Pulse or digital communications – Transmitters – Antinoise or distortion
Reexamination Certificate
1997-08-22
2001-05-08
Nguyen, Lee (Department: 2863)
Pulse or digital communications
Transmitters
Antinoise or distortion
C370S201000, C324S628000, C455S522000
Reexamination Certificate
active
06229855
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to high speed digital data networks, and more specifically refers to reducing the crosstalk between high speed data network interconnections.
2. Description of Related Art
Recently several high speed digital data services have become commercially available. These high speed digital data services are known as the ISDN (Integrated Services Digital Network) basic rate, HDSL (High Speed Digital Subscriber Loop), HDSL2 (High Speed Digital Subscriber Loop 2), ADSL (Asymmetric Digital Subscriber Loop), and T1 services. When offered by the same provider, they are typically installed in the same cable binder, and as the number of services increases, the number of cables in the cable binder increases. However, increased numbers of cables within the binder increases the crosstalk between cables, which degrades the transmission performance for each digital data service.
Each of the high speed digital data services described above typically connect two locations. The first location is typically the central office, and the other is typically a remote site. Normally, the central office site is considered the “master” and the remote site is the “slave” in terms of network synchronization. The central office is typically located at a fixed site while the remote site can be scattered over a very wide range of territory. In other words, the distance between the central office and the remote site can vary from 50 meters up to 5 kilometers depending on the location of the remote site. The cable bundle has connections distributed along the span of the cable bundle. As the number of cable pairs within the cable bundle increases, the crosstalk between pairs increases. The number of pairs within the bundle increases as the bundle gets closer to the central office, thus making the crosstalk more severe at the central office. The crosstalk interferes with proper signal transmission, thus affecting system performance.
There are two kinds of crosstalk, namely near end crosstalk (NEXT) and far end crosstalk (FEXT). In the practical network at hand where several different services are installed using different transmission methods, the NEXT is dominant over the FEXT. Therefore, methods to reduce the NEXT would have a greater effect on improving system performance.
The transmitter of the existing digital services have the nominal power required for each service. Even if the distance between the central office and the remote site is very short, the transmitter still uses the nominal transmit power and thus creates unnecessary crosstalk interference into the other pairs within the same bundle.
Although there are a few services that increase the transmit power (known as a “power burst” method) and emphasize the higher frequency content when the cable length is longer than the nominal case, the power control is not performed automatically, so additional human effort is required to maintain the service. This increases the cost of the digital service installation and maintenance. In addition, the existing power burst method is done independently, regardless of whether it is the central office or the remote site, and thus, if the power burst is performed at the central office side where the crosstalk is greater than that of the remote site, a power burst will create an even worse crosstalk environment.
It can be seen, then, that there is a need for an effective technique to reduce the crosstalk in digital data networks.
SUMMARY OF THE INVENTION
To overcome the limitations in the prior art described above, and to overcome other limitations that will become apparent upon reading and understanding the present specification, the present invention discloses a powerful and highly productive method for reducing crosstalk.
The present invention solves the above-described problems by using feedback from a high speed digital data service to determine transmission characteristics. This will raise or lower the signal strength in each cable and thus lower the interactions and crosstalk between cables in the same cable bundle.
The central office (HTU-C) receiver output signal-to-noise ratio (SNR) can be enhanced by boosting the power of the remote site (HTU-R) transmitter signal in the upstream (from remote site to central office) direction. Further improvement in performance can be achieved by reducing the HTU-C NEXT interference.
The worst case crosstalk scenario of the system is typically observed on the HDSL2 data connection at the central office (HTU-C) side of the transmission link because the number of cable pairs is larger at the central office side of the data system. The crosstalk from the ADSL is also higher at the central office side of the transmission link. The present invention measures the cable loss between the central office and the remote site, and determines from this measurement, and signal to noise measurements, when to change the transmitter power and/or frequency levels. The transmitter power level is usually increased at the remote site (HTU-R) to combat crosstalk at the central office, and is reduced at the central office to again minimize crosstalk affecting other pairs within the bundle.
A system in accordance with the principles of the present invention comprises software and hardware which provide a method for measuring the signal power from remote sites, evaluating the losses in the cable between a central office and the remote site, and adjusting the transmission power and frequency to optimize the signal transmission and reduce crosstalk.
One object of the present invention is to reduce near end crosstalk. It is a further object of the invention to increase digital data system performance. It is a further object of the invention to make it safer to operate digital data systems.
For a better understanding of the invention, its advantages, and the objects obtained by its use, reference should be made to the drawings which form a further part hereof, and to accompanying descriptive matter, in which there are illustrated and described specific examples of an apparatus in accordance with the invention.
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Shalash Ahmed F.
Takatori Hiroshi
ADC Telecommunications Inc.
Merchant & Gould P.C.
Nguyen Lee
Tran Conguen
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