of up to 640 kb/s upstream (i.e. from ATU-R a to ATU-C b) and up to 6 Mb/s downstream (i.e. from ATU-C b to ATU-R a), hence the term “asymmetric.”
At each of the various homes and businesses, ATU-R a can be linked to a PC d for the provision of high-speed Internet services, to a TV set top box e for the provision of video services (e.g. movies-on-demand), and/or to any other type of broadband device. In addition, most ADSL systems support plain old telephone service (“POTS”). In those systems, a POTS splitter f is provided to filter out the 4 kHz analog voice signal and route it to an existing analog telephone g. Alternatively, POTS splitter f may be integrated within ATU-R a.
At the telephone company central office, each ATU-C b is connected to a digital subscriber line access module (“DSLAM”) h. DSLAM h concentrates and/or switches the various data signals and routes them to their appropriate destination, such as an ATM switch i, an IP router j, or other broadband devices k. All of these devices are in turn connected to a broadband network, thereby relieving the congestion problems associated with the transmission of data signals over the PSTN. If the ADSL system supports POTS, a POTS splitter l is provided to filter out the 4 kHz analog voice signal and route it to a voice switch m of the PSTN. Alternatively, POTS splitter l may be integrated within ATU-C b.
Although various line coding techniques may be used for the transmission of information between ATU-R a and ATU-C b, the most common is discrete multi-tone (“DMT”) line coding as adopted by the ANSI T1.413 (1995) standard.
As shown in
FIG. 2
, DMT line coding is used to divide the information transmitted over twisted wire pair c between
256
subcarriers, each of which occupies 4.3125 kHz for a total bandwidth of 1.104 MHz. While most of the subcarriers are used to carry voice and data signals, some are used for network management and performance measurement functions (e.g. subcarrier #
64
at 276 kHz is reserved for a downstream pilot signal) and others are not used at all (e.g. those subcarriers affected by a bridged tap, radio-frequency noise, or impulse noise).
As shown in
FIG. 3
, the 1.104 MHz frequency spectrum is divided into four frequency bands—a voice band, a guard band, an upstream band, and a downstream band. The voice band, which occupies the lower portion of the frequency spectrum between 0 Hz and 4.3125 kHz (i.e. subcarrier #
1
), is used to carry a 4 kHz analog voice signal. The guard band, which occupies the next portion of the frequency spectrum between 4.3125 kHz and 25.875 kHz (i.e. subcarrier #'s
2
-
6
), is used to separate the voice band from the upstream and downstream bands. The upstream band, which occupies the next portion of the frequency spectrum between 25.875 kHz and 138 kHz (i.e. subcarrier #'s
7
-
32
), is used to carry data signals from ATU-R a to ATU-C b. The downstream band, which occupies the upper portion of the frequency spectrum between 25.875 kHz and 1.104 MHz (i.e. subcarrier #'s
7
-
256
), is used to carry data signals from ATU-C b to ATU-R a. Thus, with DMT line coding, ATU-R a and ATU-C b are capable of providing both POTS and high-speed Internet and other broadband services over the same twisted wire pair c.
While ADSL technology overcomes the problems associated with dial-in Internet connections, it has its own set of attendant problems. For example, current FCC regulations (i.e. FCC Part 68) limit the maximum power that can be applied to twisted wire pair c so as to control the transmission of small broadcast signals that can interfere with surrounding signals. Because of these power constraints, the distance between ATU-C b and ATU-R a is limited due to the attenuation of the signal at the far end and resulting cross-talk from other twisted wire pairs. This distance will vary according to the data rate being offered, the gauge and generation of twisted wire pair c, the number of bridged taps and other factors. As a result, homes and businesses that are located more than a certain distance from the telephone company central office are not able to receive high speed Internet and other broadband services.
One attempt to solve this problem has been to regenerate the attenuated signal by strategically placing a mid-span DSL repeater on twisted wire pair c between ATU-R a and ATU-C b. Although the cost of the DSL repeater itself is relatively small, the cost of the environmentally-hardened case surrounding the repeater and the labor required to splice the repeater into the twisted wire pair can be prohibitive. In addition to cost issues, there are other problems with powering the DSL repeater and with simply finding a mid-span location to mount the DSL repeater.
Another problem associated with ADSL technology is that ATU-R a and ATU-C b can only support data rates of up to 6 Mb/s downstream, even at the shortest distances. As such, ATU-R a and ATU-C b cannot be used to provide very high speed Internet or other broadband services that require a communication link of 6 Mb/s or greater, such as high definition television (“HDTV”), high-performance business applications and the like.
Thus, while dial-in Internet connections and ADSL technology have satisfied some of the demand for high speed data communication services, a need remains for a system that is capable of transmitting high speed Internet and other broadband services to homes and businesses that are located at greater distances from the telephone company central off
Kaplan Martin J.
White Albert L.
Sprint Communications Company L.P.
Tran Khai
Tran Khanh Cong
LandOfFree
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