Abstract:
A cable modem which can be recorded with a downstream frequency, assigned by a cable modem terminate system in a cable modem system, before the cable modem is installed in the system, and can skip searching the whole range of downstream frequency for the assigned downstream frequency in order to reduce the time for searching.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to a cable modem, which is used with a Cable Modem Terminate System (CMTS) in a cable modem system. The present invention, more particularly, relates to a cable modem capable of reducing time for searching for a downstream frequency, which is assigned to the cable modem by the CMTS, and to a method for recording the downstream frequency in the cable modem.  
         BACKGROUND AND MATERIAL INFORMATION  
         [0002]    A cable modem system, which performs data communication using a cable network constructed for a cable television (CATV), has recently become widespread. The CATV was originally mere a TV broadcasting system for providing a scriber with various services through a coaxial cable. In the cable modem system, data communication is performed using a frequency band other than frequency bands specified for TV, by a cable modem and a CMTS working in close cooperation with each other.  
           [0003]    [0003]FIG. 2 is a block diagram showing a schematic constitution of a typical cable modem system. Each of cable modems  201 , e.g.,  201   a  and  201   b  is severally connected to a CMTS  203  via a CATV cable  202 .  
           [0004]    The CMTS  203  communicates with a provider&#39;s server  205  via the provider&#39;s network, which may be composed of a local area network (LAN)  204  according to 100BASE-T or asynchronous transfer mode (ATM). The server  205 , which is disposed at a position generally called “head end”, provides each of cable modems  201  with necessary services, for example dynamic host configuration protocol (DHCP), trivial file transfer protocol (TFTP), and electric mail.  
           [0005]    The CMTS  203  coverts digital data received from the server  205  to a radio frequency (RP) signal and transmits the RF signal to each cable modem  201 . Moreover, the CMTS  203  converts an RF signal received from these cable modems  201  to digital data and transmits the digital data to the server  205 . Generally, the communication direction from the CMTS  203  to each of cable modems  201  is called “downstream” and the opposite communication direction is called “upstream.” 
           [0006]    DOCSIS (Data over cable service interface specifications) is widely known as a specification of a communication interface of the cable modems  201 .  
           [0007]    Each of cable modems  201  maybe placed at each scriber&#39;s house and be connected to each of personal computers (PCs)  206 , e.g.,  206   a  and  206   b  via a cable according to 10BASE-T or 100BASE-T, For example, the cable modem  201   a  is connected directly to the PC  206   a , and the cable modem  201   b  is connected to the PC  206   b  via a hub  207 . Each scriber transmits and receives data using each PCs  206  to obtain various services from the provider.  
           [0008]    The cable modem  201   a  picks up a carrier, which is transmitted from the CMTS  203  on a downstream frequency assigned to the cable modem  201   a  itself, from the whole range of the downstream frequency band specified for the data communication. Then, the cable modem  201  extracts necessary data from the carrier and transmits the extracted data to the PC  206   a.    
           [0009]    Accordingly, when installed in the cable modem system, the cable modem  201   a  needs to search every channel for the carrier assigned to the cable modem  201   a  itself in order to perform the data transmission to the PC  206   a . Generally, the downstream frequency band has a wide range from 88 MHz to 860 MHz and is composed of a plurality of channels each of which has a width of 6 MHz. So it takes between 2 and 6 minutes to perform the frequency search.  
           [0010]    The cable modem  201   a  determines whether or not a carrier for data communication is received on a channel. When receiving the carrier, the cable modem  201   a  determines whether or not the carrier is assigned to the cable modem  201   a  itself.  
           [0011]    When the carrier is assigned to the cable modem  201   a , the cable modem  201   a  caches the downstream frequency of the channel in an internal memory. Consequently, during a subsequent data communication, the cable modem  210   a  can pick up the down stream frequency assigned to itself using the cached down stream frequency within several tens seconds.  
           [0012]    There may be a case where the connection between cable modem  201   a  and the CATV cable  202  is disconnected for some reason, such as unplugged in error, during the operation of the cable modem  210   a  after caching the assigned downstream frequency. In this case, the CATV cable modem  201   a  needs to perform the frequency search for another downstream frequency. It takes between 2 and 6 minutes again until the cable modem become able to communicate with the CMTS  203  after the connection is restored.  
           [0013]    As mentioned above, because a cable modem does not know the downstream frequency assigned to itself at the time of installation, the cable modem needs to perform the frequency search requiring considerable time.  
           [0014]    On the other hand, the scriber or a person who installs the cable modem in the cable modem system generally knows the assigned downstream frequency at that time. Therefore, it is desired to develop a cable modem, which can be recorded with the assigned downstream frequency and can become able to communicate with a CMTS without the frequency search.  
         SUMMARY OF THE INVENTION  
         [0015]    Accordingly, the present invention is directed to a cable modem, a method for recording a downstream frequency in the cable modem, and the cable modem system that substantially obviates one or more of problems due to limitations and disadvantages of the prior art.  
           [0016]    In accordance with the purpose of the present invention, as embodied and broadly described, the present invention is directed to a cable modem, which is connected with a CMTS via a CATV cable, for enabling a computer connected with the cable modem to perform data communication using the CATV cable. The cable modem comprises a memory, means for recording a downstream frequency assigned to the cable modem in the memory according to an instruction from the computer, a table storing a downstream frequency for each channel, means for selecting a channel corresponding to the downstream frequency recorded in the memory by referring to the table, a tuner for receiving a carrier wave of the selected channel, the carrier wave being transmitted from the CMTS, means for converting data carried by the carrier wave into a predetermined data form, means for determining whether the converted data is to be transmitted by the cable modem or not, and means for transmitting the converted data to the computer when the converted data is to be transmitted by the cable modem. The selecting means changes the current channel to another channel by referring to the table when the converted data is not to be transmitted by the cable modem.  
           [0017]    Also in accordance with the present invention, there is provided a cable modem system in which a cable modem is connected with a CMTS via a CATV cable for enabling a computer connected with the cable modem to perform data communication using the CATV cable. The cable modem comprises a memory, means for recording a downstream frequency assigned to the cable modem in the memory according to an instruction from the computer, a table storing a downstream frequency for each channel, means for selecting a channel corresponding to the downstream frequency recorded in the memory by referring to the table, a tuner for receiving a carrier wave of the selected channel, the carrier wave being transmitted from the CMTS, means for converting data carried by the carrier wave into a predetermined data form, means for determining whether the converted data is to be transmitted by the cable modem or not, and means for transmitting the converted data to the computer. The selecting means changes the current channel to another channel by referring to the table when the converted data is not to be transmitted by the cable modem. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]    The accompanying drawings, which are incorporated in and constitute part of this specification, illustrate various embodiments and/or features of the invention and together with the description, serve to explain the principles of the invention. In the drawings;  
         [0019]    [0019]FIG. 1 is a block diagram showing an example of a main configuration of a cable modem consistent with the present invention; and  
         [0020]    [0020]FIG. 2 is a block diagrams showing a schematic constitution of a typical cable modem system. 
     
    
     DETAILED DESCRIPTION  
       [0021]    The embodiment of the present invention will be explained hereunder with reference to the accompanying drawings. FIG. 1 is a block diagram showing an example of a main configuration of a cable modem  101 .  
         [0022]    A cable modem  101 , which is positioned between a CMTS  102  and a PC  103 , has a tuner  104 , a physical decoder (PHY decoder)  105 , a MAC controller  106 , a memory  107 , a central processing unit (CPU)  108 , and a private management information base (private MIB)  109 , each of them as a function of performing a downstream frequency processing.  
         [0023]    The tuner  104  switches a frequency band, channel by channel, to receive a downstream frequency transmitted from the CMTS  102  according to an instruction of a tuning program  112  (described later) and sequentially receives each downstream frequency corresponding to a channel.  
         [0024]    The PHY decoder  105  converts analog data carried via the received downstream frequency to digital data of 64 QAM or 256 QAM and outputs the digital data to the MAC controller  106 .  
         [0025]    When receiving the digital data from the PHY decoder  105 , the MAC controller  106  determines whether or not the received downstream frequency is a carrier which satisfies the requirements for the specification of the cable modem  101 , such as whether DOCSIS carrier or not.  
         [0026]    The memory  107  has a frequency table  110  in which the relation between the frequency band and the channel, and a frequency cache  111  for storing a downstream frequency assigned to the cable modem  101 .  
         [0027]    The CPU  108  manages the whole control of the cable modem  101  and internally has the tuning program  112 , an IP layer  113 , and a simple network management protocol (SNMP) agent  114 . The tuning program  112  instructs the tuner  104  to switch to another channel by referring to the relation recorded in the frequency table  110  when the MAC controller  106  detects no carrier satisfying the requirements for the specification of the cable modem  101 .  
         [0028]    The MAC controller  106  performs the above detection process for each channel. When detecting the carrier satisfying the requirements, the MAC controller  106  temporally locks the current frequency band and downloads information on setting the cable modem  101 , such as IP address, from the server.  
         [0029]    When the above provisioning is completed, the cable modem  101  becomes able to communicate with the CMTS  102 . And the CPU  108  records the locked frequency band in the frequency cache  111 . Consequently, even if a power supply to the cable modem  101  is disconnected for some reason, the CPU  108  can recognize the assigned frequency band using the recorded frequency band in the frequency cache  111  when the power supply is recovered. The cable modem  101  is quickly restored to communicate with the CMTS  102  without performing the frequency search again.  
         [0030]    The SNMP, which is defined in Request For Comments (RFC)  1157 , is a protocol for monitoring a device connected to a network according to the user datagram protocol/Internet protocol (UDP/IP) via a network.  
         [0031]    The PC  103  has an SNMP manager  115 , which is a program for monitoring or managing the condition of a router or a hub and traffic condition of a network from the remote console using the SNMP.  
         [0032]    On the other hand, the device, which is managed by the SNMP, has a program called MIB. The MIB generally defines the items to be managed by the SNMP and holds a variable indicating the condition of the device itself.  
         [0033]    The NIB is to be basically installed in each device and is specified by RFC. In this embodiment, the cable modem  101  has a unique MIB, namely, private MIB  109 . The private MIB  109  is used as a new code for recording the downstream frequency in the frequency cache  111 . The basic specification of the MIB is specified by RFC  1213 . However, the specification is still being expanded even now.  
         [0034]    The SNMP agent  114 , which is a program according to the SNMP, manages the MIB including the private MIB  109  in the cable modem  101 . Also the SNMP agent  114  transmits the MIB to the SNMP manager  115  and operates the cable modem  101  according to a request from the SNMP manager  115 . The UDP is used for communicating with the SNMP manager  115  via the IP layer  113 .  
         [0035]    Next, the procedure for recording the downstream frequency with the frequency cache  111  will be explained.  
         [0036]    The cable modem  101  is given a default IP address, such as 192.168.100.1. So the customer premises equipment (CPE), such as the PC  103 , can access the cable modem  101  using the default IP address even before the cable modem  101  becomes able to communicate with the CMTS  102 .  
         [0037]    Therefore, a scriber or an installer of the cable modem  101  instructs the SNMP agent  114 , by the SNMP manager  115  on the PC  103  via the IP layer  113 , to make the private MIB  109  to record the downstream frequency assigned to the cable modem  101  in the frequency cache  111 .  
         [0038]    Consequently, the tuning program  112  can instruct the tuner  104  to select the frequency band recorded in the frequency cache  11  without executing the frequency search when the cable modem  101  is connected to the CMTS  102 .  
         [0039]    Even in a case where there are a plurality of CMTSs  102  or a plurality of downstream frequencies, the private MIB  109  records these downstream frequencies in the frequency cache  111  in the same way. The tuning program  112  instructs the tuner  104  to search the carrier for the recorded frequencies. The cable modem  101  can catch the downstream frequencies immediately.  
         [0040]    Some methods for quickly searching the assigned downstream frequencies after disconnection of the cable will be explained hereunder. Even when the downstream frequency carrier is lost, the cable modem  101  can efficiently search for the assigned downstream frequency by the following ways.  
         [0041]    (1) Searching only the channels recorded in the frequency cache  111  for the downstream frequency;  
         [0042]    (2) Searching a predetermined number of channels for the downstream frequency, searching a predetermined part of the frequency cache  111  for the downstream frequency, repeating these search by turns.  
         [0043]    (3) Searching only frequency bands recorded in the frequency cache  111  in order to reduce the width of frequency band to be searched, namely, skipping the other bands.  
         [0044]    In the above explanation, software applying to the SNMP is used as an interface for instructing the private MIB  109  to perform cache recording. However, another software using such as the HTTP may be used.