Abstract:
A data transmission apparatus using a virtual concatenated path in an SDH transmission system is capable of engaging one or more extra paths by an automated sequence execution to meet a traffic requirement that is higher than usual but up to a maximum speed of a user who is usually assigned virtual concatenated paths in a number smaller than capable of transmitting the maximum traffic.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention generally relates to a data transmission apparatus and a path control method thereof, using a virtual concatenated (VC) path in an SDH (Synchronous Digital Hierarchy) transmission system, and especially relates to the data transmission apparatus and the path control method thereof, which effectively control the number of virtual concatenated paths to be used in response to an increase and a decrease in an amount of IP data in the data transmission system using the virtual concatenated path method that is effective when the IP data is transmitted by an SDH network. 
   2. Description of the Related Art 
   A high order path capacity in the SDH transmission system has been growing with developments of transmission technology, and increased capacity of a transmission signal (payload). Specifically, from VC-3 (Virtual Container, 48,384 kbps), and VC-4 (149,760 kbps) to VC-4-4c (599,040 kbps), VC-4-16c (2,396,160 kbps), VC-4-64c (9,584,640 kbps), VC-4-256c (38,338,560 kbps), etc. have been specified (refer to  FIG. 1 ). Further, a virtual concatenated path transmission system has been developed, wherein a large-volume path signal is divided into a plurality of VC-3/VC-4 signals, transmitted, and restored to the original large-volume path signal at a receiving end (refer to related specifications, such as ITU-T Recommendation G.707 (00/10) and ITU-T Recommendation G.783 (01/1)). 
   According to this virtual concatenated path transmission system, large-volume data can be divided into 1 through 256 pieces of VC-3/4 data, enabling a transmission of the large-volume data, even if there is no large-capacity transmission-line of such as 10 G bps and 40 G bps available (refer to  FIG. 2 ). 
   A virtual concatenated path transmission system divides a path container (C-3/4-Xc) into X pieces of VC-3 or VC-4 data, and transmits the pieces via a transmission line. Here, the value of X is set up beforehand by an operator according to data volume to transmit, and VC-3/VC-4path identification numbers used for the virtual connection must also be set up at both a transmitting apparatus and a receiving apparatus. 
   When the transmitting apparatus transmits a virtual concatenated path, a multi-framing number that is used to absorb a signal transfer-lag, and a sequence number that identifies a VC-3/VC-4 path are provided to the receiving apparatus, using an H4 byte in a path-overhead (POH) of each VC-3/VC-4 (Refer to  FIG. 3 ). The receiving apparatus restores the path container (C-3/4-Xc) by arranging a plurality of received VC-3/VC-4 paths according to the sequence number, and by absorbing the signal transfer-lag of the paths by matching the multi-framing number of the H4 byte. 
   Here, an example is described. In the case that a signal to be transmitted by an SDH transmission line is a signal of Ethernet (registered trademark), and the like, the Ethernet (registered trademark) signal, unlike a conventional voice signal, tends to have an odd data volume, causing an inefficiency if a path container of SDH is to accommodate the Ethernet (registered trademark) signal. Rather than accommodating the Ethernet (registered trademark) signal in a conventional path container of an SDH transmission line, it is more advantageous in respect of efficiency to use the virtual concatenated (VC) path method that allows setting up a data transmission capacity per VC-3or VC-4. However, even if the virtual concatenated path method is used in an attempt to improve the efficiency, an operator is required to set up an X value on both transmitting side and receiving side, the X value being a number by which data to be transmitted is divided, and determined uniquely by a data volume to be transmitted. 
   Here, a feature of an Ethernet (registered trademark) signal is in that an amount of transmission traffic increases and decreases in time. In view of the feature, two methods of setting the X value are conceivable, 
   namely: 
   (1) setting the X at a value smaller than a maximum signal capacity of the Ethernet (registered trademark) signal, and 
   (2) setting the X at the maximum signal capacity of the Ethernet (registered trademark) signal. 
   The method (1) is an advantageous setting method in view of an efficient practical use of an SDH transmission line when usual traffic amount is low. Conversely, if the maximum signal capacity is required, some data will be discarded at an inputting stage to the SDH transmission line. On the other hand, if the setting method (2) is used, although the maximum signal capacity can always be offered, the maximum capacity may not always be used, resulting in useless capacity assigned, and an inefficiency of SDH transmission line usage. 
   For example, when actual traffic is 80 M bps for an Ethernet (registered trademark) signal capacity of 100 M bps (100 Base), since only a VC-3 (about 48 M bps) and a VC-4 (about 150 M bps are available in the conventional SDH transmission, a VC-4 is provided, resulting in applying the method of (2). The usual traffic is only 80 M bps in this case, resulting in a remaining capacity of about 70 M bps that is assigned uselessly. If the method (1) is applied, two VC-3 paths, also expressed as VC-3-2v and VC-3×2, will be assigned, which provide a suitable capacity for the actual traffic. However, when traffic demand reaches the maximum capacity of 100 M bps, since the capacity of VC-3-2v is 48×2=about 96 M bps, there is an insufficiency of 4 M bps, leaving part of data discarded. 
   SUMMARY OF THE INVENTION 
   Accordingly, it is a general object of the present invention to provide a path control method that adjusts a capacity of an SDH transmission line on a real-time basis, according to changes in traffic amount inputted into a transmitting apparatus. 
   It is another and more specific object of the present invention to provide the path control method that makes it possible to continue data transmission, while adjusting the capacity of the transmission line suitably by performing-a sequence of capacity adjustments between the transmitting apparatus and a receiving apparatus, without producing a loss of data, and without an operator intervening. 
   In order to achieve the above objects, the present invention provides an extra path that is additionally assigned to a virtual concatenated path, and detached from the virtual concatenated path, in a synchronous digital transmission system, wherein a signal with variable traffic is divided into two or more virtual concatenated paths. 
   Other objects and further features of the present invention will be apparent from the following detailed description when read in conjunction with the accompanying drawings. 
   By configuring in this manner, there is no need for preparing virtual concatenated paths beforehand that meet the maximum traffic capacity for a transmitting apparatus and a receiving apparatus concerned. Rather, an increase in the traffic amount in burst can be accommodated by the extra path. Accordingly, while the traffic amount is less than the maximum capacity, only a capacity sufficient to carry actual traffic is provided, without using the extra path. The extra path can be diverted to other use, such as data transmission of other apparatuses, contributing to improve usage efficiency of the transmission line (transmission path resources). When the maximum traffic capacity is required, the extra path can be used, avoiding an occurrence of data discarding. 
   In order to achieve this, change of the number of virtual concatenated paths actually used is automatically controlled by performing a predetermined sequence by the transmission apparatus and the receiving apparatus. By configuring in this manner, quick change of a transmission capacity in response to traffic change becomes possible, since a necessary process is performed in a lower layer than when an operator has to change settings. 
   Further, it is desirable that an extra path can be assigned selectively to an uplink and a downlink, corresponding to live data amounts. In this manner, an efficient use of the extra path can be provided, even if traffic of the uplink and traffic of the downlink are not balanced. 
   Further, it is desirable that the extra path is available to a plurality of groups of a plurality of virtual concatenated paths such that the extra path is efficiently used when peak traffic takes place at one of the groups of the virtual concatenated paths. 
   A priority may be assigned to each group of the virtual concatenated paths, according to which the extra path is assigned. The priority may be defined on a service type basis. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a figure showing an example of a standard format of a virtual concatenated path signal (high order path signal) which can be used in a virtual concatenated path transmission system; 
       FIG. 2  is a figure that explains an outline of a signal transmission by the virtual concatenated path transmission system; 
       FIG. 3  is a figure that explains an example of use of an H4 byte (for multi-framing identification) in the virtual concatenated path transmission system; 
       FIG. 4  is a block diagram showing a configuration of a transmitting apparatus of a data transmission system of a first embodiment of the present invention; 
       FIG. 5  is a block diagram showing a configuration of a receiving apparatus of the data transmission system of the first embodiment of the present invention; 
       FIG. 6  is a figure explaining a status of the transmitting apparatus shown in  FIG. 4  when traffic increases; 
       FIG. 7  is a figure explaining a status of the receiving apparatus shown in  FIG. 5  when traffic increases; 
       FIG. 8  is a figure explaining an example of a sequence that changes the number of virtual concatenated paths of the path transmission system of the first embodiment of the present invention; 
       FIG. 9  is a flowchart that shows an example of sequence operation on a transmitting side, which changes the number of virtual concatenated paths when traffic increases in the path transmission system of the first embodiment of the present invention; 
       FIG. 10  is a flowchart that shows an example of the sequence operation on the transmitting side, which changes the number of virtual concatenated paths when traffic decreases in the path transmission system of the first embodiment of the present invention; 
       FIG. 11  is a flowchart that shows an example of receiving-side operation of the sequence that changes the number of virtual concatenated paths when traffic increases in the path transmission system of the first embodiment of the present invention; 
       FIG. 12  is a flowchart that shows an example of receiving-side operation of the sequence that changes the number of virtual concatenated paths when traffic decreases in the path transmission system of the first embodiment of the present invention; 
       FIG. 13  is a figure for explaining operation of an information area of transmitted/received signals when a sequence message is exchanged between a transmitting apparatus and a receiving apparatus, the sequence being executed according to  FIG. 8 ; 
       FIG. 14  is a figure for explaining the concept of the second embodiment of the present invention; 
       FIG. 15  is a block diagram showing an example of a transmitting apparatus of the data transmission method of the second embodiment of the present invention; 
       FIG. 16  is a block diagram showing an example of a receiving apparatus of the data transmission method of the second embodiment of the present invention; 
       FIG. 17  is a figure for explaining an example of a method of an extra path setup in the path control method of the present invention; 
       FIG. 18  is a block diagram for explaining a configuration of a transmitting apparatus of a variation applicable to each of the first and the second embodiments of the present invention; and 
       FIG. 19  is a block diagram for explaining a configuration of a receiving apparatus of a variation applicable to each of the first embodiment and the second embodiment of the present invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   In the following, embodiments of the present invention will be described with reference to the accompanying drawings. 
   A first embodiment of the present invention is explained, referring to appended drawings. 
   This embodiment is an example of transmitting an Ethernet (registered trademark) signal, which is inputted to a transmitting apparatus, by VC-4-Xc (X=16+α).  FIG. 4  shows a configuration of the transmitting apparatus in usual operation, and  FIG. 5  shows a configuration of the receiving apparatus in usual operation. 
   A transmitting apparatus  10  shown in  FIG. 4  includes a physical terminal unit  11  to which an Ethernet (registered trademark) signal physically terminates, a packet terminal unit  12  that converts the physically terminated signal into packet data, a data buffer  13  that stores the obtained packet data temporarily, a concatenated path generating unit  14  that bundles the packet data by predetermined frames according to the virtual concatenated path method and converts into a path container format of C-4-16c , a virtual concatenated path generating unit  15  that divides the path container into a plurality (16, for example) of virtual concatenated paths (VC) and adds path overheads such as a J1 byte and an H4 byte, and a transmitting unit  16  that adds an SOH (section overhead) to the data divided into the 16 virtual concatenated paths and transmits to an SDH transmission line (optical-fiber transmission line) The transmitting apparatus  10  further includes a control unit  17  that performs the path control method of the present invention, while carrying out an overall control of the entire transmission apparatus  10 . 
   A receiving apparatus  20  shown in  FIG. 5  includes a receiving unit  21  that receives the data in the form of the plurality of virtual concatenated paths transmitted through the SDH transmission line, and terminates the data and the path overheads,  18  multi-frame phase alignment buffers marked as  22 - 1 ,  22 - 2 , and so on up to  22 - 18  that are used when aligning multi-frame numbers by aligning frame timing of each path of the received data by control of a control unit  26 , based on multi-frame information of the path overhead H4 acquired by the receiving unit  21 , a virtual concatenated path restoration unit  23  that collects data in the virtual concatenated path format input with timing (phase) aligned, and recovers the original path container form, an SDH/packet conversion unit  24  that converts this data to the original packet data, and a physical terminal unit  25  that physically terminates the data and acquires the original Ethernet (registered trademark) signal. The receiving apparatus  20  further includes a control unit  26  that performs the path control method of the present invention, while carrying out an overall control of the entire receiver apparatus  20 . 
   Here, both the transmitting apparatus  10  and the receiving apparatus  20  are installed in a communication terminal such that a bi-directional communication (exchange of a sequence message etc.) with a partner terminal is enabled. 
   In this embodiment, the number of the VC-4 paths is set at 16, as a default value, for both the transmitting apparatus  10  and the receiving apparatus  20 , as shown in  FIG. 4  and  FIG. 5 . Additionally, 2 VC-4paths are prepared as extra paths (VC-4# 17 , and VC-4# 18  in  FIG. 4  and  FIG. 5 ) so that increased data traffic may be accommodated. As a default status, an AIS or UNEQP signal is inserted to the extra paths by the control unit  17 . Further, the AIS signal and the like are inserted in the buffers  22 - 17  and  22 - 18  that correspond to the extra paths by the control unit  26 . 
   Usually, i.e., in the default status, an Ethernet (registered trademark) signal is inserted into a path container of C-4-16c, and divided into virtual concatenated paths, and transmitted via 16 paths VC-4# 1  through VC-4# 16  by the transmitting apparatus  10 . The control unit  17  monitors a traffic amount (the amount of live data) of the Ethernet (registered trademark) signal in the packet terminal unit  12  of the transmitting apparatus  10 . Specifically, the traffic amount is based on the number of packets generated, and in the case of a variable-length packet, information relative to packet length is additionally monitored. On the side of the receiving apparatus  20 , the C-4-16c is restored by multiplexing the virtual concatenated paths, after aligning phases of the 16 paths, treating the 16 paths of VC-4# 1  through VC-4# 16  in the buffers  22 - 1  through  22 - 16 , respectively, as a virtual concatenated path in the default status, as shown in  FIG. 5   
   Next, the case where the traffic amount (amount of live data) of the Ethernet (registered trademark) signal increases is explained. As mentioned above, the control unit  17  keeps monitoring the traffic amount of the packet terminal unit  12  of the transmitting apparatus  10 . When the control unit  17  determines that “traffic increases”, the concatenated path generating unit  14  and the virtual concatenated path generating unit  15  are directed such that an extra path, e.g., VC-4# 17  is engaged, leaving the VC-4# 18  as an extra path still available.  FIG. 6  shows the state of the transmitting apparatus  10  in this case. 
   On the side of the receiving apparatus  20 , the added path VC-4# 17  is joined to the 16 default paths. Then, all the 17 paths are processed such that the concatenated path is recovered, and then, the Ethernet (registered trademark) signal is recovered, without having to discard the increased traffic, as shown in  FIG. 7 . 
   Here, the transmitting apparatus  10  and the receiving apparatus  20  must be synchronized when changing the number of the virtual concatenated paths, failing which, a data error or discarding may occur. In order to prevent the problem from occurring, the present invention provides a procedure (sequence) that is executed by the transmitting apparatus  10  and the receiving apparatus  20  such that the change takes place simultaneously, assuring divided and transmitted data to be correctly received and recovered. 
   The details of this sequence (procedure) are shown in  FIG. 8 , and a flow of operation of the transmitting apparatus  10  when adding an extra path to meet the traffic increase is shown in  FIG. 9 . Further, a flow of operation of the transmitting apparatus  10  when returning to the default status, pursuant to a traffic decrease is shown in  FIG. 10 . A flow of operation of the receiving apparatus  20  when the traffic increases is shown in  FIG. 11 , and, similarly a flow of operation when the traffic decreases is shown in  FIG. 12 . Hereafter, when a step number of a flowchart is described, the word “step” may be omitted and only S 1 , S 2  and the like may appear. 
   First, whether a traffic amount is above or below a default value is checked (S 1 , S 21 ). 
   1) If it is determined that the traffic amount is “above” the default value, whether an extra path is available to the transmitting apparatus  10  is checked (S 2 ). 
   2) If an extra path is available, the extra path is set up (cross-connected) by the transmitting apparatus  10  (S 3 ). 
   At this time, a content of the extra path remains “unused”, that is, all bits of an AU-AIS or C 2  byte of POH (path overhead) are set at “0” (UNEQP). 
   3) Next, the transmitting apparatus  10  transmits a “change demand” to the receiving apparatus  20  such that the number of virtual concatenated paths is changed (S 4 , S 22 ). 
   Here, the “change demand” to change the number of virtual concatenated paths includes following information: 
   (1) Demand path number: identification information of a virtual concatenated path that demands the change, 
   (2) Increase/decrease identification code: a code that discriminates whether the change demand is an increase demand or a decrease demand, 
   (3) The number of paths demanded: quantity information as to how many paths are to be increased/decreased in terms of the number of VC-3paths or VC-4 paths, and 
   (4) Increase/decrease path number: identification information, indicating which extra paths are to be increased/decreased where a plurality of extra paths are available. 
   In this embodiment, these pieces of information are included in a J1 byte as shown in  FIG. 13 . However, any other unused byte in POH can be used for this purpose. 
   4) When the receiving apparatus  20  receives from transmitting apparatus  10  a “change demand” to change the number of virtual concatenated paths (YES at S 31  and S 51 ), a content of the demand is checked. If the content indicates “above” the default value, that is, an increase in traffic capacity is demanded, a status of extra paths of the receiving apparatus  20  is checked (S 32 ). 
   5) If an extra path is available, the receiving apparatus  20  sets up the extra path (crossing connection) as demanded (S 34 ). 
   At this time, the extra path is not joined yet to the virtual concatenated paths, but rather the path is treated as “unused”, in the case of increasing the capacity. Conversely, in the case of decreasing the capacity, the extra path in use remains joined in the virtual concatenated paths. 
   6) The receiving apparatus  20  transmits a “demand response” to the transmitting apparatus  10  (S 36 , S 52 ). 
   The “demand response” includes following information: 
   (1) Demand path number: identification information of the virtual concatenated path for which the change demand is issued, which is the same as what is received, 
   (2) Response to the change demand: results of a checking/changing in the receiving apparatus  20 :
         Normal,   Abnormal,   Demand NG: The change demand content is not sufficient or illegal,   Extra path NG: An extra path fails, and   Set-up NG: Set-up (cross connection) of an extra path fails.       

   The above pieces of information may be transmitted by using, e.g., a J1 byte of the path overhead (POH) of the path signal, as shown in  FIG. 13 . 
   7) If the transmitting apparatus  10  receives a “normal” response from the receiving apparatus  20  (YES at S 6  and S 24 ), the following process is performed. 
   If the number of the virtual concatenated paths is to be increased, a multi-framing number and a sequence number are inserted in the H4 byte of the extra path (S 8 ). 
   At this time, live data is not yet inserted into the extra path. 
   Conversely, if the number of the virtual concatenated paths is to be decreased, the process progresses to S 9 . 
   8) The receiving apparatus  20  performs as follows. 
   If the number of the virtual concatenated paths is to be increased, a phase of the extra path is aligned with present virtual concatenated paths, using a multi-framing number indicated by the H4 byte of the extra path. 
   At this time, since there are no live data inserted in the extra path, the extra path is not used in a restoration (multiplex) process of a virtual concatenated path. 
   Conversely, if the number of the virtual concatenated paths is to be decreased, the process progresses to S 10 . 
   9) The transmitting apparatus  10  transmits a “change from next multi-frame” flag (S 9 ), indicating that the number of the concatenated paths is changed from the first frame of the next multi-frame, if the flag is “1”. The “change from next multi-frame” flag is set up in one of vacant bit spaces, for example, Bit  1  of the H4 byte of the second frame in  FIG. 3 . Then, the following process is performed. 
   If the number of the virtual concatenated paths is to be increased, inserting live data to the extra path is started (S 10 ). 
   Conversely, if the number of the virtual concatenated paths is to be decreased, inserting an UNEQP signal or all-“1” data to the extra path is started (S 27 ). 
   10) When the receiving apparatus  20  receives the “change from the next multi-frame” flag by the H4 byte (YES at S 37  and S 53 ), the number of the virtual concatenated paths is changed from the first frame of the next multi-frame (S 38 -S 39 ), and the following process is performed. 
   If the number of the virtual concatenated paths is to be increased, restoration now includes the extra path (S 40 ). 
   Conversely, if the number of the virtual concatenated paths is to be decreased, the extra path is put into an “unused” status and the restoration is performed to remaining concatenated paths (S 54 ). 
   Time required to exchange the sequence message, as shown in  FIG. 8 , between the transmission apparatus  10  and the receiving apparatus  20  is about one second at the maximum, which compares with a manual operation, likely taking tens of seconds. By providing the automatic setting at the lower layer, setting time is saved, error occurrence is eliminated, an immediate compliance to traffic variation is possible, and, as a result, an efficient path control is realized. 
   Here, the change of the number of the concatenated paths by the sequence is performed to an uplink and a downlink, selectively, such that traffic status in each direction is properly reflected in configuring the transmission paths. In this manner, a higher efficiency of the transmission usage is attained. 
   Next, the second embodiment of the present invention is explained. 
   In the second embodiment, one or more extra paths are shared by two or more virtual concatenated paths between the same two nodes. Concept of this system is explained, using  FIG. 14 . In  FIG. 14 , two Ethernet (registered trademark) signals (A) and (B) are transmitted from a node ( 4 ) to a node ( 2 ). Suppose a total of 24 VC-4 paths are required by (A) and (B), i.e., 18 VC-4 and 6 VC-4, respectively, to meet each maximum capacity in a conventional system. The present embodiment provides a total of 22 VC-4 paths, i.e., 16 VC-4 and 4 VC-4 paths to (A) and (B), respectively, and two VC-4 extra paths. The extra paths (VC-4×2) are to be shared by virtual concatenated paths (A) and (B). In this manner, efficiency of path usage is enhanced without causing data discarding when data traffic increases. 
     FIG. 15  shows a configuration of a transmitting apparatus of the second embodiment, wherein one or more extra paths are shared.  FIG. 16  shows a configuration of a corresponding receiving apparatus. To components that correspond to those in  FIG. 4  and  FIG. 5 , a quotation mark (”) is attached. 
   In the second embodiment, two virtual concatenated paths (A) and (B) share two extra paths (VC-4# 17  and VC-4# 18 ). By sharing the extra paths, when a traffic amount of an Ethernet (registered trademark) signal (A) in  FIG. 15  is modest, i.e., less than a default value, and a traffic amount of an Ethernet-(registered trademark) signal (B) becomes high, namely, a path container of C-4-4c (about 600 M bps becomes no longer sufficient, the extra paths are joined to virtual concatenated paths of the Ethernet (registered trademark) signal (B). When the traffic amount of the Ethernet (registered trademark) signal (B) returns to a modest status, the extra paths are freed. Afterwards, when a traffic amount of the Ethernet (registered trademark) signal (A) exceeds a capacity of C-4-16c (about 2.4 G bps, the extra paths are joined to virtual concatenated paths of the Ethernet (registered trademark) signal (A). In this manner, the extra paths are used by a signal of which traffic amount temporarily increases, enhancing an accommodation efficiency of an SDH transmission line. 
   Further, in the second embodiment, a priority can be assigned to the two virtual concatenated paths. For example, a higher priority is assigned to the signal (A), and a lower priority to the signal (B). In this case, when the traffic amount of the signal (A) becomes high while the signal (B) is using the extra path, the signal (B) is deprived of the extra paths, which are assigned to the signal (A) that has the higher priority. The above configuration is effective where priorities are assigned to each of Ethernet (registered trademark) signals accommodated in an SDH transmission line. 
   The second embodiment can also be configured such that an extra path is assigned on a first-come-first-served basis. A late comer has to wait until the extra path is released. Data that exceeds a transmission capacity may be stored in a data buffer until the extra path becomes available. 
   A sequence message, such as shown  FIG. 8 , of each of the first embodiment and the second embodiment can be exchanged by using the J1 byte in the path overhead. Although  FIG. 13  shows an example of the sequence message when the J1 byte is used, the sequence message may use any other available bytes, such as an N1 byte and other unused bytes in POH. 
   Further, even if the J1 byte and the like are used by a 64-multi-frame for hitless path switching, it is possible to transmit and receive the sequence message of the present invention using an unused byte in the 64-multi-frame. 
   Further, it is also possible to transmit and receive the sequence message of the first embodiment and the second embodiment by an unused bit of the multi-frame of the H4 byte. 
   Further, a configuration of the first embodiment and the second embodiment can be such that an operator manually performs various set-ups to make the VC-4# 17  and VC-4# 18  the extra paths. 
   The first embodiment and the second embodiment can also be structured such that an installation position of an interface panel for processing an Ethernet (registered trademark) signal uniquely defines an extra path (refer to  FIG. 17 ). In this case, the extra path is automatically defined, and the path control operation of the present invention is performed without an operator&#39;s intervention. 
   Further, the first embodiment and the second embodiment can be structured as follows. The concatenated path generating unit  14  is divided into a concatenated path generating unit for default paths  14   a  and a concatenated path generating unit for extra paths  14   b , and the virtual concatenated path generating unit  15  is divided into a virtual concatenated path generating unit for default paths  15   a , and a virtual concatenated path generating unit for extra paths  15   b  in the transmitting apparatus. Similarly, on the receiving apparatus side, the virtual concatenated path restoration unit  23  is divided into a virtual concatenated path restoration unit for default paths  23   a  and a virtual concatenated path restoration unit for extra paths  23   b , and the SDH/packet conversion unit  24  is divided into an SDH/packet conversion unit for default paths  24   a  and an SDH/packet conversion unit for extra paths  24   b  as shown in  FIG. 19 . In this manner, the number of the concatenated paths can be changed by switching selectors  31  and  32  at a later stage. This configuration dispenses with a need to provide a different clock speed to each of above units. (In the case of  FIGS. 4 ,  5 ,  6  and  7 , different clock frequencies have to be provided to the concatenated path generating unit  14 , the virtual concatenated path generating unit  15 , the virtual concatenated path restoring unit  23  and the SDH/packet conversion unit  24 , according to whether or not an extra path is engaged.) 
   In this manner, the present invention enhances efficiency of transmission line resources using a virtual concatenated path, enabling an adaptive arrangement of the transmission resources, without accompanying data discarding and error occurring when data traffic increases. 
   Further, the present invention is not limited to these embodiments, but various variations and modifications may be made without departing from the scope of the present invention. 
   The present application is based on Japanese priority application No. 2001-264704 filed on Aug. 31, 2001 with the Japanese Patent Office, the entire contents of which are hereby incorporated by reference.