Abstract:
The present invention provides a system and method for improving the performance of a data communication system, particularly a CATV system, by dividing the functionality of the cable modem termination systems (CMTSs) into functional units, and distributing these functional units among fiber nodes or other locations throughout the data communication system. This approach provides passive return signal paths and their associated benefits.

Description:
PRIORITY CLAIM  
       [0001]     The present application claims priority as a continuation of United States patent application HYBRID FIBER OPTIC AND COAXIAL CABLE NETWORK NODE THAT CONTAINS A CABLE MODEM TERMINATION SYSTEM, having application Ser. No. 10/029,810, filed on Thursday, Dec. 27, 2001. 
     
    
     TECHNICAL FIELD  
       [0002]     The present invention relates to data communication systems, and more particularly to high speed broadband data communication delivered via multi-channel shared cable television (CATV) systems.  
       BACKGROUND  
       [0003]     Data communication systems, such as cable television systems, are well known. A typical cable television (CATV) systems is comprised of a physical entity at a central location known as a headend, with one or more trunk lines extending therefrom. Each trunk line has a plurality of feeder lines extending therefrom into subscriber areas, where each subscriber is attached via a line tap onto the feeder or service line. If the distances between the headend and subscriber areas are substantial, intervening distribution hubs may be located along the trunk lines to replenish the strength and quality of the signal being provided to subscribers.  
         [0004]     The trunk, feeder and service lines of many existing CATV systems are all coaxial cables. Since the signals carried by these coaxial cables are electrical, these systems are susceptible to electrical and magnetic noise from natural phenomenon as well as other electrical and magnetic sources. In order to improve the clarity of the signals carried over a CATV system, the coaxial cables used for trunk and feeder lines are being replaced by fiber optic cables. Fiber optic cables carry light signals which are inherently less susceptible to electrical and electromagnetic noise from external sources. In addition, fiber optic cables carry signals for longer distances without appreciable signal strength loss than coaxial cable. However, the cost of replacing existing coaxial cables with fiber optic cables prevents many companies from converting their service lines to fiber optic cables. CATV systems having both fiber optic trunk and feeder lines along with coaxial service lines are typically called hybrid fiber cable (HFC) systems. In HFC systems, the service sites where the light signal from a fiber optic cable is converted to an electrical signal for a coaxial service line is called a fiber conversion node, fiber node, or simply a node.  
         [0005]     The utilization of high speed data services over all-coaxial or HFC systems has recently included implementation of headend controllers known as Cable Modem Termination Systems (CMTSs). A CMTS standard is defined in the Data Over Cable Service Interface Specification (DOCSIS) published by Cable Television Laboratories (incorporated herein by reference). A CMTS is described in this document as being normally embodied as a physical entity at a central location, e.g., the system&#39;s headend. However, widespread use of this system architecture has produced unforeseen and challenging system engineering issues when new services are deployed within HFC systems. For example, having the entire functionality of the CMTS at the one headend location means that passive return paths are not possible with these existing systems.  
         [0006]     In a CATV system, passive return paths are desirable because they provide the Inherent benefits of, inter alia, reduced cost of return path hardware, since return path amplifiers are not required; return path loss improvements; increased system reliability; increased return path capacity; improved noise funneling; decreased cost of return path optical transmitters; and the capability of carrying forward and return signals on a single fiber optic cable.  
         [0007]     References of possible interest in the field of CATV include U.S. Pat. No. 6,100,883, WO 01/45305 A, WO 01/17168 A and WO 02/061979 A.  
         [0008]     U.S. Pat. No. 6,100,883 describes an interactive television information system that is coupled to a cable television system having a headend for supplying information services and an information service distribution network for delivering information services to subscriber televisions. With this arrangement, each subscriber television is associated with a home interface controller. The home interface controllers receive the television information signals and include a data transceiver for data communications. A subscriber selection device associated with a home interface controller permits subscriber Interaction through the data transceiver with an assigned interactive controller from a plurality of interactive controllers. The assigned interactive controller is in communication with the information sources and in television communication with its assigned home interface controller. Selection of an information source is made through channel selection of an apparent channel from any of a first group of apparent channels and a second group of apparent channels. Different information services on different apparent channels from the first group of apparent channels are provided to a given home interface controller via the same television information signal as the subscriber changes channel selection from one of the apparent channels in the first group of apparent channels to another apparent channel in the first group of apparent channels. To receive apparent channels from the second group of apparent channels, a home interface controller selects the television information signal at its input corresponding to the selected channel.  
         [0009]     WO 01/45305 A relates to an optical distribution node that includes a laser transceiver that may be coupled to at least one fiber optic link. The optical distribution node communicates upstream and downstream digital data with the head end over the at least one fiber optic link. The optical distribution node further includes a data concentrator coupled to the laser transceiver. Further, for each of at least one coaxial cable link, the optical distribution node includes a frequency translator and a node modem. The frequency translator receives and translates the upstream digital data from modems on the at least one coaxial cable link to a different carrier to provide a signal to the modems on the at least one coaxial cable link for collision detection. The node modem is coupled between the coaxial cable link and the data concentrator. The node modem demodulates upstream digital data for the data concentrator and modulates downstream digital data for transmission over the coaxial cable link.  
         [0010]     WO 01/17168 A pertains to an arrangement in which upstream requests such a bandwidth request, are processed by a cable modem termination system (CMTS) out of order on a priority basis to reduce latency in responding to the request. With this configuration, CMTS is connected to a plurality of cable modems by a cable plant The CMTS has a burst receiver adapted to be connected to the cable plant to process upstream data packet units and bandwidth requests transmitted by the cable modems. Each packet includes a header that uniquely distinguishes the bandwidth requests from other data types. Packet data units are arranged in a first memory queue. Bandwidth requests are arranged in a second memory queue. The headers of the packets processed by the burst receiver are inspected as they arrive at the CMTS to determine if the packets are packet data units or bandwidth requests. Packet data units are routed to the first memory queue. Bandwidth requests are routed to the second memory queue. Bandwidth allocation MAP messages are generated from the bandwidth requests stored in the second queue and transmitted downstream to the cable modems. The packet data units stored in the first queue are coupled to a data output for further distribution.  
         [0011]     WO 02/061979 A describes a hybrid fiber-coax network having a head end and at least one fiber node in two-way communication therewith. With this network, a cable modem is located within the fiber node that provides a communication channel. The communication channel is adapted to transmit at least one informational signal that is indicative of a condition of the fiber node to the head end and is adapted to receive at least one control signal from the head end.  
       SUMMARY  
       [0012]     In a first embodiment, the invention provides a data communication system comprising: a headend for generating a transmission signal for transmission to a plurality of subscriber sites and for processing return signals from the subscriber sites; a plurality of distribution hubs operationally coupled to said headend; a plurality of fiber nodes operationally coupled to at least one of said distribution hubs; a plurality of service lines operationally coupled between at least one of said fiber nodes and the plurality of subscriber sites, wherein the hubs, fiber nodes and services lines transmit the transmission signal from the headend to the plurality of subscriber sites and transmit the return signals from subscriber sites to the headend along at least one return path. The data communication system also includes a cable modem termination functional unit or package operationally coupled along the at least one return path to either a hub, a fiber node or a service line. The cable modem termination functional unit comprises at least one receiver configured to receive a plurality of return signals generated by the plurality of subscriber sites; at least one signal demodulator configured to demodulate the plurality of received signals into an equal plurality of baseband digital signals; a time division multiplexor circuit configured to multiplex the plurality of baseband digital signals to form one serial digital bit stream; and a transmitter configured to transmit the digital bit stream toward the headend along the at least one return path. Typically, the transmission signal includes a cable television (CATV) signal.  
         [0013]     In a second embodiment, the invention provides a method for use by a cable modem termination functional unit or package within a data communication system having a headend for generating a transmission signal for transmission to a plurality of subscriber sites and for processing return signals from the subscriber sites, a plurality of distribution hubs operationally coupled to said headend, a plurality of fiber nodes operationally coupled to at least one of said distribution hubs, a plurality of service lines operationally coupled between at least one of said fiber nodes and the plurality of subscriber sites, wherein the hubs, fiber nodes and services lines transmit the transmission signal from the headend to the plurality of subscriber sites and transmit the return signals from subscriber sites to the headend along at least one return path. The method comprises the steps of: receiving a plurality of return signals generated by the plurality of subscriber sites; demodulating the plurality of received signals into an equal plurality of baseband digital signals; time division multiplexing the plurality of baseband digital signals to form one serial digital bit stream; and transmitting the digital bit stream toward the headend along the at least one return path.  
         [0014]     In a third embodiment, the invention provides a cable modem termination functional unit or package for use with a data communication system interconnecting a headend and a plurality of subscriber sites. The cable modem termination functional unit is operationally coupled along return paths between the headend and the plurality of subscriber sites. The cable modem termination functional unit comprises at least one receiver configured to receive a plurality of return signals generated by the plurality of subscriber sites; at least one signal demodulator configured to demodulate the plurality of received signals into an equal plurality of baseband digital signals; a time division multiplexor circuit configured to multiplex the plurality of baseband digital signals to form one serial digital bit stream; and a transmitter configured to transmit the digital bit stream toward the headend along the at least one return path.  
         [0015]     Hence, in at least some embodiments, the invention improves the performance of a HFC CATV system by dividing the functionality of the CMTSs and distributing this functionality throughout the network. This approach effectively provides passive return paths and their associated benefits. CMTS functionality is split so that a certain CMTS functionality is locate at the system&#39;s headend, and the remaining functionality is distributed around the HFC CATV system and is contained in the optical/electrical conversion, or fiber, nodes.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]     The exemplary embodiments of this invention will be described in detail, with reference to the accompanying figures, wherein like designations denote like elements, and wherein:  
         [0017]      FIG. 1  is a diagram illustrating an exemplary embodiment of a data communication system of the present invention;  
         [0018]      FIG. 2  is a diagram illustrating an exemplary embodiment of a fiber node of the present invention;  
         [0019]      FIG. 3  is a diagram illustrating a CMTP of an exemplary embodiment of the present invention; and  
         [0020]      FIG. 4  is a diagram illustrating a flow diagram of the signal processing steps occurring in a CMTP in an exemplary embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0021]     The following is a detailed explanation of the method and system for a data communication system which utilizes Cable Modem Termination Systems (CMTSs), and which provides for passive return paths. The inventive data communication system of the present invention divides the functionality of the CMTS into functional units, hereinafter known as Cable Modem Termination Packages (CMTPs), and distributes the CMTPs to various points within the overall data communication system.  
         [0022]     Referring to  FIG. 1 , a diagram of a data communication system according to the present invention is shown. Data communication system  100  comprises a headend  105 , a plurality of distribution hubs  110  coupled to the headend  105 , and a plurality of fiber nodes  115  coupled to the distribution hubs  110 . Each fiber node  115  is coupled to one or more service lines  120  to which a plurality of service subscribers are coupled through subscriber taps  125 . Coupling each fiber node  115  to a corresponding distribution hub  110  is a transmission cable  132  and a receive cable  137 . These transmission cables  132 ,  137  are typically fiber optic cables, while service lines  120  are typically coaxial cables. The optical transmission system  100  of the present invention may employ either Wavelength Division Multiplexing (WDM) or Dense Wavelength Division Multiplexing (DWDM), or both technologies.  
         [0023]     The term “fiber node” is commonly used to describe a service site or similar component wherein signals carried by fiber optic cables from a higher level are converted to electrical signals (e.g., RF signals) for transmission along coaxial cables. Each fiber node  115  connected to a distribution hub  110  has its own transmission cable  132  and receive cable  137  to couple the fiber node  115  to the distribution hub  110 . Headend  105  is similarly coupled to each distribution hub  110  by transmission cables  130  and receive cables  135 .  
         [0024]     Referring to  FIG. 2 , each fiber node  115  commonly includes one or more optical converters  240 , wherein the electrical signals received on coaxial cables  120  are converted into optical signals for transmission to a distribution hub  110  along upstream fiber optic cables  137 . Similarly, optical signals received via upstream fiber optic cable  132  are processed by additional optical converters  245  into electrical signals for transmission along coaxial cables  120 . The optical signals are also processed as necessary with optical multiplexor  260  and an optical demultiplexor  265 .  
         [0025]     Fiber node  115  may also include a wavelength stabilized source  250 , an oscillator  255 , an optical multiplexor  260 , and an optical demultiplexor  265 . The wavelength stabilized source  250  is useful for providing additional processing of the wavelengths of the optical signals before the signals are transmitted on the upstream fiber optic cable  137 . The oscillator  255  is also used to process signals for transmission on the fiber optic cables.  
         [0026]     According to the present invention, each fiber node  115  also contains a cable modem termination package (CMTP)  300 , as shown in detail in  FIG. 3 . The CMTP may include, inter alia, electronic devices, optical devices, microprocessors, and related operational software. For instance, use of optical devices such as, inter alia, optical transmitters is desirable since the optical transmitters of digital signals need not have linear performance characteristics. Therefore a significantly lower cost optical transmitter can be used. The CMTP also includes a demodulator circuit  301 , a multiplexor circuit  302 , a demultiplexor circuit  303 , at least one optical transmitter  304 , at least one optical receiver  305 , and connection devices  306  for operationally connecting said cable termination package to a data communication package.  
         [0027]     Referring now to  FIG. 4 , a flow diagram  400  of the signal processing steps occurring in a CMTP is illustrated. Data signals, from each of the return paths  120  (see  FIG. 2 ), are received at an optical receiver in an initial step  401 . These signals are processed in a signal demodulator step  405  so that their frequencies are demodulated to their baseband digital signals. These baseband digital signals  410  are then processed by a time division multiplexor circuit  415  to form one serial digital bit stream  420 . The serial digital bit stream  420  is processed in a pulse code modulator circuit  425 , and is then fed to an optical transmitter  430  on a different signal wavelength than that of the signal wavelength being carried by the fiber optic cable  440 . Alternatively, a wavelength division multiplexer circuit  435  can be employed to mix the return optical signal for transmission on the same fiber optic cable as that which carries the forward signals.  
         [0028]     The embodiment of the present invention as described herein makes possible a passive return path, since the input signal level required by the presence of the CMTP  300  in the fiber node  115  is lower (e.g., 20 to 30 dB lower) than that required for a fiber node not having the CMTS functions embodied in a CMTP in that node. A derivative feature of this decreased input signal level is that the quantity of signal amplifiers in the return path may be reduced, or the signal amplifiers may be eliminated entirely. A second derivative feature of this feature is that the reduction in, or lack of, return path signal amplifiers results in lower equipment costs for the overall data communication system. Similarly, hardware maintenance expenses are reduced due to the decreased amount of hardware. The decreased amount of hardware also yields increased system reliability, since each piece of hardware that is removed also removes a potential point of failure.  
         [0029]     Another improvement provided by the present invention relates to an increase in the signal carrying capacity of the return path. Known HFC fiber nodes typically have four return path inputs that are combined into a single signal for transmission upstream towards the headend. In such a case, the frequency of the return signals from each of the four individual return paths cannot be duplicated. However, the present invention allows the utilization of the frequencies on each of the return paths. Therefore, if there are, for example, four return paths to a particular fiber node, this invention provides an increase of four times the signal carrying capacity for transmitting data signals back upstream towards the headend.  
         [0030]     Another benefit resulting from the above feature is related to external noise (e.g., thermal noise and ingress noise). In the related art, when four return paths are combined into a single return path, the noise level is additive. This represents the phenomenon known as noise funneling. In noise funneling, therefore, the noise worsens by a factor of four. In the present invention, each return path is kept separate, so that noise funneling cannot occur.  
         [0031]     An additional feature of the present invention is directed towards making larger node serving areas practical. Without the inventive concept of the present invention, the node serving areas are limited because of the noise accumulation effects (i.e., noise funneling) and the limited return path carrying capacity. Since the present inventive concept reduces the noise accumulation by a factor of four, and simultaneously increases the traffic capacity by a factor of four, it follows that the node serving area may be increased by four times.  
         [0032]     Although certain exemplary embodiments of the present invention have been shown and described in detail, it should be understood that various changes and modifications may be made without departing from the scope of the present invention. The scope of the present invention will in no way be limited to the number of constituting components, the materials thereof, the shapes thereof, the relative arrangement thereof, etc., and are disclosed simply as an example of the exemplary embodiments.