Multiplex communications – Channel assignment techniques – Details of circuit or interface for connecting user to the...
Reexamination Certificate
2001-03-23
2004-12-07
Phan, Man (Department: 2665)
Multiplex communications
Channel assignment techniques
Details of circuit or interface for connecting user to the...
C370S242000, C370S468000, C375S222000, C375S225000, C725S100000, C725S106000
Reexamination Certificate
active
06829246
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention relates generally to the field of high-speed digital signals and services provided to subscribers via installed twisted-pair circuits. More specifically, the present invention is related to a system and method of using optical fiber technology to extend the range over which such signals and services may be provided.
2. Discussion of Prior Art
The most ubiquitous medium for transport of electronic communications is the Public Switched Telephone Network (PSTN), which was designed to provide two-way analog voice communications. This same network is capable of being used for digital communications by employing traditional “modems,” but fundamental physical constraints severely limit the speed reliably attained by such devices. These restraints include the bandwidth limitations (under 4 KHz) of the PSTN and the signal-to-noise ratio (about 30 dB) of the medium carrying the signals.
The signal-to-noise ratio is derived from physical properties of subscriber lines utilized to transport the analog signals between the subscriber's premises and the Central Office (CO), namely Unshielded Twisted Pairs (UTP) of copper wire. The attenuation of the desired signals in these wires, and the pickup of various undesired noises by these wires, result in limited signal-to-noise-ratio, and ultimately to restriction of the length of UTP which can be deployed.
The bandwidth limitation is derived from the fact that the PSTN was designed for voice transmission. Early telephones could only provide 4 KHz of bandwidth, a bandwidth sufficient for basic speech communications. Enforcing low bandwidth increases the efficiency of both Frequency Domain Multiplexing (FDM) analog systems and Time Domain Multiplexing (TDM) digital systems which carry multiple conversations on a single cable.
The 4 KHz bandwidth constraint is imposed by the network and is not a limitation of the subscriber lines themselves. Direct connection to the UTP affords much higher bandwidth and hence support for much higher rate digital communications. Recently, technologies termed collectively Digital Subscriber Line (DSL) have been employed to exploit these higher rates. The different variants of DSL are collectively referred to as xDSL (e.g. HDSL, ADSL and VDSL).
A typical xDSL system is illustrated in FIG.
1
. At the CO there is a DSL access multiplexer (DSLAM)
100
, which contains a bank of xDSL Terminal Units (xTU) and a mechanism for combining all the digital information to and from these xTUs into a single information stream in order to interface with a high-speed network, such as the Internet. At the other end of the subscriber lines
102
a-
102
e
are individual remote Terminal Units
104
a-
104
e
located at the subscriber's premises. In order to differentiate between xTUs based on location, CO-based xTUs are typically designated xTU-C, while remote xTUs are designated xTU-R. Each xTU-R communicates with a corresponding xTU-C of DSLAM
100
, extracting the digital information contained in the xDSL signal and forwarding it to the premises distribution network.
As aforementioned, xDSL technologies provide high-speed digital communications by exploiting frequencies within the physical bandwidth of the subscriber lines
102
a-
102
e,
but above the 4 KHz utilized by the Plain Old Telephone Service (POTS). For example, standard ADSL uses frequencies between about 30 KHz to about 1104 KHz; the lower portion of this spectrum (up to 138 KHz) being for upstream transmission from the subscriber, and the upper portion for downstream transmission to the subscriber.
Due to the attenuation of signals in UTP lines becoming stronger with increasing frequency, xDSL technologies which utilize very high frequencies are subject to extremely high attenuation factors. In addition, unwanted pickup of radiation from adjacent lines, a phenomenon known as cross-talk, also becomes more pronounced with increasing frequency. Because of these effects the signal-to-noise-ratio declines rapidly with increasing frequency. Conversely, as the distance from the subscriber to the CO increases, achievable data rates diminish. The maximum distance obtainable by the xDSL technology at a given data rate is called its maximum reach. The maximum reach of ADSL at 1.5 Mbps downstream is typically about 18 Kft (5.5 Km); for ADSL at 8 Mbps downstream this is reduced to about 11 Kft (3.3 Km); while VDSL at 52 Mbps downstream has a maximum reach of only 1 Kft (300 m).
xDSL service providers wish to provide the highest data rates to as many customers as possible. Unfortunately, because of the physical properties of UTP subscriber lines just described, these two aims are mutually incompatible. Higher data rates can only be achieved for shorter distances, thus restricting the number of reachable subscribers. Present xDSL technologies can supply the highest rates (e.g. VDSL) only to subscribers serviced by short lengths of UTP, lower rates (ADSL, HDSL) to more distant customers, and must declare distant customers ineligible for any type of xDSL access.
One solution proposed to this problem is so-called Fiber To The Cabinet (FTTC), scenario depicted in FIG.
2
. With FTTC, digital data is transferred over fiber optic cable
202
from the CO
200
to a street cabinet containing the DSLAM
204
. Subscriber lines
206
a-
206
e
from the cabinet to the xTU-Rs
208
a-
208
e
at the subscribers' premises are kept to minimal lengths, thus enabling the highest data rates to be obtained. This scenario, and similar ones such as Fiber To The Basement (FTTB), are commonly used for VDSL service.
However, FTTC is not a fully satisfactory solution because of the inhospitality of the street cabinet environment. These cabinets obviously place restrictions on the physical size of equipment, and due to limited power and inadequate heat dissipation they severely restrict power consumption. These constraints have impeded successful mass deployment of FTTC-DSL solutions since high-speed DSL modems require sophisticated signal processing and high power digital and analog circuitry.
ADSL was originally designed for video on demand services. In this context U.S. Pat. No. 5,534,912 to Kostreski et al. suggests utilizing a fiber optic cable to extend ADSL reach. A plurality of video channels is arranged at a Central Office into ADSL format and, together with a provisioning channel, multiplexed into a composite spectrum. This composite spectrum is then transmitted to an intermediate distribution point, remote from the Central Office, over analog optical fiber. The composite spectrum is split and applied individually to channel selection mixers associated with the subscribers serviced by the intermediate distribution point. However, Kostreski et al. does not teach the use of the optical fiber to extend the range that general xDSL services (i.e., the transmission of arbitrary data) can be provided to allow additional subscribers to be reached while providing the high data rates available to users proximate to the CO to these additional subscribers. Rather, Kostreski et al., teaches the streaming of multiple video feeds via the channels to a subscriber and the provision of a video library for streaming video on demand to a subscriber utilizing one of the channels. Kostreski et al. also does not teach the connection of the DSLAM at the CO to a high-speed general-purpose network, such as the Internet, to provide subscribers the capability of general-purpose communications between their subscriber premises equipment, such as personal computers, with equipment connected to the high-speed general-purpose network.
Whatever the precise merits, features and advantages of the above cited reference, it does not achieve nor fulfill the purposes of the present invention.
SUMMARY OF THE INVENTION
A system and method for extending the range over which high-speed digital data from arbitrary communications networks can be transmitted using pairs of copper wire such as lines serving subscribers of the conventional telephone system. The digit
Eitane Tsvi (Henri)
Silberman Hugo
Stein Ysakov
Katten Muchin Zavis & Rosenman
Phan Man
Rad Data Communications Ltd.
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