Abstract:
In a cable data system, frequency division multiplexed upstream data traffic is first split for subsequent signal processing. In the course of performing an analog RF signal splitting operation, signal splitter output amplitude is decreased as an artifact of an RF signal splitter. A signal splitter indicator type that is used per signal splitter, provides an indication to the CMTS channel interface cards to control the amount of compensating gain required to restore the original signal to its original amplitude.

Description:
FIELD OF THE INVENTION 
     This invention relates to a cable data system by which two-way data communications, such as Internet access, is provided via a cable televisions system. In particular, this invention relates to an improved method and apparatus for connecting the physical cable media to a cable modem termination system (CMTS) so as to expedite installation and provide a more serviceable CMTS to maintenance personnel. 
     BACKGROUND OF THE INVENTION 
     Cable television systems are well known. In such a system, several different frequency-division multiplexed television channels are distributed to subscribers over a coaxial cable. Each television channel is typically allocated a frequency band, (typically 6 MHz.) in which audio and video information for a television channel is carried. Data signals can also be modulated onto an RF carrier and be transmitted in one or more of the pre-allocated television frequency bands. By allocating one or more T.V. channels for data, the cable television network can readily carry data, such as the data exchanged between computers. Cable data systems provide Internet access to subscribers at speeds that are far greater than dial-up modems. 
     A cable communications system topology resembles an inverted tree or a directed acyclical graph. The top or upper-most node in a directed acyclical graph (DAG) representing a cable distribution system is the node from which signal information is distributed and is frequently referred to as the cable system head end. Each link in the DAG represents a coaxial cable on which there might be several different frequency-division multiplexed signals. 
     One or more cable modem termination systems (CMTS) at or near the head end direct the distribution and collection of data to, and from, cable data system subscribers. At the head end of a cable data system, there are typically hundreds of physical cables that branch out from the head end to and from the system subscribers′ homes over a hybrid-fiber coaxial system. Downstream signals are transported on a hybrid-fiber coaxial cable with carrier frequencies centered above the 50 MHz point in the cable spectrum, while upstream signals are transported on a hybrid-fiber coaxial cable with carrier frequencies centered in the 5-42 MHz region of the cable spectrum. 
     Several different upstream channels can be frequency division multiplexed onto a single cable. In order to recover each channel, the upstream signal must be divided (or split) so that it can be coupled into separate RF band pass filters before being terminated at a unique Physical Interface (PHY) chip. Each PHY chip filters and demodulates the upstream signal for a particular channel to re-create the digital data stream for that channel. 
     If each upstream channel is transported on a different upstream cable, then a different cable must be used to inject each upstream channel into a unique CMTS upstream connector. In the high-capacity, high-bandwidth CMTS systems of the future, there will be many upstream channels supported by a single CMTS, so this will result in a large number of cables connecting to the CMTS in a relatively small area (yielding very high cable densities). These high cable densities can be difficult to manage. 
     However, if several upstream channels are frequency-division multiplexed on a single upstream cable, then it is possible to reduce the high cable densities at the CMTS by injecting multiples of these upstream channels over a single cable into a single CMTS upstream connector. For example, if four upstream channels are always frequency-division multiplexed on every upstream cable, then a system that originally required  100  separate upstream cables could be re-cabled using only  25  upstream cables, where each of the upstream cables carries four upstream channels into the CMTS. However, in order to permit this desirable decrease in cable density, the CMTS must be able to provide the further splitting required to route each of the upstream channels to a unique CMTS RF band pass filter and PHY chip. Since the actual number of upstream channels that will be multiplexed together is oftentimes unknown until the cable system is being assembled, the amount of integrated splitting provided by the CMTS must be configurable by the installer. This can be accomplished by providing several circuit cards with different splitting ratios so that the CMTS equipage can be fine-tuned to match the needs of the installer. 
     Although this solution solves the cable density problem, it leads to another problem, because installation of circuit cards with different splitting ratios will lead to different amounts of loss in the upstream signal paths. To maintain acceptable signal levels at the PHY chips, the CMTS should probably provide suitable gain to compensate for the splitting loss. Unfortunately, the required gain cannot be predicted until the installer has inserted the actual splitter circuit card (with a particular splitting ratio pre-defined on the circuit card). Thus, it would be beneficial if the CMTS could sense the splitting ratio on the installed splitter circuit card and provide adequate gain to directly compensate for the resulting splitter loss. 
     SUMMARY OF THE INVENTION 
     In a cable data system, upstream frequency-division multiplexed signals that received at a CMTS on a single cable are divided (i.e. split) using an analog radio frequency (RF) splitter so that individual frequency bands can be selectively filtered by CMTS channel interface cards that include band pass filters, which in at least one embodiment are programmably tuned to the center frequency of a particular pass band. 
     The CMTS signal splitter card is inserted into a card edge connector, preferably at the rear of the CMTS and includes on the card, one or more resistive networks which, by way of card edge connector wiring and splitter card wiring, automatically identifies to the CMTS and the channel interface cards (which filter and amplify signals in predetermined frequency bands) the signal splitting ratio of the signal splitter card. 
     A known artifact of splitting an RF cable signal, using an analog signal splitter, is a corresponding reduction in the split signal amplitude that is proportional to the signal division factor. By way of example, splitting an RF signal into two (2) separate signals causes each of the two, separate output signals from the splitter to be 3 dB (decibels) below the level of the signal input to the splitter. Splitting a signal into four (4) signals causes each one of the four separate outputs to be 6 dB below the signal level input to the splitter. Splitting a signal into eight (8) signals causes each one of the eight separate outputs to be 9 dB below the signal level input to the splitter. 
     In the preferred embodiment, the CMTS channel interface cards recognize the division ratio of the signal splitter card in the CMTS back plane and automatically increase the gain of programmable gain compensation stage (i.e. an amplifier) which is preferably an amplifier that is either band-pass tunable, or having a wide enough frequency response to amplify the entire spectrum of signals that it will be expected to amplify, so as to restore the signal amplitude output from the splitter to the amplitude that was input to the splitter. Different signal splitter cards with different signal division factors use different resistive indicator networks to identify to the CMTS and the channel interface cards, the amount of amplification required to compensate for the signal splitting. Increased system reliability is achieved by automatically adjusting RF signal amplification after splitting a frequency-division multiplexed signal into its respective components for subsequent recovery of data signals from subscribers. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a simplified drawing of a system for splitting cable data signals and automatically amplifying the split signals to compensate for the signal splitting division ratio. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 shows a simplified representation of a system of a signal splitter with automatic gain amplification compensation for use with a cable modem termination system (CMTS). The system  100  includes an integrated signal splitter card  102  for use with an upstream cable data system wherein frequency division multiplexed signals are received at a first RF input port or terminal  104  on the integrated splitter card  102  such that the frequency division multiplexed signals input at input terminal  104  are divided and appear at multiple output terminals or ports  106 a-d on the signal splitter card  102  for connection to a mid-plane connector  108  for the CMTS. The mid-plane is analogous to a back plane that supports card edge connectors, however, the mid-plane  108  has connectors on both sides (not shown for clarity) such that circuit cards can be inserted into the CMTS from both the front and rear sides, into connectors of the mid-plane  108 . 
     The outputs at the terminals  106   a-d  from the signal splitter card  102  are routed through the mid-plane  108  connector to a CMTS channel interface card  110 . In the preferred embodiment, each channel interface card  110  includes an RF front end  112  comprised of a frequency selective filter  114  and a variable gain stage  116 . The frequency selective filter  114  is preferably a passband filter that selectively passes only the frequency of a cable data channel of interest, and on which data is carried, to a programmable gain stage  116  that is responsive to signals received from the integrated splitter card  102 . Typical embodiments for the passband filter (not shown) include active or passive filters as well as suitably fast digital signal processors. 
     Input signals on the upstream cable  104 , which are frequency division multiplexed, are coupled into an analog signal splitter  118  having an input  120  and outputs  106   a-d  as shown. The analog splitter  118  functions as any radio frequency analog signal splitter functions and it produces at each of the output terminals  106   a-d  a copy of the signal input at terminal  104 . 
     It is well known that an artifact of a signal splitter is a reduction in the signal amplitude output at each terminal  106   a-d  of the splitter  118 . In the case of a 1:4 splitter, i.e. a splitter with a single input and four separate outputs, the process of splitting the signals into four different outputs produces a single loss of 6dB. Stated alternatively, the amplitude of the outputs at the terminals  106   a-d  will be six dB below the amplitude input at terminal  104 . 
     As shown in FIG. 1, the integrated splitter card  102  also includes a 1:8 splitter  122 , which is preferably an RF signal splitter suitable for use with cable data-frequency signals, i.e. capable of splitting RF signals up to approximately 800 MHz. or higher. Signals input at input terminal  124  and which are output at terminals  126   a-g  will be reduced in amplitude by an amount equal to 10dB. 
     In a cable data system, the prior art solution to processing the different frequency channels was to incorporate a discreet signal splitter on an upstream cable followed by discreet amplification/gain stages. In FIG. 1, the integrated signal splitting card  102  is equipped with a card type indicator  128 , which in the preferred embodiment is a pad of resistors between ground potential and coupled to a multi-wired bus  130  through the mid-plane connector  108  to the channel processing card  110 . Alternate embodiments of the card type indicator would include so-called DIP switches, selectable jumpers or even hardwired jumpers by which a signal or combination of voltages, resistances or other measurable electrical signal is generated to identify the signal splitting performed in the signal splitter card. By means of the signals provided by the card type indicator  128 , the CMTS as well as the CMTS channel interface card  110  can automatically know the type of signal splitter card being used to couple the upstream data channels to the CMTS. 
     As shown in FIG. 1, the signal splitter card  102  uses a 1:4 signal splitter  118  the outputs of which appear at terminals  106   a-d . For purposes of a 1:4 signal splitter ratio, each gain stage of the receiver front end  116  is required to boost the signal level from the splitter by a factor of 6 dB. By reading the card type through the connections provided at the card type indicator  128 , which are coupled through the mid-plane  108  and wired directly to the channel interface card front ends  112 , the channel interface cards can automatically apply an appropriate gain factor to restore the original amplitude of the signal input at terminal  104  to the integrated splitter card. 
     In the preferred embodiment, connections between the card type indicator  128 , which path through the mid-plane directly to the channel interface card  110  are decoded by the channel interface cards automatically to hard wiring. No processor overhead is required because the circuitry on both the integrated splitter card and the channel interface card are designed to automatically adjust the amplification provided according to the signals provided by the channel indicator  128 . 
     By providing an automatic gain increase  116  according to the splitter card type, the proper amount of signal amplification is automatically provided to the upstream cable data channel signals after they are divided into separate, but reduced amplitude copies, by the signal splitter  118  and selectively filtered by the channel interface card programmable filters  114 . In operation, after the particular passband signals are selectively filtered to isolate the upstream data, that data can be amplified by an amount equal to the signal loss attributable to the splitting operation performed in the integrated splitter card circuit. 
     In the preferred embodiment, the integrated signal splitter card  102  employs similar division ratio splitters. Depending upon the splitting factor used on a particular card, the gain compensation by the channel interface card  110  might have to be adjusted upwardly or downwardly. An appropriate high-frequency operational amplifier is one example of a gain stage embodiment. Those skilled in the art will recognize that by selecting differently-valued feedback, the gain of such a device can be adjusted as needed. Differently-valued feedback resistors can be selected under program control by opening and closing relays so as switch in and switch out, more or less negative feedback to control gain. The selection of an amplification factor is preferably done by the card type indicator  128  signals. In an alternate embodiment, a processor (not shown) can select the gain factor and is considered herein to be an equivalent embodiment and providing the programmable gain. In at least one alternate embodiment, the programmable gain stage also provide frequency-selective gain, such that only the signals in a particular frequency band or spectrum are amplified. An active filter providing a positive gain would achieve such a result. 
     In the case of a 1:4 splitter, each channel interface card front end processor (including the programmable gain stage) should restore 6dB of gain to the frequency divided signals. In the case of a 1:8 splitter, the gain that must be restored by the channel interface card is higher, well known to those in the art to be approximately 10dB. 
     In the preferred embodiment, in the cable modem termination system of Cadant, Inc., the channel interface cards are plugged into the front side of the CMTS card cage. These channel interface cards  110  are inserted to a so-called mid-plane edge connector and receive the contacts on the edge connector of the channel interface cards  110 . The mid-plane  108  has coupled to its opposite side, another card edge connector into which the integrated signal splitter card is coupled. In this fashion, the myriad of cables coming up from a cable data system, can be wired into a so-called “back card” that is the integrated signal splitter card thereby keeping the maze of wires and cables coming up from a cable data system out of sight from the front side of the cable modem termination system. 
     By automatically sensing the type of integrated splitter card used, cable data systems that multiplex  4  signals onto a cable or 8 signals onto a cable or 2 signals onto a cable can have the channel interface card  110  automatically provide the appropriate amount of gain compensation, without relying upon external circuitry separate and apart from the CMTS hardware. 
     Assembling the components for a cable data system is simplified by using an integrated signal splitter card which together with the wiring provided by the mid-plane and channel interface cards provide a system by which cable signal splitting and gain compensation is automatically provided. System maintainability is improved because craft persons do not have to track the type of gain required to compensate for various signal splitting ratios used in the prior art.