Abstract:
Disclosed is a method of switching between redundant routes of a communication system in which terminal stations are connected by an uplink transmission line and a downlink transmission line that are devoid of redundancy. The method includes adopting working/protection redundancy for first and second apparatuses constituting each terminal station, detecting line failure, which has occurred on the side of the first terminal station, by the second terminal station, multiplexing additional data, which includes a line failure alarm, onto main signal sent from the second terminal station to the first terminal station (or inserting the line failure alarm in the form of overhead bytes), and causing the first terminal station to perform line switching upon detecting the alarm included in the additional data or overhead bytes.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a method of switching between redundant routes of a communication system not having redundant transmission lines. More particularly, the invention relates to a method of switching between redundant routes of a communication system in which terminal stations having a redundant structure are connected by two transmission lines, namely uplink and downlink transmission lines, not having a redundant structure. 
     In an optical transmission system, a 1+1 line switching point-to-point system is constructed by adding a switch controller to a 1+1 configuration having one working line and one protection line, or a 1:N line switching point-to-point system is constructed by adding a switch controller to a 1:N configuration having N working lines and one protection line. When a working line fails, a changeover is effected to a protection line so that communication can continue. 
     The sending and receiving of information relating to the switching of optical signal lines is stipulated by the international standard (SONET) regarding SDH (Synchronous Digital Hierarchy). It is so arranged that K1/K2 bytes are used in overhead bytes OHB. 
     FIG. 10A is a diagram useful in describing a SONET STS-3 (OC-3) frame format. One frame is composed of 9×270 bytes. The first 9×9 bytes constitute section overhead (SOH), and the remaining bytes constitute path overhead (POH) and payload (PL). The section overhead SOH is the part of the frame that sends information (a frame synchronizing signal) representing the beginning of the frame, information specific to the transmission line (such as information which checks for error at the time of transmission and information for maintenance of the network), and a pointer which indicates the position of the path overhead POH. Further, path overhead POH is the part of the frame that sends end-to-end monitoring information in the network. The payload PL sends information at 150 Mbps. 
     The section overhead SOH is composed of repeater section overhead of 3×9 bytes, a pointer of 1×9 bytes and multiplex section overhead of 5×9 bytes. As shown in FIG. 10B, the repeater section overhead has bytes A 1 ˜A 2 , C 1 , B 1 , E 1 , F 1 , D 1  ˜D 3 , and the multiplex section overhead has bytes B 2 , K 1 ˜K 2 , D 4  D 12 , S 1 , Z 1 ˜Z 2 . The repeater section overhead and the multiplex section overhead each have a large number of undefined bytes the use of which is left to the particular communications concern. 
     The K 1  byte among the overhead bytes is used mainly to request switching and designates the level of the switch request and the switched line. The K 2  byte is used mainly to respond to the K 1  byte and is employed also to represent system architecture, switching mode and AIS (Alarm Indication Signal)/FERF (Far End Receive Failure). Switching requests include, in addition to a request for switching at the time of signal failure, a switching request based upon forced switching and manual switching. FIGS. 11 and 12 illustrate the bits of the K 1 /K 2  bytes stipulated by the SONET standard, as well as the meanings thereof. 
     K 1  Byte 
     The first four bits b 1 ˜b 4  of the K 1  byte represent the switching request, and the remaining four bits b 5 ˜b 8  represent the switched line and are capable of specifying a maximum of 14 transmission lines. “Lockout of Protection” is a switch request that prohibits switching to a protection transmission line. “Forced Switching” is a request for switching of an artificially designated transmission line. If a switch has been made, a switch will not be made to another a line even if a fault has occurred there. “SF” (Signal Failure) is a switching request for when a transmission line loses it signal. This request has two priorities, namely high and low. “SD” (Signal Degradation) is a switch request based upon signal degradation of a transmission line and has two priorities, namely high and low. The SF switching request has a higher priority than that of the SD switching request. “Manual Switching” is an artificial switching request. If a fault occurs somewhere, priority is given to switching of this location. “Wait to Restore” is a request wherein, even if a switch-back request is issued following restoration of a faulty line, switch-back is performed upon elapse of a predetermined period of time. “Exerciser” performs an actual switching to self-diagnose whether switching will be performed normally. “No Request” is sent when the situation is normal or when bridging is canceled. 
     K 2  Byte 
     The first four bits b 1 ˜b 4  of the K 2  byte specify a transmission line number and are nulled (0000) in a case where the bits b 5 ˜b 8  of a received K 1  byte are null. In other cases these bits represent the number of the transmission line to which a changeover has been made. The b 5  bit indicates the network configuration; “1” indicates a 1+1 system and “0” a 1:N system. The b 6 ˜b 8  bits indicate the category of the switching mode, the specifics of the fault, etc. There are two types of switching modes, namely a unidirectional mode, in which only a signal in one direction is changed over, and a bidirectional mode, in which signals in both directions are changed over simultaneously. 
     Switching Sequence Using K 1 , K 2  Bytes 
     In the case of the unidirectional mode, the K 1  byte (switching request) is sent to a station A if a station B detects SF, as shown in FIG.  13 A. The station A performs bridging control in regard to the line specified by the K 1  byte (switching request) that has been received. Bridging control is control for sending identical signals to both working and protection lines. After performing bridging control, the station A transmits the K 2  byte (switching response), which is in response to the received K 1  byte, to the opposing station (station B). Upon receiving the K 2  byte, the station B performs switching control. Switching control is control for switching a designated line signal in the receiving direction to a protection line. 
     In the case of the bidirectional mode, the K 1  byte (switching request) is sent to station A if station B detects SF, as shown in FIG.  13 B. Station A performs bridging control in regard to the line specified by the K 1  byte (switching request) that has been received, sends back the K 2  byte (switching response) in the same manner as in the unidirectional mode and, at the same time, sends the K 1  byte designating “reverse request” (RR). Upon receiving RR, station B performs switching control and bridging control in regard to the line that was designated by the K 1  byte sent by the B station itself and sends the K 2  byte (switching response) to the opposing station (station A). Upon receiving the K 2  byte, station A performs switching control. 
     FIG. 14 is a diagram useful in describing the details of switching based upon line protection using the K 1  and K 2  bytes. Shown in FIG. 14 are a terminal station (station A)  1 , a terminal station (station B)  2  opposing the station A, a working transmission line  3  comprising a working uplink line  3   a  and a working downlink line  3   b , and a protection transmission line  4  comprising a protection uplink line  4   a  and a protection downlink line  4   b.    
     The terminal station (station A)  1  includes a multiplexer/demultiplexer  1   a,  working and protection transmitters (TX-W, TX-P)  1   b,    1   c , respectively, which send exactly identical signals, and working and protection receivers (RX-W, RX-P)  1   d ,  1   e , respectively, which receive exactly identical signals. The terminal station (station B)  2  includes a multiplexer/demultiplexer  2   a , working and protection transmitters (TXW, TX-P)  2   b ,  2   c , respectively, which send exactly identical signals, and working and protection receivers (RX-W, RX-P)  2   d ,  2   e , respectively, which receive exactly identical signals. 
     The working transmitter  1   b  of terminal station  1  is connected to the working receiver  2   d  of the terminal station  2  via the working uplink line  3   a , and the protection transmitter  1   c  of terminal station  1  is connected to the protection receiver  2   e  of the terminal station  2  via the protection uplink line  4   a . Similarly, the working transmitter  2   b  of terminal station  2  is connected to the working receiver  1   d  of the terminal station  1  via the working downlink line  3   b , and the protection transmitter  2   c  of terminal station  2  is connected to the protection receiver  1   e  of the terminal station  1  via the protection downlink line  4   b.    
     More specifically, in the system shown in FIG. 14, the terminal stations and transmission lines are both duplexed for redundancy so that when a fault develops in the working uplink line  3   a , the system switches to the protection uplink line  4   a . When a fault develops in the working downlink line  3   b , the system switches to the protection downlink line  4   b . For example, if the uplink line  3   a  develops a fault at the “x” mark and signal failure or signal degradation occurs, then the terminal station (station B)  2  detects SF or SD and sends the K 1  byte (switching request) to the terminal station (station A)  1  via the protection line  4   b . On the basis of the K 1  byte which it has received, the station A performs bridging control, sends identical signals to both the working line  3   a  and protection line  4   a  and transmits the K 2  byte (switching response), which is in response to the received K 1  byte, to the opposing station (station B). Upon receiving the K 2  byte, station B switches the line from the working line  3   a  to the protection line  4   a  by switching control. Thus, when an alarm is detected on a working or protection line, a changeover is made from the faulty line to the normal line. 
     When line redundancy is employed in a communication system covering a long distance and entailing a high transmission line cost (e.g., a submerged system required for international communications), providing a large number of lines and repeaters raises the cost of the communication system and is unrealistic. For this reason, use can be made of a communication system in which only the terminal equipment is provided with redundancy and not the lines. 
     FIG. 15 is an example of a communication system in which lines are not redundant. Components identical with those shown in FIG. 14 are designated by like reference characters. Shown in FIG. 15 are an additional unit  5  on the side of terminal A, an additional unit  6  on the side of terminal B, an uplink transmission line  7  and a downlink transmission line  8 . The transmission lines  8  and  9  are not furnished with redundancy. The additional unit  5  has a switch  5   a  for selecting and sending the uplink transmission line  7  one of the signals that enter from the working and protection transmitters  1   b,    1   c , a hybrid circuit  5   b  for distributing a signal, which enters from the downlink transmission line  8  to the working and protection receivers  1   d ,  1   e , and a controller  5   c  for designating the signal to be selected by the switch  5   a . The additional unit  6  has a switch  6   a  for selecting and sending the downlink transmission line  8  one of the signals that enter from the working and protection transmitters  2   b ,  2   c , a hybrid circuit  6   b  for distributing a signal, which enters from the uplink transmission line  7  to the working and protection receivers  2   d ,  2   e , and a controller  6   c  for designating the signal to be selected by the switch  6   a.    
     Assume that the signal of the working route is selected by the switch  5   a  in this communication and that this signal is being distributed to the working and protection receivers  2   d ,  2   e  by the hybrid circuit  6   b . In the event that a fault develops at point x under these circumstances, the terminal station (station B)  2  detects Signal Failure SF or Signal Degradation SD in working and protection lines  3   a ′,  4   a ′ simultaneously. If SF or SD is sensed simultaneously in the working and protection lines, line switching is not performed according to the stipulations of the international standard since such switching would be meaningless. With the arrangement of FIG. 15, however, continuation of communication becomes possible if the line is switched from a working line  3   a  to a protection line  4   a . This means that with a communication system not having line redundancy, it is necessary to arrange it so that communication can be continued by line switching even in a case where SF or SD is sensed simultaneously in the working and protection lines. 
     Progress has recently been made in communication techniques employing ultra-high-speed bit rates, communication techniques (optical amplification, code correction, etc.) using relay span extension means and optical wavelength multiplexed communication techniques, and various devices have been developed on the basis of these techniques. Such devices are placed between an SDH unit and a transmission line in an effort to reduce communication costs even if only marginally. However, there are instances where not all transmission line alarms can be detected in these devices, in which case it is necessary to deal with this by performing detection on the side of the SDH unit. 
     The communication system shown in FIG. 16 has been proposed to meet this requirement. This communication system has two 2.5-Gbps SDH units (SDH MUX A, SDH MUX C)  11 ,  12  and a 5-Gbps higher level unit (SLTE-A)  13  provided on the side of station A, and two 2.5-Gbps SDH units (SDH MUX B, SDH MUX D)  14 ,  15  and a 5-Gbps higher level unit (SLTE-B)  16  provided on the side of station B. The higher level units  13  and  16  are connected by uplink and downlink transmission lines  17 ,  18 . The higher level unit  13  multiplexes the 2.5-Gbps signals from the SDH units (SDH MUX)  11 ,  12  to obtain a 5-Gbps signal, sends this signal to the transmission line  17 , branches a 5-Gbps multiplexed signal from the transmission line  18  to the working and protection routes, demultiplexes each of these branched signals to a 2.5-Gbps signal and inputs the 2.5-Gbps signals to the SDH units (SDH MUX)  11 ,  12 . The higher level unit  16  multiplexes the 2.5-Gbps signals from the SDH units (SDH MUX)  14 ,  15  to obtain a 5-Gbps signal, sends this signal to the transmission line  18 , distributes a 5-Gbps multiplexed signal from the transmission line  17  to the working and protection routes, demultiplexes each of these signals to a 2.5-Gbps signal and inputs the resulting signals to the SDH units (SDH MUX)  14 ,  15 . 
     The SDH units  11 ,  12  and  14 ,  15  have identical structures and each has the components indicated below. Specifically, each SDH unit (SDH MUX) includes: 
     (1) a working transmitter (WTX)  21 ; 
     (2) a working receiver (WRX)  22 ; 
     (3) a protection transmitter (PTX)  23 ; 
     (4) a protection receiver (PRX)  24 ; 
     (5) a multiplexer (MUX)  25  for multiplexing signals that enter from in lines #1˜#11 and distributing the multiplexed signal to the working/protection transmitters  21 ,  23 ; 
     (6) a demultiplexer (DMUX)  26  for demultiplexing one of the multiplexed signals that enter from the working/protection receivers  22 ,  24 ; 
     (7) an alarm detector (ALM)  27  for detecting an alarm in the opposing station sent from the opposing station and outputting a line switching request; 
     (8) a fault detector (ALM)  28  for detecting signal failure SF or signal degradation SD and designating transmission of an opposing station alarm; and 
     (9) a controller (CONT)  29  for sending an opposing station alarm in response to an opposing station alarm transmission trigger. 
     It should be noted that not all of the above-mentioned components are shown in the SDH units  11 ,  12  and  14 ,  15 ; only those components necessary for descriptive purposes are illustrated. 
     The higher level units (SDH MUX)  13 ,  16  have identical structures and each has the components indicated below. For example, the higher level unit (SDH MUX)  13  includes: 
     (1) a working multiplexer/demultiplexer (WMLDM)  31  which multiplexes 2.5-Gbps signals from the working transmitters  21  of the SDH (SDH MUX) units  11 ,  12  to obtain a 5-Gbps signal, sends this signal to the transmission line  17  via a switch, demultiplexes a 5-Gbps multiplexed signal that enters from the transmission line  18  via a hybrid circuit to 2.5-Gbps signals and inputs these signals to the working receivers  22  of the SDH units (SDH MUX)  11 ,  12 ; 
     (2) a protection multiplexer/demultiplexer (PMLDM)  32  which multiplexes 2.5-Gbps signals from the protection transmitters  23  of the SDH (SDH MUX) units  11 ,  12  to obtain a 5-Gbps signal, sends this signal to the transmission line  17  via a switch, demultiplexes a 5-Gbps multiplexed signal that enters from the transmission line  18  via a hybrid circuit to 2.5-Gbps signals and inputs these signals to the protection receivers  24  of the SDH units (SDH MUX)  11 ,  12 ; 
     (3) a switch (SW)  33  for selecting, and sending to the transmission line  17 , one of the multiplexed signals that enters from the working multiplexer/demultiplexer (WMLDM)  31  and protection multiplexer/demultiplexer (PMLDM)  32 ; 
     (4) a hybrid circuit (HYB)  34  for distributing the 5-Gbps multiplexed signal that enters from the transmission line  18  to the working and protection multiplexer/demultiplexers  31 ,  32 ; and 
     (5) a switch controller (SW CONT)  35  for controlling the switch  33 , in response to a line switching request specified by the alarm detector (DET)  27 , to send a working or protection signal to the transmission line  17 . 
     It should be noted that not all of the above-mentioned components are shown in the higher level units  13 ,  16 ; only those components necessary for descriptive purposes are illustrated. 
     Described next will be a line switching procedure in a case where a line failure occurs at point x when a working signal is being transmitted in the uplink direction via the switch  33  and the transmission line  17  along the path indicated by the dashed line and, similarly, when a working signal is being transmitted in the downlink direction via the switch  33  and the transmission line  18  along the path indicated by the dot-and-dash line. In this case, (a) the higher level unit (SLTE MUX A)  13  implements switching, (b) the SDH unit (SDH MUX B) detects the transmission line alarm and transmits the opposing station information, and (c) the SDH unit (SDH MUX A)  11  detects the alarm (opposing station alarm) from the opposing station and outputs the line switching request. 
     (1) If a failure occurs at the location indicated at x, the signal on the line indicated by the dashed line is lost (SF) or degraded (SD). 
     (2) As a result, the transmission line alarm (SF, SD) is detected simultaneously by the working and protection receivers  22 ,  24  in the SDH unit (SDH MUX B)  14 . 
     (3) In response to detection of the alarm (signal failure SF or signal degradation SD) simultaneously in both the working and protection routes, the fault detector (ALM)  28  generates the opposing station alarm transmission trigger. In the event that the alarm is detected in only one route, the opposing station alarm transmission trigger is not generated. 
     For example, if a failure occurs at point b between the multiplexer/demultiplexer  31  of the higher level  16  and the working receiver  22  of the SDH unit  14 , the fault detector (ALM)  28  detects an alarm (signal failure SF or signal degradation SD) solely in the working route. In such case the fault detector (ALM)  28  so notifies the controller  29 . As a result, the controller  29  controls a demultiplexer (not shown) so that the demultiplexer demultiplexes the signal from the protection receiver  24  instead of the signal from the working receiver  22  and sends the demultiplexed signals to the prescribed lines #1˜#n. 
     (4) Upon receiving the opposing station alarm transmission trigger, the controller (CONT)  29  transmits the opposing station alarm to the SDH unit  11  along the path of the dot-and-dash line using the K 2  byte of the overhead OHB. 
     (5) The working and protection receivers  22 ,  24  of the SDH unit  11  detect the opposing station alarm sent from the SDH unit  14  and notify the alarm detector (DET)  27 . 
     (6) In response to detection of the opposing station alarm in either the working or protection route, the alarm detector (DET)  27  issues a switching request to the higher level unit  13 . 
     (7) Upon receiving the switching request, the higher level unit  13  instructs the switch  33  to switch lines. In response to this indication to switch, the switch  33  selects the signal from the protection multiplexer/demultiplexer  32  instead of the signal from the working multiplexer/demultiplexer  31  and sends the selected signal to the transmission line  17 . As the result of these operations, communication is allowed to continue. 
     Thus, with the arrangement shown in FIG. 16, line switching can be controlled even in a case where an alarm (signal failure SF or signal degradation SD) is detected simultaneously in both the working and protection routes of a communication system not having line redundancy. 
     However, a problem which arises is that the prior-art SDH unit, in contravention of the stipulation of the international standard, performs line switching in a case where an alarm is detected in working and protection routes simultaneously. 
     Further, when a switching request is issued, the prior art is such that switching is implemented in toggled fashion; whether the switching request is from the working side to the protection side or from the protection side to the working side cannot be determined. Consequently, if switching occurs frequently, a problem which arises in the prior art is confusion and an inability to perform switching correctly. 
     In an effort to solve these problems, the insertion of an alarm on the side of the opposing station is canceled after the completion of a switching operation, guard time is provided for the period of time required for the canceled signal to return to the station that issued it, and completion of switching is verified. However, communication is interrupted for the duration of switching verification. 
     Further, line switching in the event of failures in the transmission lines  17  and  18  is meaningless and unnecessary. Nevertheless, needless line switching is performed in such case according to the prior art. 
     SUMMARY OF THE INVENTION 
     Accordingly, an object of the present invention is to construct and standardize an SDH unit (a multiplexer serving as a lower order unit) in conformity with the stipulation of the international standard, and to positively detect faults that require switching, thereby improving system reliability. 
     Another object of the present invention is to make it possible to determine whether a switching request is from the working side to the protection side or from the protection side to the working side. 
     A further object of the present invention is to avoid unnecessary line switching in a case where transmission line failure has occurred. 
     According to the present invention, the foregoing objects are attained by providing a method of switching between redundant routes of a communication system provided with two terminal stations each having a first apparatus for sending working/protection main signals toward the opposing station and receiving working/protection main signals from the opposing station, and a second apparatus for selecting, and sending to the opposing station, one of the working/protection main signals output from the first apparatus toward the opposing station, and for branching a main signal, which has been sent from the opposing station, and inputting the branched signals to the first apparatus as working/protection main signals, the second apparatuses of the two terminal stations being connected by an uplink transmission line and a downlink transmission line that are devoid of redundancy, the method comprising the steps of: adopting working/protection redundancy for the first and second apparatuses constituting each terminal station; detecting line failure, which has occurred on the side of the first terminal station, by the second terminal station; multiplexing additional data, which includes a line failure alarm, onto the main signal sent from the second terminal station to the first terminal station (or inserting overhead bytes, which notify of a line failure alarm, into the main signals); and causing the first terminal station to perform line switching upon detecting the alarm from the additional data or overhead bytes. 
     Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram showing the configuration of a communication system to which the line switching method of the present invention is applied; 
     FIGS. 2A and 2B are diagrams useful in describing the multiplexer section of a multiplexer/demultiplexer; 
     FIGS. 3A and 3B are diagrams useful in describing the demultiplexer section of the multiplexer/demultiplexer; 
     FIG. 4 is a diagram showing a line switching arrangement according to a first embodiment of the present invention; 
     FIG. 5 is a diagram showing a line switching arrangement according to a second embodiment of the present invention; 
     FIG. 6 is a diagram showing a line switching arrangement according to a third embodiment of the present invention; 
     FIG. 7 is a diagram showing a line switching arrangement according to a fourth embodiment of the present invention; 
     FIG. 8 is a diagram showing a line switching arrangement according to a fifth embodiment of the present invention; 
     FIG. 9 is a diagram showing a line switching arrangement according to a sixth embodiment of the present invention; 
     FIGS. 10A and 10B are diagrams useful in describing the format of a SONET OC-3 frame; 
     FIG. 11 is a diagram useful in describing a K 1  byte; 
     FIG. 12 is a diagram useful in describing a K 2  byte; 
     FIGS. 13A and 13B are sequences for sending and receiving K 1 , K 2  bytes; 
     FIG. 14 is a diagram showing a switching scheme based upon line protection; 
     FIG. 15 is a communication system not having line redundancy; and 
     FIG. 16 is a diagram showing the configuration of a prior-art communication system in which line switching is made possible even if failure is detected in both working and protection lines simultaneously. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     (A) Communication System to Which Line Switching Method of the Invention is Applicable 
     (a) Configuration 
     FIG. 1 is a diagram showing the configuration of a communication system to which the line switching method of the present invention is capable of applied. 
     Shown in FIG. 1 are a terminal station  50 A on an A side and a terminal station B on a B side. The terminal station  50 A includes 2.5-Gbps SDH units (SDH MUX A, SDH MUX C)  51 ,  52 , and a 5-Gbps higher level unit (SLTE-A)  53  for multiplexing and demultiplexing. The terminal station  50 B includes 2.5-Gbps SDH units (SDH MUX B, SDH MUX D)  54 ,  55 , and a 5-Gbps higher level unit (SLTE-B)  56  for multiplexing and demultiplexing. An uplink transmission line  57  sends a signal from the higher level unit  53  of the terminal station  50 A to the higher level unit  56  of the terminal station  50 B, and an downlink transmission line  58  sends a signal from the higher level unit  56  to the higher level unit  53 . 
     The higher level unit  53  multiplexes the 2.5-Gbps signals from the SDH units (SDH MUX)  51 ,  52  to obtain a 5-Gbps signal, sends this signal to the transmission line  57 , branches a 5-Gbps multiplexed signal from the transmission line  58 , demultiplexes each of these branched signals to a 2.5-Gbps signal and inputs the 2.5-Gbps signals to the SDH units (SDH MUX)  51 ,  52 . The higher level unit  56  multiplexes the 2.5-Gbps signals from the SDH units (SDH MUX)  54 ,  55  to obtain a 5-Gbps signal, sends this signal to the transmission line  58 , branches a 5-Gbps multiplexed signal from the transmission line  57 , demultiplexes each of these branched signals to a 2.5-Gbps signal and inputs the 2.5-Gbps signals to the SDH units (SDH MUX)  54 ,  55 . 
     The SDH units  51 ,  52  and  54 ,  55  are identical in structure and each has (1) a working transmitter (WTX)  61 , (2) a working receiver (WRX)  62 , (3) a protection transmitter (PTX)  63  and (4) a protection receiver (PRX)  64 . 
     The higher level unit (SDH MUX)  53  includes the following components: 
     (1) a working multiplexer/demultiplexer (WMLDM)  71  which multiplexes 2.5-Gbps signals from the working transmitters  61  of the SDH (SDH MUX) units  51 ,  52  to obtain a 5-Gbps signal, sends this 5-Gbps signal to the transmission line  57  via a switch, demultiplexes a 5-Gbps multiplexed signal that enters from the transmission line  58  via a hybrid circuit to 2.5-Gbps signals and inputs these 2.5-Gbps signals to the working receivers  62  of the SDH units (SDH MUX)  51 ,  52 ; 
     (2) a protection multiplexer/demultiplexer (PMLDM)  72  which multiplexes 2.5-Gbps signals from the protection transmitters  63  of the SDH (SDH MUX) units  51 ,  52  to obtain a 5-Gbps signal, sends this 5-Gbps signal to the transmission line  57  via a switch, demultiplexes a 5-Gbps multiplexed signal that enters from the transmission line  58  via a hybrid circuit to 2.5-Gbps signals and inputs these 2.5-Gbps signals to the protection receivers  64  of the SDH units (SDH MUX)  51 ,  52 ; 
     (3) a switch (SW)  73  for selecting, and sending to the transmission line  57 , one of the 5-Gbps multiplexed signals that enters from the working multiplexer/demultiplexer (WMLDM)  71  and protection multiplexer/demultiplexer (PMLDM)  72 ; and 
     (4) a hybrid circuit (HYB)  74  for distributing the 5-Gbps multiplexed signal that enters from the transmission line  58  to the working and protection multiplexer/demultiplexers  71 ,  72 . 
     The higher level unit (SDH MUX)  56  has the same structure as that of the higher level unit  53  though the components of the latter are a mirror image of those of the former. 
     (b) Signal Transmission 
     Transmission of signals from the side of station A to the side of station B will now be described. 
     Working signals that have been externally applied to the SDH units  51 ,  52  of station A enter the working multiplexer/demultiplexer  71  of the higher level unit  53  via the working transmitters  61  of the respective SDH units  51 ,  52 , whereby the working signals are time-division multiplexed and input to the switch  73 . In parallel with this operation, protection signals that have entered the SDH units  51 ,  52  of station A enter the protection multiplexer/demultiplexer  72  of the higher level unit  53  via the protection transmitters  63  of the respective SDH units  51 ,  52 , whereby the protection signals are time-division multiplexed and input to the switch  73 . The switch  73  selects the signal from the working route, for example, and sends the selected signal to the transmission line  57 . 
     The hybrid circuit  74  of the higher level unit  56  of station B branches a multiplexed signal, which has been transmitted from station A via the transmission line  57 , to the working and protection multiplexer/demultiplexers  71 ,  72 . The working multiplexer/demultiplexer  71  demultiplexes the input multiplexed signal and inputs the demultiplexed signals to the working receivers  62  of the respective SDH units  54 ,  55 . Each of the working receivers  62  sends its input signal to the outside as a working signal. The protection multiplexer/demultiplexer  72  demultiplexes the input multiplexed signal and inputs the demultiplexed signals to the protection receivers  64  of the respective SDH units  54 ,  55 . Each of the protection receivers  64  sends its input signal to the outside as a protection signal. 
     If a fault occurs at, say, point a in the working line during the transmission of these signals, the switch  73  is controlled by the line switching arrangement of the present invention, described later, in such a manner that the protection multiplexed signal instead of the working multiplexed signal is sent to the transmission line  57  to allow communication to continue. 
     Though the foregoing describes transmission of signals from station A to station B, operation is similar when signals are transmitted from station B to station A. 
     (c) Construction of Multiplexer/Demultiplexer 
     There are cases where the multiplexer/demultiplexers  71 ,  72  do and do not have a function capable of sending signals other than the main signals at the same time as the main signals. 
     (c-1) Arrangement with Simultaneous Transmission Function 
     In a case where the multiplexer/demultiplexers  71 ,  72  do not have a function that enables signals other than the main signals to be transmitted at the same time as the main signals, the main signals that enter from the SDH units  51 ,  52  are simply time-division multiplexed, and then sent to the switch  73 , by the multiplexer/demultiplexers  71 ,  72  at a bit rate twice that of the main signals. In demultiplexing, the multiplexed signal branched from the hybrid circuit  74  is demultiplexed and the demultiplexed signals are sent to the SDH units  51 ,  52  at a bit rate half that of the multiplexed signal. 
     (c-2) Arrangement Without simultaneous Transmission Function 
     In a case where the multiplexer/demultiplexers  71 ,  72  have a function that enables signals other than the main signals to be transmitted at the same time as the main signals, the multiplexer/demultiplexers time-division multiplex the main signals at a speed higher than twice the bit rate of the main signals to produce a vacant time slot, place additional data ADDT in the time slot and send the result. FIG. 2A is a diagram useful in describing the principle of such multiplexing. A vacant time slot is generated by multiplexing data items DATA A and DATA B, which have entered from the SDH units  51 ,  52 , respectively, at a speed higher than twice the bit rate of this data, additional data ADDT, which has been generated by an additional data generator  75 , is multiplexed onto this time slot and the result is sent to the switch  73 . 
     FIG. 2B is a diagram showing the construction of the multiplexer section of each multiplexer/demultiplexer. Data items DATA A and DATA C, which have been sent from the SDH units  51 ,  52 , respectively, are stored in first and second memories  76   a ,  76   b , respectively, in synchronization with a low-speed clock LCLK. A PLL  76   c  uses the low-speed clock to produce a high-speed clock HCLK, the speed of which is twice that of the low-speed clock LCLK, and inputs the high-speed clock HCLK to a multiplex controller  76   d . Whenever the high-speed clock HCLK is generated, the multiplex controller  76   d  reads the data out of the memories in the order ADDT→DATA A→DATA C. That is, in synchronization with the high-speed clock HCLK, the multiplex controller  76   d  first sends the additional data ADDT, then sends one frame of the data DATA A and then sends one frame of the data DATA C, after which the multiplex controller  76  sends, by multiplexing, the additional data ADDT, the data DATA A and the data DATA C in the above-mentioned order. 
     Further, at the time of demultiplexing, as shown in FIG. 3A, the multiplexer/demultiplexers  71 ,  72  demultiplex a multiplexed signal DATA, which has entered from the hybrid circuit  74 , to additional data ADDT′, data DATA A′ and data DATA C′ in synchronization with the high-speed clock HCLK, and sends these items of data to an additional data analyzer  75 ′ and the SDH units  51 ,  52 . 
     FIG. 3B is a diagram showing the construction of the demultiplexer section of each multiplexer/demultiplexer. Multiplexed data DATA that has entered from the hybrid circuit  74  is demultiplexed to additional data ADDT′, data DATA A′ and data DATA C′ by a demultiplex controller  77   a  in synchronization with the high-speed clock HCLK. The additional data ADDT′ enters an ADDT analyzer  75 ′, and the items of data DATA A′, DATA C′ are written to first and second memories  77   b ,  77   c , respectively. PLLs  77   d ,  77   e  produce the low-speed clock LCLK from the high-speed clock, and the memories  77   b    77   d  send the data DATA A′, DATA C′ to the SDH units  51 ,  52 , respectively, in synchronization with the lowspeed clock. 
     (B) Embodiments 
     Embodiments of line switching when line failure has occurred in the communication system of FIG. 1 will now be described. It should be noted that the SDH units  52 ,  55  have been deleted from the diagrams showing the construction of each embodiment. Further, the working multiplexer/demultiplexer  71  and the protection multiplexer/demultiplexer  72  of FIG. 1 have each been divided into multiplexer and demultiplexer sections, the multiplexer sections have been united and are illustrated as a single multiplexer section (MUX)  81 , and the demultiplexer sections have been united and are illustrated as a single demultiplexer section (DMUX)  82 . 
     (a) First Embodiment 
     (a-1) Construction 
     FIG. 4 is a diagram showing the construction of a line switching arrangement according to a first embodiment of the present invention. Components identical with those shown in FIG. 1 are designated by like reference characters. Further, the multiplexer section  81  has a function for multiplexing the additional data ADDT onto the main signals and then sending the multiplexed signal, and the demultiplexer section  82  has a function for demultiplexing a multiplexed signal into main signals and the additional data ADDT and then outputting the demultiplexed signals. 
     The first embodiment shown in FIG. 4 includes an AND gate  83  provided in the higher level unit  56  on the side of station B for computing the AND between alarm detection signals ALM 1 , ALM 2  of the working and protection routes. When the result of the AND operation is “1”, the AND gate  83  inputs an opposing station alarm to the multiplexer section  81  as the additional data ADDT. The first embodiment further includes an OR gate  84  provided in the higher level unit  53  on the side of station A. The OR gate  84  performs an OR operation to determine whether the additional data ADDT of one of the working and protection multiplexed signals sent from station B contains the opposing station alarm. When the result of the OR operation is “1”, the switch  73  is controlled to execute line switching. 
     (a-2) Line Switching Control 
     A working signal that has been externally applied to the SDH unit  51  of station A enters a working multiplexer  81   a  of the higher level unit  53  via the working transmitter  61 , whereby the working signal is time-division multiplexed with the working signal from the SDH unit  52  (not shown). The resulting multiplexed signal is input to the switch  73 . A protection signal that has entered the SDH unit  51  of station A enters a protection multiplexer  81   b  of the higher level unit  53  via the protection transmitter  63  of the respective SDH unit  51 , whereby the protection signal is time-division multiplexed with the protection signal from the SDH unit  52  (not shown). The resulting multiplexed signal is input to the switch  73 . The switch  73  selects the signal from the working route, for example, and sends the selected signal to the transmission line  57 . 
     The hybrid circuit  74  of the higher level unit  56  of station B inputs a multiplexed signal, which has been transmitted from station A via the transmission line  57 , to working and protection demultiplexers  82   a ,  82   b . The working demultiplexer  82   a  demultiplexes the input multiplexed signal and inputs the demultiplexed signals to the working receivers  62  of the respective SDH units  54 ,  55  (the latter of which is not shown). The working receiver  62  sends this signal to the outside as the working signal. A protection demultiplexer  82   b  demultiplexes the input multiplexed signal and inputs the demultiplexed signals to the protection receivers  64  of the respective SDH units  54 ,  55  (the latter of which is not shown). The protection receiver  64  sends this signal to the outside as the protection signal. 
     Though the foregoing describes transmission of signals from station A to station B, operation is similar when signals are transmitted from station B to station A. 
     Under these normal transmission conditions, the working and protection receivers  62 ,  64  of SDH unit  54  send the higher level unit  56  the alarm detection signals ALM 1 , ALM 2  of logical “0”, which indicates the absence of an alarm (i.e., that line failure has not occurred). Since the output of the AND gate  83  in higher level unit  56  will be “0” in this case, absence of the alarms is input to the multiplexers  81   a ,  81   b . The latter include the fact that alarms are non-existent in the additional data ADDT that is multiplexed onto the main signals and then transmit the data toward the station A. 
     The demultiplexer section  82  of the higher level unit  53  in station A performs monitoring to determine whether the additional data ADDT sent from station A includes alarm signals and enters the results of monitoring into the OR gate  84 . Since the output of the OR gate  84  will be “0” if the additional data ADDT does not contain alarm signals, communication continues without the line being switched. 
     If the working line develops a fault at point a under these conditions, the working and protection receivers  62 ,  64  of the SDH unit  54  of station B detect signal failure SF or line degradation SD and therefore output high-level alarm detection signals ALM 1 , ALM 2 , respectively. As a result, the logical output of the AND gate  83  rises to “1”. Consequently, the AND gate  83  inputs an opposing station alarm to the working and protection multiplexers  81   a ,  81   b  as the additional data ADDT. The working and protection multiplexers  81   a ,  81   b  add the opposing station alarm onto the respective main signals and then send the results toward station A. The working and protection demultiplexers  82   a ,  82   b  of the SDH unit  53  in station A check to determine whether working and protection multiplexed signals contain the opposing station alarm. If this is the case, then the demultiplexers  82   a ,  82   b  output high-level opposing station alarm detection signals D 1 , D 2 . The OR gate  84  computes the logical sum of the signals D 1 , D 2  indicating whether the additional data ADDT of the working and protection multiplexed signals contains the opposing station alarm. When the result of OR operation is “1”, i.e., when either of the demultiplexers has received the opposing station alarm, the OR gate  84  instructs the switch  73  effect line switching. As a result, the switch  73  selects the protection multiplexed signal, which is being output by the protection multiplexer  81   b , instead of the working multiplexed signal from the working multiplexer  81   a , and sends the selected signal to the transmission line  57  so that communication may continue. 
     Thus, in accordance with the first embodiment, the higher level units  53  and  56  are allowed to perform the transmission and detection of the opposing station alarm, which are operations that have heretofore been performed by the SDH units. As a result, the SDH units can be standardized while the procedure involving the opposing station alarm is left to be executed in conformity with the international standard. 
     (b) Second Embodiment 
     (b-1) Construction 
     FIG. 5 is a diagram showing the construction of a line switching arrangement according to a second embodiment of the present invention. Components identical with those shown in FIG. 1 are designated by like reference characters. 
     The multiplexer section  81  has a function for multiplexing the additional data ADDT onto the main signals and then sending the multiplexed signal, and the demultiplexer section  82  has a function for demultiplexing a multiplexed signal into main signals and the additional data ADDT and then outputting the demultiplexed signals. The working multiplexer  81   a  provided in the higher level unit  53  of station A multiplexes a working route identification pattern (W pattern) onto the main signals and then sends the multiplexed signal. The protection multiplexer  81   b  multiplexes a protection route identification pattern (P pattern) onto the main signals and then sends the multiplexed signal. 
     The working demultiplexer  82   a  provided in the higher level unit  56  of station B extracts the route identification pattern that has been multiplexed onto the working main signal and inputs the pattern to an additional data producing unit (MIX)  86 , described later. The protection demultiplexer  82   b  extracts the route identification pattern that has been multiplexed onto the protection main signal and inputs the pattern to the additional data producing unit  86 . 
     An alarm detector (ALM DET)  85 , which is provided in the higher level unit  56  of station B, outputs an alarm signal ALM when the working and protection alarm detection signals ALM 1 , ALM 2  attain the high level simultaneously. The additional data producing unit (MIX)  86  produces the additional data ADDT, which comprises (1) the opposing station alarm and (2) the route identification patterns entering from the demultiplexers  82   a ,  82   b , and inputs the additional data ADDT to the working and protection multiplexers  81   a ,  81   b.    
     An opposing station alarm detector (DET/CONT)  87 , which is provided in the higher level unit  53  of station A, identifies the opposing station alarm and faulty route (working or protection route) from the additional data ADDT that has been multiplexed onto the working and protection signals sent from station B, and instructs the switch  73  to perform line switching based upon the results of identification. 
     (b-2) Line Switching Control 
     A working signal that has been externally applied to the SDH unit  51  of station A enters the working multiplexer  81   a  of the higher level unit  53  via the working transmitter  61 , whereby the working signal is time-division multiplexed with the working signal from the SDH unit  52  (not shown) and the W pattern. The resulting multiplexed signal is input to the switch  73 . A protection signal that has entered the SDH unit  51  of station A enters the protection multiplexer  81   b  of the higher level unit  53  via the protection transmitter  63  of the respective SDH unit  51 , whereby the protection signal is time-division multiplexed with the protection signal from the SDH unit  52  (not shown) and the P pattern. The resulting multiplexed signal is input to the switch  73 . The switch  73  selects the signal (onto which the W pattern has been multiplexed) from the working route, for example, and sends the selected signal to the transmission line  57 . 
     The hybrid circuit  74  of the higher level unit  56  of station B inputs a multiplexed signal, which has been transmitted from station A via the transmission line  57 , to the working and protection demultiplexers  82   a ,  82   b . The working demultiplexer  82   a  demultiplexes the input multiplexed signal, inputs the demultiplexed W pattern to the additional data producing unit (MIX)  86  and inputs the demultiplexed main signals to the working receivers  62  of the respective SDH units  54 ,  55  (the latter of which is not shown). The working receiver  62  sends this signal to the outside as the working signal. 
     The protection demultiplexer  82   b  demultiplexes the input multiplexed signal and inputs the demultiplexed main signals to the protection receivers  64  of the respective SDH units  54 ,  55  (the latter of which is not shown). The protection receiver  64  sends this signal to the outside as the protection signal. 
     Though the foregoing describes transmission of signals from station A to station B, operation is similar when signals are transmitted from station B to station A. 
     Under these normal transmission conditions, the working and protection receivers  62 ,  64  of SDH unit  54  notify the alarm detection unit  85  of the higher level unit  56  of the absence of an alarm. As a result, the alarm detector  85  inputs the absence of an alarm to the additional data producing unit  86 . The latter creates the additional data ADDT, which includes the W pattern but not the opposing station alarm, and inputs this data to the multiplexers  81   a ,  81   b . The multiplexers  81   a ,  81   b  multiplex this additional data ADDT onto the main signals and then transmit the data toward the station A. 
     The demultiplexer section  82  in the higher level unit  53  of station A extracts the additional data ADDT sent from the B station and inputs this data to the opposing station alarm detector  87 . Since the additional data ADDT does not contain the opposing station alarm, the opposing station alarm detector  87  does not switch lines. 
     If the working line develops a fault at point a under these conditions, the working and protection receivers  62 ,  64  of the SDH unit  54  of station B detect signal failure SF or line degradation SD and therefore output the high-level alarm detection signals ALM 1 , ALM 2 , respectively. As a result, the alarm detector  85  inputs the high-level alarm signal ALM to the additional data producing unit  86 . 
     It should be noted that even though a fault has occurred at point a, the working multiplexer  81   a  of the higher level unit  53  is multiplexing the W pattern onto the working main signal and sending the multiplexed signal. Consequently, the working demultiplexer  82   a  of the higher level unit  56  in station B extracts the W pattern and inputs the pattern to the additional data producing unit  86 . 
     When the alarm signal ALM attains the high level, the additional data producing unit  86  creates the additional data ADDT that includes the opposing station alarm and the W pattern and inputs the additional data ADDT to the working and protection multiplexers  81   a ,  81   b . The latter multiplex the additional data ADDT onto the main signals and then transmit the multiplexed signal toward station A. 
     The opposing station alarm detector  87  identifies the opposing station alarm and the faulty route (the main route in this case) from the additional data ADDT extracted by the working and protection demultiplexers  82   a ,  82   b  and instructs the switch  73  to perform line switching based upon the results of identification. 
     When the line switching operation ends, the route identification pattern is changed from the W pattern to the P pattern automatically and the P pattern is sent back to the higher level unit  53  of station A via the higher level unit  56  of station B. At this time switching is performed reliably owing to verification of the presence or absence of an alarm, whereby it can be confirmed whether the alarm has been eliminated. 
     The foregoing relates to a case where the working line failed at point a. However, there are instances where a failure occurs at a point b along the transmission line. In such case line switching would be meaningless and therefore unnecessary. Here it should be noted that a route identification pattern will not enter the additional data producing unit  86  when the transmission line  57  fails. Accordingly, the occurrence of an alarm unaccompanied by input of a route identification pattern is judged to be a failure in the transmission line  57  and the additional data producing unit  86  does not include the opposing station alarm in the additional data ADDT. Line switching, therefore, is not carried out. 
     Thus, in accordance with the second embodiment, effects similar to those of the first embodiment can be obtained. Moreover, the faulty route can be identified. Further, according to the second embodiment, it can be so arranged that unnecessary line switching is not performed when a fault develops in the transmission line. 
     (c) Third Embodiment 
     (c-1) Construction 
     FIG. 6 is a diagram showing the construction of a line switching arrangement according to a third embodiment of the present invention. Components identical with those shown in FIG. 1 are designated by like reference characters. 
     This arrangement includes the opposing station alarm detector (DET/CONT)  87  provided in the higher level unit  53  of station A for identifying the opposing station alarm from the additional data ADDT that has been multiplexed onto the working and protection signals sent from station B, and instructing the switch  73  to perform line switching based upon the results of identification. 
     The arrangement of FIG. 6 further includes a pattern generator  91  provided in the higher level unit  53  of station A for generating a predetermined pattern and inputting the pattern to the working and protection transmitters  61 ,  63  of the SDH unit  51 . The transmitters  61 ,  63  each send the above-mentioned pattern to station B using the undefined bytes D 1 ˜D 12 , F 1 , E 1 , E 2  among the overhead bytes OHB of the main signals. 
     This embodiment further includes a pattern detector (DET)  92  provided in the higher level unit  56  of station B. The working and protection receivers  62 ,  64  of station B identify the patterns, which have been sent by the undefined bytes D 1 ˜D 12 , F 1 , E 1 , E 2  among the overhead bytes OHB of the main signals, and input the patterns to the pattern detector  92 . The latter compares the patterns that have entered from the working and protection receivers  62 ,  64  with an already known pattern and determines whether the compared patterns match. If neither pattern matches the known pattern, the pattern detector  92  judges that a line failure has occurred, creates the additional data ADDT inclusive of the opposing station alarm and inputs this data to the working and protection multiplexers  81   a ,  81   b.    
     (c-2) Line Switching Control 
     The working and protection transmitters  61 ,  63  of the SDH unit  51  of station A insert the pattern entering from the pattern generator  91  into the working and protection main signals using the undefined bytes of the overhead bytes OHB and input the main signals to the working and protection multiplexers  81   a ,  81   b  of the higher level unit  53 . The working multiplexer  81   a  time-division multiplexes the working signals from the SDH units  51  and  52  (the latter of which is not shown) and inputs the resulting multiplexed signal to the switch  73 . Further, the protection multiplexer  81   b  timedivision multiplexes the protection signals from the SDH units  51  and  52  (the latter of which is not shown) and inputs the resulting multiplexed signal to the switch  73 . The switch  73  selects the signal from the working route, for example, and sends the selected signal to the transmission line  57 . 
     The hybrid circuit  74  of the higher level unit  56  of station B inputs the multiplexed signal, which has been transmitted from station A via the transmission line  57 , to the working and protection demultiplexers  82   a ,  82   b . The working demultiplexer  82   a  demultiplexes the input multiplexed signal and inputs the demultiplexed main signals to the working receivers  62  of the respective SDH units  54 ,  55  (the latter of which is not shown). The working receiver  62  sends this signal to the outside as the working signal, extracts the pattern from the undefined bytes of the overhead bytes of the main signals and inputs the pattern to the pattern detector  92 . The protection demultiplexer  82   b  demultiplexes the input multiplexed signal and inputs the demultiplexed main signals to the protection receivers  64  of the respective SDH units  54 ,  55  (the latter of which is not shown). The protection receiver  64  sends this signal to the outside as the protection signal, extracts the pattern from the undefined bytes of the overhead bytes of the main signals and inputs the pattern to the pattern detector  92 . 
     Under these normal operating conditions, the patterns extracted by the working and protection receivers  62 ,  64  match the pattern generated by the pattern generator  91 . Accordingly, the pattern detector  92  creates the additional data ADDT, which does not include the opposing station alarm, and inputs this data to the multiplexers  81   a ,  81   b . The multiplexers  81   a ,  81   b  multiplex this additional data ADDT onto the main signals and then transmit the data toward the station A. 
     The demultiplexer section  82  in the higher level unit  53  of station A extracts the additional data ADDT sent from the B station and inputs this data to the opposing station alarm detector  87 . Since the additional data ADDT does not contain the opposing station alarm, the opposing station alarm detector  87  does not switch lines. 
     If the working line develops a fault at point a under these conditions, the working and protection receivers  62 ,  64  of the SDH unit  54  of station B cannot extract the pattern generated by the pattern generator  91  and, as a consequence, the patterns output by the working and protection receivers  62 ,  64  will no longer match the known pattern. If this occurs, the pattern detector  92  construes this as being indicative of line failure, creates the additional data ADDT inclusive of the opposing station alarm and inputs the data to the working and protection multiplexers  81   a ,  81   b . The latter multiplex the additional data ADDT onto the main signals and then transmit the multiplexed signal toward station A. 
     The opposing station alarm detector  87  identifies the opposing station alarm from the additional data ADDT extracted by the working and protection demultiplexers  82   a ,  82   b  and instructs the switch  73  to perform line switching based upon the results of identification. 
     When the line switching operation ends, the above-mentioned pattern is sent back to the higher level unit  53  of station A via the higher level unit  56  of station B. At this time switching is performed reliably owing to verification of the presence or absence of an alarm, whereby it can be confirmed whether the alarm has been eliminated. 
     Thus, in accordance with the third embodiment, effects similar to those of the first embodiment can be obtained. In addition, even if line failure cannot be definitely detected based upon signal failure SF or signal degradation SD, line failure can be detected and line switching performed promptly and reliably based upon non-matching of the compared patterns. 
     (d) Fourth Embodiment 
     (d-1) Construction 
     FIG. 7 is a diagram showing the construction of a line switching arrangement according to a fourth embodiment of the present invention. Components identical with those shown in FIG. 1 are designated by like reference characters. 
     This arrangement includes the opposing station alarm detector (DET/CONT)  87  provided in the higher level unit  53  of station A for identifying the opposing station alarm from the additional data ADDT that has been multiplexed onto the working and protection signals sent from station B, and instructing the switch  73  to perform line switching based upon the results of identification. 
     The arrangement of FIG. 7 further includes a pattern generator  93  provided in the higher level unit  53  of station A for constantly generating W and P patterns and inputting the W and P patterns to the working and protection transmitters  61 ,  63 , respectively, of the SDH unit  51 . The working and protection transmitters  61 ,  63  insert the W and P patterns into the main signals using the undefined bytes D 1 ˜D 12 , F 1 , E 1 , E 2  among the overhead bytes OHB of the main signals and send the resulting signals to station B. 
     This embodiment further includes a pattern detector (DET)  94  provided in the higher level unit  56  of station B. The working and protection receivers  62 ,  64  of station B identify the patterns, which have been sent by the undefined bytes D 1 ˜D 12 , F 1 , E 1 , E 2  among the overhead bytes OHB of the main signals, and input the patterns to the pattern detector  92 . The latter compares the patterns that have entered from the working and protection receivers  62 ,  64  with an already known pattern (W or P pattern) and determines whether the compared patterns match. If neither pattern matches the known pattern, the pattern detector  94  judges that a line failure has occurred, creates the additional data ADDT inclusive of the opposing station alarm and inputs this data to the working and protection multiplexers  81   a ,  81   b.    
     (d-2) Line Switching Control 
     The working and protection transmitters  61 ,  63  of the SDH unit  51  of station A insert the W and P patterns entering from the pattern generator  93  into the working and protection main signals, respectively, using the undefined bytes of the overhead bytes OHB and input the main signals to the working and protection multiplexers  81   a ,  81   b  of the higher level unit  53 . The working multiplexer  81   a  time-division multiplexes the working signals from the SDH units  51  and  52  (the latter of which is not shown) and inputs the resulting multiplexed signal to the switch  73 . Further, the protection multiplexer  81   b  time-division multiplexes the protection signals from the SDH units  51  and  52  (the latter of which is not shown) and inputs the resulting multiplexed signal to the switch  73 . The switch  73  selects the signal from the working multiplexer  81   a  (the signal in which the W pattern has been inserted), for example, and sends the selected signal to the transmission line  57 . 
     The hybrid circuit  74  of the higher level unit  56  of station B inputs the working-route multiplexed signal, which has been transmitted from station A via the transmission line  57 , to the working and protection demultiplexers  82   a ,  82   b . The working demultiplexer  82   a  demultiplexes the input working-route multiplexed signal and inputs the demultiplexed main signals to the working receivers  62  of the respective SDH units  54 ,  55  (the latter of which is not shown). The working receiver  62  sends this signal to the outside as the working signal, extracts the W pattern from the undefined bytes of the overhead bytes OHB of the main signals and inputs the W pattern to the pattern detector  94 . 
     Under these normal operating conditions, the patterns extracted by the working and protection receivers  62 ,  64  match the W pattern. Accordingly, the pattern detector  94  creates the additional data ADDT, which does not include the opposing station alarm, and inputs this data to the multiplexers  81   a ,  81   b . The multiplexers  81   a ,  81   b  multiplex this additional data ADDT onto the main signals and then transmit the data toward the station A. 
     The working and protection demultiplexers  82   a ,  82   b  of the higher level unit  53  in station A extract the additional data ADDT sent from the B station and input this data to the opposing station alarm detector  87 . Since the additional data ADDT does not contain the opposing station alarm, the opposing station alarm detector  87  does not switch lines. 
     If the working line develops a fault at point a under these conditions, the working and protection receivers  62 ,  64  can no longer extract the W pattern. If this occurs, the pattern detector  94  construes this as being indicative of line failure, creates the additional data ADDT inclusive of the opposing station alarm and inputs the data to the working and protection multiplexers  81   a ,  81   b . The latter multiplex the additional data ADDT onto the main signals and then transmit the multiplexed signal toward station A. 
     The opposing station alarm detector  87  identifies the opposing station alarm from the additional data ADDT extracted by the working and protection demultiplexers  82   a ,  82   b  and instructs the switch  73  to perform line switching from the working to the protection line based upon the results of identification. 
     When the line switching operation ends, the route identification pattern is changed from the W pattern to the P pattern and the P pattern is sent back to the higher level unit  53  of station A via the higher level unit  56  of station B. At this time switching is performed reliably owing to verification of the presence or absence of an alarm, whereby it can be confirmed whether the alarm has been eliminated. 
     Thus, in accordance with the third embodiment, effects similar to those of the first embodiment can be obtained. Moreover, the faulty route can be identified. 
     Further, according to the fourth embodiment, even if line failure cannot be definitely detected based upon signal failure SF or signal degradation SD, line failure can be detected and line switching performed promptly and reliably based upon non-matching of the compared patterns. 
     (e) Fifth Embodiment 
     (e-1) Construction 
     FIG. 8 is a diagram showing the construction of a line switching arrangement according to a fifth embodiment of the present invention. Components identical with those shown in FIG. 1 are designated by like reference characters. 
     This arrangement includes an opposing station alarm detector (DET/CONT)  88  provided in the higher level unit  53  of station A for determining whether the opposing station alarm is being sent by the K 1  byte of the overhead bytes OHB in the working and protection signals sent from station B, and instructing the switch  73  to perform line switching based upon the results of the determination. 
     This arrangement further includes an alarm signal generator (ALM GEN)  90  for sending an alarm signal to the working and protection transmitters  61 ,  63  when line failure has been detected. Upon receiving the alarm signal applied thereto, the transmitters  61 ,  63  notify station A of the occurrence of failure using the K 1  byte of the overhead bytes OHB in the working and protection signals. 
     The pattern generator  91  is provided in the higher level unit  53  of station A, generates a predetermined pattern and inputs the pattern to the working and protection transmitters  61 ,  63  of the SDH unit  51 . The transmitters  61 ,  63  each send the above-mentioned pattern to station B using the undefined bytes D 1 ˜D 12 , F 1 , E 1 , E 2  among the overhead bytes OHB of the main signals. 
     The pattern detector  92  is provided in the higher level unit  56  of station B. The working and protection receivers  62 ,  64  of station B identify the patterns, which have been sent by the undefined bytes D 1 ˜D 12 , F 1 , E 1 , E 2  among the overhead bytes OHB of the main signals, and input the patterns to the pattern detector  92 . The latter compares the patterns that have entered from the working and protection receivers  62 ,  64  with an already known pattern and determines whether the compared patterns match. If neither pattern matches the known pattern, the pattern detector  92  judges that a line failure has occurred and enters a signal indicative of this fact into the alarm signal generator (ALM GEN)  90 . 
     (e-2) Line Switching Control 
     The working and protection transmitters  61 ,  63  of the SDH unit  51  of station A insert the fixed pattern entering from the pattern generator  91  into the working and protection main signals using the undefined bytes of the overhead bytes OHB and input the main signals to the working and protection multiplexers  81   a ,  81   b  of the higher level unit  53 . The working multiplexer  81   a  time-division multiplexes the working signals from the SDH units  51  and  52  (the latter of which is not shown) and inputs the resulting multiplexed signal to the switch  73 . Further, the protection multiplexer  81   b  time-division multiplexes the protection signals from the SDH units  51  and  52  (the latter of which is not shown) and inputs the resulting multiplexed signal to the switch  73 . The switch  73  selects the signal from the working route, for example, and sends the selected signal to the transmission line  57 . 
     The hybrid circuit  74  of the higher level unit  56  of station B inputs the multiplexed signal, which has been transmitted from station A via the transmission line  57 , to the working and protection demultiplexers  82   a ,  82   b . The working demultiplexer  82   a  demultiplexes the input multiplexed signal and inputs the demultiplexed main signals to the working receivers  62  of the respective SDH units  54 ,  55  (the latter of which is not shown). The working receiver  62  sends this signal to the outside as the working signal, extracts the pattern from the undefined bytes of the overhead bytes OHB of the main signals and inputs the pattern to the pattern detector  92 . The protection demultiplexer  82   b  demultiplexes the input multiplexed signal and inputs the demultiplexed main signals to the protection receivers  64  of the respective SDH units  54 ,  55  (the latter of which is not shown). The protection receiver  64  sends this signal to the outside as the protection signal, extracts the pattern from the undefined bytes of the overhead bytes of the main signals and inputs the pattern to the pattern detector  92 . 
     Under these normal operating conditions, the patterns extracted by the working and protection receivers  62 ,  64  match the pattern generated by the pattern generator  91 . Accordingly, the pattern detector  92  inputs a signal indicative of pattern matching to the alarm signal generator  90  and the alarm signal generator  90  recognizes the fact that a line failure has not occurred. As a result, notification of line failure is not indicated by the overhead bytes and, hence, the opposing station alarm detector  88  does not switch lines. 
     If the working line develops a fault at point a under these conditions, the patterns output by the working and protection receivers  62 ,  64  will no longer match the known pattern owing to signal failure or signal degradation. As a result, the pattern detector  92  inputs a signal indicative of non-matching to the alarm signal generator  90 . In response, the alarm signal generator  90  notifies the working and protection transmitters  61 ,  63  of the SDH unit  54  of the occurrence of line failure. In response to such notification, the working and protection transmitters  61 ,  63  transmit the opposing station alarm by the overhead bytes OHB (the K 1  byte). 
     The working and protection receivers  62 ,  64  of station A each extract the K 1  byte of the overhead bytes OHB and input the K 1  byte to the opposing station alarm detector  88 . In response to the K 1  byte of the overhead bytes OHB, the opposing station alarm detector  88  detects the fact that notification has been given of line failure and instructs the switch  73  to switch lines. 
     Thus, in accordance with the fifth embodiment, effects similar to those of the first embodiment can be obtained. Moreover, even if a terminal station does not possess a function for multiplexing the additional data ADDT onto main signals and then transmitting the same, a pattern or opposing station alarm can be sent to the terminal station of the other party by the overhead bytes OHB so that line switching can be carried out. 
     (f) Sixth Embodiment 
     (f-1) Construction 
     FIG. 9 is a diagram showing the construction of a line switching arrangement according to a fourth embodiment of the present invention. Components identical with those shown in FIG. 1 are designated by like reference characters. 
     This arrangement includes the opposing station alarm detector (DET/CONT)  88  provided in the higher level unit  53  of station A for determining whether the opposing station alarm is being sent by the K 1  byte of the overhead bytes OHB in the working and protection main signals sent from station B, and instructing the switch  73  to perform line switching based upon the results of the determination. 
     This arrangement further includes the alarm signal generator (ALM GEN)  90  for notifying the working and protection transmitters  61 ,  63  of the SDH unit  54  that a line failure has occurred. Upon receiving notification of line failure, the transmitters  61 ,  63  notify station A of the occurrence of the failure using the K 1  byte of the overhead bytes OHB in the working and protection main signals. 
     The arrangement of FIG. 9 further includes the pattern generator  93  provided in the higher level unit  53  of station A for constantly generating W and P patterns and inputting the W and P patterns to the working and protection transmitters  61 ,  63 , respectively, of the SDH unit  51 . The working and protection transmitters  61 ,  63  insert the W and P patterns into the working and protection main signals using the undefined bytes D 1 ˜D 12 , F 1 , E 1 , E 2  among the overhead bytes OHB of the main signals and send the resulting signals to station B. 
     This embodiment further includes the pattern detector (DET)  94  provided in the higher level unit  56  of station B. The working and protection receivers  62 ,  64  of station B identify the patterns, which have been sent by the undefined bytes D 1 ˜D 12 , F 1 , E 1 , E 2  among the overhead bytes OHB of the main signals, and input the patterns to the pattern detector  92 . The latter compares the patterns that have entered from the working and protection receivers  62 ,  64  with an already known pattern (W or P pattern) and determines whether the compared patterns match. If neither pattern matches the known pattern, the pattern detector  94  inputs a signal indicative of non-matching to the alarm signal generator  90 . 
     (f-2) Line Switching Control 
     The working and protection transmitters  61 ,  63  of the SDH unit  51  of station A insert the W and P patterns entering from the pattern generator  93  into the working and protection main signals using the undefined bytes of the overhead bytes OHB and input the main signals to the working and protection multiplexers  81   a ,  81   b  of the higher level unit  53 . The working multiplexer  81   a  time-division multiplexes the working signals from the SDH units  51  and  52  (the latter of which is not shown) and inputs the resulting multiplexed signal to the switch  73 . Further, the protection multiplexer  81   b  time-division multiplexes the protection signals from the SDH units  51  and  52  (the latter of which is not shown) and inputs the resulting multiplexed signal to the switch  73 . The switch  73  selects the signal from the working multiplexer  81   a  (the signal in which the W pattern has been inserted), for example, and sends the selected signal to the transmission line  57 . 
     The hybrid circuit  74  of the higher level unit  56  of station B inputs the multiplexed signal, which has been transmitted from station A via the transmission line  57 , to the working and protection demultiplexers  82   a ,  82   b . The working demultiplexer  82   a  demultiplexes the input working multiplexed signal and inputs the demultiplexed main signals to the working receivers  62  of the respective SDH units  54 ,  55  (the latter of which is not shown). The working receiver  62  sends this signal to the outside as the working signal, extracts the W pattern from the undefined bytes of the overhead bytes OHB of the main signals and inputs the W pattern to the pattern detector  94 . 
     Under these normal operating conditions, the patterns extracted by the working and protection receivers  62 ,  64  match the W pattern. Accordingly, the pattern detector  94  inputs a signal indicative of pattern matching to the alarm signal generator  90  and the alarm signal generator  90  recognizes the fact that a line failure has not occurred. As a result, notification of line failure is not indicated by the overhead bytes (K 1  byte) and, hence, the opposing station alarm detector  88  does not switch lines. 
     If the working line develops a fault at point a under these conditions, the patterns output by the working and protection receivers  62 ,  64  will no longer match the known W pattern owing to signal failure or signal degradation. As a result, the pattern detector  94  inputs a signal indicative of non-matching to the alarm signal generator  90 . In response, the alarm signal generator  90  notifies the working and protection transmitters  61 ,  63  of the SDH unit  54  of the occurrence of line failure. In response to such notification, the working and protection transmitters  61 ,  63  transmit the opposing station alarm by the overhead bytes OHB. 
     The working and protection receivers  62 ,  64  of station A each extract the K 1  byte of the overhead bytes OHB and input the K 1  byte to the opposing station alarm detector  88 . In response to the K 1  byte of the overhead bytes OHB, the opposing station alarm detector  88  is notified of line failure and instructs the switch  73  to switch lines. 
     It should be noted that when operation is normal, station B sends station A the identified pattern (the pattern to be received by station B) by the overhead bytes OHB. When notification of line failure is given, station B sends station A the pattern to be received, along with the opposing station alarm, by the overhead bytes OHB. As a result, station A is capable of identifying the faulty route when the opposing station alarm is received and instructs the switch  73  to switch lines from the faulty line to the normal line based upon the identification made. When line switching is completed, the line identification pattern is changed over and this pattern is sent back to the higher level unit  53  of station A via the higher level unit  56  of station B. At this time, therefore, switching is performed reliably owing to verification of the presence or absence of an alarm, whereby it can be confirmed whether the alarm has been eliminated. 
     Thus, in accordance with the sixth embodiment, effects similar to those of the first embodiment can be obtained. Moreover, even if a terminal station does not possess a function for multiplexing the additional data ADDT onto main signals and then transmitting the same, a pattern or opposing station alarm can be sent to the terminal station of the other party by the overhead bytes OHB so that line switching can be carried out. 
     Thus, in accordance with the present invention as described above, the higher level units are allowed to perform the transmission and detection of the opposing station alarm, which are operations that have heretofore been performed by the SDH units. As a result, the SDH units can be standardized while the procedure involving the opposing station alarm is left to be executed in conformity with the international standard. 
     Further, in accordance with the present invention, working and protection route identification patterns are sent along with main signals, thereby making it possible to perform line switching correctly by identifying the faulty route. Moreover, unnecessary line switching is not carried out when a failure occurs in the transmission line of a communication system not having transmission line redundancy. 
     In accordance with the present invention, line switching can be performed upon detecting line failure quickly and accurately based upon non-matching of inserted and detected patterns even in a case where line failure cannot be definitely detected based upon signal failure or signal degradation. 
     In accordance with the present invention, a pattern or opposing station alarm can be sent to the terminal station of the other party by overhead bytes so that line switching can be carried out even if a terminal station does not possess a function for multiplexing the additional data onto main signals and then transmitting the same. 
     As many apparently widely different embodiments of the present invention can be made without departing from the spirit and scope thereof, it is to be understood that the invention is not limited to the specific embodiments thereof except as defined in the appended claims.