Abstract:
A system for communication via cable-television communication lines includes a center device which generates a loop-detection packet for detecting a loop that is a defect of a network configuration, and a cable modem which is situated between said center device and a subscriber end, and detects the loop by regarding a receipt of the loop-detection packet from the subscriber end as a detection of the loop.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to cable modems and cable-modem systems, and particularly relates to a cable modem and a cable modem system which provide a connection to a LAN via a cable television transmission line. 
     2. Description of the Related Art 
     A cable television system (hereinafter referred to as a CATV system) distributes broadcast signals from a headend, which is a center of a CATV provider, to subscribers via trunk lines and branch lines, which are configured in a tree structure or in a star structure. The trunk lines and branch lines are used for transmission of broadband broadcast signals, and, thus, are implemented by using a coaxial cable or an optical fiber cable. This renders a CATV system a superior broadband transmission capacity. 
     In recent years, the Internet has made a significant market progress. Against this background, a superior broadband transmission capacity of a CATV system has been attracting attention, and efforts are underway to utilize CATV transmission lines as an access network to the Internet. A cable-modem system, in particular, is increasingly used in practice, connecting subscriber households to the Internet via a LAN interface utilizing CATV lines. 
     FIG. 1 is an illustrative drawing showing a configuration of a cable modem system. A description of the cable modem system will be given with reference to FIG.  1 . 
     The cable modem system of FIG. 1 includes a CATV center  10 , a CATV delivery facility  20 , and a subscriber household  30 . The CATV center  10  includes a center device  11 , a headend device  12 , photoelectric converters  13 , a router  14 , a server  15 , personal computers  16 , and bridges  17 . The center device  11  is connected to an intra-CATV-center LAN at one end and to the headend device  12  at the other end. The headend device  12  transmits video signals and the like after demodulation and mixing of signals. The router  14  provides a connection to the Internet. The server  15  stores data therein to provide various services. The bridges  17  are used for connecting different LANs. The bridges  17  monitor packets, and reduces the number of packets in the LANs via a filtering function to dispose of unnecessary packets, thereby insuring transmission efficiency. 
     The CATV delivery facility  20  includes optical cables  21 , photoelectric converters  22 , coaxial cables  23 , amplifiers  24 , and tap-offs  25 . Here, the tap-offs  25  are nodes or branching devices that connect between the CATV delivery facility  20  and the subscriber household  30 . 
     The subscriber household  30  includes cable modems  31   a  through  31   c , terminal devices  32   a  through  32   d  such as personal computers, and a bridge  17 . The terminal device  32   a , operating standalone, is connected to the CATV delivery facility  20  via the cable modem  31   a . Further, a subscriber-household LAN  33  including the terminal devices  32   b  through  32   d  and the bridge  17  is connected to the CATV delivery facility  20  via the cable modems  31   b  and  31   c.    
     When a user accesses the Internet via the terminal device  32   c , the terminal device  32   c  supplies packets to the cable modem  31   b  or  31   c , where the packets are converted into RF signals to be supplied to the CATV delivery facility  20  via the tap-offs  25 . The RF signals attenuate as they propagate through the CATV delivery facility  20 , so that the amplifiers  24  are provided to boost signal levels. The RF signals are converted into optical signals by the photoelectric converters  22 , and the optical signals are then sent via the optical cables  21 . 
     The photoelectric converters  13  convert the optical signals received from the optical cables  21  into electrical signals, and supplies reconstructed RF signals to the center device  11  via the headend device  12 . The center device  11  selects a frequency band that is used as an uplink band for the cable modems  31   a  through  31   c , and converts the RF signals of this frequency band into packet signals. The reconstructed packets are sent to the Internet via the router  14 . When packets are supplied from the Internet, on the other hand, transfer of the packets is carried out by following steps of the same procedure as described above in a reversed order. In this manner, the user can access the Internet by using the cable modems  31   a  through  31   c.    
     Unfortunately, there is a case in which a mistake in wire connections ends up causing the subscriber-household LAN  33  to create a closed loop, trapping packets inside. Such a loop may leads to a shutdown of the entire cable-modem system. In consideration of this, the bridge  17  is typically provided with a function to detect and sever a loop. 
     The bridge  17  in a conventional LAN system exchanges BPDU messages (packets) with other bridges by attaching its own MAC (media access control) address to these packets for the purpose of automatically detecting a loop. When a loop is detected, a bridge constituting part of the loop closes a relevant port thereof in order to sever the loop. This scheme is called a spanning tree scheme. 
     A cable-modem system is subject to standardization, and is required to serve as a bridge in compliance with standards set forth in the United States. A typical cable-modem system, therefore, adapts the spanning tree scheme in order to provide a function to detect and sever a loop in the same fashion as does a bridge of a conventional LAN system. 
     FIG. 2 is an illustrative drawing showing a logical configuration of a cable-modem system such as that shown in FIG.  1 . With reference to FIG. 2, a description will be given with regard to a function to detect and sever a loop in a cable-modem system. 
     In FIG. 2, a center device  40  and cable modems  43   a  through  43   e  are each configured to serve as a bridge, and, for this purpose, are each provided with a filtering database  44 . The filtering database  44  stores a definition as to whether to transit or dispose of a packet with respect to each MAC address. Such a definition is temporarily stored for a predetermined time period. 
     In the cable-modem system of FIG. 2, the center device  40  serves as a bridge BRIDGE 0 , and the cable modems  43   a  through  43   n  serve as bridges BRIDGE 1  through BRIDGEn, respectively. The center device  40  has a port PORT 0  thereof connected to the Internet via a router  41  and a port PORT 1  thereof connected to a port PORT 0  of each of the cable modems  43   a  through  43   n . A port PORT 1  of each of the cable modems  43   a  through  43   n  is connected to a terminal device  45  either directly or via a hub  46  and/or a bridge  47 . 
     As described above, the center device  40  and the cable modems  43   a  through  43   e  are configured to serve as logically separate bridges, and are required to have a function to detect and sever a loop as such a loop may become a problem when implementing a LAN. Namely, each of the center device  40  and the cable modems  43   a  through  43   e  is so designed as to perform the spanning tree scheme. 
     Another configuration that can take care of the problem of a closed loop in a cable-modem system is to regulate communications between the cable modems  43   a  through  43   e  by requiring all the packets from the cable modems  43   a  through  43   e  to always make a transit at the center device  40 . This can prevent formation of a closed loop. 
     FIG. 3 is an illustrative drawing showing another example of a logical configuration of a cable-modem system. The configuration of FIG. 3 has advantages in reduction of the load and conservation of resources. 
     The cable-modem system of FIG. 3 includes a center device  51  and cable modems  52   a  through  52   n , all of which together constitute a logical bridge  50 . The logical bridge  50  is equivalent to a bridge that has a port PORT 0  on the side of the intra-CATV-center LAN and ports PORT 1  through PORTn on the side of the subscriber-household LAN at the end of the cable modems  52   a  through  52   n.    
     FIG. 4 is a block diagram showing a configuration of the center device of FIG.  3 . 
     As shown in FIG. 4, the center device  51  serving as part of the logical bridge  50  includes a LAN-interface unit  55 , a bridge processing unit  56 , and an RF-interface unit  57 . 
     FIG. 5 is a block diagram showing a configuration of one of the cable modems shown in FIG.  3 . 
     As shown in FIG. 5, the cable modem serving as part of the logical bridge  50  includes an RF-interface unit  57  and a LAN-interface unit  55 . In the logical bridge  50  of FIG. 3, the cable modems  52   a  through  52   n  serve simply as repeaters, each of which transfers signals by boosting signal levels. 
     The filtering database  44  is provided only in the center device  51 , and controls transit processing of the logical bridge  50 . The bridge processing unit  56  consults the filtering database  44 , and determines whether to dispose of a received packet or to transfer a received packet to the LAN-interface unit  55  or the RF-interface unit  57 . When it is decided that a received packet is to be supplied to the RF-interface unit  57 , an ID number of one of the cable modems  52   a  through  52   n  and an MAC address of a terminal device attached thereto are paired and stored in a cache table  53 . Here, the one of-the cable modems  52   a  through  52   n  registered in the cache table  53  is a destination of the received packet. 
     By registering an ID number of a destination cable modem and an MAC address of a terminal device attached thereto in the cache table  53 , an ID number of a cable modem can be searched for based on an MAC address of a packet destination when communication is effected between the cable modems  52   a  through  52   n  or between the center device  51  and one of the cable modems  52   a  through  52   n.    
     If the cache table  53  does not have an ID number and a MAC address stored therein, a packet is supplied to all the cable modems  52   a  through  52   n . In this configuration shown in FIG. 3, therefore, no MAC address needs to be assigned to any one of the cable modems  52   a  through  52   n.    
     Related-art cable-modem systems are designed and implemented as described above. 
     In cable-mode systems having each cable modem serving as a bridge as those developed and subject to standardization in the United States, a mechanism for implementing the spanning-tree scheme is necessary in addition to the mechanism for the bridge function. This requires highly complex data processing, and, also, requires each cable modem to have an MAC address attached thereto. Another problem is a cost increase. 
     Further, packets transmitted from the cable modems are always required to make a transit at the center device in some, systems in order to regulate communication between the cable modems. Such a system has a drawback in that services options are rather limited. 
     A cable system that has a center device and cable modems together constituting a single logical bridge is not equipped with a function to automatically detect and sever a loop. If a loop is created in a subscriber-household LAM, therefore, an entire system may run a risk of having to be shutdown. 
     When a loop develops in the cable modem system of FIG. 3, for example, an entire system is shutdown when a broadcast MAC packet or a multicast MAC packet is supplied to the loop. This is because the bridge  47  is designed to transfer a received broadcast or multicast MAC packet to a port on the opposite side. When broadcast or multicast MAC packets are continuously transferred in the loop, therefore, this results in a sudden increase in traffic. 
     In order to provide a function to automatically detect and sever a loop, each one of the cable modems  52   a  through  52   n  needs to have an MAC address assigned thereto. Since the cable modems  52   a  through  52   n  merely serve as repeaters, no MAC addresses are normally assigned. When considering a current situation where there is an explosive increase in use of the Internet, it is desirable to detect and sever a loop without assigning to an MAC address to each of the cable modems  52   a  through  52   n.    
     Accordingly, there is a need for a cable modem and a cable-modem system which can detect and sever a loop without having MAC addresses assigned to cable modems. 
     SUMMARY OF THE INVENTION 
     Accordingly it is a general object of the present invention to provide a cable modem and a cable-modem system which can satisfy the need described above. 
     It is another and more specific object of the present invention to provide a cable modem and a cable-modem system which can detect and sever a loop without having MAC addresses assigned to cable modems. 
     In order to achieve the above needs according to the present invention, a system for communication via cable-television communication lines includes a center device which generates a loop-detection packet for detecting a loop that is a defect of a network configuration, and a cable modem which is situated between the center device and a subscriber end, and detects the loop by regarding a receipt of the loop-detection packet from the subscriber end as a detection of the loop. 
     The system described above can detect a loop created in a subscriber-household LAN. A principle underlying this loop detection is a premise that the loop-detection packet generated by the center device cannot be supplied to the cable modem from the subscriber end unless there is a loop at the subscriber end. Based on this principle, the present invention can detect and sever a loop. 
     According to another aspect of the present invention, a system for communication via cable-television communication lines includes a cable modem which is connected to a subscriber end, and counts broadcast or multicast packets supplied from the subscriber end to obtain a packet count, the cable modem detecting a loop that is a defect of a network configuration by regarding the packet count exceeding a predetermined number within a given constant interval as a detection of the loop. 
     The system described above can detect a loop created in a subscriber-household LAN. A principle underlying this loop detection is a premise that the number of broadcast or multicast packets supplied from the subscriber end within a given constant interval is not likely to exceed the predetermined number unless there is a loop at the subscriber end. Based on this principle, the present invention can detect and sever a loop. 
     According to another aspect of the present invention, a system for communication via cable-television communication lines includes a cable modem which is connected to a subscriber end, and stores source addresses of packets supplied from the subscriber end, the cable modem disposing of a packet supplied from the subscriber end if a destination address of the packet is one of the stored source addresses. 
     In the system as described above, the cable modem disposes of packets so as not to allow the packets to reach the center device when these packets are used for communication in a subscriber-household LAN, thereby insuring the privy of the communication. 
     According to another aspect of the present invention, a system for communication via cable-television communication lines includes a center device which stores an address of a source terminal device and an identifier of a source cable modem when receiving a packet from the source terminal device via the source cable modem, the address and the identifier being paired, and a cable modem, connected between the center device and a subscriber end, which stores in a memory an address of a source terminal device of a packet supplied from the subscriber end, and monitors addresses of packets supplied from the center device so as to dispose of a packet supplied from the center device and having a destination address not stored in the memory. 
     In the system described above, the cable modem disposes of the packet that is not supposed to be delivered to the terminal device connected to the cable modem. This insures that packets used for communication between a given terminal device and the Internet are not forwarded to another terminal device, thereby providing the privy of communication. 
     According to another aspect of the present invention, a system for communication via cable-television communication lines includes a center device, and a cable modem connected between the center device and a subscriber end, wherein both the center device and the cable modem dispose of a packet generated and supplied from a bridge if the packet has an address identifying the bridge. 
     The system as described above can prevent packets used for loop detection in the spanning-tree scheme from being exchanged between an intra-center LAN and a subscriber-household LAN. 
     In the spanning-tree scheme, the number of bridges between two ends needs to be fewer than seven in order to insure normal operations of the bridges. With such a requirement, a system having a change in the number of bridges in the subscriber-household LAN may encounter abnormal behavior of the bridges. 
     The system described above makes sure that the packets used for the spanning-tree scheme are not exchanged between the intra-center LAN and the subscriber-household LAN, so that the spanning-tree scheme should properly work within the confines of each LAN without being affected by other LAN systems. 
     Other objects and further features of the present invention will be apparent from the following detailed description when read in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an illustrative drawing showing a configuration of a cable modem system; 
     FIG. 2 is an illustrative drawing showing a logical configuration of a cable-modem system such as that shown in FIG. 1; 
     FIG. 3 is an illustrative drawing showing another example of a logical configuration of a cable-modem system; 
     FIG. 4 is a block diagram showing a configuration of a center device of FIG. 3; 
     FIG. 5 is a block diagram showing a configuration of one of cable modems shown in FIG. 3; 
     FIG. 6 is a block diagram of a cable-modem system according to a first embodiment of the present invention; 
     FIG. 7 is a block diagram of a center device of FIG. 6 according to the first embodiment of the present invention; 
     FIG. 8 is a block diagram of a cable modem of FIG. 6 according to the first embodiment of the present invention; 
     FIG. 9 is an illustrative drawing showing a structure of a filtering database; 
     FIG. 10 is an illustrative drawing showing a structure of a cache table; 
     FIG. 11 is an illustrative drawing showing a structure of another cache table; 
     FIG. 12 is an illustrative drawing for explaining a basic operation of a loop detection process; 
     FIG. 13 is an illustrative drawing showing an example of a loop-detection-packet format; 
     FIG. 14 is a sequence diagram showing a loop-detection operation of the cable-modem system of FIG. 6 according to the first embodiment of the present invention; 
     FIG. 15 is a block diagram of a variation of a cable modem according to the first embodiment of the present invention; 
     FIG. 16 is an illustrative drawing for explaining a basic operation of a loop detection process; 
     FIG. 17 is a sequence diagram showing a loop-detection operation of the cable-modem system using the cable modem of FIG. 15 according to the present invention; 
     FIG. 18 is a block diagram showing a cable-modem system according to a second embodiment of the present invention; 
     FIG. 19 is a sequence diagram showing an operation of the cable-modem system of FIG. 18 according to the present invention; 
     FIG. 20 is a block diagram showing a cable-modem system according to a third embodiment of the present invention; 
     FIG. 21 is a sequence diagram showing an operation of the cable-modem system of FIG. 20 according to the third embodiment of the present invention; and 
     FIG. 22 is a block diagram showing a cable-modem system according to a fourth embodiment of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following, embodiments of the present invention will be described with reference to the accompanying drawings. 
     FIG. 6 is a block diagram of a cable-modem system according to a first embodiment of the present invention. FIG. 7 is a block diagram of a center device of FIG. 6 according to the first embodiment of the present invention. FIG. 8 is a block diagram of a cable modem of FIG. 6 according to the first embodiment of the present invention. 
     The cable-modem system of FIG. 6 includes a router  60 , a server  61 , a center device  62 , a CATV delivery facility  63 , cable modems  64   a  and  64   b , a bridge  65 , and terminal devices  66   a  and  66   b . The router  60  serves to connect between LANs. The server  61  stores data therein to provide various services. The center device  62 -connects between the CATV delivery facility  63  and intra-center LAN. The cable modems  64   a  and  64   b  are connected to the CATV delivery facility  63 , and the bridge  65  is connected to the cable modems  64   a  and  64   b . The terminal devices  66   a  and  66   b  such as personal computers are hooked up to the bridge  65 . The center device  62  and the cable modems  64   a  and  64   b  together make up a logical bridge. Because of this configuration, the cable modems  64   a  and  64   b  do not have MAC addresses assigned thereto. Rather than MAC addresses, ID numbers for identification purposes are allocated to the cable modems  64   a  and  64   b.    
     As shown in FIG. 7, the center device  62  includes a LAN-interface unit  68 , BPDU-packet-filtering unit  69 , a bridge processing unit  70 , a loop-detection-packet supplying unit  71 , and a RF-interface unit  72 . The LAN-interface unit  68  is used for establishing contact with the intra-center LAN. The BPDU-packet-filtering unit  69  disposes of BPUD packets that are used for implementing the spanning-tree scheme. The bridge processing unit  70  determines whether to dispose of a received packet, and further determines, if the packet is not to be disposed of, whether to send the packet to the LAN-interface unit  68  or to the RF-interface unit  72 . The bridge processing unit  70  includes a filtering database  73  and a cache table  74 . The loop-detection-packet supplying unit  71  supplies loop-detection packets to the bridge processing unit  70 . The RF-interface unit  72  establishes connection with the CATV delivery facility  63 . 
     As shown in FIG. 8, the cable modems  64   a  and  64   b  include a RF-interface unit  72 , a downward-packet-filtering unit  75 , a passing/disposing-switch unit  76 , a loop-detection unit  77 , an upward-packet-filtering unit  78 , a BPDU-packet-filtering unit  79 , and a LAN-interface unit  68 . The RF-interface unit  72  is used for establish connection with the CATV delivery facility  63 . The downward-packet-filtering unit  75  disposes of packets received from the CATV delivery facility  63  after consulting a cache-table copy  81 . The passing/disposing-switch unit  76  allows a passage of or disposes of a received packet by operating under the control of the loop-detection unit  77 . The upward-packet-filtering unit  78  disposes of packets received from a subscriber-household LAN after consulting a cache table  80 . The BPDU-packet-filtering unit  79  disposes of BPDU packets used in the spanning-tree scheme. The LAN-interface unit  68  establishes connection with the subscriber-household LAN. The contents of the cache table  80  are copied to the cache-table copy  81  such that consistency between the original and the copy is maintained at all times. 
     In the following, a description will be given with regard to operations of the filtering database  73 , the cache table  74 , and the cache table  80 . 
     FIG. 9 is an illustrative drawing showing a structure of the filtering database. FIG. 10 is an illustrative drawing showing a structure of the cache table  74 . FIG. 11 is an illustrative drawing showing a structure of the cache table  80 . 
     As shown in FIG. 9, the filtering database  73  included in the bridge processing unit  70  stores therein MAC addresses of received-packet destinations. As entries each paired with an entry of an MAC address, the filtering database  73  further stores a receipt port, a transmission port, and an aging timer, which indicates a time period that passes before the corresponding MAC address is removed. 
     As shown in FIG. 10, the cache table  74  stores therein an MAC address identifying a destination of a received packet, an ID number of a cable modem connected to the destination terminal, and a aging timer, which indicates a time period that passes before the corresponding MAC address is removed. Hereinafter, the ID number of a cable modem will be referred to as a CID number. 
     As shown in FIG. 11, the cache table  80  included in the upward-packet-filtering unit  78  stores therein MAC addresses of the terminal devices  66   a  and  66   b  connected to the subscriber-household LAN. Further, the cache table  80  stores aging timers each paired with a corresponding MAC address, so that a time period that passes before deletion of an MAC address is specified. 
     In what follows, a description will be given with regard to how a loop is detected in the cable-modem system as described above. 
     FIG. 12 is an illustrative drawing for explaining a basic operation of a loop detection process. FIG. 13 is an illustrative drawing showing an example of a loop-detection-packet format. 
     In an example of FIG. 12, a loop is created along the center device  62 , the cable modem  64   a , the bridge  65 , and the cable modem  64   b.    
     The loop-detection-packet supplying unit  71  of the center device  62  supplies loop-detection packets to the bridge processing unit  70  at constant intervals. The loop-detection packets have a format as shown in FIG.  13 . In each loop-detection packet, a dedicated unicast MAC address, which is not assigned to any terminals, is used as a destination MAC address. 
     As the center device  62  supplies a loop-detection packet to the cable modem  64   a , the cable modem  64   a  transfers the loop-detection packet to the bridge  65 . The bridge  65  checks a destination MAC address of the loop-detection packet, and finds that the destination is the dedicated unicast MAC address that is not assigned to any terminals. Failing to identify a terminal device corresponding to the destination MAC address, the bridge  65  sends the loop-detection packet to each of the cable modem  64   b  and the terminal devices  66   a  and  66   b.    
     The cable modem  64   b  receives this loop-detection packet at the LAN-interface unit  68  as the loop-detection packet is supplied from the end of the subscriber-household LAN. The received loop-detection packet is supplied to the loop-detection unit  77  via the BPDU-packet-filtering unit  79  and the upward-packet-filtering unit  78 . The loop-detection unit  77  continuously monitors destination MAC addresses of packets as the packets are supplied from the end of the subscriber-household LAN. When detecting the unicast MAC address that is specifically used for the loop-detection purpose, the loop-detection unit  77  ascertains that there is a loop, and instructs the passing/disposing-switch unit  76  to stop transferring of incoming packets. 
     The present invention detects a loop by utilizing the fact that in the absence of a loop, a loop-detection packet transmitted from the center device  62  is not supplied to either the cable modem  64   a  or  64   b  from the end of the subscriber-household-LAN. Based on this fact, the cable modems  64   a  and  64   b  monitor destination MAC addresses of the packets supplied from the end of the subscriber-household LAN, and check if any one of the destination MAC addresses is the unicast MAC address. This makes it possible to detect a loop. 
     As described above, the loop-detection packets have the destination MAC address thereof being the dedicated unicast MAC address, which is not assigned to any terminals. This insures that terminal devices of the intra-center LAN and the subscriber-household LAN are not affected, and that the loop-detection packets are not transferred to other networks via the router  60 . 
     In what follows, a loop-detection operation will be described step by step with regard to the cable-modem system of FIG.  6 . 
     FIG. 14 is a sequence diagram showing a loop-detection operation of the cable-modem system of FIG. 6 according to the first embodiment of the present invention. In FIG. 14, “Loop-Det(M lp , M c )” represents a loop-detection packet that has a source MAC address indicating the center device  62  and a destination MAC address being the unicast MAC address. 
     “ARP-REQ(M br , M pc1 )” represents a packet that has a source MAC address indicating the terminal device  66   a  and a destination MAC address being a broadcast MAC  3 address. Further, “CM=# 1 ”, for example, represents a packet that is transmitted from the cable modem  64   a  to the center device  62  via the CATV delivery facility  63  or a packet that is transmitted from the center device  62  to the cable modem  64   a  via the CATV delivery facility  63 . 
     The center device  62  of FIG. 6 periodically transmits loop-detection packets to the router  60 , the cable modem  64   a , and the cable modem  64   b . The loop-detection packets that are sent to the intra-center LAN do not affect operations of the intra-center LAN because the MAC addresses of these packets do not coincide with any one of terminal devices in the intra-center LAN. Also, none of these loop-detection packets are transferred to other networks via the router  60 . 
     The cable modems  64   a  and  64   b , on the other hand, transfer the loop-detection packets to the bridge  65  as they receive the packets via the CATV delivery facility  63 . The bridge  65  checks destination MAC addresses of the loop-detection packets, and finds that the destinations are the dedicated unicast MAC address that is not assigned to any terminals. Failing to identify a terminal device corresponding to the destination MAC address, the bridge  65  sends the loop-detection packets supplied from the cable modem  64   a  to each of the cable modem  64   b  and the terminal devices  66   a  and  66   b , and, also, sends the loop-detection packets supplied from the cable modem  64   b  to each of the cable modem  64   a  and the terminal devices  66   a  and  66   b.    
     The cable modems  64   a  and  64   b  receive these loop-detection packets from the end of the subscriber-household LAN, and the received packets are supplied to the loop-detection unit  77  via the BPDU-packet-filtering unit  79  and&amp;the upward-packet-filtering unit  78 . The loop-detection unit  77  continuously monitors destination MAC addresses of packets as these packets are supplied from the end of the subscriber-household LAN. When detecting the unicast MAC address that is specifically used for the loop-detection purpose, the loop-detection unit  77  ascertains that there is a loop, and instructs the passing/disposing-switch unit  76  to stop transferring of incoming packets. As a result, the cable modems  64   a  and  64   b  stop transferring received packets. 
     In this manner, the cable-modem system of FIG. 6 checks if there, is a loop at constant intervals by utilizing loop-detection packets, and stops packet transfer at the cable modems  64   a  and  64   b  if a loop is detected. This prevents a system shutdown. 
     The cable modem  64   a , for example, may be reactivated thereafter, in order to resume transfer of packets. Even when a loop-detection packet is supplied from the center device  62 , the cable modem  64   a  does not stop transferring of packets since the packet transferring at the cable modem  64   b  is being halted. 
     Since the dedicated unicast MAC address of a loop-detection packet is not assigned to any terminals, there is no influence on the operations of the terminal devices  66   a  and  66   b  in the subscriber-household LAN. 
     In the following, a description will be given with regard to a variation of a cable modem according to the first embodiment of the present invention. 
     FIG. 15 is a block diagram of a variation of a cable modem according to the first embodiment of the present invention. The cable modem of FIG. 15 has the same structure as that of FIG. 8 except for some details. The same elements of FIG. 15 as those of FIG. 8 are referred to by the same numerals, and a description thereof will be omitted. 
     Cable modems  83   a  and  83   b  of FIG. 15 are implemented in the cable-modem system of FIG. 6 in the same manner as the cable modems  64   a  and  64   b . The cable modems  83   a  and  83   b  of FIG. 15 include the RF-interface unit  72 , the downward-packet-filtering unit  75 , the passing/disposing-switch unit  76 , the upward-packet-filtering unit  78 , the BPDU-packet-filtering unit  79 , the LAN-interface unit  68 , and a broadcast-packet/multicast-packet detection unit  82 , which detects a loop by counting the number of broadcast packets or multicast packets. 
     The contents of the cache table  80  are copied to the cache-table copy  81  such that consistency between the original and the copy is maintained at all times. The broadcast-packet/multicast-packet detection unit  82  is provided in place of the loop-detection unit  77  of FIG. 8, receiving packets from the upward-packet-filtering unit  78  and transferring the packets to the passing/disposing-switch unit  76 . 
     In what follows, a description will be given with regard to how a loop is detected in the cable-modem system as described above. 
     FIG. 16 is an illustrative drawing for explaining a basic operation of a loop detection process. In this example, a loop is created along the center device  62 , the cable modem  83   a , the bridge  65 , and the cable modem  83   b  as shown in FIG.  16 . 
     When a user attempts to access the Internet from the terminal device  66   a , for example, the terminal device  66   a  transmits broadcast MAC packets to the cable modems  83   a  and  83   b  in response to the user operation. The broadcast MAC packets are transferred from the cable modem  83   a  to the center device  62  and also from the cable modem  83   b  to the center device  62 . Here, the broadcast MAC packets have a destination MAC address thereof set to a broadcast MAC address. 
     Since the received packets have the broadcast MAC address, the center device  62  transfers the packets received from the cable modem  83   a  to the cable modems  83   a  and  83   b , and transfers the packets received from the cable modem  83   b  to the cable modems  83   a  and  83   b . In this manner, the broadcast MAC packets are repeatedly transferred between the center device  62  and the bridge  65  to increase in number. 
     The broadcast-packet/multicast-packet detection unit  82  included in each of the cable modems  83   a  and  83   b  monitors a destination MAC address of a packet received from the end of the subscriber-household LAN. While doing so, the broadcast-packet/multicast-packet detection unit  82  counts the number of broadcast MAC packets that have the broadcast MAC address as their destination MAC address. 
     When the number of the broadcast MAC packets supplied within a given constant timeframe exceeds a predetermined number, the broadcast-packet/multicast-packet detection unit  82  ascertains that there is a loop, and instructs the passing/disposing-switch unit  76  to stop transfer of packets. 
     The loop detection as described above draws on that fact that the number of broadcast MAC packets supplied from the end of the subscriber-household LAN within a given constant timeframe is not likely to exceed a predetermined number if there is no loop. Utilizing this fact, the cable modems  83   a  and  83   b  detect and sever a loop by counting the number of broadcast MAC packets supplied from the end of the subscriber-household, LAN within a given constant timeframe. Here, the same result is obtained even if multicast packets are used in place of the broadcast MAC packets. 
     In what follows, a loop-detection operation will be described with regard to the cable-modem system of FIG. 6 having the cable modems of FIG.  15 . 
     FIG. 17 is a sequence diagram showing a loop-detection operation of the cable-modem system using the cable-modem of FIG. 15, according to the present invention. Legends used in FIG. 17 such as “Loop-Det(M lp , M c )” are the same as those of FIG. 14, and a description thereof will be omitted. 
     When a user is to access the Internet from the terminal device  66   a  via the router  60 , the terminal device  66   a  transmits an ARP-REQ packet as a broadcast MAC packet to the bridge  65 . Since the supplied packet is that of a broadcast nature, the bridge  65  transfers the supplied broadcast MAC packet to each of the cable modem  83   a , the cable modem  83   b , and the terminal device  66   b.    
     The cable modems  83   a  and  83   b  monitor destination MAC addresses of packets as it receives the packets from the end of the subscriber-household LAN. Since the number of broadcast MAC packets supplied from the end of the subscriber-household LAN within a given constant timeframe T does not exceed a predetermined number (e.g., 5), transferring of the packets continues at this time. 
     Each of the cable modems  83   a  and  83   b  transfers the broadcast MAC packet to the center device  62 . Since the supplied packets are those of a broadcast nature, the center device  62  transfers the broadcast MAC packet supplied from the cable modem  83   a  to the cable modems  83   a  and  83   b , and, also, transfers the broadcast MAC packet supplied from the cable modem  83   b  to the cable modems  83   a  and  83   b.    
     Having received the two broadcast MAC packets from the center device  62 , the cable modem  83   a  transfers these two packets to the bridge  65 . By the same token, having received its two broadcast MAC packets from the center device  62 , the cable modem  83   b  transfers these two packets to the bridge  65 . 
     Since the supplied packets are those of a broadcast nature, the bridge  65  transfers the two broadcast MAC packets supplied form the cable modem  83   a  to each of the cable modem  83   b  and the terminal devices  66   a  and  66   b , and, also, transfers the two broadcast MAC packets supplied form the cable modem  83   b  to each of the cable modem  83   a  and the terminal devices  66   a  and  66   b.    
     At this time, each of the cable modems  83   a  and  83   b  receives two broadcast MAC packets. Since the number of the broadcast MAC packets received from the end of the subscriber-household LAN within a given constant timeframe does not exceed the predetermined number (e.g., 5), the transfer of packets still continues. 
     Each of the cable modems  83   a  and  83   b  transfers the two broadcast MAC packets to the center device  62 . Having received the packets of a broadcast nature, the center device  62  transfers the two broadcast MAC packets supplied from the cable modem  83   a  to the cable modems  83   a  and  83   b , and, also, transfers the two broadcast MAC packets supplied from the cable modem  83   b  to the cable modems  83   a  and  83   b . Each of the cable modems  83   a  and  83   b  thus receives four broadcast MAC packets. 
     In this manner, broadcast MAC packets increase in number between the center device  62  and the bridge  65  because of the presence of a loop. As the number increases, there will be a point when the number of broadcast MAC packets supplied to the broadcast-packet/multicast-packet detection unit  82  from the end of the subscriber-household LAN during a given constant timeframe exceeds the predetermined number (e.g., 5). When this happens, the broadcast-packet/multicast-packet detection unit  82  ascertains that there is a loop, and instructs the passing/disposing-switch unit  76  to halt the transfer of packets. 
     In this manner, each of the cable modems  83   a  and  83   b  counts the number of the broadcast MAC packets arriving par predetermined interval from the end of the subscriber-household LAN, thereby detecting a loop and stopping transfer of packets. This results in prevention of system shutdown. 
     In the following, a cable-modem system according to a second embodiment of the present invention will be described. 
     FIG. 18 is a block diagram showing a cable-modem system according to a second embodiment of the present invention. The cable-modem system of FIG. 18 has a configuration closely similar to that of FIG.  6 . In FIG. 18, the same elements as those of FIG. 6 are referred to by the same numerals, and a description thereof will be omitted. 
     The cable-modem system of FIG. 18 includes the router  60 , the server  61 , the center device  62 , a cable modem  64 , a hub  85  connected to the cable modem  64 , and the terminal devices  66   a  and  66   b  connected to the hub  85 . The cable modem  64  has either the configuration of FIG. 8 or the configuration of FIG. 15, and includes the upward-packet-filtering unit  78 . Further, the hub  85  is a repeater hub. 
     In what follows, an operation of the cable-modem system of FIG. 18 will be described. 
     FIG. 19 is a sequence diagram showing an operation of the cable-modem system of FIG. 18 according to the present invention. Legends used in FIG. 19 such as “Loop-Det(M lp , M c )” are the same as those of FIG. 14, and a description thereof will be omitted. 
     When a user attempts to establish communication in the subscriber-household LAN by connecting the terminal devices  66   a  and  66   b  together, the terminal device  66   a  transmits an ARP-REQ packet as a broadcast MAC packet to the hub  85  with an aim of detecting an MAC address of the terminal device  66   b.    
     Being a repeater hub, the hub  85  forwards the ARP-REQ packet supplied from the terminal device  66   a  to each of the cable modem  64  and the terminal device  66   b . Since the destination MAC address of the supplied ARP-REQ packet is a broadcast MAC address, the cable modem  64  forwards the ARP-REQ packet to the center device  62 . Also, the cable modem  64  stores the source MAC address M pc1  of the ARP-REQ packet in the cache table  80 . Having received the ARP-REQ packet, the terminal device  66   b  transmits an ARP-REP packet to the hub  85  towards an ultimate destination of the terminal device  66   a.    
     Being a repeater hub, the hub  85  forwards the ARP-REP packet supplied from the terminal device  66   b  to each of the cable modem  64  and the terminal device  66   a . The upward-packet-filtering unit  78  of the cable modem  64  monitors destination MAC addresses of packets that are supplied from the end of the subscriber-household LAN, and checks if the destination MAC address M pc1  of the ARP-REP packet is stored in the cache table  80 . 
     Since the cache table  80  already has the MAC address M pc1  stored therein, the cable modem  64  dispose of the supplied ARP-REP packet, and stores the source MAC address M pc2  of this ARP-REP packet in the cache table  80 . In the meantime, the terminal device  66   a  receives the ARP-REP packet, and learns the MAC address of the terminal device  66   b.    
     When the terminal devices  66   a  and  66   b  communicate with each-other thereafter, data packets transmitted from these devices have the destination MAC address M pc1  or M pc2 , which is stored in the cache table  80  of the upward-packet-filtering unit  78 . These data packets are thus disposed of at the cable modem  64 , and do not reach the center device  62 . 
     In this manner, communication packets used in the subscriber-household LAN are not forwarded from the cable modem  64  to the center device  62 , but are properly disposed of. This insures privy of the communication in the subscriber-household LAN. 
     In the following, a cable-modem system according to a third embodiment of the present invention will be described. 
     FIG. 20 is a block diagram showing a cable-modem system according to the third embodiment of the present invention. The cable-modem system of FIG. 20 has a configuration closely similar to that of FIG.  6 . In FIG. 20, the same elements as those of FIG. 6 are referred to by the same numerals, and a description thereof will be omitted. 
     The cable-modem system of FIG. 20 includes the router  60 , the server  61 , the center device  62 , the cable modems  64   a  and  64   b , and the terminal devices  66   a  and  66   b , which are connected to the cable modems  64   a  and  64   b , respectively. The cable modems  64   a  and  64   b  have either the configuration of FIG. 8 or the configuration of FIG. 15, and include the upward-packet-filtering unit  78  and the downward-packet-filtering unit  75 . 
     In what follows, an operation of the cable-modem system of FIG. 20 will be described. 
     FIG. 21 is a sequence diagram showing an operation of the cable-modem system of FIG. 20 according to the third embodiment of the present invention. Legends used in FIG. 21 such as “Loop-Det(M lp , M c )” are the same as those of FIG. 14, and a description thereof will be omitted. 
     The cache table  80  of the cable modem  64   a  has an MAC address M pc1  of the terminal device  66   a  already stored therein. When a user accesses the Internet from the terminal device  66   b  via the router  60 , the terminal device  66   b  transmits an ARP-REQ packet as a broadcast MAC packet to the cable modem  64   b.    
     The cable modem  64   b  forwards the received ARP-REQ packet to the center device  62  since the ARP-REQ packet has a broadcast MAC address as its destination MAC address. Further, the cable modem  64   b  stores a source MAC address M pc2  of the packet in the cache table  80 . 
     The center device  62  stores the source MAC address M pc2  of the received ARP-REQ packet and a CID # 2  of the source cable modem  64   b  in the cache table  74 , and, also, transfers the ARP-REQ packet to the router  60 . Further, the center device  62  attaches a broadcast CID number to the received ARP-REQ packet, and sends the ARP-REQ packet with this attachment to the CATV delivery facility  63 . 
     Having received the ARP-REQ packet from the center device  62 , the router  60  sends an ARP-REP packet to the center device  62  towards an ultimate destination of the terminal device  66   b . The bridge processing unit  70  of the center device  62  monitors destination MAC addresses of packets supplied from the end of the intra-center LAN. Having received the ARP-REP packet from the router  60 , the bridge processing unit  70  identifies a CID number corresponding to the destination MAC address M pc2  of this ARP-REP packet in the cache table  74 . Finding the CID # 2  corresponding to the destination MAC address M pc2 , the center device  62  attaches the CID # 2  as a header to the ARP-REP packet, and sends this packet to the cable modem  64   b.    
     The cable modem  64   b  forwards the received ARP-REP packet to the terminal device  66   b . As a result, the terminal device  66   b  learns the MAC address of the router  60  from the supplied ARP-REP packet. 
     Thereafter, the terminal device  66   b  sends DATA packets to the cable modem  64   b  by setting destination MAC addresses of these DATA packets to the MAC address of the router  60 . The cable modem  64   b  forwards the DATA packets to the center device  62  via the CATV delivery facility  63  as these packets are supplied from the terminal device  66   b . The center device  62  then forwards the DATA packets to the router  60 . The router  60  transfers the DATA packets to the Internet, and receives reply DATA packets from the Internet. The reply DATA packets are sent to the center device  62 . 
     The bridge processing unit  70  of the center device  62  identifies a CID number corresponding to the destination MAC address pc2  of the DATA packets in the cache table  74 , and attaches the identified CID # 2  as a header to the DATA packets. The DATA packets with the attached header are sent to the cable modem  64   b . The cable modem  64   b  then forwards the DATA packets to the terminal device  66   b.    
     After this, when no communication is observed for a substantially long period of time between the center device  62  and the cable modems  64   a  and  64   b , the MAC address M pc2  and CID # 2  are deleted from the cache table  74  as their allocated time period expires. When DATA packets directed to the terminal device  66   b  are supplied from the Internet, the router  60  transfers the DATA packets to the center device  62 . 
     When the bridge processing unit  70  of the center device  62  searches in the cache table  74  for a CID number corresponding to the destination MAC address M pc2  of these DATA packets, no CID number is obtained because the corresponding entry was already deleted. Then, the center device  62  attaches a broadcast CID Br to the DATA packets as a header, and sends them to the cable modems  64   a  and  64   b.    
     When receiving the DATA packets, the cable modem  64   b  searches in the cache-table copy  81  of the downward-packet-filtering unit  75  to check if an MAC address corresponding to the destination MAC address of the DATA packets is stored therein. 
     Since such a corresponding MAC address is found in the cache-table copy  81 , the cable modem  64   b  forwards the DATA packets to the terminal device  66   b . In the meantime, the cable modem  64   a  cannot find an MAC address corresponding to the destination MAC address of the DATA packets in the cache-table copy  81 , and, thus, does not forward the DATA packets to the terminal device  66   a.    
     In this manner, the cable modems  64   a  and  64   b  store the MAC addresses of the respective terminal devices  66   a  and  66   b  in the cache-table copy  81  of the downward-packet-filtering unit  75 . If the MAC address stored in the cache-table copy  81  differs from the destination MAC address of a received packet, the packet will be disposed of. This insures privy of the communication with the Internet. 
     In the following, a cable-modem system according to a fourth embodiment of the present invention will be described. 
     FIG. 22 is a block diagram showing a cable-modem system according to the fourth embodiment of the present invention. The cable-modem system of FIG. 22 has a configuration closely similar to that of FIG.  6 . In FIG. 22, the same elements as those of FIG. 6 are referred to by the same numerals, and a description thereof will be omitted. 
     The cable-modem system of FIG. 22 includes the router  60 , the server  61 , the center device  62 , bridges  87   a  through  87   h , and the cable modem  64 . The bridges  87   a  through  87   d  are connected to the intra-center LAN, and the remaining bridges  87   e  through  87   h  are connected to the subscriber-household LAN. 
     Among the bridges  87   e  through  87   h  which are connected to the subscriber-household LAN, the bridge  87   e  is part of an original configuration of this system, and the other bridges  87   f  through  87   h  are those later added as extensions. The center device  62  has the configuration of FIG. 7, for example, and includes the BPDU-packet-filtering unit  69 . The cable modem  64  has either the configuration of FIG. 8 or the configuration of FIG. 15, and includes the BPDU-packet-filtering unit  79 . 
     In a single network that is comprised of bridges for exchanging BPDU packets and supporting spanning-tree protocols, the number of bridges between two end bridges needs to be fewer than seven according to the recommendations of the IEEE 802.1D. This requirement insures that the spanning-tree function works correctly provided that each bridge is set to its default settings. Hereinafter, the number of bridges is referred to as a bridge diameter. 
     The cable-modem system of FIG. 22 has a bridge diameter of 5 before the bridges  87   f  through  87   h  are added as extensions to the subscriber-household LAN, so that the spanning-tree function works normally in this system. After the addition of the bridges  87   f  through  87   h  to the subscriber-household LAN, however, the bridge diameter of the cable-modem system becomes  8 , which may result in the spanning-tree function not working correctly. 
     To obviate this problem the cable-modem system of FIG. 22 has the BPDU-packet-filtering unit  69  and the BPDU-packet-filtering unit  79  in the center device  62  and the cable modem  64 , respectively, for the purpose of disposing of BPDU packets. The center device  62  uses the BPDU-packet-filtering unit  69  to dispose of BPDU packets supplied from the bridge  87   a . The cable modem  64  uses the BPDU-packet-filtering unit  79  to dispose of BPDU packets supplied from the bridge  87   e.    
     This insures that no exchange of BPDU packets is conducted between the intra-center LAN and the subscriber-household LAN. The intra-center LAN and the subscriber-household LAN thus cause no effect on other LANs connected to the cable-modem system as far as a bridge diameter thereof is concerned. 
     Further, the present invention is not limited to these embodiments, but various variations and modifications may be made without departing from the scope of the present invention. 
     The present application is based on Japanese priority application No. 10-359437 filed on Dec. 17, 1998, with the Japanese Patent Office, the entire contents of which are hereby incorporated by reference.