Abstract:
A communication device includes a memory to store data and a processor to receive data, to convert the received data and a same data of the received data among the stored data into a predetermined format, and to transmit data resulting from the data conversion.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2010-270635, filed on Dec. 3, 2010, the entire contents of which are incorporated herein by reference. 
     FIELD 
     The disclosure herein relates to a communication device and a communication method. 
     BACKGROUND 
     With widespread use of the Internet and computers, the amounts of traffic of data systems increase in communication networks. Examples of the traffic of data systems include IP (internet protocol) packets and LAN (local area network) packets of Ethernet and so on. 
     Transfer of traffic for data systems for VPN (virtual private network) services and so on has brought about increases in the transfer speed and in the transfer data. 
     The reliability of communication devices for use in the VPN services has improved with a redundant function. The redundant function is to duplicate processing. During failure or maintenance of a working side, the communication device having the redundant function is capable of switching its operation to a protection side. Thus, the redundant function improves the reliability of the communication device. 
     For example, LAN links and TDM (time division multiplexing) links are available to transmit traffic. The TDM links employ, for example, SONET (synchronous optical network), SDH (synchronous digital hierarchy), PDH (plesiochronous digital hierarchy), and so on. OTN (Optical transport network) is a new TDM technology. 
     The LAN links are capable of directly transferring packets. The TDM links, however, use a mapper function to convey packets. 
     The mapper function provides a function for mapping packets onto TDM frames at an entrance of a TDM network. The mapper function further provides a function for retrieving packets from the TDM frame at an exit at the opposite end of the TDM network. 
     Examples of devices having a redundant function and a mapper function as described above will be described below. 
     Japanese Unexamined Patent Application Publication No. 2010-74629 discloses a packet synchronization switching method for a gateway device. This gateway device has an audio processing blade redundantly configured between an IP network and an SDH network. The gateway device further has a function for receiving a VoIP packet from the IP network, performing audio processing on the packet, and outputting an I-TDM packet to the SDH network. Such a gateway device receives reference timing and time information, performs audio processing in accordance with the reference timing, and performs system switching between audio processing blades in accordance with the time information. 
     Japanese Unexamined Patent Application Publication No. 2008-178060 discloses a technology in which any of gateway devices that receive packets supplies a TDM signal to a TDM network. Each gateway device receives a duplicated same packet and writes the packet to a buffer in accordance with a common write pointer for the packet. In addition, each gateway device sequentially reads written packets from the buffer and generates a TDM signal. 
     Japanese Unexamined Patent Application Publication No. 2003-134074 discloses a technology in which a SONET/SDH transmission device selectively switches between transmission paths. The SONET/SDH transmission device has redundant transmission lines constituted by two transmission lines. The SONET/SDH transmission device transmits a packet representing transmission-line switching control information to another transmission device through the transmission line(s). In accordance with a state of packet reception from the other transmission device, the SONET/SDH transmission device switches the transmission paths for transmitting a transmission signal. 
     Japanese Unexamined Patent Application Publication No. 2006-67040 discloses an interface converting device for converting a synchronization transfer signal into a packet signal. The interface converting device has a pair of a working port and a protection port adjacent to a packet network. The interface converting device further has duplicated pairs of path transmission lines. The interface converting device includes switching information in added information to be added to a TDM signal after multiplexing. The interface converting device then adds a packet-network destination header to a signal to which the added information is added. 
     SUMMARY 
     According to an aspect of the embodiment, a communication device includes a memory to store data and a processor to receive data, to convert the received data and a same data of the received data among the stored data into a predetermined format, and to transmit data resulting from the data conversion. 
     The object and advantages of the embodiment will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the embodiment, as claimed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram of one example of a communication device; 
         FIG. 2A  illustrates examples of TDM frames output by working and protection mappers; 
         FIG. 2B  illustrates examples of TDM frames output by the working and protection mappers; 
         FIG. 3  is a block diagram of a communication device according to a first embodiment; 
         FIG. 4  is a block diagram of a communication device according to a second embodiment; 
         FIG. 5  illustrates an in-device frame according to the second embodiment; 
         FIG. 6  is a block diagram according to a QOS unit of the second embodiment; 
         FIG. 7A  illustrates a main signal according to the QOS unit of the second embodiment; 
         FIG. 7B  illustrates packet information for writing according to the QOS unit of the second embodiment; 
         FIG. 7C  illustrates packet information for reading according to the QOS unit of the second embodiment; 
         FIG. 8  is a block diagram according to a synchronization managing unit of the second embodiment; 
         FIG. 9  is a flowchart illustrating a first example of synchronization determination processing according to the second embodiment; 
         FIG. 10A  illustrates working packet information according to the synchronization determination processing according to the second embodiment; 
         FIG. 10B  illustrates protection packet information according to the synchronization determination processing according to the second embodiment; 
         FIG. 10C  illustrates a packet information table according to the synchronization determination processing according to the second embodiment; 
         FIG. 11A  is a diagram illustrating a state before the QOS control unit according to the synchronization managing unit performs processing according to the second embodiment; 
         FIG. 11B  is a diagram illustrating a state after the QOS control unit according to the synchronization managing unit performs processing according to the second embodiment; 
         FIG. 12A  is a diagram illustrating one example of the QOS control unit according to according to the second embodiment; 
         FIG. 12B  is a diagram illustrating one example of the QOS control unit according to the second embodiment; 
         FIG. 13  illustrates one example of mapping-position calculation according to the synchronization determination processing according to the second embodiment; 
         FIG. 14A  illustrates one example of packet information according to the synchronization determination processing according to the second embodiment; 
         FIG. 14B  illustrates one example of packet information according to the synchronization determination processing according to the second embodiment; 
         FIG. 14C  illustrates one example of packet information according to the synchronization determination processing according to the second embodiment; 
         FIG. 15  is a flowchart illustrating a second example of the synchronization determination processing according to the second embodiment; and 
         FIG. 16  illustrates encapsulation using an HDLC method and encapsulation using a GFP. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Preferred embodiments of the present invention will be explained with reference to accompanying drawings. 
     [a] Exemplary Communication Device 
     In a VPN (virtual private network) service or the like, various data may coexist on a network. For example, audio, video, and so on generally require real-time transmission, which is highly susceptible to packet loss. In contrast, file transfer, web viewing, and so on permit certain degrees of delay and packet loss. 
     Accordingly, it is desired that a communication device that handles traffic for a data system be capable of dealing with various qualities of services. 
       FIG. 1  is a block diagram of a chassis-type communication device having a function for linking a packet network and a TDM (time-division multiplexing) network. A packet interface  11 , a packet switch  12 , a working mapper  13 , a protection mapper  14 , a TDM switch  15 , a TDM interface  16 , and a control unit  17  are incorporated into respective cards. 
     In the communication device, QOS units consider a quality of service (QOS). Each QOS unit rearranges input packets in order of priority and outputs the rearranged packets. 
     In a mapping direction, the packet interface  11  receives packets from a packet network  20  and the packet switch  12  copies each of the received packets and supplies the copied packet to the working mapper  13  and the protection mapper  14 . The working mapper  13  and the protection mapper  14  map the packets onto TDM frames and output the resulting TDM frames, respectively. The TDM switch  15  selects one of the TDM frames and the TDM interface  16  transmits the selected TDM frame to a TDM network  21 . 
     In a demapping direction, the TDM interface  16  receives a TDM frame from the TDM network  21  and the TDM switch  15  copies the TDM frame and supplies the TDM frame to the working mapper  13  and the protection mapper  14 . The mappers  13  and  14  demap packets from the TDM frames and outputs the packets. The packet switch  12  selects the packet and the packet interface  11  transmits the selected packet to the packet network  20 . The control unit  17  performs setting for each card by performing software operation and generates and supplies a low-speed clock and a frame pulse for each card by performing hardware operation. Lastly, the software determines which of the mappers  13  and  14  is to be used as a working mapper. 
     In the mapping direction, the communication device has the following problems. 
     The TDM switch  15  executes switching processing at timing of the border of TDM frames or the like. However, there is no guarantee that payloads of the TDM frames output from the two mappers  13  and  14  match each other exactly. 
     A first reason for the mismatch is that packets that flow through the mappers  13  and  14  are out of sync with each other. 
     Suppose, for example, that the communication device initially operates using only the working mapper  13  and then performs redundant operation using both of the working mapper  13  and the protection mapper  14 . 
     In this case, a QOS unit  13   a  in the working mapper  13  is filled with packets. However, a QOS unit  14   a  in the working mapper  14  is empty. Thus, packets output to downstream mapping units  13   b  and  14   b  are not always the same packet. 
     Suppose, for example, that the communication device is in a state in which a buffer in the QOS unit  13   a  in the working mapper  13  is full. 
     In this case, the communication device performs back pressure control on the packet switch  12  to stop inputting. As a result of the back pressure control, there is a possibility that packets output from the QOS unit  13   a  and the QOS unit  14   a  differ from each other. 
     Suppose, for example, that after the redundant operation of the communication device, the control unit  17  changes the QOS setting. 
     During the change in the QOS setting, the control unit  17  does not perform setting for the working mapper  13  and the protection mapper  14  at the same time. The control unit  17  first performs setting for the working mapper  13  and then performs setting for the protection mapper  14 . As a result of the change in the QOS setting, there is a possibility that packets output from the QOS units  13   a  and  14   a  in the mappers  13  and  14  differ from each other. 
     A second reason for the above-noted mismatch is that high-speed clocks used by the mappers  13  and  14  are out of sync with each other. 
     For example, in a chassis-type communication device that performs high-speed, large-volume processing, each card internally generates a high-speed clock (with a frequency of 300 MHz or the like) by multiplying a low-speed clock (with a frequency of 38 MHz or the like) supplied from the control unit  17 . 
     Thus, although the low-speed clocks supplied from the control unit  17  synchronize with each other, not all of the phases of the high-speed clocks perfectly synchronize with each other. 
     Suppose, in this case, certain processing is externally triggered on the working and protection mappers  13  and  14  at the same time. 
     If, however, the mappers  13  and  14  are out of sync with each other, the mappers  13  and  14  do not necessarily always capture the processing at the same clock phase. When the high-speed clocks of the mappers  13  and  14  are out of sync with each other, the working mapper  13  and the protection mapper  14  perform packet processing involving QOS at exactly the same phase (timing). 
       FIG. 2A  illustrates TDM frames respectively output from the mappers  13  and  14 . The first reason discussed above yields a difference between the TDM frame output from the working mapper  13  and the TDM frame output from the protection mapper  14 , as illustrated in  FIG. 2A . 
       FIG. 2B  illustrates TDM frames respectively output from the mappers  13  and  14 . The second reason discussed above yields a difference between the TDM frame output from the working mapper  13  and the TDM frame output from the protection mapper  14 , as illustrated in  FIG. 2B . 
     As described above, there is no guarantee that payloads of the TDM frames output from the two mappers  13  and  14  match each other exactly. Thus, the above-described communication device involves interruptions during switching between the working mapper and the protection mapper. 
     The problems discussed above are not problems found only in communication devices used for VPN services. The problems discussed above could occur in any communication devices that perform communication. 
     In addition, communication devices that encounter the above-described problems are not limited to chassis-type communication devices having a function for linking a packet network and a TDM network. The above-described problems could similarly occur in any communication devices that perform communication. 
     [b] First Embodiment of the Communication Device 
       FIG. 3  is a block diagram illustrating a first embodiment of the communication device. The embodiment illustrated in  FIG. 3  corresponds to a case in the mapping direction. 
     A packet interface  31 , a packet switch  32 , mappers  33  and  34 , a TDM switch  35 , a TDM interface  36 , and a control unit  37  are incorporated into respective cards. 
     For example, ICs (integrated circuit) composed of processors, memories, etc. execute functions of the packet interface  31 , the packet switch  32 , the mapper  33 , the mapper  34 , the TDM switch  35 , the TDM interface  36 , and the control unit  37  respectively and are installed on the respective cards. 
     The packet interface  31  receives packets from a packet network  40  and supplies the received packets to the packet switch  32 . 
     In the packet switch  32 , the received packets are supplied to an identifier giving unit  322  via a switching unit  321  and the identifier giving unit  322  gives an identifier to each packet. In the packet switch  32 , a copying unit  323  copies each packet given the identifier and supplies the packet to the mappers  33  and  34 . 
     In this communication device, the mapper  33  serves as a working mapper and the mapper  34  serves as a protection mapper. The identifier giving unit  322  may also be eliminated as described below. 
     The mappers  33  and  34  store, in corresponding buffers in QOS units  331  and  341 , the packets supplied from the packet switch  32 . The QOS units  331  and  341  in the mappers  33  and  34  retrieve pieces of packet information for writing with respect to the corresponding packets stored in the corresponding buffers. Examples of each piece of packet information for writing include the identifier, QOS information, and the packet length of the corresponding packet. Each of the mappers  33  and  34  then supplies, for each packet, the piece of packet information to a synchronization managing unit  351  in the TDM switch  35 . The synchronization managing unit  351  may be provided in another card, for example, the card of the control unit  37 , not in the card of the TDM switch  35 . 
     The synchronization managing unit  351  stores the packet information supplied from the mappers  33  and  34 . When it is determined that the same packet is stored in the mappers  33  and  34 , the synchronization managing unit  351  determines that this packet is readable from the mappers  33  and  34 . 
     However, when multiple readable packets exist, the synchronization managing unit  351  determines which packet is to be first read considering the priority of classes, the fairness of users, or the like, on the basis of the QOS information of each packet. The synchronization managing unit  351  regards the result of the determination as a final readable packet. 
     The synchronization managing unit  351  calculates a mapping position (a byte position) for mapping each packet into the payload of the TDM frame. This calculation is based on the packet length of each packet permitted to be read by the mapper  33  or  34 . The synchronization managing unit  351  supplies packet designation information and mapping position information to the mappers  33  and  34  as a piece of packet information for reading. The packet designation information designates a packet to be read. The mapping position information specifies the mapping position of the packet. 
     The card of the control unit  37  distributes a common low-speed frame pulse to both of the mappers  33  and  34 . Thus, the TDM frames output from the mappers  33  and  34  are synchronous with each other. 
     The packets are read from the respective QOS units  331  and  341  in accordance with the packet designation information and the mapping units  332  and  342  map the read packets on the TDM frames, respectively, in accordance with the mapping position information in the packet information. 
     The TDM frames output from the respective mappers  33  and  34  are supplied to a selector (SEL)  352  in the TDM switch  35 . The selector  352  selects one of the “working” TDM frame and the “protection” TDM frame and outputs the selected TDM frame. The TDM frame output from the selector  352  is switched by a switching unit  353  and is sent from the TDM interface  36  to a TDM network  41 . 
     The control unit  37  performs setting for each card by performing software operation and generates and supplies a low-speed clock and a frame pulse for each card by performing hardware operation. Lastly, the software determines which of the mappers  33  and  34  is to be used as a working mapper. 
     As described above, the communication device causes synchronization of the timings of packets processed by the mappers  33  and  34  and the timings at which the packets are mapped onto corresponding TDM frames, thereby allowing the payloads of the TDM frames output from the working and protection mappers  33  and  34  to match each other. This allows the communication device to uninterruptedly perform duplicated switching between the working mapper  33  and the protection mapper  34  by using the TDM switch  35 . 
     For such uninterrupted switching, the synchronization managing unit  351  may store and process packet information of about a few to ten bytes per packet. Thus, the amount of buffer in the synchronization managing unit  351  is reduced. 
     In the communication device, the packet information for writing and the packet information for reading are exchanged between the mappers  33  and  34  and the TDM switch  35 . However, the amount of the packet information for writing and reading is considerably small compared to the amount of information of main signals (the received packets). Accordingly, the exchange of the packet information is sufficiently feasible using signals having lower speed than the main signals. 
     [c] Second Embodiment of the Communication Device 
       FIG. 4  is a block diagram illustrating a second embodiment of the communication device. The embodiment illustrated in  FIG. 4  corresponds to a case in the mapping direction. 
     The communication device in  FIG. 4  is different from the communication device in  FIG. 3  in that the packet network  40  is implemented by a LAN (local area network)  40 A such as Ethernet, the TDM network  41  is implemented by a SONET network  41 A, and the packet interface  31  is implemented by a LAN interface  31 A. 
     The identifier giving unit  322  is replaced with an SN giving unit  322 A. The SN giving unit  322 A generates a sequence number (SN) for each packet, determines a packet length, and gives the sequence number serving as an identifier, along with the packet length, to each packet. 
     Further, the mapping units  332  and  342  in the mappers are replaced with mapping units  332 A and  342 A. Each of the mapping units  332 A and  342 A maps a LAN packet onto a SONET STS3c frame. 
     Synchronization managing unit  351 A is substantially similar to the synchronization managing unit  351  disclosed in  FIG. 3 . However, for example, synchronization managing unit  351 A calculates a mapping position (a byte position) for mapping each packet into the payload of the SONET STS frame. This calculation is based on the packet length of each packet permitted to be read by the mapper  33  or  34 . The synchronization managing unit  351  supplies packet designation information and mapping position information to the mappers  33  and  34  as a piece of packet information for reading. The packet designation information designates a packet to be read. The mapping position information specifies the mapping position of the packet. 
     The TDM switch  35  is replaced with an STS (synchronous transport signal) switch  35 A. 
     The TDM interface  36  is replaced with a SONET interface  36 A. The selector  352 A selects one of the “working” SONET frame and the “protection” SONET frame and outputs the selected SONET frame. The SONET frame output from the selector  352 A is switched by a switching unit  353  and is sent from the SONET interface  36 A to a SONET network  41 A. 
     The LAN interface  31 A, the packet switch  32 , the mapper  33 , the mapper  34 , the STS switch  35 A, and the SONET interface  36 A are incorporated into respective cards. Respective ICs composed of processors, memories, etc. execute functions of the cards. In addition, respective functions of the SN giving unit  322 A, the QOS unit  331 A, the QOS unit  341 A, the mapping unit  332 A, the mapping unit  342 A, and the selector  352 A may be executed, for example, by a processor, such as a microprocessor or the like. 
       FIG. 5  is a diagram illustrating the format of an in-device frame according to the second embodiment of the communication device. 
     The in-device frame corresponds to a packet output by the SN giving unit  322 A. 
     An in-device header at the beginning of the frame has a sequence number (SN) and a frame length given by the SN giving unit  322 A. 
     A MAC DA is a MAC destination address and a MAC SA is a MAC source address. 
     A VLAN (virtual local area network) tag has a TPID (type identifier), a PCP (class), a CFI (canonical format indicator), and a VID (VLAN value). 
     A type/frame length is a type value or a frame length. Payload is arbitrary data. FCS is frame-error check data. 
     [c.1] One Example of QOS Unit According to Second Embodiment 
       FIG. 6  is a block diagram illustrating a first example of the QOS units  331 A and  341 A. The QOS unit illustrated in  FIG. 6  is applicable to the second embodiment ( FIG. 4 ) of the communication device. 
     The packet information extraction unit  53 , the writing control unit  54 , the reading control unit  56 , and the mapping position giving unit are, for instance, processors, such as microprocessors or the like. In addition, respective processes, executed by the packet information extraction unit  53 , the writing control unit  54 , the reading control unit  56 , and the mapping position adding unit  58 , are executed by processors such as microprocessors or the like. Moreover, the packet storage memory  52  is composed of memories, such as flash memories or the like. 
     A main signal (a packet of the in-device frame) is supplied from the copying unit  323  to a terminal  51 . The packet supplied to the terminal  51  is further supplied to a packet-storing memory  52  and a packet-information extracting unit  53 . 
       FIG. 7A  illustrates a main signal supplied to the QOS units  331 A and  341 A. That is, a main signal as illustrated in  FIG. 7A  is supplied to the terminal  51 . 
     The packet-information extracting unit  53  extracts the packet sequence number (SN), the frame length, the class, and the VLAN value from the supplied packet. The packet-information extracting unit  53  adds a working/protection flag to the extracted information to generate a piece of packet information. The working/protection flag indicates whether the supplied packet is a working packet or a protection packet. The packet-information extracting unit  53  supplies the generated packet information to the synchronization managing unit  351 A (not illustrated in  FIG. 6 ) via a terminal  55 . 
     The packet-information extracting unit  53  also supplies the sequence number of each packet to a writing control unit  54 . 
       FIG. 7B  illustrates the format of the packet information for writing, the packet information being output by the packet-information extracting unit  53 . 
     The writing control unit  54  generates a write address for the packet-storing memory  52  on the basis of the sequence number supplied from the packet-information extracting unit  53 . The writing control unit  54  supplies the generated write address to the packet-storing memory  52 . This allows the packet-storing memory  52  to regard the sequence number as an address. That is, the packets of main signals are sequentially stored at addresses corresponding to the sequence numbers, the addresses being included in the packet-storing memory  52 . 
     The packet information for reading is supplied from the synchronization managing unit  351 A to the reading control unit  56  via a terminal  57 . The reading control unit  56  generates an address based on the above-described packet designation information in the packet information. The reading control unit  56  reads the packet corresponding to the address from the packet-storing memory  52 . 
     The packet read from the packet-storing memory  52  is supplied to a mapping-position adding unit  58 . The mapping-position adding unit  58  adds the mapping position information to the read packet. The mapping-position adding unit  58  outputs the packet information with the mapping position information to the mapping unit via a terminal  59 . 
       FIG. 7C  illustrates the format of the packet information for reading according to the first configuration of the QOS units  331 A and  341 A. The packet information for reading is supplied from the synchronization managing unit  351 A. The packet information for reading contains a working/protection flag (ACT/STBY), a sequence number (SN) serving as the packet designation information, a frame number (a SONET frame number) serving as the mapping position information, and an in-frame mapping position. 
     [c.2] One Example of the Synchronization Managing Unit According to Second Embodiment 
       FIG. 8  is a diagram illustrating a first configuration of the synchronization managing unit  351 A. The synchronization managing unit illustrated in  FIG. 8  is applicable to the second embodiment ( FIG. 4 ) of the communication device. 
     The synchronous judgment unit  63 , the QOS control unit  65 , the mapping position calculation unit  68 , and the scheduling and reading control unit  67  are composed of processors, such as microprocessors or the like. Respective processes executed by the synchronous judgment unit  63 , the QOS control unit  65 , the mapping position calculation unit  68 , the scheduling, and reading control unit  67  are executed, for instance, by processors, such as microprocessors or the like. Moreover, the packet information table  64  is composed of memories, such as flash memories or the like. 
     The pieces of packet information for writing are supplied from the working and protection mappers to terminals  61  and  62 , respectively. The pieces of packet information for writing are further supplied from the terminals  61  and  62  to a synchronization determination unit  63 . 
     The synchronization determination unit  63  has a packet information table  64 . The packet information table  64  contains pieces of packet information, with the sequence numbers thereof being used as search keys. The synchronization determination unit  63  writes the pieces of packet information, output from the working and protection mappers to the packet information table  64 . 
     In this case, both of the working and protection mappers output pieces of packet information regarding the same packet. Thus, the synchronization determination unit  63  writes, to the packet information table  64 , the piece of packet information that arrives earlier out of the pieces of packet information having the same sequence number. Upon arrival of the other piece of packet information later, the synchronization determination unit  63  reads the earlier piece of packet information from the packet information table  64  and outputs the read piece of packet information to a downstream QOS control unit  65 . 
     [c.2.1a] One Example of Synchronization Determination Processing According to Second Embodiment 
       FIG. 9  is a flowchart of a first example of synchronization determination processing executed by a synchronization determination unit  63 . This processing may be executed by the synchronization determination unit  63  in the first configuration ( FIG. 8 ) of the synchronization managing unit  351 A. 
     The following process that the synchronous judgment unit  63  executes is executed by processors, such as microprocessors or the like. 
     This processing is started each time the packet information is supplied to the synchronization determination unit  63 . 
     In operation S 11 , the synchronization determination unit  63  determines whether or not the communication device is in a duplicated operation state. The duplicated operation state refers to a state in which the same packet is supplied from the mappers  33  and  34 . In other words, the duplicated operation state refers to a state in which uninterrupted switching is possible after the communication device enters the synchronous state. 
     When the communication device is not in the duplicated operation state, i.e., the communication device is in a single-channel operation state, the process proceeds to operation S 12  in which the synchronization determination unit  63  writes the supplied packet information to the packet information table  64 , since packet information having the same sequence number as the sequence number of the supplied packet information does not exist in the packet information table  64 . 
     In operation S 13 , since the communication device is in the single-channel operation, the synchronization determination unit  63  reads the protection packet information from the packet information table  64  and outputs the read packet information to the downstream QOS control unit  65 . 
     Although the supplied packet information is temporarily written to the packet information table  64  in operation S 12  described above, operation S 12  may be eliminated so that the supplied packet information is directly output to the downstream QOS control unit  65 . 
     On the other hand, when the communication device is in the duplicated operation state, the process proceeds to operation S 14  in which the synchronization determination unit  63  uses the working/protection flag in the supplied packet information to determine whether the packet information is working packet information or protection packet information. 
     When the supplied packet information is working packet information, the process proceeds to operation S 15  in which the synchronization determination unit  63  determines whether or not protection packet information having the same sequence number as the sequence number of the supplied packet information exists in the packet information table  64 . 
     When protection packet information having the same sequence number exists in the packet information table  64 , the process proceeds to operation S 16  in which the synchronization determination unit  63  enters the synchronous state and issues a notification to the control unit  37 . The synchronization determination unit  63  reads the protection packet information from the packet information table  64  and outputs the read packet information to the downstream QOS control unit  65 . 
     When protection packet information having the same sequence number does not exist in the packet information table  64 , the process proceeds to operation S 17  in which the synchronization determination unit  63  writes the supplied working packet information to the packet information table  64 . 
     On the other hand, when it is determined in operation S 14  that the supplied packet information is protection packet information, the process proceeds to operation S 18  in which the synchronization determination unit  63  determines whether or not working packet information having the same sequence number as the sequence number of the supplied packet information exists in the packet information table  64 . 
     When working packet information having the same sequence number exists in the packet information table  64 , the process proceeds to operation S 19  in which the synchronization determination unit  63  enters the synchronous state and issues a notification to the control unit  37 . The synchronization determination unit  63  reads the working packet information from the packet information table  64  and outputs the read packet information to the downstream QOS control unit  65 . 
     When working packet information having the same sequence number does not exist in the packet information table  64 , the process proceeds to operation S 20  in which the synchronization determination unit  63  writes the supplied protection packet information to the packet information table  64 . 
     The first example ( FIG. 9 ) of the synchronization determination processing will now be described with reference to examples illustrated in  FIGS. 10A to 10C .  FIGS. 10A and 10B  illustrate examples of the pieces of packet information in the first embodiment of the synchronization determination processing and  FIG. 10C  illustrates an example of the packet information table. The pieces of packet information and the packet information table are not limited to those illustrated in  FIGS. 10A to 10C . 
       FIG. 10A  illustrates the working packet information in operation S 15  in the first example of the synchronization determination processing. 
       FIG. 10B  illustrates the protection packet information in operation S 18  in the first example of the synchronization determination processing. 
       FIG. 10C  illustrates the packet information table in operations S 15  and S 18  in the first example of the synchronization determination processing. 
     At time t 1 , working packet information with sequence number  101  and protection packet information with sequence number  100  are supplied to the synchronization determination unit  63 . The synchronization determination unit  63  stores the received pieces of packet information in the packet information table  64 . 
     At time t 2 , working packet information with sequence number  100  is supplied to the synchronization determination unit  63 . At this point, the synchronization determination unit  63  reads the protection packet information with sequence number  100  from the packet information table  64 . The synchronization determination unit  63  supplies the read information to the QOS control unit  65 . 
     At time t 3 , protection packet information with sequence number  101  is supplied to the synchronization determination unit  63 . At this point, the synchronization determination unit  63  reads the working packet information with sequence number  101  from the packet information table  64 . The synchronization determination unit  63  supplies the read packet information to the QOS control unit  65 . 
     Similarly, at time t 4 , working packet information with sequence number  102  and protection packet information with sequence number  103  are supplied to the synchronization determination unit  63 . The synchronization determination unit  63  stores the received pieces of packet information in the packet information table  64 . 
     At time t 5 , working packet information with sequence number  105  is supplied to the synchronization determination unit  63 . The synchronization determination unit  63  stores the received piece of packet information in the packet information table  64 . 
     At time t 6 , working packet information with sequence number  103  and protection packet information with sequence number  105  are supplied to the synchronization determination unit  63 . At this point, the synchronization determination unit  63  reads, from the packet information table  64 , the protection packet information with sequence number  103  and the working packet information with sequence number  105 . The synchronization determination unit  63  supplies the read pieces of packet information to the QOS control unit  65 . 
     At time t 7 , protection packet information with sequence number  102  is supplied to the synchronization determination unit  63 . At this point, the synchronization determination unit  63  reads the working packet information with sequence number  102  from the packet information table  64 . The synchronization determination unit  63  supplies the read packet information to the QOS control unit  65 . 
     [c.2.2] One Example of QOS Control Unit According to Second Embodiment 
     Referring back to  FIG. 8 , the packet information output by the synchronization determination unit  63  is supplied to the QOS control unit  65  and a writing control unit  66 . 
     The QOS control unit  65  sorts the pieces of packet information according to the VLAN values. The QOS control unit  65  has buffers for the respective classes. 
     Under the control of the writing control unit  66 , each piece of packet information supplied to the QOS control unit  65  is written to the buffer corresponding to the VLAN value and the class in the packet information. 
     A scheduling and reading control unit  67  reads one piece of packet information from the pieces of packet information, stored in the buffers in the QOS control unit  65 , in accordance with scheduling of giving priority to a higher-priority class. The scheduling and reading control unit  67  supplies the read packet information to a mapping-position calculating unit  68 . 
     The order of priority of the classes is class A, class B, and class C. The priority of class A is the highest. 
     The QOS control unit (the QOS control unit  65  in  FIG. 8 ) will now be described with reference to  FIGS. 11A and 11B . 
       FIG. 11A  is a diagram illustrating a state before the QOS control unit in the first configuration of the synchronization managing unit performs processing. 
       FIG. 11B  is a diagram illustrating a state after the QOS control unit in the first configuration of the synchronization managing unit performs processing. 
     As illustrated in  FIG. 11A , the QOS control unit  65  stores packet information with sequence number  103  in the buffer for VLAN “10” and class B. The QOS control unit  65  stores packet information with sequence number  105  in the buffer for VLAN “50” and class C. 
     In this case, as illustrated in  FIG. 11B , under the control of the scheduling and reading control unit  67 , the QOS control unit  65  first reads the packet information with sequence number  103  from the higher-priority buffer for VLAN “10” and class B and outputs the read packet information. 
     The QOS control unit (the QOS control unit  65  in  FIG. 8 ) will be further described with reference to  FIGS. 12A and 12B . 
     As illustrated in  FIG. 12A , the QOS control unit  65  stores packet information with sequence number  102  in the buffer for VLAN “50” and class A. The QOS control unit  65  stores packet information with sequence number  103  in the buffer for VLAN “50” and class B. 
     In this case, as illustrated in  FIG. 12B , under the control of the scheduling and reading control unit  67 , the QOS control unit  65  first reads the packet information with sequence number  102  from the higher-priority buffer for VLAN “50” and class A and outputs the read packet information. 
     [c.2.3] One Example Mapping-Position Calculating Unit According to Second Embodiment 
     Referring now to  FIG. 8 , for example, a frame pulse with a period of 125 μsec is supplied from the control unit  37  to the mapping-position calculating unit  68  via a terminal  69 . The frame pulse with a period of 125 μsec indicates a front end of an STS3c frame. 
     On the basis of the frame length of the pieces of packet information sequentially supplied from the QOS control unit  65 , the mapping-position calculating unit  68  calculates the frame number of the STS3c frame and a first position at which packets of the STS3c frame are to be mapped. That is, the mapping-position calculating unit  68  calculates a mapping position on the basis of the frame length. The mapping-position calculating unit  68  adds the frame number and the mapping position to the corresponding packet information supplied from the QOS control unit  65  and supplies the resulting packet information to the mappers  33  and  34  via a terminal  70 . 
       FIG. 13  illustrates the mapping-position calculation processing performed by the mapping-position calculating unit  68  in the first configuration of the synchronization managing unit  351 A. 
     In this case, the packets with sequence numbers  100 ,  101 ,  103 ,  102 , and  105  are encapsulated into GFP (generic framing procedure) frames in that order and are mapped. 
     The packet with sequence number  100  (with a frame length of 64 bytes) has a mapping position at the 13th byte in a frame with frame number  1 . The next packet with sequence number  101  (with a frame length of 1500 bytes) has a mapping position at the 85th byte in the frame with frame number  1 . 
     This mapping position is obtained by adding the overhead (8 bytes) of the GFP frame to the frame length (64 bytes) of the packet with sequence number  100 . 
     The packet with sequence number  101  is followed by a blank area (GFP idle). Subsequent to the blank area, the packet with sequence number  103  (with a frame length of 1500 bytes) has a mapping position at the 2173rd byte in the frame with frame number  1 . In addition, the packet with sequence number  102  (with a frame length of 1500 bytes) has a mapping position at the 4321st byte in a frame with frame number  2 . The packet with sequence number  105  (with a frame length of 64 bytes) has a mapping position at the 737th byte in a frame with frame number  3 . 
     The processing (in  FIG. 13 ) performed by the mapping-position calculating unit  68  in the first configuration of the synchronization determination unit  63  will now be described with reference to  FIGS. 14A to 14C . 
       FIG. 14A  illustrates working packet information supplied to the synchronization determination unit  63  in the first configuration of the synchronization managing unit  351 . 
       FIG. 14B  illustrates protection packet information supplied to the synchronization determination unit  63  in the first configuration of the synchronization managing unit  351 . 
       FIG. 14C  illustrates packet information output from the mapping-position calculating unit  68  in the first configuration of the synchronization managing unit  351 . 
       FIGS. 14A and 14B  are substantially the same as  FIGS. 10A and 10B . 
     As illustrated in  FIG. 14C , when the same packet information is supplied from the working and protection mappers  33  and  34  to the synchronization determination unit  63 , it outputs the packet information to the QOS control unit  65 . 
     The QOS control unit  65  sequentially retrieves the packet information in descending order of priority. 
     The mapping-position calculating unit  68  gives a frame number and a mapping position to each piece of the packet information and outputs the resulting packet information. Mapping position=1-13 (in  FIG. 14C ) of the packet with sequence number  100  represents the 13th byte in the frame with frame number  1 . 
     [c.2.1b] One Example of Synchronization Determination Processing According to Second Embodiment 
       FIG. 15  is a flowchart of a second example of the synchronization determination processing executed by the synchronization determination unit  63 . This processing is started each time the packet information is supplied to the synchronization determination unit  63  via the terminals  61  and  62 . 
     For instance, the following process that the synchronous judgment unit  63  executes is executed by processors, such as microprocessors or the like. Moreover, the packet information table  64  is composed of memories, such as flash memories or the like. 
     In operation S 31 , the synchronization determination unit  63  determines whether or not the communication device is in a duplicated operation state. 
     When the communication device is not in the duplicated operation state, i.e., the communication device is in a single-channel operation state, the process proceeds to operation S 32  in which the synchronization determination unit  63  writes the supplied packet information to the packet information table  64 , since packet information having the same sequence number as the sequence number of the supplied packet information does not exist in the packet information table  64 . 
     In operation S 33 , since the communication device is in the single-channel operation state, the synchronization determination unit  63  reads the packet information from the packet information table  64  and outputs the read packet information to the downstream QOS control unit  65 . 
     Although the supplied packet information is temporarily written to the packet information table  64  in operation S 32  described above, operation S 32  may be eliminated so that the supplied packet information is directly output to the downstream QOS control unit  65 . 
     On the other hand, when the communication device is in the duplicated operation state, the process proceeds to operation S 34  in which the synchronization determination unit  63  uses the working/protection flag in the supplied packet information to determine whether the packet information is working packet information or protection packet information. 
     When the supplied packet information is working packet information, the process proceeds to operation S 35  in which the synchronization determination unit  63  determines whether or not protection packet information having the same sequence number as the sequence number of the supplied packet information exists in the packet information table  64 . 
     When protection packet information having the same sequence number exists in the packet information table  64 , the process proceeds to operation S 36  in which the synchronization determination unit  63  enters the synchronous state and issues a notification to the control unit  37 . Thereafter, the synchronization determination unit  63  reads the protection packet information from the packet information table  64  and outputs the read packet information to the downstream QOS control unit  65 . 
     When protection packet information having the same sequence number does not exist in the packet information table  64 , the process proceeds to operation S 37  in which the synchronization determination unit  63  writes the supplied working packet information to the packet information table  64 . 
     In operation S 38 , the synchronization determination unit  63  starts a timer. In operation S 39 , the synchronization determination unit  63  determines whether or not the time is expired. When protection packet information having the same sequence number is supplied within a predetermined period of time after the completion of the processing in operation S 37 , the synchronization determination unit  63  maintains the synchronous state without time expiration and finishes the processing. 
     When protection packet information having the same sequence number is not supplied within a predetermined period of time, the process proceeds to operation S 40  in which the synchronization determining unit  63  enters an asynchronous state and issues an alarm to the control unit  37 . The synchronization determination unit  63  then reads the packet information from the packet information table  64  and outputs the read packet information to the downstream QOS control unit  65 . 
     On the other hand, when it is determined in operation S 34  that the supplied packet information is protection packet information, the process proceeds to operation S 41  in which the synchronization determination unit  63  determines whether or not working packet information having the same sequence number as the sequence number of the supplied packet information exists in the packet information table  64 . 
     When working packet information having the same sequence number exists in the packet information table  64 , the process proceeds to operation S 42  in which the synchronization determination unit  63  enters the synchronous state and issues a notification to the control unit  37 . The synchronization determination unit  63  reads the working packet information from the packet information table  64  and outputs the read packet information to the downstream QOS control unit  65 . 
     When working packet information having the same sequence number does not exist in the packet information table  64 , the process proceeds to operation S 43  in which the synchronization determination unit  63  writes the supplied protection packet information to the packet information table  64 . 
     In operation S 44 , the synchronization determination unit  63  starts a timer. In operation S 45 , the synchronization determination unit  63  determines whether or not the time is expired. When working packet information having the same sequence number is supplied within a predetermined period of time after the completion of the processing in operation S 43 , the synchronization determination unit  63  maintains the synchronous state without time expiration and finishes the processing. 
     When working packet information having the same sequence number is not supplied within the predetermined period of time, the time is expired and the process proceeds to operation S 46  in which the synchronization determining unit  63  enters the asynchronous state and issues an alarm to the control unit  37 . The synchronization determination unit  63  reads the packet information from the packet information table  64  and outputs the read packet information to the downstream QOS control unit  65 . 
     Upon receiving the alarm, the control unit  37  performs duplicated switching control so that the mapper that sent the packet information to the synchronization determination unit  63  earlier becomes a working mapper. In this case, when the mapper that sent the packet information earlier is already a working mapper, the control unit  37  does not perform switching. Conversely, when the mapper that sent the packet information earlier is a protection mapper, the control unit  37  performs switching. 
     [d] Modification of Packet Identifier 
     In the embodiment described above, the sequence number is used as the packet identifier. 
     The packet identifier, however, may be an FCS (frame check sequence). 
     The FCS is error detection information contained in each LAN packet. When the value of the header or payload in the LAN packet is different, the FCS value also varies. Thus, the FCS may also be used as the packet identifier. 
     The FCS is a value originally included in each LAN packet. Thus, when the FCS is used, the SN giving unit  322 A ( FIG. 4 ) in the communication device may be eliminated. In such a case, the QOS units  331  and  341  and the synchronization managing unit  351  perform processing using the FCS in place of the sequence number. 
     The packet identifier may also be a hash value. The hash value is obtained by performing hash calculation using a part of the received packet. 
     In this case, SN giving unit  322 A ( FIG. 4 ) of the communication device is eliminated and corresponding hash calculating units are provided immediately subsequent to the terminals  51  of the mappers  33  and  34 . Each hash calculating unit adds a hash value. 
     An IP packet lacks the FCS, unlike the LAN packet. Thus, when the hash values obtained by the hash calculating units provided in the mappers  33  and  34  are used to identify packets, the identifier giving unit  322  may be eliminated from the communication device. 
     [e] Modification of Packet Information 
     In the above-described embodiment, during mapping of LAN packets onto a SONET frame, the communication device encapsulates the packets into a GFP frame. In this case, an overhead of an 8-byte fixed length is added to each LAN packet. 
     However, an overhead for encapsulation using an HDLC (high level data link control procedures) method, such as X.86, is 9 bytes. The HDLC method employs a 0x7E pattern (0x indicates a hexadecimal representation) at the beginning and the end. Thus, in the HDLC method, 1 byte of 0x7E in a LAN packet to be encapsulated is replaced with 2 bytes of 0x7D and 0x5E. 
     In this case, the frame length after the encapsulation is not equal to a value obtained by simply adding 9 bytes to the LAN frame length. That is, the frame length is further increased according to the number of 0x7E patterns contained in the LAN packet. 
     When the mappers  33  and  34  perform encapsulation using the HDLC method to map packets onto SONET frames, each of the packet-information extracting units  53  for the QOS units  331  and  341  in the mappers  33  and  34  counts the number of appearances of the 0x7E pattern in the LAN packet. The packet-information extracting unit  53  then adds the counted number of appearances to the packet information and transfers the resulting information to the synchronization managing unit  351 . 
     The synchronization managing unit  351  calculates a mapping position, by taking the number of appearances of the 0x7E pattern into account.  FIG. 16  illustrates LAN packet encapsulation using the HDLC method and encapsulation using the GFP. In  FIG. 16 , the 0x7E pattern is represented as 7Eh. 
     All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the principles of the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiments of the present invention have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.