Abstract:
A method and an apparatus are employed for individually monitoring the connectivity status of cables connected at a cable modem termination system (CMTS), where the cables conduct upstream and downstream RF communication signals. The monitoring is self-contained within the CMTS. The monitoring is achieved by producing a reference signal having a frequency outside the frequency range of the RF communication signals. The reference signal is injected onto the RF communication signal. The power level of the reference signal is detected within the CMTS, whereby the power level correlates with an expected cable load impedance. A DC control voltage based on the detected power level of the reference signal is generated, which allows a controller to determine the connectivity statuses of the connected cables.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention generally relates to switching of upstream and downstream RF communications among redundant primary and backup routers in HFC access networks, and more particularly, to cable connection monitoring at the routers.  
         BACKGROUND  
         [0002]    In an IP or VoIP network using cable routers and hybrid fiber coaxial (HFC) access networks, a cable interface such as a cable modem termination system (CMTS) is necessary. FIG. 1 shows a diagram of communication system  10 , comprising the IP network  70 , system headend  75 , including primary routers  21 , backup router  22  and cable modem termination system (CMTS)  20 ; HFC access network  40 , PSTN  15  and end users  25 ,  26 . Detection and status reporting of CMTS cable connections can prevent a switchover from a primary router  21  to a backup or redundant router  22  from occurring under network failures caused by external cable problems, such as improper connection, removed cable connection to one or more of its RF ports, cable break, etc. Switching to backup equipment during external faults does not solve the problem, and effectively ties up both the primary and backup equipment needlessly. This reduces the overall reliability of the system. It is also desirable to have the ability to quickly detect and locate a fault in the HFC cable network to allow prompt repair and system recovery.  
           [0003]    Prior art solutions include injection of a signal across the center conductor or coaxial shield at the source of a cable connection. The presence of the known signal is then detected by either a dedicated detector placed at one or more points along the signal path or through detection of the radiated signal through some type of inductive coupling. Connection of specific continuity detectors to cable or near the cable in the HFC network is a not viable option where a CMTS designer has little control over the externally coupled cable network. In addition, even in cable plants where it would be possible to connect external cable monitoring equipment, the CMTS&#39;s need for such equipment and its maintenance may be unfavorable to network managers.  
           [0004]    An alternative solution is time domain reflectometry (TDR) which can sense cable discontinuities when the cable length is great enough. However, implementing TDR in the CMTS or matrix switch is prohibitive with respect to cost, size and complexity. TDR is also an intrusive test and is ineffective on short cable lengths.  
         SUMMARY  
         [0005]    A continuously operating non-intrusive, self-contained system of determining cable connectivity between a cable modem termination system (CMTS) and the remainder of the HFC cable plant is employed. A reference signal having an out-of-band power signal is placed onto the cable network for detection by an onboard power detector. A correctly terminated cable produces a power measurement that correlates with an expected cable load impedance. The power detector generates a DC voltage proportional to the power level applied to its input. A comparator verifies that the voltage falls within the expected range. A detector reading that is outside the range of normal connectivity indicates a fault condition. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]    [0006]FIG. 1 shows a block diagram of an HFC access network.  
         [0007]    [0007]FIG. 2 shows a block diagram of an RF switch used in a cable modem termination system.  
         [0008]    [0008]FIG. 3 shows a block diagram of a cable connection monitoring circuit for a downstream communication path from an RF switch to an HFC network.  
         [0009]    [0009]FIG. 4 shows a block diagram of an alternative embodiment to the cable connection monitoring circuit of FIG. 3.  
         [0010]    [0010]FIG. 5 shows a block diagram of an alternative embodiment to the cable connection monitoring circuit of FIG. 4.  
         [0011]    [0011]FIG. 6 shows a block diagram of a cable connection monitoring circuit for an upstream communication path from an HFC network to an RF switch and from an RF switch to a router receiver.  
         [0012]    [0012]FIG. 7 shows a block diagram of a cable connection monitoring circuit for downstream cables from a router transmitter to an RF switch. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0013]    [0013]FIG. 2 shows a block diagram of an RF switch  50 , used within a CMTS as an interface between primary stackable broadband access routers (PSBARs)  31 - 35 , secondary stackable broadband access router (SSBAR)  36 , and HFC access network  40 . Each PSBAR  31 - 35  and SSBAR  36  preferably comprises one transmitter Tx and eight receivers Rx 0 -Rx 7  (not shown).  
         [0014]    RF switch  50  provides the switching capability to allow the SSBAR  36  to function as a PSBAR, when it has been determined that any of the PSBARs  31 - 35  have malfunctioned. The RF switch  50  provides the inter-connect between PSBARs  31 - 35 , SSBAR  36 , and the HFC network  40  residing at the cable operator&#39;s distribution hub or headend operation.  
         [0015]    RF switch  50  comprises an RF backplane  90  connected to various modules: RF switch controller card  56 ; an optional backup RF switch controller card  57 ; preferably five primary switch cards  51 - 55 ; and one secondary switch card  59 . Although RF switch  50  is described comprising a particular number of switch cards and switch controller cards, the number of cards is preferable, but not intended to be limiting. Alternative embodiments include, but are no limited to, more or less primary switch cards, secondary switch cards and switch controller cards as deemed necessary for interface with any number of PSBARs and SSBARs.  
         [0016]    RF switch controllers  56 ,  57  provide a communications link between SSBAR  36  and RF Switch  50 . Including a second RF switch controller  57  in addition to controller  56  enables the RF switch hardware to meet the “five nines” (99.999%) high availability (HA) standard. The redundancy of two switch controllers  56 ,  57  also serves a benefit of allowing either controller card to serve as the master controller. The SSBAR  36  monitors a first controller, say  56 , to ensure it is operating properly. If the SSBAR  36  detects an error with controller  56 , it will send commands to controller  57 . If the first controller card fails, the second controller can be used to manipulate and monitor RF switch  50 .  
         [0017]    Each primary switch card  51 - 55  is connected to one of the PSBARs  31 - 35 , respectively. The purpose of the primary switch cards  51 - 55  is to provide interconnect and switching capability between the RF circuitry of PSBARs  31 - 35  and SSBAR  36 . A primary switch card  51 - 55  is used in conjunction with the secondary switch card  59  to provide the complete switchover between a malfunctioning PSBAR  31 - 35  and the SSBAR  36 . The primary function is to switch the malfunctioning PSBAR&#39;s RF transmitter and receiver connections to the SSBAR&#39;s RF transmitter and receivers. RF switch controller  56 ,  57  programs the primary switch cards  51 - 55  via connections  61 - 67  with RF backplane  90 . Relays are used to switch between ports on the primary switch cards  51 - 55  upon control commands dispatched by switch controllers  56 ,  57 . Connections  61 - 67  include serial peripheral interfaces (SPIs) as well as parallel cables that carry signals from transmitters Tx and receivers Rx 0 -Rx 7  of PSBAR  31 - 35  and SSBAR  36 . There is cable detection circuitry on each of the primary switch cards  51 - 55  to detect proper cable connectivity to its corresponding activated PSBAR  31 - 35  or SSBAR  36 , as will be discussed in further detail.  
         [0018]    Secondary switch card  59  is used to route signals between SSBAR  36  to each primary switch card  51 - 55 .  
         [0019]    RF switch controllers  56 ,  57  store settings for primary switch cards  51 - 55  and secondary switch card  59 . Deciphered messages containing switch setting information is interpreted by RF switch controller  56  or  57  and relayed to the switch cards via the SPI connections. Polling by SSBAR  36  determines the current state of all status information on RF switch controllers  56 ,  57 .  
         [0020]    [0020]FIG. 3 shows an interconnection diagram of cable connection monitoring circuit  100  for a single primary switch card representative of primary switch cards  51 - 55 . Each primary switch card  51 - 55  has circuitry for continuously performing self-contained detection of the connectivity of cables attached to HFC network  40 . Monitoring circuit  100  determines if a monitored cable has been removed, cut in the immediate vicinity, or short circuited, which permits an appropriate decision for primary and secondary resources of the CMTS that maintains best availability. Any cable fault condition detected by circuit  100  indicates that a switch from the PSBAR  31 - 35  to the SSBAR  36  is unnecessary. Switch controllers  56 ,  57  supply a sinusoidal control signal of preferably 4.8 MHz used in the cable detection circuitry of the primary switch cards  51 - 55 . A frequency of 4.8 MHz is preferable because it is non-intrusively below the standard upstream frequency range (5-60 MHz), but with close enough proximity to ensure an impedance close to the network nominal impedance, typically 75 Ohms. As a non-intrusive signal, the injection of the 4.8 MHz signal out onto the cable does not impair, interrupt, or otherwise reduce the available/usable spectrum available for any programming or other services that may be carried on that cable. Each controller  56 ,  57  drives this signal on a separate line for each primary switch card  51 - 55  and monitors the 4.8 MHz signal to ensure it is operating properly.  
         [0021]    For cable connection monitoring on the HFC network  40  downstream signal cable  41  connection to the CMTS at connector  42 , the 4.8 MHz signal is injected onto the main RF signal path via directional coupler  151 . Preferably, coupler  151  has a soldered connection onto a trace of the primary switch card  51 - 55 , which maintains the nominal impedance rating of the RF signal path, (i.e., preferably 75 Ohms). The RF signal RF IN  originates from either PSBAR  31 - 35  or SSBAR  36  transmitter Tx connected to RF switch  50  in the CMTS. Power detector  120  and window comparator  130  test for the presence of the nominal network impedance. Power detector  120  receives the reference signal from the backplane  90  through directional coupler  152 , an equivalent counterpart to coupler  151 . The received 4.8 MHz reference signal is converted by power detector  120 , which generates a DC voltage proportional to the power level of the received reference signal. A predefined window of acceptable readings is stored by window comparator  130  for comparison to the measured values. Window comparator  130  verifies whether the measured voltage is within the expected range. A detector reading that is outside the window of normal connectivity indicates a fault or open cable condition. A disconnected or open cable will produce a higher than normal power reading due to the high impedance as seen at connector  42 . A connected cable with a short circuit or ground fault condition will produce a lower than normal power reading as there is a low impedance condition on the cable. Accordingly, comparator  130  sends a cable status indicator signal to backplane  90 . The status indicator is an alarm signal for either of the two possible types of detected cable fault conditions. Otherwise, the status indicator is an acknowledgement signal that the cable connectivity is satisfactory. Controller  56 ,  57  receives the status indicator signal and thereby maintains the continuous cable connectivity monitoring status for cable  41 . Similar connectivity status is maintained for the cables associated with each primary switch card  51 - 55 .  
         [0022]    [0022]FIG. 4 shows an alternative embodiment  200  in which power detector  120  measures differential power across a series source resistor R that is connected between the 4.8 MHz signal source and directional coupler  151  at the RF signal path. Parallel power detector  121  acts as a reference signal monitor as it directly measures the 4.8 MHz reference signal power. Difference amplifier  123  determines the differential power between power detectors  120  and  121 . Window comparator  130  compares the measured power difference value to a stored range of predetermined acceptable power values. A detection of differential power that is within a predetermined window for normal differential power indicates normal connectivity. However, detection of a less than normal power differential indicates a high impedance, which is caused by either a broken or disconnected cable  41 . If a significant power drop, or power differential, is detected between the reference source power measured by detector  121  and the power present at RF signal cable measured by detector  120 , it indicates a possible short or ground fault condition. The advantage of this embodiment compared with that shown in FIG. 3 is that output power for the 4.8 MHz reference signal does not need to be as tightly controlled because it is not directly used as the reference for comparison. Thus, fluctuations in the 4.8 MHz signal are less troublesome.  
         [0023]    [0023]FIG. 5 shows an alternative embodiment in circuit  300  which uses a transmitter  110  to produce a 3 kHz output signal from the reference 4.8 MHz signal. Preferably, transmitter  110  comprises an IC modem having significantly higher impedance than the nominal 75 Ohm system impedance. The preferred embodiment includes a 600 Ohms rated modem, but modems having other rated impedance values may be used. The advantage of the higher impedance is to eliminate the need for high isolation directional couplers  151  and  152 , which introduce insertion loss. Instead, the 3 kHz signal is injected onto the main RF signal path via non-directional coupler  251  on the primary switch card  51 - 55 . Receiver  126 , which may also comprise a modem IC, receives the 3 kHz reference signal through non-directional coupler  252 . When an HFC cable  41  is disconnected at CMTS connector  42 , the level of the 3 kHz signal drops below the carrier detect threshold level of the receiver  126 . The output of receiver  126  is converted to an SPI compatible signal at converter  136 , which signals controller  56 ,  57  via backplane  90  that a cable has been disconnected. Depending on how receiver  126  is implemented, detector  136  may be unnecessary, or it can be a digital level translator, or a digital SPI interface. For example, receiver  126  may be implemented to produce simply either a digital true or false signal that can be easily read by controller  56 ,  57  to mean either connectivity is good, or there is a cable fault. In such a case, detector  136  is not needed. Similar to detector  120  in circuits  100  and  200 , detector  126  of circuit  300  monitors the reference signal for high impedance faults, short circuit and ground faults on cable  41 .  
         [0024]    [0024]FIG. 6 shows a block diagram of the HFC cable connection monitoring circuitry  400  for an upstream signal cable  241  at connector  242  to one receiver Rx from among eight receivers Rx 0 -Rx 7  in each PSBAR  31 - 35  and SSBAR  36 . Transmitter  110  produces a 3 kHz output signal from the reference 4.8 MHz reference signal. The 3 kHz signal is transmitted across the monitored RF signal path via non-directional coupler  251  on the primary switch card  51 - 55 . Receiver  226 , which may also comprise a modem IC, receives the 3 kHz reference signal through non-directional coupler  252 . When an RF port&#39;s upstream HFC cable  241  is disconnected at CMTS connector  242 , the level of the 3 kHz signal drops below the carrier detect threshold level of the receiver  226 . The output of receiver  226  is converted to an SPI compatible signal at converter  236 , which signals controller  56 ,  57  via backplane  90  that a cable has been disconnected.  
         [0025]    The cable connectivity monitoring circuitry  400  shown in FIG. 6 also includes continuity detection for an upstream signal cable on the PSBAR  31 - 35  and SSBAR  36  receiver side. A 4.8 MHz sine wave is driven down the RF backplane  90  of a primary switch card  51 - 55  to connector  243 , to which a cable between one of PSBARs  31 - 35  or SSBAR  36  and the RF switch  50  are attached. Cable  244  represents a single cable connected to one receiver Rx from among eight receivers Rx 0 -Rx 7  of PSBARs  31 - 35  and SSBAR  36 . Each PSBAR  31 - 35  and SSBAR  36  have their respective detectors that monitor presence of this 4.8 MHz signal and notify RF switch controller  56 ,  57  through SPI connections with backplane  90 . The RF switch controller  56 ,  57  and SSBAR  36  maintain continuous communication to facilitate seamless switchover from primary to secondary operation or vice-versa. All cable detection status, regardless of whether detection is at PSBAR  31 - 35 , SSBAR  36  or RF switch  50 , is communicated to the common control point at RF switch controller  56 ,  57 .  
         [0026]    [0026]FIG. 7 shows a block diagram of cable connection monitoring circuitry  500  for the downstream cable connections from a PSBAR  31 - 35  or SSBAR  36  transmitter onto the RF switch  50 . An RF signal detector  135  checks for presence of RF signal power in the downstream frequency band extracted from the RF signal path at directional coupler  152 . If there is presence of an RF signal, it follows that there must be a cable connected from the router transmitter Tx to the RF switch  50  of the CMTS. Converter  130  reads the RF signal indication signal from detector  135 , and provides indication of cable connectivity to switch controller  56 ,  57  through backplane  90 . A power detector at Tx output concurrently monitors cable  44  power on the SBAR side of cable  44  so the status is known on both ends of the cable  44 .