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. In the first case, although the Internet access is provided, it is not of sufficient bandwidth to provide the graphic-rich service which typically is desired. 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. Also in the latter case, the ISDN installation on existing cable pairs is limited to about 70% of the installed base due to the way in which ISDN transport was designed specifically for non-loaded cable plants. The existing copper cable outside plant was constructed in accordance with design rules 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-Hz (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. In summary, 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 media, such as fiber optic cable, is the ultimate solution for the digital access, but 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.
Another solution to this problem is to utilize TV cable networks and cable modems instead of telephone networks. The cable modems allow cable TV providers to offer data services and Internet connectivity. Because coaxial cable plants were designed for unidirectional analog broadcast services, they could not carry bi-directional data flows. The cable systems used a cable looping around the service area with taps at every home or small office. For the efficient bi-directional data transport, the cable networks must be upgraded to take on the characteristics of the star topology, but the reconfiguration process is prohibitively expensive.
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 provides only analog dial tone is called integrated service digital network (ISDN). ISDN was developed in the 1980's, when state-of-the-art digital encoding technology resulted in the standards as described in Bellcore documents TR-TSY-00393 (ISDN Basic Access Digital Subscriber Lines, May 1988) and TA-TSY-00397 (ISDN Basic Access Transport System Requirements, October 1986), disclosures of which are incorporated herein by reference. The basic transmission speed 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 kilo-feet (kft) but only 85% of all subscribers on average. This results in a problem with respect to reaching all subscribers wishing 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, and 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. does 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. Furthermore, 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 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. This invention is subsequently referred to as a "multiple ISDN carrier system" or abbreviated as "MICS."