Multiple ISDN and Pots Carrier System

The field of the invention relates generally to high speed digital communication. More particularly, the field of the invention relates to transportation of two ISDN signals and one POTS signal over one twisted cable pair from a front end and regeneration of the ISDN and POTS signals at a remote end.

The recent growth of the Internet has created both a tremendous demand for additional subscriber access to public switched telephone networks (PSTN) and a demand for additional bandwidth for the access. The former demand is being met by deploying additional analog access lines so that modems can be used for the Internet access, and the latter demand is being met by deploying integrated service digital network (ISDN) lines over twisted cables. In the first case, although Internet access is provided, it is not of sufficient bandwidth to provide the graphic-rich service which typically is desired. In the latter case, a subscriber's main standard plain old telephone service (POTS) line is not converted to ISDN in order to maintain the subscriber's POTS access in the event of a local power outage or electronic system failure because the ISDN service is dependent upon the residence power and proper function of the ISDN electronics. In both cases, the installation of an additional copper-pair based access line is expensive and time consuming due to a general shortage of pre-existing installed cable pairs.

In the latter case, the ISDN installation on existing cable pairs is limited to about 70% of the installed base due to the manner in which ISDN transport was designed specifically for non-loaded cable plants. The existing copper cable outside plant was constructed in accordance with design requirements specifying that for local loops exceeding 13 kilo-ohms (k.OMEGA.), or approximately 18 kilo-feet (kft) which is equivalent to 5,486 meters, loading coils or filter capacitors are added to remove voice frequencies shifted above 4 kilo-Hertz (kHz) due to the loop resistance. The REA loop survey of 1986 indicates that for the US as a whole, approximately 85% of all loops are non-loaded. Since ISDN uses a digital signal operating at a center frequency of 40 kHz, it will not transmit in the presence of a load coil. Bridged taps or branches attached to a primary cable run further reduce the reach of an ISDN signal, with the net result being that only about 70% of all existing subscribers can have ISDN service added without additional construction expenses, as reported by Pacific Bell in early 1996. Therefore, providing ubiquitous digital access for all telephone subscribers is limited by both the number of pre-existing cable pairs and limitations imposed by the design of the telephone outside plant.

One solution is for the telephone company to simply install more copper cables. In fact, record amounts of copper cables are being installed in response to the huge demand for added lines. But this is not a financially viable alternative for the telephone companies due to the long depreciation schedule for these cables. It is generally recognized that a higher-bandwidth medium, such as fiber optic cable, is the ultimate solution for the digital access though the fiber connectivity precludes the lifeline access in the event of local power failures. While the technical and financial issues related to fiber installation are being worked out, installing copper cables only consumes capital and delays the day for fiberization.

One other approach is to more efficiently utilize existing phone lines for high-speed digital transmissions. The phone lines are made of twisted copper pairs and are configured in a star-like architecture that is suitable for bi-directional communications. The principal technology for placing a digital signal onto a copper pair that originally provided only analog dial tone is called integrated service digital network (ISDN). ISDN was developed in the 1980's, when the state-of-the-art digital encoding technology resulted in the standards as described in Bellcore documents TR-TSY-000393 and TR-TSY-000397. The basic transmission speed, called BRI for ISDN is 160 kilobits per second (kbps). This digital rate and its corresponding communication method are digital subscriber line (DSL). It is significant that ISDN was designed specifically for a non-loaded telephone plant since loading capacitors effectively attenuated high frequency digital signals. The non-loaded cable plant reaches 18 kft but only 85% of all subscribers on average. This results in a problem with respect to reaching all subscribers desiring ISDN services. Since 1990, the development of microprocessors has significantly improved the performance of communication chipsets. High bit rate subscriber line (HDSL) chipsets can run at 784 kbps or even 1 Mbps to transport one half of a T1/E1 digital loop carrier signal in an application called "Repeaterless T1/E1." Other types of high speed communication technologies for the twisted pairs, such as asymmetric DSL (ADSL), are emerging from labs but are still too expensive for wide range applications. HDSL technology can be used to transport either one high speed signal or several lower speed signals through multiplexing and demultiplexing. Installing one high bit line for multiple lower bit signals is more cost effective than installing several lower bit lines. This approach was explored by several inventors in the past.

By way of example, Carse et al., U.S. Pat. No. 4,730,311 describe a multiplexer for use in a telephone system in which a plurality of subscriber locations are connected to a central office by a single subscriber loop. Carse et al. focus on the design of the multiplexer rather than the entire communication system. Their technique applies generally to any methods of digital transmission, consequently the transmission rate is arbitrary. The subscribers are defined to be locally powered and backed-up with battery power. The battery back-up can only last for a limited period of time in the case of local power loss. For the design of the multiplexer, Carse et al. do not define either a digital interface or standard of loopback testing. Also, the configuration of the central office is not described.

Litteral et al., U.S. Pat. No. 5,247,347 and Coddington et al., U.S. Pat. No. 5,410,343 define how to provide digital video signals from a video information provider to one or more of a plurality of subscriber premises. However, the multiplexers used in both systems mainly perform frequency domain multiplexing/demultiplexing which is inherently disadvantageous with respect to time domain multiplexing/demultiplexing. The power source of the multiplexers is not specified. In addition, Litteral et al. and Coddington et al. only describe transport and encoding of specific video signals rather than generic digital signals. Bliven, U.S. Pat. No. 5,459,729 describes a method and apparatus for transmitting and receiving multiple telephone signals over a single twisted pair. Two conventional telephone signals are converted into one digital signal and then transported over a single twisted pair at a rate of 160 kbps. Creating a multiplicity of telephone channels in this way is sufficient for analog POTS but is too low to provide adequate Internet access.

Accordingly, it is a primary object of the present invention to provide a communication system that transports multiple ISDN signals and a POTS channel over a single twisted cable pair at a high bit rate. It is a further object of the invention to provide line powering to a remote terminal to avoid dependence upon local power and to provide for a metallic POTS access in the event of electronic failures. This invention is subsequently referred to as a "multiple ISDN and POTS carrier system" or abbreviated as "MIPCS."

These and other objects and advantages are attained by connecting a signal provider to a signal subscriber through a twisted cable pair. The maximum length of the cable is approximately 16 kft depending upon the wire gauge, which offers roughly 1.2 k.OMEGA. loop resistance, from the signal provider to the signal subscriber, back to the signal provider. The signal provider comprises one MIPCS card powered by a 48-Volt power supply. The MIPCS card connects two ISDN cards and one POTS line card to the twisted cable pair. The ISDN cards provide a digital interface between a PSTN and a local ISDN network at a transmission rate of 160 kbps. The components of the MIPCS card comprise a transceiver, a digital interface module, a subscriber line interface (SLI) module, a bypass relay, and a utility microprocessor. The transceiver uses a 2B1Q transport mechanism and operates at a minimum rate of 384 kbps. The digital interface module uses two ISDN U-interfaces. The bypass relay ensures the subscriber's POTS access in the event of a local power outage or electronic failure. The utility microprocessor performs loopback testing in accordance with the National ISDN Standards.

The signal subscriber comprises a MIPCS card, one POTS line, and two ISDN cables. The MIPCS card connects the twisted cable pair to the two ISDN cables and one POTS line and is powered via the twisted cable pair. The components of the MIPCS card contain a transceiver, a digital interface module, a SLI module, a bypass relay, and a utility microprocessor which are characterized by the same structure as those of the MIPCS card of the signal provider. The two ISDN cables carry the signals to a single remote premise or multiple remote premises where networking devices (NT1's) are located. The transmission rate of the cables is 160 kbps, and their maximum length is 18 kft. The SLI module supports a POTS loop with a normal resistance of 560 Ohms and provides five ringer equivalents.

FIG. 1 is a schematic diagram of a high bit communication system with a single twisted cable pair.

FIG. 2 is a simplified block diagram for the MIPCS cards illustrated in FIG. 1.

FIG. 3 is a schematic diagram according to an embodiment called office-to-house.

FIG. 4 is a schematic diagram according to an embodiment called corporate-intranet.

A schematic diagram of an aspect of the invention is shown in FIG. 1. A signal provider 1 comprises a MIPCS card 6 which connects two ISDN cards 4 and 5 and a POTS card 10 to a first end of a twisted cable pair 2. The MIPCS card 6 is plugged into an equipment shelf, constituting a signal provider terminal. The ISDN cards 4 and 5 convert an E1 or T1 signal on a PSTN into a 160-kbps ISDN signal on a local ISDN network. The ISDN and POTS signals are then multiplexed by the MIPCS card 6, and the multiplexed signal is sent to a signal subscriber 3 over the twisted cable pair 2. The MIPCS card 6 is powered by a 48-Volt power supply 31. Since each of the ISDN signals has a rate of 160 kbps and a digitized POTS channel occupies 64 kbps, the twisted cable pair 2 is required to provide a minimum speed of 384 kbps. The maximum length of the cable is limited by either the MIPCS system's ability to transmit the powering voltage, or the signal processing ability of the digital encoding chip. For example, the BROOKTREE 8960 chip has an advertised reach of 16.5 kft. Both of these limitations are due to a combination of loop resistance, loop capacitance, and other signal attenuation and degrading influences. The reach of a digital system may be increased by removing bridged taps, or by renewing cable splices, or by replacing cables having water intrusion, so the system reach is not only variable but also controllable to some extent, depending on the value of providing the digital service to the customer. In summary, one practical limit of the reach of the system is the loop resistance which governs powering as well as signal. Given the state of the art in digital encoding, as exemplified by the BROOKTREE 8960 chip, 1.2 k.OMEGA. is roughly a practical limit, and this corresponds to approximately 16.5 kft on mixed 26 and 24 gauge cable plant. It is anticipated that the practical reach of the MIPCS system may increase as improvements are made in subsequently available digital encoding chips which replace the BROOKTREE 8960. Similarly, if the loop is comprised of 24 AWG wire, the reach would exceed 20 Kft.

The signal subscriber 3 contains a MIPCS card 7, one POTS line 42, and two ISDN cables 8 and 9. The MIPCS card 7 is powered via the twisted cable pair 2. The amount of power which needs to be transmitted does not exceed 80 Volts on either wire in accordance with the U.S. safety standards. Internationally, the safety requirement for line-powering voltage varies and can go as low as 50 Volts on each wire. The digital signal transported over the twisted cable pair is demultiplexed and regenerated into two individual ISDN signals and one POTS channel. The regenerated ISDN signals have the same signal quality and distance capability as the original ones, and they are sent to remote premises over the two ISDN cables 8 and 9. The POTS signal is sent to the remote premises over the POTS line 42.

Each of the MIPCS cards 6 and 7 comprises a digital data interface module 10, a utility microprocessor 11, a transceiver 12, a SLI module 44, and a frame processor 33, as shown in FIG. 2. The digital data interface module 10 uses two ISDN U-interfaces, such as Motorola MC145572. The utility microprocessor 11 such as Motorola M68HC05 family provides loopback testing in accordance with the National ISDN Standards. The transceiver 12 uses a 2B1Q transport mechanism and operates at a speed of at least 384 kbps. One of such transceivers is Bt8960 DSL Transceiver by BROOKTREE Corporation with a transmission rate of 416 kbps in full duplex mode. The SLI module 44 in the MIPCS card for the signal provider emulates a telephone set for signaling purposes and converts the analog POTS channel to a digital signal which is fed to the frame processor; while the SLI module for the signal subscriber converts the digitized POTS signal to analog and provides all standard functionality, such as ringing, off-hook detection, caller identification tones, and other features. The bypass relay 46 ensures the subscriber's POTS access in the event of a electronic failure or local power outage.

A first embodiment of the invention is an office-to-house system, as illustrated in FIG. 3. A MIPCS card 7 is installed on a pole, on a house, or in a cabinet. Two customers 18 and 19 in a same house or two different houses each have one ISDN access, and one of them has a POTS access. They maybe kept in close proximity for maintenance purposes. Because the MIPCS card 7 regenerates high quality ISDN signals, the two ISDN lines can be 18 kft long from the MIPCS card, but the POTS line is limited to a 130-Ohm loop or about 2-kft-long on standard 24 AWG wire.

A second embodiment of the invention is a corporate-intranet system, as shown in FIG. 4. In this case, a corporate ISDN user needs ISDN accesses at a remote office 22 whose location is not permanently fixed or which is wired with only one twisted pair. The twisted cable pair 2, normally used for POTS, is temporarily or permanently converted by adding a MIPCS card 6 in a business wiring closet 32 and a MIPCS card 7 in the remote office 22 to provide two ISDN connections, in addition to the original POTS channel.

It will be clear to one skilled in the art that the above embodiments may be altered in many ways without departing from the scope of the invention. For example, more than one twisted cable pair can be used to transport more than two ISDN signals and more than one POTS channel. Accordingly, the scope of the invention should be determined by the-following claims and their legal equivalents.