Abstract:
A system to convert first data signals into second data signals is provided. The first system may receive one of the first data signals associated with a data frame, determine first synchronization information corresponding to one of the second data signals associated with the data frame, and output the first synchronization information. The second system may receive the first synchronization information from the first system, determine second synchronization information corresponding to another one of the second data signals associated with a subsequent data frame based on the first synchronization information, and allocate a position in the subsequent data frame based on the second synchronization information.

Description:
RELATED APPLICATION 
   This application is a continuation of U.S. patent application Ser. No. 11/071,578 filed Mar. 4, 2005, which is a continuation of U.S. patent application Ser. No. 09/858,637, filed May 17, 2001, now U.S. Pat. No. 6,882,650 which are all incorporated herein by reference. 

   BACKGROUNDS OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a dual AAL1 device and a synchronization method used therewith and particularly relates to a synchronization method for the operation status between the act system and the standby system when an AAL1 (ATM Adaptation Layer Type 1) device to convert the STM (Synchronous Transfer Mode) data into ATM (Asynchronous Transfer Mode) cells has a dual configuration consisting of an act system (Current operation system) and a standby system (Reserved system). 
   2. Description of the Related Art 
   Conventionally, the STM data input to an AAL1 device to convert the STM data into ATM cells has the structure as shown in  FIG. 10 . As shown in  FIG. 10 , the STM data is given by time division multiplexing, where the channel is identified by the time position in the frame, and transferred for each time slot (TS; 1 TS=1 octet). 
   Further, an ATM cell converted by the above AAL1 device has the structure as shown in  FIGS. 11 to 14 .  FIG. 11  shows a format of the SAR (Segmentation and Re-assembly) sublayer. The ATM cell payload of 48 octets as a whole represents an SAR-PDU (SAR Protocol Data Unit). The first octet of the SAR-PDU is allocated to the SAR-PDU header and the remaining 47 octets are the SAR-PDU. payload. 
     FIG. 12  shows a format of the SAR-PDU header and  FIG. 13  shows a non-p format and  FIG. 14  shows a P format. In  FIG. 12 , the first 4 bits of the SAR-PDU header are called “SN” (Sequence Number) and the latter 4 bits are called “SNP” (Sequence Number Protection). 
   The bits of the SAR-PDU header are used as follows: CSI (Convergence Sublayer Indication) is the bit used by the CS (Convergence Sublayer) and the utilization method differs for each CS. 
   The SC (Sequence Count) is the number indicating the order of the cell (which ranges from 0 to 7) and is given by the CS. The CRC (Cyclic Redundancy Check=Error correction detection code) and the P (Parity) are inspection bits for error control of the SAR-PDU header. By combining the CRC and the P, any error in the SAR-PDU header can be corrected or detected. 
   The structure of the ATM cell as shown in  FIGS. 11 to 14  is designed so that it can support transfer of the STM signal frame structure to the receiver when required and the protocol for the structured data transfer is specified. In this case, as shown in  FIG. 14 , a pointer (which ranges from 0 to 93) is inserted to the cell to clearly indicate the first time slot of the STM data frame. 
   The pointer is inserted to the beginning of the SAR-PDU payload. The cells to which the pointer can be inserted are limited to those with even SC. The pointer is inserted once for one cycle of SC (0 to 7) and causes the CSI bit to be “1”. The cell structure to which the pointer is thus inserted is called the P format and, as shown in  FIG. 13 , the cell structure to which the pointer is not inserted is called the non-P format. In addition, the case without using the protocol for the structured data transfer is called the UDT (Unstructured Data Transfer) and the case using the structured data. transfer protocol is called the SDT (Structured Data Transfer). 
   The functional blocks in an ATM switch for the ATM network have dual redundant configurations to assure a high reliability with confounding among the functional blocks. A dual-structure functional block uses one set of equipment as the act system and the other set of equipment as the standby system. It may be sometimes necessary to switch between the equipment serving as the act system and the equipment serving as the standby system for maintenance requirements 
   If the timing to form ATM cells is not synchronized in the AAL1 device with the dual configuration, there will be phase differences between the payload of the cells output from the AAL1 device of the act system and that of the cells output from the AAL1 device of the standby system as shown in  FIG. 15 . This may result in lack or redundancy of data in system switching processing. 
   Japanese Patent Application Laid-open Print (Kokai) No. Heisei 9-55752 discloses a synchronization method for a dual AAL1 device to enable switching between the equipment serving as the act system and the equipment serving as the standby system without causing any lack or redundancy of information or other momentary shutdown. 
   The dual AAL1 device as described in the above patent application comprises inter-system signal lines connecting the act system AAL1 device and the standby system AAL1 device as well as a cell assembly buffer for one or more unit of channels provided for each system. 
   For synchronization, the cell assembly buffer number to execute synchronization is notified from the act system AAL1 device to the standby system AAL1 device by each unit of cell assembly buffer via the inter-system signal line at the timing when the storage data in the cell assembly buffer is the final octet of the SAR-PDU payload, i.e. the time slot input to the cell assembly buffer next is the first data of the SAR-PDU payload and when the SC of the SAR-PDU header becomes the predetermined value (“0”, for example). 
   Upon receipt of such notice, the standby system AAL1 device starts STM data taking into the cell assembly buffer starting from the applicable time slot of the next STM frame and thereby has the output ATM cell of the act system AAL1 device coincide with the output ATM cell of the standby system AAL1 device. By conducting the same processing for all channels one by one, the act system AAL1 device and the standby system AAL1 device are synchronized. 
   As described above, this method is characterized by that the cell forming is not started until the timing when a particular time slot of the STM frame can be allocated from the first octet of the SAR-PDU payload section having the predetermined SC value (“0”, for example) for the SAR-PDU header. By this method, the cell forming statuses of the act system and the standby system can be made identical without causing momentary shutdown of the STM signal and, without any loss or redundancy of data, switching between the act system and the standby system can be achieved. 
   The conventional dual AAL1 device as described above waits for the timing when the particular time slot of the STM frame can be allocated from the first octet of the SAR-PDU payload section having the predetermined SC (“0”, for example) for the SAR-PDU header to start cell forming and it takes time to synchronize the standby system AAL1 device. 
   In addition, the conventional dual AAL1 device waits for the timing when the particular time slot of the STM frame can be allocated from the first octet of the SAR-PDU payload section having the predetermined SC (“0”, for example) for the SAR-PDU header to start cell forming and the frame cycle where the standby system AAL1 device can be synchronized varies depending on the time slot count in one channel of the STM data. 
   Details of these drawbacks are described below with referring to  FIGS. 16 and 17 .  FIGS. 16 and 17  show the SAR-PDU payload extracted and laid out for every cycle (0 to 7) of SC for the SAR-PDU header. The section surrounded by a thick line shows the data for the same frame. 
   Suppose that a particular time slot of the STM frame is the first time slot of the frame and the first time slot of the frame is allocated from the first octet of the SAR-PDU payload having “0” for the SC of the SAR-PDU header. The frame cycle enabling allocation of the first time slot of the frame to the first octet of the SAR-PDU payload in the ATM cell having “0” as the SC is represented by “the least common multiple for the octet count of the SAR-PDU payload in one cycle of SC and the time slot count in one channel within one frame of the STM data divided by the time slot count in one channel within one frame of the STM data”. 
   In other words, different time slot counts in one channel within one frame of the STM data result in different frame cycles to enable allocating the first time slot of the frame to the first octet of the SAR-PDU payload in the ATM cell having “0” as the SC. 
     FIG. 16  shows the allocation to the ATM cell when the time slot count in one channel within one frame of the STM data is 470 (data for ten SAR-PDU payloads). Since the octet count in the SAR-PDU payload for one SC cycle is 376 octets and the time slot count in one frame of the STM data is 470, the least common multiple for them is 1880. When 1880 is divided by 470, the time slot count in one channel within one frame of the STM data, the quotient is 4. 
   Specifically, as shown in  FIG. 16 , the frame cycle in which the first time slot of a frame can be allocated to the first octet of the SAR-PDU payload of the ATM cell having “0” as SC is 4. 
     FIG. 17  shows the allocation to the ATM cell when the time slot count in one channel within one frame of the STM data is 423 (data for nine SAR-PDU payloads). Since the octet count in the SAR-PDU payload for one SC cycle is 376 octets and the time slot count in one frame of the STM data is 423, the least common multiple for them is 3384. When 3384 is divided by 423, the time slot count in one frame of the STM data, the quotient is 8. 
   Specifically, as shown in  FIG. 17 , the frame cycle in which the first time slot of a frame can be allocated to the first octet of the SAR-PDU payload of the ATM cell having “0” for SC is 8. 
   As described above, depending on the time slot count in one channel within one frame of the STM data, the frame cycle in which a particular time slot of a frame can be allocated from the first octet of the SAR-PDU payload section having the predetermined SC (“0”, for example) for the SAR-PDU header varies. Thus, depending on the time slot count in one channel within one frame of the STM data, the frame cycle in which a particular time slot of a frame can be allocated from the first octet of the SAR-PDU payload section having the predetermined SC (“0”, for example) for the SAR-PDU header may have several hundreds of frames. It takes time to synchronize the standby system AAL1 device. 
   SUMMARY OF THE INVENTION 
   An aspect of the present invention is directed to a dual AAL1 device and a synchronized method to be used therewith which can eliminate the above drawbacks, reduce the time needed by synchronization for system switching without any loss or redundancy of data and make the time required for synchronization constant regardless of the time slot count in one channel within one frame of STM data. 
   According to the first aspect of the invention, a dual AAL1 (ATM Adaptation Layer Type 1) device with a dual configuration consisting of an act system and a standby system to convert the STM (Synchronous Transfer Mode) data into ATM (Asynchronous Transfer Mode) cells in the act system and the standby system respectively comprising, for the act system and the standby system respectively, 
   means for extracting the SC (Sequence Count) information of the ATM cell where a particular time slot of the STM data is allocated and the allocation position information for the SAR-PDU (Segmentation And Re-assembly-Protocol Data Unit) payload when this means belong to the act system, and 
   means for calculating the SC of the ATM cell where a particular time slot of the STM data in the next frame is allocated and the allocation position for the SAR-PDU payload based on the SC information and the allocation position information extracted by the act system when this means belongs to the standby system. 
   In one construction, the act system extracts the SC information of the ATM cell to which a particular time slot of the STM data in the applicable frame is allocated and the allocation position information for the SAR-PDU payload upon receipt of a synchronization request and notifies such information to the standby system, and the standby system uses the SC information and the allocation position information from the act system to calculate the SC of the ATM cell to which a particular time slot of the STM data in the next frame is allocated and the allocation position for the SAR-PDU payload and then allocates the particular time slot of the STM data in the next frame to that position. 
   According to the second aspect of the invention, a dual AAL1 device with a dual configuration consisting of an act system and a standby system to convert the STM data into ATM cells in the act system and the standby system respectively comprising, for the act system and the standby system respectively, 
   means for extracting the SC information of the ATM cell to which the pointer is inserted and the pointer value information when this means belongs to the act system, and 
   means for calculating the SC of the ATM cell to which a particular time slot of the STM data in the next frame is allocated and the allocation position for the SAR-PDU payload based on the SC information and the pointer value information extracted by the act system when this means belongs to the standby system. 
   In one construction, the act system extracts the SC information of the ATM cell to which the pointer is inserted and the pointer value information upon receipt of a synchronization request and notifies the information to the standby system, and the standby system calculates the SC of the ATM cell to which a particular time slot of the STM data in the next frame is allocated and the allocation position for the SAR-PDU payload based on the SC information of the ATM cell to which the pointer is inserted and the pointer value information from the act system and allocates the particular time slot of the STM data in the next frame at that position. 
   According to the third aspect of the invention, a dual AAL1 device with a dual configuration consisting of an act system and a standby system to convert the STM data into ATM cells in the act system and the standby system respectively comprising, for the act system and the standby system respectively, 
   means for extracting the SC information of the ATM cell to which a particular time slot of the STM data is allocated and the allocation position information for the SAR-PDU payload, and 
   means for calculating the SC of the ATM cell to which a particular time slot of the STM data in the next frame is allocated and the allocation position for the SAR-PDU payload based on the extracted SC information and the allocation position information, 
   the SC of the ATM cell and the allocation position for the SAR-PDU payload are calculated and notified to the standby system when this means belongs to the act system. 
   In one construction, the act system extracts the SC information of the ATM cell to which a particular time slot of the STM data in the applicable frame is allocated and the allocation position information for the SAR-PDU payload upon receipt of a synchronization request, calculates the SC of the ATM cell to which a particular time slot of the STM data in the next frame is allocated and the allocation position for the SAR-PDU payload and notifies the calculation results to the standby system, and 
   the standby system allocates the particular time slot of the STM data in the next frame to the allocation position obtained from the act system. 
   According to another aspect of the invention, a dual AAL1 device with a dual configuration consisting of an act system and a standby system to convert the STM data into ATM cells in the act system and the standby system respectively comprising, for the act system and the standby system respectively, 
   means for extracting the SC information of the ATM cell to which the pointer is inserted and the pointer value information, and 
   means for calculating the SC of the ATM cell to which a particular time slot of the STM data in the next frame is allocated and the allocation position for the SAR-PDU payload based on the extracted SC information and pointer value information and, 
   in case of the act system, the SC of the ATM cell and the allocation position of the SAR-PDU payload being calculated and notified to the standby system. 
   In one construction, the act system extracts the SC information of the ATM cell to which the pointer is inserted and the pointer value information upon receipt of a synchronization request, calculates the SC of the ATM cell to which a particular time slot of the STM data in the next frame is allocated and the allocation position for the SAR-PDU payload and notifies the calculation results to the standby system and 
   According to another aspect of the invention, a synchronization method for a dual AAL1 device with a dual configuration consisting of an act system and a standby system to convert the STM data into ATM cells in the act system and the standby system respectively comprising: 
   a step for the act system to extract the SC of the ATM cell to which a particular time slot of the STM data in the applicable frame is allocated and the allocation position information for the SAR-PDU payload upon receipt of a synchronization request and to notify the information to the standby system, and 
   a step for the standby system to calculate the SC of the ATM cell in which a particular time slot of the STM data in the next frame is allocated and the allocation position for the SAR-PDU payload based on the SC information and the allocation position information from the act system and to allocate the particular time slot of the STM data in the next frame at that position. 
   According to another aspect of the invention, a synchronization method for a dual AAL1 device with a dual configuration consisting of an act system and a standby system to convert the STM data into ATM cells in the act system and the standby system respectively comprising: 
   a step for the act system 
   extracting the SC information of the ATM cell to which the pointer is inserted and the pointer value information upon receipt of a synchronization request and notifying the information to the standby system, and 
   a step for the standby system 
   calculating the SC of the ATM cell to which a particular time slot of the STM data in the next frame is allocated and the allocation position for the SAR-PDU payload based on the SC information of the ATM cell to which the pointer is inserted and the pointer value information from the act system and allocating the particular time slot of the STM data in the next frame to that position. 
   According to another aspect of the invention, a synchronization method for a dual AAL1 device with a dual configuration consisting of an act system and a standby system to convert the STM data into ATM cells in the act system and the standby system respectively comprising: 
   a step for the act system 
   extracting the SC information of the ATM cell to which a particular time slot of the STM data in the applicable frame is allocated and the allocation position information for the SAR-PDU payload upon receipt of a synchronization request, calculating the SC of the ATM cell to which a particular time slot of the STM data in the next frame is allocated and the allocation position for the SAR-PDU payload and to notifying the calculation result to the standby system, and 
   a step for the standby system 
   allocating the particular time slot of the STM data in the next frame to the allocation position from the act system. 
   According to another aspect of the invention, a synchronization method for a dual AAL1 device with a dual configuration consisting of an act system and a standby system to convert the STM data into ATM cells in the act system and the standby system respectively comprising: 
   a step for the act system 
   extracting the SC information of the ATM cell to which the pointer is inserted and the pointer value information upon receipt of a synchronization request, to calculate the SC of the ATM cell to which a particular time slot of the STM data in the next frame is allocated and the allocation position for the SAR-PDU payload and notifying the calculation result to the standby system, and 
   a step for the standby system 
   allocating the particular time slot of the STM data in the next frame to the allocation position from the act system. 
   According to a further aspect of the invention, a dual AAL1 device with a dual configuration consisting of an act system and a standby system to convert the STM data into ATM cells in the act system and the standby system respectively comprising for the act system and the standby system respectively, 
   a cell forming section which, when it belongs to the act system, extracts the SC information of the ATM cell where a particular time slot of the STM data is allocated and the allocation position information for the SAR-PDU payload and 
   an operation processing section which, when it belongs to the standby system, uses the SC information and the allocation position information extracted by the act system to calculate the SC of the ATM cell where a particular time slot of the STM data in the next frame is allocated and the allocation position for the SAR-PDU payload. 
   In one construction, the act system extracts the SC information of the ATM cell to which a particular time slot of the STM data in the applicable frame is allocated and the allocation position information for the SAR-PDU payload upon receipt of a synchronization request and notifies the information to the standby system, and the standby system uses the SC information and the allocation position information from the act system to calculate the SC of the ATM cell to which a particular time slot of the STM data in the next frame is allocated and the allocation position for the SAR-PDU payload and allocates the particular time slot of the STM data in the next frame to that position. 
   According to a further aspect of the invention, a dual AAL1 device with a dual configuration consisting of an act system and a standby system to convert the STM data into ATM cells in the act system and the standby system respectively comprising, for the act system and the standby system respectively, 
   a cell forming section which, when it belongs to the act system, extracts the SC information of the ATM cell where the pointer is inserted and the pointer value information, and 
   an operation processing section which, when it belongs to the standby system, uses the SC information and the pointer value information extracted by the act system to calculate the SC of the ATM cell where particular time slot of the STM data in the next frame is allocated and the allocation position for the SAR-PDU payload. 
   In one construction, the act system extracts the SC information of the ATM cell to which the pointer is inserted and the pointer value information upon receipt of a synchronization request and notifies the information to the standby system and 
   the standby system uses the SC information of the ATM cell to which the pointer is inserted and the pointer value information obtained from the act system to calculate the SC of the ATM cell to which a particular time slot of the STM data in the next frame is allocated and the allocation position for the SAR-PDU payload and allocates the particular time slot of the STM data in the next frame to that position. 
   According to a still further aspect of the invention, a dual AAL1 device with a dual configuration consisting of an act system and a standby system to convert the STM data into ATM cells in the act system and the standby system respectively comprising, 
   for the act system and the standby system respectively, 
   a cell forming section which extracts the SC information of the ATM cell where a particular time slot of the STM data is allocated and the allocation position information for the SAR-PDU payload and 
   an operation processing section which uses the extracted SC information and the allocation position information to calculate the SC of the ATM cell where a particular time slot of the STM data in the next frame is allocated and the allocation position for the SAR-PDU payload and, 
   in case of the act system, the SC of the ATM cell and the allocation position for the SAR-PDU payload being calculated and notified to the standby system. 
   In one construction, the act system extracts the SC information of the ATM cell to which a particular time slot of the STM data in the applicable frame is allocated and the allocation position information for the SAR-PDU payload upon receipt of a synchronization request and calculates the SC of the ATM cell to which a particular time slot of the STM data in the next frame is allocated and the allocation position for the SAR-PDU payload and notifies the calculation results to the standby system and 
   the standby system allocates the particular time slot of the STM data in the next frame to the allocation position obtained from the act system. 
   According to a still further aspect of the invention, a dual AAL1 device with a dual configuration consisting of an act system and a standby system to convert the STM data into ATM cells in the act system and the standby system respectively comprising, in the act system and the standby system respectively, 
   a cell forming section which extracts the SC information of the ATM cell where the pointer is inserted and the pointer value information 
   an operation processing section which uses the extracted SC information and the pointer value information to calculate the SC of the ATM cell where a particular time slot of the STM data in the next frame is allocated and the allocation position for the SAR-PDU payload and, 
   in case of the act system, the SC of the ATM cell and the allocation position for the SAR-PDU payload being calculated and notified to the standby system. 
   In one construction, the act system extracts the SC information of the ATM cell to which the pointer is inserted and the pointer value information upon receipt of a synchronization request and calculates the SC of the ATM cell to which a particular time slot of the STM data in the next frame is allocated and the allocation position for the SAR-PDU payload and notifies the calculation results to the standby system, and the standby system allocates the particular time slot of the STM data in the next frame to the allocation position obtained from the act system. 
   Specifically, a dual AAL1 device according to the present invention comprises means to extract the SC information of the ATM cell to which a particular time slot of the STM data is allocated and the allocation position information for the SAR-PDU payload (the SC information and the allocation position information for the SAR-PDU payload are hereinafter referred to as the synchronization information), means to send or receive the extracted synchronization information between both systems of AAL1 devices, and means to calculate the SC of the cell to which the applicable time slot of the next STM data is allocated and the allocation position for the SAR-PDU payload based on the received synchronization information. 
   In addition, for the dual AAL1 device according to the present invention, the SC information of the cell to which the pointer is inserted and the pointer value information can be also used as the synchronization information in case the protocol of the structured data transfer is applied. 
   In the dual AAL1 device according to the present invention, the SC information of the ATM cell to which a particular time slot of the STM data is allocated and the allocation position information for the SAR-PDU payload or, in case the protocol of the structured data transfer is applied, the SC information of the ATM cell to which the pointer is inserted and the pointer value information are used for calculation of the SC of the cell to which the applicable time slot of the next STM data is allocated and the allocation position for the SAR-PDU payload. 
   Thus, it is not necessary to wait for the timing when the particular time slot of an STM frame is allocated from the first octet of the SAR-PDU payload section having the SC with the predetermined SAR-PDU header (“0” for example). In addition, it becomes possible to make the time required for synchronization constant regardless of the time slot count in a channel within one frame of the STM data. 
   Other objects, features and advantages of the present invention will become clear from the detailed description given herebelow. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be understood more fully from the detailed description given herebelow and from the accompanying drawings of the preferred embodiment of the invention, which, however, should not be taken to be limitative to the invention, but are for explanation and understanding only. 
     In the drawings: 
       FIG. 1  is a block diagram showing the configuration of a dual AAL1 device according to an embodiment of the present invention; 
       FIG. 2  is a diagram to show the definition of the synchronization information according to an embodiment of the present invention; 
       FIG. 3  is a diagram showing the configuration of the STM data to illustrate the octet count in one channel within one frame according to an embodiment of the present invention; 
       FIG. 4  is a diagram to show the STM data octet count in one cycle during the unstructured data transfer according to an embodiment of the present invention; 
       FIG. 5  is a diagram to show the STM data octet count in one cycle during the structured data transfer according to an embodiment of the present invention; 
       FIG. 6  is a diagram showing the synchronization processing for a dual AAL1 device according to an embodiment of the present invention; 
       FIG. 7  is a diagram showing the structure where the STM data allocated to the ATM cells are extracted and connected for every cycle of ATM cells according to an embodiment of the present invention; 
       FIG. 8  is a block diagram showing the configuration of a dual AAL1 device according to another embodiment of the present invention; 
       FIG. 9  is a diagram showing the synchronization processing for a dual AAL1 device according to another embodiment of the present invention; 
       FIG. 10  is a diagram showing the conventional STM data structure; 
       FIG. 11  is a diagram showing the structure of a conventional SAR sublayer; 
       FIG. 12  is a diagram showing the structure of a conventional SAR-PDU header; 
       FIG. 13  is a diagram showing the structure of a conventional non-P format; 
       FIG. 14  is a diagram showing the structure of a conventional P format; 
       FIG. 15  is a diagram showing drawbacks of a conventional dual AAL1 device without synchronization; 
       FIG. 16  is a diagram showing the conventional allocation to the ATM cells when the time slot count in one frame of STM data is 470; and 
       FIG. 17  is a diagram showing the conventional allocation to the ATM cells when the time slot count in one frame of STM data is 423. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   The preferred embodiment of the present invention will be discussed hereinafter in detail with reference to the accompanying drawings. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be obvious, however, to those skilled in the art that the present invention may be practiced without these specific details. In other instance, well-known structures are not shown in detail in order to unnecessary obscure the present invention. 
     FIG. 1  is a block diagram showing the configuration of a dual AAL1 device according to an embodiment of the present invention. In  FIG. 1 , a dual AAL1 device according to an embodiment of the present invention has a dual configuration comprising an act system AAL1 device  1  and a standby system AAL1 device  2 . Each of the act system AAL1 device  1  and the standby system AAL1 device  2  respectively has the same circuit configuration with an AAL1 processing function to convert the STM format signal into the ATM format signal. 
   Each of the act system AAL1 device  1  and the standby system AAL1 device  2  is connected to a CPU  3  and comprises an upper device interface section  11  or  21 , a synchronization information send/receive section  12  or  22 , an operation processing section  13  or  23  and a cell forming section  14  or  24  as well as an STM signal input terminal  111  or  112  and an ATM signal output terminal  113  or  114 . 
   The upper device interface section  11 / 21  transfers the instruction from the CPU  3  to the functional blocks in the same device. The cell forming section  14 / 24  converts the STM data into ATM cells. The synchronization information send/receive section  12 / 22  extracts the synchronization information from the cell forming section  14 / 24  according to the instruction from the upper device interface section  11 / 21  and sends or receives the synchronization information to or from the other system. 
   The operation processing section  13 / 23  calculates, based on the synchronization information sent from the synchronization information send/receive section  12 / 22 , the SC of the cell to which the applicable time slot of the next STM data is allocated and the allocation position for the SAR-PDU payload. The STM signal input terminal  111 / 112  receives input of the STM signal (Data signal, frame signal and clock signal) and the ATM signal output terminal  113 / 114  outputs the ATM signal (Data signal, frame signal and clock signal). 
     FIG. 2  is a diagram showing the definition of the synchronization information according to an embodiment of the present invention. In  FIG. 2 , the synchronization information comprises SC information  601  of the ATM cell to which a particular time slot of the STM data is allocated and allocation position information  602  for the SAR-PDU payload  604 . In case the protocol for the structured data transfer is used, the SC information  601  of the cell to which the pointer is inserted and its pointer value information  603  can be treated as the synchronization information. 
   Next, the definition of the allocation position information  602  for the SAR-PDU payload  604  is described. Suppose that “0” is allocated to the first octet position of the SAR-PDU payload  604 , “1” to the subsequent octet and so on so that a octet position number  605  is allocated to a octet position in the SAR-PDU payload  604 . The octet position number  605  of the position to which a particular time slot of the STM data is allocated is defined as the allocation position information  602  for the SAR-PDU payload  604 . 
     FIG. 3  is a diagram showing the STM data structure to illustrate the octet count in one channel within one frame according to an embodiment of the present invention.  FIG. 3  shows the elements of the STM data required for the operation processing section  13 / 23 . 
   According to an embodiment of the present invention, the time slot count (hereinafter referred to as TBC)  701  in one channel within one frame is required for the operation processing section  13 / 23 . When all time slots in one frame are treated as one channel, the TBC  701  becomes equal to the total number of time slots. 
     FIGS. 4 and 5  are diagrams to illustrate the STM data octet count within one cycle of ATM cells according to an embodiment of the present invention.  FIG. 4  shows the STM data octet count within one cycle for the unstructured data transfer (UDT) and  FIG. 5  shows the STM data octet count within one cycle for the structured data transfer (SDT).  FIGS. 4 and 5  show the elements of the ATM cells required for the operation processing section  13 / 23 . The AAL1 has a structure in which eight ATM cells form one cycle. 
   In the unstructured data transfer, the SAR-PDU payload is fixed to 47 octets for all ATM cells. The STM data octet count in one cycle of ATM cells is 376 (=8×47) octets. 
   In the structured data transfer, an ATM cell containing a pointer is always inserted to one cycle of ATM cells. The STM data octet count in one cycle of ATM cells is always 375 (=7×47+46) octets. 
   According to an embodiment of the present invention, the STM data octet count (hereinafter referred to as the SBC)  801  for one ATM cycle is required for the operation processing section  13 / 23 . Therefore, the SBC  801  becomes  376  for the unstructured data transfer and the SBC  801  becomes  375  for the structured data transfer. 
     FIG. 6  is a diagram showing the synchronization processing for a dual AAL1 device according to an embodiment of the present invention and  FIG. 7  is a diagram showing the structure where the STM data allocated to ATM cells are extracted and connected for every cycle of ATM cells according to an embodiment of the present invention. With referring to  FIGS. 1 to 7 , a synchronization method used for cell forming processing according to an embodiment of the present invention will be described. 
   Described below is the synchronization method for cell forming processing when all time slots of the STM data are treated as one channel and the SC information  601  of the cell to which the first time slot of the frame is allocated and the allocation position information  602  for the SAR-PDU payload are treated as the synchronization information. 
   When a synchronization request  901  is input from the CPU  3  to the upper interface section  11  of the act system AAL1 device  1 , the upper interface section  11  gives a synchronization instruction  902  to the synchronization information send/receive section  12 . Upon receipt of the synchronization instruction  902 , the synchronization information send/receive section  12  gives a synchronization information extraction instruction  903  to the cell forming section  14 . Upon receipt of the synchronization information extraction instruction  903 , the cell forming section  14  waits for input of the next frame pulse (“AF2”  907  in  FIG. 6 ). 
   Upon receipt of the next frame pulse (“AF2”  907  in  FIG. 6 ), the cell forming section  14  extracts the SC information  601  of the cell to which the first octet “TS0”  908  of the “AF2” frame is allocated and the allocation position information  602  for the SAR-PDU payload (Synchronization information extraction  904 ). 
   The synchronization information is transferred to the synchronization information send/receive section  22  of the standby system AAL1 device  2  via the synchronization information send/receive section  12 . Upon receipt of the synchronization information, the synchronization information send/receive section  22  of the standby system AAL1 device  2  transfers the synchronization information to the operation processing section  23 . 
   The operation method at the operation processing section  23  (Operation processing  905 ) will be described with referring to  FIG. 7 . In  FIG. 7 , the section surrounded by a thick line represents the AF2 frame. The shaded part shows a position  1001  to which the first time slot “TS0”  908  of the AF2 frame is allocated and the horizontal line section shows a position  1002  to which the first time slot “TS0”  910  of the SF3 frame is allocated and RBC  1003  shows the time slot count in the AF2 frame within the cycle (n+3). Based on the synchronization information and the elements TBC  701  and SBC  801 , the RBC  1003  can be expressed by the formula below:
 
 RBC=MOD ([ TBC −{(7 −SC )×47+( n+ 1)}] /SBC )
 
where MOD([TBC−{(7−SC)×47+(n+1)}]/SBC)
 
represents the “remainder when [TBC−{(7−SC)×47+(n+1)}] is divided by SBC”, SC represents the SC information  601  of the synchronization information and n represents the allocation position information  602  for the SAR-PDU payload.
 
   Based on this RBC  1003 , the SC of the ATM cell to which the first time slot “TS0”  910  of the SF3 frame is allocated and the allocation position for the SAR-PDU payload in the unstructured data transfer can be expressed as follows:
 
 SC=MOD (QUTIENT( RBC/ 47)/8); and
 
Allocation position for the SAR-PDU payload= MOD ( RBC/ 47)
 
   where QUTIENT (RBC/47) show the “integer section of the quotient when RBC is divided by 47”. 
   In case of the structured data transfer, the SC of the ATM cell to which the first time slot “TS0”  910  of the SF3 frame is allocated and the allocation position for the SAR-PDU payload can be expressed as follows:
 
 SC=MOD (QUTIENT(( RBC/ 47)+1/8); and
 
Allocation position for the SAR-PDU payload= MOD ( RBC/ 47)+1
 
   Then, the SC of the ATM cell to which the first time slot “TS0”  910  of the SF3 frame is allocated and the allocation position for the SAR-PDU payload as the operation processing results of the operation processing section  23  are transferred to the cell forming section  24 . Upon receipt of the SF3 frame pulse  909 , the cell device section  24  allocates the STM data from the first time slot “TS0”  910  of the “SF3” frame starting from the allocation position for the SAR-PDU payload of the ATM cell having the SC obtained as the result of the above operation processing (Start of cell forming  906 ) and thereby synchronizes the act system AAL1 device and the standby system AAL1 device  2 . 
     FIG. 8  is a block diagram showing the configuration of a dual AAL1 device according to another embodiment of the present invention. In  FIG. 8 , the dual AAL1 device according to another embodiment of the present invention has the same configuration as the dual AAL1 device according to the first embodiment of the present invention shown in  FIG. 1  except that the synchronization instruction  902  is input to the operation processing section  13  and the synchronization information extraction instruction  903  is output from the operation processing section  13  and input to the cell forming device  14 . The same configuration elements are provided with the same reference numbers. 
     FIG. 9  shows the synchronization processing of the dual AAL1 device according to another embodiment of the present invention. With referring to  FIGS. 8 and 9 , a synchronization method in the cell forming according to another embodiment of the present invention will be described. 
   Described below is the synchronization method in the cell forming when all time slots of the STM data are processed as one channel and the SC information  601  of the cell to which the first time slot of the frame is allocated and the allocation position information  602  for the SAR-PDU payload are used as the synchronization information. 
   When a synchronization request  901  from the CPU  3  is input to the upper device interface section  11  of the act system AAL1 device  1 , the upper device interface section  11  gives a synchronization instruction  902  to the operation processing section  13 . Upon receipt of the synchronization instruction  902 , the operation processing section  13  gives a synchronization information extraction instruction  903  to the cell forming section  14 . Upon receipt of the synchronization information extraction instruction  903 , the dell forming section  14  waits for input of the next frame pulse (“AF2”  907  in  FIG. 9 ). 
   Upon receipt of the next frame pulse (“AF2”  907  in  FIG. 9 ), the cell forming section  14  extracts the SC information  601  of the cell to which the first octet “TS0”  908  of the “AF2” frame is allocated and the allocation position information  602  for the SAR-PDU payload (Synchronization information extraction  904 ). 
   The synchronization information is transferred to the operation processing section  13  and the operation processing section  13  operates the SC of the ATM cell to which the first time slot “TS0”  910  of the “SF3” frame is allocated and the allocation position for the SAR-PDU payload. The operation results obtained by the operation processing section  13  are the same as those in the first embodiment of the present invention and explanation about them is omitted here. 
   The SC of the ATM cell to which the first time slot “TS0”  910  of the “SF3” frame is allocated and the allocation position for the SAR-PDU payload operated by the operation processing section  13  of the act system AAL1 device  1  are transferred to the synchronization information send/receive section  22  of the standby system AAL1 device  2  via the synchronization information send/receive section  12 . 
   The synchronization information send/receive section  22  of the standby system AAL1 device  2  transfers the received calculation results to the cell forming section  24 . Upon receipt of the “SF3” frame pulse  909 , the cell forming section  24  allocates the STM data from the first time slot “TS0”  910  of the “SF3” frame starting from the allocation position for the SAR-PDU payload of the ATM cell having the SC obtained as the result of above operation processing (Start of cell forming  906 ) and thereby synchronizes the act system AAL1 device  1  and the standby system AAL1 device  2 . 
   When the standby system AAL1 device  2  starts the processing to convert the STM data into ATM cells upon receipt of the synchronization request by having the operation processing section  23  calculate the allocation position of a particular time slot of the next frame based on the allocation position information of a particular time slot of the current frame, it can synchronize the two frames after the receipt of the synchronization request. Thus, the time required for synchronization between the act system AAL1 device  1  and the standby system AAL1 device  2  can be reduced than that in the prior art. Further, regardless of the time slot count in one channel within one frame of the STM data, the time required for synchronization can be made constant. 
   Though the description above is about the case where the SC information of the ATM cell to which a particular time slot of the STM data is allocated and the allocation position information for the SAR-PDU payload are used as the synchronization information, the synchronization information may be the SC value information of the ATM cell to which the pointer is allocated and the pointer value information in case of the structured data transfer. 
   Further, the above description is about the case where all time slots in a frame are processed as one channel, but the present invention can be also applied to a case where one frame has several channels with time slots [64 kbps data or multiplied (64 k×n) data] and each channel is formed into ATM cells. 
   As described above, according to the present invention, a dual AAL1 device has a dual structure comprising an act system and a standby system and converts the STM data into ATM cells in each of the act system and the standby system. Upon receipt of a synchronization request, the act system extracts the SC information of the ATM cell to which a particular time slot of the STM data in the applicable frame is allocated and the allocation position information for the SAR-PDU payload and notifies the standby system of such information. The standby system calculates the SC of the ATM cell to which a particular time slot of the STM data in the next frame is allocated and the allocation position for the SAR-PDU payload based on the SC information and the allocation position information obtained from the act system so that the particular time slot of the STM data in the next frame is allocated to that position. Thus, the time required for synchronization in system switching can be reduced without any loss or redundancy of data and the time required for synchronization can be made constant regardless of the time slot count in the one channel within one STM data frame.