Abstract:
A method for adjusting bandwidth allocation in a cable modem termination system is provided. The method includes detecting an overflow condition at a primary transceiver of the cable modem termination system, determining whether a back-up transceiver is available in the cable modem termination system, and when the back-up transceiver is available, establishing at least one channel for the back-up transceiver to communicate over a common physical medium with at least one channel of the primary transceiver to effectively increase the bandwidth of the cable modem termination system.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is related to U.S. patent application Ser. No. 09/995,167 entitled PASSIVE CMTS REDUNDANCY, filed Nov. 26, 2001 (the &#39;167 Application). The &#39;167 Application is incorporated herein by reference. 
     
    
     
       TECHNICAL FIELD  
         [0002]    The present invention relates generally to telecommunications, and in particular to backup for cable modem termination systems (CMTS).  
         BACKGROUND  
         [0003]    Coaxial cable networks have been used to deliver high quality video programming to subscribers for many years. Conventionally, these networks have been unidirectional, broadcast networks with a limited number of channels and a limited variety of content provided to the subscribers. In recent years, cable companies have developed systems to provide bi-directional communication over their existing networks to provide a wider variety of services and content to their subscribers. For example, many cable companies now provide connection to the Internet through the use of cable modems.  
           [0004]    The cable industry has developed a number of standards for delivering data over their networks to provide a uniform basis for the design and development of the equipment necessary to support these services. For example, a consortium of cable companies developed the Data Over Cable Service Interface Specifications (DOCSIS) standard. The DOCSIS standard specifies the necessary interfaces to allow for transparent, bi-directional transfer of Internet Protocol (IP) traffic between a cable head end and customer equipment over a cable network, such as a coaxial cable network or hybrid fiber/coax (HFC) network.  
           [0005]    A cable modem termination system (CMTS) is included in the head end of the cable network for processing the upstream and downstream transmission of data. In the upstream, the CMTS down converts data signals from cable modems to base band or a low intermediate frequency. The CMTS then demodulates the signals and provides the data to a network, e.g., the Internet. In the downstream, the CMTS receives data for a plurality of modems at a network interface. The CMTS modulates a carrier with this data and transmits it downstream over a shared medium to the plurality of modems.  
           [0006]    Problems frequently occur during high usage times when the amount of data received and/or transmitted by a CMTS reaches capacity limits of the CMTS. Such problems often include reduced data flow rates, data contention, etc. Moreover, users can even be denied access during high usage times.  
           [0007]    For the reasons stated above, and for other reasons stated below that will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for increasing capacity of cable modem termination systems.  
         SUMMARY  
         [0008]    The above-mentioned problems with cable modem termination systems and other problems are addressed by embodiments of the present invention and will be understood by reading and studying the following specification. In one embodiment, the capacity of the cable modem termination system is increased on an as-needed basis by using bandwidth of a backup transceiver when not in use carrying signals for a failed primary transceiver. Thus, the capacity of the system is intermittently increased to meet demand without requiring dedication of an additional transceiver and still providing the backup function.  
           [0009]    In one embodiment, a method for adjusting bandwidth allocation in a cable modem termination system is provided. The method includes detecting an overflow condition at a primary transceiver of the cable modem termination system, determining whether a back-up transceiver is available in the cable modem termination system, and when the back-up transceiver is available, establishing at least one channel for the back-up transceiver to communicate over a common physical medium with at least one channel of the primary transceiver to effectively increase the bandwidth of the cable modem termination system.  
           [0010]    Further embodiments of the invention include apparatus and methods of varying scope. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    [0011]FIG. 1 is a schematic of a communications network in accordance with an embodiment of the invention.  
         [0012]    [0012]FIG. 2 is a schematic of a cable modem termination system (CMTS) in accordance with an embodiment of the invention.  
         [0013]    [0013]FIG. 3 is a schematic showing detail of a primary CMTS transceiver and an associated interface adapter in accordance with an embodiment of the invention.  
         [0014]    FIGS.  4 A- 4 B are schematics of an upstream switch module including optional directional couplers and pilot tone generator for use in testing of the CMTS in accordance with an embodiment of the invention.  
         [0015]    FIGS.  5 A- 5 B are schematics of a downstream switch module including optional directional couplers and RF level detector for use in testing of the CMTS in accordance with an embodiment of the invention.  
         [0016]    [0016]FIG. 6 is a block diagram of a CMTS showing connectivity of various components in accordance with an embodiment of the invention.  
         [0017]    [0017]FIG. 7 is a flow chart of a process for operating a CMTS in an overflow mode according to an embodiment of the invention. 
     
    
     DETAILED DESCRIPTION  
       [0018]    In the following detailed description of the present embodiments, reference is made the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that process, electrical or mechanical changes may be made without departing from the scope of the present invention. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined only by the appended claims and equivalents thereof.  
         [0019]    The various embodiments provide passive backup for cable modem termination systems (CMTS). Passive backup allows a backup transceiver to be activated to handle overflow data traffic, as needed. The various embodiments further facilitate elimination of the use of switches or any active components in the primary signal path that can reduce the reliability of the system.  
         [0020]    Various embodiments utilize directional couplers in the primary signal path for each CMTS transceiver to be backed up. Signals sent between subscriber equipment and the primary CMTS transceivers pass through directional couplers with relatively low loss. Signals associated with data overflow are selectively routed through a backup CMTS transceiver outside of the primary signal path.  
         [0021]    [0021]FIG. 1 is a schematic of a communications network  100  in accordance with an embodiment of the invention. The communications network  100  carries communication signals between a head end  102 , e.g., the Internet, and subscriber equipment  104 , e.g., cable modems, through an access network  106 , e.g., a coaxial cable network or hybrid fiber/coax (HFC) network using the Data Over Cable Service Interface Specifications (DOCSIS) standard.  
         [0022]    Communication signals between the head end  102  and the subscriber equipment  104  is facilitated through a cable modem termination system (CMTS)  110  connected between the access network  106  and the head end  102 . As used herein, the term “connected” in its various forms refers to establishing an ability to convey communication signals and does not require a direct physical or electrical connection.  
         [0023]    The CMTS  110  includes at least one primary CMTS transceiver  120  and at least one backup CMTS transceiver  130 . While the ratio of primary CMTS transceivers  120  to backup CMTS transceivers  130  is not limited by the invention, it is typically preferred to have some ratio greater than one. For one embodiment, there is one backup CMTS transceiver  130  for every six primary CMTS transceivers  120 . For another embodiment, there is one backup CMTS transceiver  130  for every ten primary CMTS transceivers  120 .  
         [0024]    Passive directional couplers  140  are connected between the access network  106  and the primary CMTS transceivers. In a similar fashion, further directional couplers (not shown in FIG. 1 for clarity) are connected between the head end  102  and the primary CMTS transceivers  120 . The directional couplers  140  are configured to incur relatively low loss in the primary signal path, i.e., the path between the head end  102  and the subscriber equipment  104  through the primary CMTS transceivers  120 , yet permit a portion of the signal to pass to the backup CMTS transceiver  130 . The use of passive devices such as the directional couplers  140  permit switching to the backup CMTS transceiver  130  without having an active switching device in the primary signal path.  
         [0025]    [0025]FIG. 2 is a schematic of a CMTS  110  in accordance with an embodiment of the invention. The CMTS  110  includes a plurality of active or primary CMTS transceivers  120   1  to  120   N  for every one backup CMTS transceiver  130 . Each of primary CMTS transceivers  120   1  to  120   N  has a number of communication ports. Similarly, each backup CMTS transceiver  130  has a number of communication ports. As used herein, communication ports for connecting to the subscriber equipment  104  will be termed upstream communication ports and communication ports for connecting to the head end  102  will be termed downstream communication ports.  
         [0026]    Upstream communication ports for primary CMTS transceivers are designated as reference numbers  122   1  to  122   n  in FIG. 2 while upstream communication ports for backup CMTS transceivers are designated as reference numbers  132   1  to  132   n  in FIG. 2. Similarly, downstream communication ports for primary CMTS transceivers are designated as reference numbers  124   1  to  124   P  in FIG. 2 while downstream communication ports for backup CMTS transceivers are designated as reference numbers  134   1  to  134   P  in FIG. 2.  
         [0027]    In general, each CMTS transceiver has one or more upstream communication ports and one or more downstream communication ports. A CMTS transceiver will typically have more upstream communication ports than downstream communication ports. However, such is not required. For one embodiment, each CMTS transceiver has eight upstream communication ports and two downstream communication ports. For ease of implementation and inventory, it is preferable that backup CMTS transceivers are either similarly configured, containing a corresponding communication port for each communication port of the primary CMTS transceivers for which it provides redundancy, or they contain at least as many downstream and upstream ports as any primary CMTS in the system.  
         [0028]    Each of primary CMTS transceivers  120   1  to  120   N  is respectively connected to interface adapters  250   1  to  250   N . Each of interface adapters  250   1  to  250   N  includes upstream communication ports  252   1  to  252   n  and downstream communication ports  254   1  to  254   P . A directional coupler  140   up  is connected between each of upstream communication ports  252   1  to  252   n  and upstream communications ports  122   1  to  122   n , as illustrated for CMTS transceiver  120   1  and interface adapter  250   1 . A directional coupler  140   DN  is connected between each of downstream communication ports  254   1 , to  254   P  and downstream communications ports  124   1  to  124   P , as illustrated for CMTS transceiver  120   1  and interface adapter  250   1 .  
         [0029]    Each directional coupler  140  is further connected to a switch module  260 . Such connection may be made through an optional intermediary device, designated in FIG. 2 as mezzanine board  261 . The switch module  260  acts as a multiplexer, selectively connecting a directional coupler  140  with an associated communication port of the backup CMTS transceiver  130 .  
         [0030]    For one embodiment, the switch module  260  includes one upstream switching module  262  for each of upstream communications ports  132   1  to  132   n  of backup CMTS transceiver  130 . Each upstream switching module  262  selectively connects one of upstream communications ports  132   1  to  132   n , e.g., upstream communications port  132   1 , to the respective one of upstream communications ports  252   1  to  252   n , e.g., upstream communications port  252   1 , of each of interface adapters  250   1  to  250   N  via the corresponding directional coupler  140   UP .  
         [0031]    For a further embodiment, the switch module  260  further includes one downstream switching module  264  for each of downstream communications ports  134   1  to  134   P  of backup CMTS transceiver  130 . Each downstream switching module  264  selectively connects one of downstream communications ports  134   1  to  134   P , e.g., downstream communications port  134   1 , to the respective one of downstream communications ports  254   1  to  254   P , e.g., downstream communications port  254   1 , of each of interface adapters  250   1  to  250   N  via the corresponding directional coupler  140   DN .  
         [0032]    To maintain near unity gain when using the backup CMTS transceiver  130 , the signal path between the backup CMTS transceiver  130  and the directional couplers  140  will need to be amplified. Because the directional couplers  140  are configured to have relatively low losses in the primary signal path, preferably on the order of 1.5 dB or less, losses between the backup CMTS transceiver  130  and the directional couplers  140  will generally be relatively high, e.g., 7 dB or more. Thus, to insure transparency to the end users when a switch is made to the backup CMTS transceiver  130  or when the backup CMTS transceiver is used to increase the bandwidth of the CMTS in an “overflow” mode as described below, this signal path must compensate for such losses. For one embodiment, an amplifier  263  is connected between each upstream switching module  262  and its associated upstream communication port  132 . For a further embodiment, an amplifier  265  is connected between each downstream switching module  264  and its associated downstream communication port  134 . While the amplifiers  262  and  264  could be connected on the opposite sides of the switching modules  262  and  264 , respectively, it is more economical to have a one-to-one relationship with the communication ports of the backup CMTS transceiver  130  than with all of the primary CMTS transceivers  120 .  
         [0033]    The backup CMTS transceiver  130  may be connected to the switch modules  262 / 264  through an interface adapter  255 . Although the interface adapter  255  could have the same configuration as the interface adapters  250 , there is no need for additional directional couplers connected to the backup CMTS transceiver  130  through the interface adapter  255 .  
         [0034]    The inherent isolation between ports of the directional couplers  140  in the primary CMTS transceiver signal paths adds to the performance of the CMTS  110 . Potential crosstalk between primary CMTS transceivers  120  through imperfections of the switching modules  262 / 264  is minimized. Additionally, when a primary CMTS transceiver  120  is removed for maintenance, the effect on the system performance when the backup CMTS transceiver  130  is active is negligible.  
         [0035]    [0035]FIG. 3 is a schematic showing additional detail of a primary CMTS transceiver  120  and its associated interface adapter  250 . The primary CMTS transceiver  120  depicted in FIG. 3 includes one downstream communication port  124  connected to a downsteam communication port  254  through a directional coupler  140   DN . Primary CMTS transceiver  120  also includes six upstream communication ports  122  respectively connected to six upstream communications ports  252  through six directional couplers  140   UP . For one embodiment, the directional couplers exhibit −1.5 dB in the downstream direction and −1.5 dB in the upstream direction for the primary signal path. For a further embodiment, the directional couplers exhibit −10 dB in the downstream direction and −10 dB in the upstream direction for the backup signal path, i.e., the path between the directional couplers  140  and the backup CMTS transceiver  130 . Directional couplers  140  are generally 4-port devices. These ports are commonly referred to as an “in” port, an “out” port, a “forward coupled” port and a “reverse coupled” port. A signal in the primary signal path passes from the “in” port to the “out” port with relatively low loss. A signal in the backup signal path passes from the “in” port to the “forward coupled” port attenuated by the coupling value of the directional coupler. The unconnected port of each directional coupler  140 , i.e., the “reverse coupled” port, is preferably resistance terminated. For one embodiment, a 75 ohm resistance is used to terminate each unconnected port.  
         [0036]    Testing of the backup CMTS transceiver  130  is also possible with this scheme without interrupting primary CMTS service and without removing the backup CMTS transceiver  130  from the CMTS  110 . Through the use of additional directional couplers in the backup signal path (not shown in FIGS.  2  or  3 ), internally generated test signals can be detected and measured. By exercising the switch modules  262 / 264 , the service availability of the switches can be determined by detecting the test signals. This process in non-invasive to the primary CMTS transceiver signal paths.  
         [0037]    FIGS.  4 A- 4 B are schematics of an upstream switch module  262  including optional directional couplers  440  and pilot tone generator  475  for use in testing of the CMTS  110 . The upstream switch module  262  may further include circuitry  473  for detecting the RF level of the backup signal path. Such can be used to assure that the RF level is within operating limits when the intended switches are activated.  
         [0038]    The upstream switching module  262  includes a plurality of switches, or switching matrix, SW 1 -SW 19  of FIG. 4A for selectively connecting an upstream communication port of the backup CMTS transceiver  130  to its associated directional coupler  140  through a directional coupler  440 . Switches SW 1 -SW 19  of FIG. 4A are preferably RF switches or relays. An additional port of each directional coupler  440  is selectively connected to pilot tone generator  475 , such as through electronic switches SW 21 -SW 30  of FIG. 4B. The unconnected port of each directional coupler  440  is preferably resistance terminated.  
         [0039]    The pilot tone generator  475  may be selectively activated, such as through switch SW 31 . Similarly, as the amplifier  263  is not needed unless the backup CMTS transceiver  130  is active, it may be selectively activated, such as through switch SW 20 . The pilot tone generator  475  permits testing of the receive portion of the backup CMTS transceiver  130 , as well as the switches and amplifier that pass the upstream signals to the backup CMTS transceiver  130 . For one embodiment, the pilot tone generator  275  resides in the backup CMTS transceiver  130 .  
         [0040]    FIGS.  5 A- 5 B are schematics of a downstream switch module  264  including optional directional couplers  540  and RF level detector  580  for use in testing all components in the signal path starting at the transmit portion of the Backup CMTS transceiver  130  and ending with the RF level detector  473  in the upstream switch module  262 . The downstream switch module  264  may further include circuitry  573  for detecting the RF level of the backup signal path. Such can be used to assure that the RF level is within operating limits when the intended switches are activated. The RF level detector  580  may be used to adjust the gain of the amplifier  265  in order to provide near unity gain in the backup signal path. For one embodiment, the directional couplers  540  are 20 dB couplers.  
         [0041]    The downstream switching module  264  includes a plurality of switches SW 1 -SW 19  of FIG. 5A for selectively connecting a downstream communication port of the backup CMTS transceiver  130  to its associated directional coupler  140  through a directional coupler  540 . An additional port of each directional coupler  540  is selectively connected to RF level detector  580 , such as through switches SW 21 -SW 29  of FIG. 5B. The unconnected port of each directional coupler  540  is preferably resistance terminated. An additional amplifier  581  may be connected to the RF level detector  580  for use in bringing the signal power to a desired level.  
         [0042]    [0042]FIG. 6 is a block diagram of a CMTS  110  showing connectivity of various components. For clarity, signal lines are shown only for the first primary CMTS transceiver  120  and for the backup CMTS transceiver  130 . Identical signal lines may run from each primary CMTS transceiver  120  to the upstream switch modules  262  and the downstream switch modules  264 . Note that the CMTS  110  of FIG. 6 is depicted as having ten primary CMTS transceivers  120  and one backup CMTS transceiver  130 , with each CMTS transceiver having twelve upstream communication ports and three downstream communication ports.  
         [0043]    In one embodiment, CMTS  110  includes a controller card  160 , as shown in FIG. 1. Controller card  160  is adapted to perform methods of operating CMTS  110  in accordance with embodiments of the invention in response to machine-readable instructions. In another embodiment, these instructions are in the form of hardware and are hard coded as part of controller card  160 , e.g., an application-specific integrated circuit (ASIC) chip. In other embodiments, the machine-readable instructions are in the form of firmware or software stored on a machine-usable media  170  associated with controller card  160  for retrieval by a processor on controller card  160 . In another embodiment, machine-usable media  170  includes static or dynamic random access memory (SRAM or DRAM), read-only memory (ROM), electrically-erasable programmable ROM (EEPROM or flash memory), magnetic media and optical media, whether permanent or removable, or the like.  
         [0044]    In some embodiments, each of primary CMTS transceivers  120   1 to  120   N  and backup CMTS transceiver  130  includes machine-readable instructions in the form of firmware or software stored on a machine-usable medium  180 , as shown in FIG. 2, such as ROM, EEPROM, flash memory or the like. In other embodiments, controller card  160  communicates with machine-usable media  180 .  
       OPERATING MODES  
       [0045]    In one embodiment, CMTS  110  operates in two basic operating modes: a normal mode and an “overflow” mode. In the normal mode, the primary CMTS transceivers  120   1  to  120   N  carry signals between the head end  102  and subscribers  104 . When operational failures are detected during normal mode, backup CMTS transceiver  130  is logically inserted in place of the failed primary CMTS transceiver  120 . In this capacity, backup CMTS transceiver communicates signals between subscribers  104  and head end  102  in place of the failed primary CMTS transceiver  120 . Thus, in normal mode, backup CMTS transceiver  130  is placed in standby and does not carry signals between the head end  102  and subscribers  104  absent a failure of one of the primary CMTS transceivers.  
         [0046]    In the overflow mode, backup CMTS transceiver  130  is provisioned to carry signals between the head end  102  and subscribers  104  when any one of the primary CMTS transceivers experiences an “overflow condition.” The term “overflow condition” is defined below. In the overflow mode, backup CMTS transceiver  130  is used to provide additional bandwidth for any of the primary CMTS transceivers  120 . When the primary CMTS transceivers  120  are operational, the processing capabilities of the backup CMTS transceiver are available to meet intermittent demand for bandwidth not addressable by the individual primary CMTS transceivers. Thus, when an overflow condition is detected, the primary CMTS transceiver  120  continues to carry traffic over its associated physical media, e.g., coaxial cable connections, on its assigned CMTS channel or channels. Further, the backup CMTS transceiver  130  carries the additional signals corresponding to the intermittent demand over the same physical media as the CMTS channels of the primary CMTS transceiver  120 . The backup CMTS transceiver  130  uses, for example, a different CMTS channel than the associated primary CMTS transceiver  120 .  
         [0047]    In one embodiment, when a failure in a primary CMTS transceiver  120  occurs during an overflow condition, the CMTS returns to normal operation mode and the backup CMTS transceiver  130  ceases to carry the overflow signals and begins to carry traffic in place of the failed, primary CMTS transceiver.  
       NORMAL OPERATING MODE  
       [0048]    For one embodiment of the normal operating mode, communication signals are transferred bi-directionally respectively between downstream communications ports  254   1  to  254   P  of each of interface adapters  250   1  to  125   N  and downstream communications ports  124   1  to  124   P  of each of primary CMTS transceivers  120   1  to  120   N  through the directional couplers  140   DN . Communication signals are also transferred bi-directionally respectively between upstream communications ports  252   1 , to  252   n  of each of interface adapters  250   1  to  250   N  and upstream communications ports  122   1  to  122   n  of each of primary CMTS transceivers  120   1  to  120   N  through the directional couplers  140   UP .  
         [0049]    In one embodiment, controller card  160  monitors operation of each of primary CMTS transceivers  120   1  to  120   N  via machine-usable media  180 . When one of CMTS transceivers  120   1  to  120   N  fails, controller card  160  logically removes the failed CMTS transceiver from operation and logically replaces it with backup CMTS transceiver  130 . More specifically, controller card  160  prevents transfer of communication signals to and from the failed transceiver. Controller card  160  instructs switch module  260  to respectively connect each of upstream communications ports  132   1  to  132   n  of backup CMTS transceiver  130  with the directional couplers  140   UP  respectively connected to the upstream communications ports  122   1  to  122   n  of the failed transceiver. This respectively connects each of upstream communications ports  132   1  to  132   n  to each of upstream communications ports  252   1  to  252   n  of the one of interface adapters  250   1  to  250   N  associated with the failed transceiver.  
         [0050]    Controller card  160  also instructs switch module  260  to respectively connect each of downstream communications ports  134   1  to  134   P  of backup CMTS transceiver  130  with the directional couplers  140   DN  respectively connected to the downstream communications ports  124   1  to  124   P  of the failed transceiver. This respectively connects each of downstream communications ports  134   1  to  134   P  to each of downstream communications ports  254   1  to  254   P  of the one of interface adapters  250   1  to  250   N  associated with the failed transceiver.  
       OVERFLOW MODE  
       [0051]    In one embodiment, CMTS  110  operates in an overflow mode which allows the backup CMTS transceiver  130  to carry signals when the backup CMTS transceiver  130  is not operating as a backup to a failed primary transceiver  120 . In overflow mode, the backup CMTS transceiver  130  effectively increases the bandwidth of one of the primary CMTS transceivers  120  on an intermittent basis. The backup CMTS transceiver  130  carries signals on one or more additional CMTS channels between subscribers  104  and the head end  102  over a shared physical medium with the primary CMTS transceiver  120 . Advantageously, this is accomplished on an as-needed basis to utilize the available capacity of the backup CMTS transceiver  130 .  
         [0052]    In overflow mode, controller card  160  monitors the flow of signals for each primary CMTS transceiver  120  to determine when additional capacity is needed or requested. This need or request for additional capacity is referred to as an “overflow condition.” In one embodiment, an overflow condition is determined by controller card  160 . FIG. 7 provides one embodiment of a process for controller card  160  to operate in overflow mode. The process begins at block  700 . At block  702 , the process determines when an overflow condition exists. For example, in one embodiment, controller card  160  monitors the flow of signals at each of upstream communications ports  252   1  to  252   n  and at each of downstream communications ports  254   1  to  254   P  of each of interface adapters  250   1  to  250   N  via machine-usable media  180 . In one embodiment, additional bandwidth is needed for an overflow condition when the flow rate of communication signals at any of the upstream communication ports  252   1  to  252   n  and/or at any of the downstream communication ports  254   1  to  254   P  meets a selected criterion. In one embodiment, the selected criterion includes exceeding a particular flow rate of communication signals. In other embodiments, the selected criterion includes exceeding a particular flow rate of communication signals for a given period of time. In other embodiments, other selection criteria are used to determine an overflow condition. Essentially, an overflow condition is detected at block  702  based on any appropriate criteria or criterion indicating that backup CMTS transceiver  130  can be used to provide additional bandwidth.  
         [0053]    In one embodiment, when an overflow condition is detected at block  702 , controller card  160  determines whether the backup CMTS transceiver is available at block  703 . For example, controller card  160  determines whether the backup CMTS transceiver is currently operating in place of a failed primary transceiver. If not, controller card  160  identifies CMTS channels for the primary CMTS transceiver to be rerouted to the backup CMTS transceiver at block  704 . At block  706 , the process issues commands to move the channels. In one embodiment, approximately one-third of the channels of a primary CMTS transceiver are moved to be handled by the backup CMTS transceiver. To implement this, in one embodiment, controller card  160  instructs switch module  260  to connect a corresponding upstream/downstream communications port  132 / 134  of backup CMTS transceiver  130  to the directional coupler  140   UP / 140   DN  connected to the upstream/downstream communications port  252 / 254 . A first portion of communication signals, e.g., one or more CMTS channels, flows between the upstream/downstream communications port  122 / 124  and the corresponding upstream/downstream communications port  252 / 254  via the directional coupler  140   UP / 140   DN , and a second portion of communication signals, e.g., one or more additional CMTS channels, flows between upstream upstream/downstream communications port  132 / 134  and the corresponding upstream/downstream communications port  252 / 254  via the directional coupler  140   UP / 140   DN .  
         [0054]    In one embodiment, controller card  160  switches from the overflow mode to the normal mode upon detecting that one of primary CMTS transceivers  120   1  to  120   N  has failed. When this occurs, the subscribers using CMTS channels associated with the backup CMTS transceiver are redirected to channels associated with their respective primary CMTS tranceiver. Meanwhile, controller card  160  logically removes the failed CMTS transceiver from operation and replaces it with backup CMTS transceiver  130 , as described above.  
         [0055]    For another embodiment, controller card  160  switches from the overflow mode to the normal mode upon detecting that the one of CMTS transceivers  120   1  to  120   N  having the overflow condition has failed. When this occurs, controller card  160  logically removes the failed CMTS transceiver from operation, as described above. The second portion of the communication signals is combined with the first portion, and the combined first and second portions flow between the upstream/downstream communications port  132 / 134  and the corresponding upstream/downstream communications port  252 / 254  via the corresponding directional coupler  140   UP / 140   DN . The remaining upstream/downstream communications ports  132 / 134  of backup CMTS transceiver  130  are connected to the corresponding upstream/downstream communications ports  252 / 254  via the corresponding directional couplers  140   UP / 140   DN  to replace the corresponding upstream/downstream communications ports  122 / 124 .  
         [0056]    For another embodiment, controller card  160  switches from the overflow mode to the normal mode upon detecting that the overflow condition no longer occurs, e.g., when the flow rate of the communication signals no longer meets the selected criterion indicative of the overflow condition. When this occurs, the subscribers assigned to CMTS channels serviced by the backup CMTS transceiver are instructed to switch to a CMTS channel assigned to the primary CMTS transceiver and backup CMTS transceiver  130  is placed in a standby mode.  
         [0057]    Cable modem termination systems (CMTS) have been described to facilitate redundancy without the need for active components, e.g., switches or amplifiers, in the primary signal path, and with a low signal loss in the primary signal path incurred by the redundancy components. Such elimination of active components in the primary signal path is made possible through the use of passive directional couplers in the primary signal path.  
         [0058]    Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement that is calculated to achieve the same purpose may be substituted for the specific embodiments shown. Many adaptations of the invention will be apparent to those of ordinary skill in the art. Accordingly, this application is intended to cover any such adaptations or variations of the invention. It is manifestly intended that this invention be limited only by the following claims and equivalents thereof.