Patent Publication Number: US-6219336-B1

Title: Terminal multiplexer and method of constructing the terminal multiplexer

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. application Ser. No. 08/884,137, filed Jun. 27,1997, hereby incorporated by reference now U.S. Pat. 6,049,525. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a terminal multiplexer which performs multiplexing and demultiplexing of digital signals between a plurality of low speed transmission lines and a high speed transmission line in a digital communication network and relates to a terminal multiplexer such as a channel rearrange equipment, for example ADM (Add Drop Multiplexer), having a cross connect function. 
     2. Description of Related Art 
     As a transmission system using a terminal multiplexer which performs multiplex conversion of signals between a plurality of low speed transmission lines and a high speed transmission line, there is known a system in which line terminating equipments (hereinafter, referred to as “LTE”) are connected in a point-to-point manner, perform time division multiplexing of low speed signals received from a plurality of low speed transmission lines to send them as high speed signals onto a high speed transmission line, and perform demultiplexing of high speed signals received from the high speed transmission line into a plurality of low speed signals to send the demultiplexed signals onto respective low speed transmission lines, as shown in FIG.  13 A. 
     As a configuration for realizing automatic protection switching in a transmission system using such LTEs, there are known 1:1 configuration and 1:n configuration. In the 1:1 configuration, a set of two working high speed transmission lines which transmit signals in opposite directions to each other and a set of two protection (i.e., preparatory) high speed transmission lines which transmit signals in opposite directions to each other are provided between the LTEs at both ends. In the 1:n configuration, between the LTEs at both ends, there are provided a plurality of working high speed transmission lines in sets of pairs, which transmit signals in the opposite directions to each other, and a set of two protection high speed transmission lines which are used in common with the plurality of high speed transmission lines and transmit signals in opposite directions to each other. 
     In the present description, working lines will be represented by the symbol “W” (working), and protection lines will be represented by the symbol “P” (Protection), Further, as for terminal multiplexer connected between two other terminal multiplexers, one side of the two other terminal multiplexers will be described as “West” and the other side will be described as “East”. 
     Now, in LTEs in 1:1 and 1:n configurations, when a problem arises in a working high speed transmission line, automatic protection switching is performed so that the high speed transmission line used for transmitting signals is switched from the faulty one to a protection high speed transmission line. As a method for out this switching, the bi-directional switching method and the uni-directional switching method are known. In the bi-directional switching method, as shown in FIG. 13B, both of the two high speed transmission lines in the faulty set are switched to two protection high speed transmission lines. In the uni-directional switching method, as shown in FIG. 18C, only the faulty high speed transmission line is switched to a line having the same direction of transmitting signals as the faulty line out of the protection high speed transmission lines. 
     Further, as a terminal multiplexer which performs multiple conversion between a plurality of low speed transmission lines and a high speed transmission line, as shown in FIG. 14A, there is known an ADM which performs demultiplexing of some high speed signals received from the high speed transmission line (West side) into a plurality of low speed signals to send them onto the respective low speed transmission lines, and performs time division multiplexing of the remaining high speed signals received from the high speed transmission line and low speed signals received from the low speed transmission lines to send the multiplexed signals onto the other high speed transmission line (East side), or performs similar operations in the reverse direction from the East side to the West side. 
     As a configuration of a transmission system using such an ADM, there are known a linear configuration in which ADMs are located between LTEs as shown in FIG. 14B, and a ring configuration in which a plurality of ADMs are connected in a ring shape with high speed transmission lines as shown in FIG.  14 C. 
     As the linear configuration using ADMs, there are known two configurations corresponding respectively to the above-described 1:1 and 1:n configurations of LTEs. In the 1:1 linear configuration, as shown in FIG. 15A, on each of the West and East sides, there are provided a set of two working high speed transmission lines for transmitting signals in opposite directions to each other and a set of two protection high speed transmission lines for transmitting signals in opposite directions to each other. In the 1:n linear configuration, as shown in FIG. 15B, on each of the West and East sides, there are provided a plurality of working high speed transmission lines in sets of two, transmitting signals in opposite directions to each other, and a set of two protection high speed transmission lines which are used in common with the plurality of high speed transmission lines and transmit signals in opposite directions to each other. Here, at the time of the automatic protection switching of ADM in 1:1 and 1:n linear configurations, switching from the working high speed transmission lines to the protection high speed transmission lines is performed on each of the West and East sides, similarly to the above-described switching in LTE in 1:1 and 1:n configurations. 
     On the other hand, as the ring configuration in which ADMs are connected in a ring shape, there have been proposed a 2 fiber configuration in which each pair of adjacent ADMs are connected with a set of two optical fiber transmission lines transmitting signals in opposite directions to each other, and a 4 fiber configuration in which each pair of adjacent ADMs are connected with two sets of two optical fiber transmission lines transmitting signals in opposite directions to each other. 
     Further, as the 4 fiber ring configuration, there is known 4-Fiber BLSR (Bi-directional Line Switched Ring) in which, as shown in FIG. 16A, out of two sets of optical fiber transmission lines connecting each pair of ADMs, one set is used as working lines and the other set is used as protection lines. As the 2 fiber ring configuration as shown in FIG. 17A, there are known 2-Fiber UPSR (Uni-directional Path Switched Ring) and 2-Fiber BLSR. In 2-Fiber UPSR, optical fiber transmission lines transmitting signals in one rotational direction are used as working lines and optical fiber transmission lines transmitting signals in the other rotational direction are used as protection lines, and switching is performed for each path. In 2-Fiber BLSR, instead of setting a working or protection line for each optical fiber transmission line, some time slots on each optical fiber transmission line are used as working slots and the other time slots are used as protection slots. 
     Now, switching from working lines to protection lines at the time of automatic protection switching in 4-Fiber BLSR is illustrated in FIGS. 16B and 16C. As shown, as the switching performed by ADMs adjacent to a faulty portion in 4-Fiber BLSR, there are two kinds of switching, i.e., (1) switching from a set of working optical fiber transmission lines on the side of the faulty portion to a set of protection optical fiber transmission lines on the side of the faulty portion (FIG.  16 B), and (2) turning back of signal flows from a set of working optical transmission lines on the opposite side to the faulty portion to a set of protection optical fiber transmission lines on the opposite side to the faulty portion (FIG.  16 C), the former being called Span Switch, and the later Ring Switch. 
     FIG. 17B illustrates the switching from the working time slots to the protection time slots at the time of automatic protection switching in 2-Fiber BLSR, and FIG,  17 C illustrates the switching from the working path to the protection path at the time of automatic protection switching in 2-Fiber UPSR. 
     As shown, switching in 2-Fiber BLSR is performed in such a manner that ADMs adjacent to a fault portion turn back signal flow in working time slots of two optical fiber transmission lines on the opposite side to the faulty portion into protection time slots of two optical fiber transmission on the opposite side to the faulty portion. In FIG. 17B, in the case that time slots A-F of #1 optical fiber transmission lines and time slots A-F of #2 optical fiber transmission lines are used as working time slots, and time slots G-L of #1 optical fiber transmission lines and time slots G-L of #2 optical fiber transmission lines are used as protection time slots, ADM A, B adjacent to the faulty portion turn back signal flow in the time slots A-F of #1 optical fiber transmission lines into the time slots G-F of #2 optical fiber transmission lies, and turn back signal flow in the time slots A-F of #2 optical fiber transmission lines into the time slots G-F of #1 optical fiber transmission lines. 
     Further, switching of 2-Fiber UPSR is performed as shown in FIG.  17 C. Namely, each ADM transmits signals to other ADMs, using both the working optical fiber transmission lines and protection optical fiber transmission lines. In a normal condition, each ADM receives signals from other ADMs through working optical fiber transmission line and processes them, and when it can not receive from a particular ADM through the working optical fiber transmission line, it receives signals from that particular ADM through protection optical fiber transmission line and processes them. 
     As described above, functions required for a terminal multiplexer vary according to LTE used in the 1:1 configuration, LTE used in the 1:n configuration, ADM used in the 1:1 linear configuration, ADM used in the 1:n linear configuration, ADM used in 4-Fiber BLSR, ADM used in 2-Fiber BLSR, and ADM used in 2-Fiber UPSR. Accordingly, LTEs or ADMs have, conventionally, been made as dedicated equipments for each particular configuration of transmission system. 
     Sometimes, it is desired to change a configuration of a transmission system, for example, in order to make the transmission system advance after the start of its operation. For example, it may be desired that, in order to increase transmission capacity, a transmission system using LTEs connected in 1:1 configuration in a point-to-point manner is changed to a transmission stem using LTEs connected in 1:n configuration in a point-to-point manner, that a transmission system using LTEs connected in 1:1 configuration in a point-to-point manner is changed to 2-Fiber BLSR or 4-Fiber BLSR, in accordance with increase in connected points. 
     Conventionally, however, each LTE or ADM is a dedicated equipment for a transmission system before change, and therefore, when configuration of such a transmission system is to be changed, LTEs or ADMs should be exchanged, so that the burden of introducing equipments is large at the time of changing the configuration of a transmission system. Further, when LTEs or ADMs are exchanged, a transmission system must be taken down once, and communication must be stopped. 
     On the other hand, in accordance with recent increase in transmission capacity of a transmission system, a multiplex conversion equipment becomes of large scale. Accordingly, for example, it is, now, difficult to construct a terminal multiplexer adapted for OC-192 optical carrier 192 using an optical fiber transmission line with 10 G of transmission capacity as a high speed transmission line, in one rack. Here, a “track” is a case which houses electronic boards constituting a terminal multiplexer, and is provided with printed circuits connecting between electronic boards. A rack is limited in its size from the viewpoint of handling requirements such as transportation and installation. Thus, when a terminal multiplexer can not be constructed with single rack but with a plurality of racks, signals should be sent and received among the racks. To send and receive signals among the racks, cables should be used instead of printed circuit on an electronic board. Accordingly, and for other reasons, there are some limitations in terms of number and speed of signals, differently from sending and receipt of signals within a rack. Thus, for example, it is difficult to employ such a configuration that, in LTE etc. of 1:n configurations, n working high speed transmission lines and one protection high speed transmission line are connected to inputs of a single selector, and that selector switches the above-described working and protection lines. 
     SUMMARY OF THE INVENTION 
     A first object of the present invention is to provide a method of constructing a terminal multiplexer, by which a configuration of a transmission system can be changed by upgrading terminal multiplexers such as LTEs or ADMs used in a conventional transmission system. 
     A second object of the present invention is to provide a terminal multiplexer having a configuration suited for using a plurality of racks to construct a terminal multiplexer. 
     To accomplish the above-described first object, the present invention provides a method of constructing a terminal multiplexer, comprises steps of providing a high speed transmission line interface unit responsible for signal input-output interface with a set of sending and receiving high speed transmission lines; a low speed transmission line interface unit responsible for signal input-output interface with a set of sending and receiving low speed transmission lines; a multiplex converting unit for performing multiplexing and demultiplexing between high speed signals transmitted on the high speed transmission lines and low speed signals transmitted on the low speed transmission lines; and a switching unit for performing switching between the high speed signals transmitted on the high speed transmission lines and the low speed signals transmitted on the low speed transmission lines, which has an interface for signals outside of the unit, which is made common with an interface for signals outside of the multiplex converting unit; 
     combining the high speed transmission line interface unit, and the multiplex converting unit, the low speed transmission line interface unit to construct a terminal multiplexer; and 
     constructing a channel rearrange equipment by substituting the switching unit for the multiplex converting unit of the terminal multiplexer. 
     According to such a construction method, a terminal multiplexer such as LTE can be upgraded to a channel rearrange equipment such as ADM, simply by substituting a switching unit for a terminal multiplexer. Thus, by such upgrading, the configuration of a transmission system can be changed. 
     Further, to accomplish the above-described second object, the present invention provides a terminal multiplexer for transmitting signals to an apparatus at each side to be connected to the terminal multiplexer, using, for example, n (n is an integer greater than or equal to 1) sets of working high speed transmission lines and one set of sending and receiving protection high speed transmission lines, comprising: 
     n working equipments, i.e. 1st to n-th working equipments and one protection equipment; wherein each of the working equipments comprises: a high speed transmission line interface unit responsible for signal input-output interface with a set of working high speed transmission lines; 
     a plurality of low speed transmission line interface units responsible for signal input-output interface with a set of sending and receiving low speed transmission lines; 
     a multiplex converting unit which performs demultiplexing of high speed signals received by the high speed transmission line interface unit from the working high speed transmission lines, to distribute the demultiplexed signals to respective the low speed transmission line interface units as signals to be sent to the low speed transmission lines, and performs multiplexing of low speed signals received by the respective low speed transmission line interface units from low speed transmission lines, to send the multiplexed signals to the high speed transmission line interface unit as high speed signals to be sent to the working high speed transmission lines, and 
     a first forwarding unit connected to the multiplex converting unit; 
     the protection equipment comprising at least: 
     a high speed transmission line interface unit responsible for signal input-output interface with a set of protection high speed transmission lines; and 
     a second forwarding unit connected to the high speed transmission line interface unit; 
     the second forwarding unit of the protection equipment being connected to the first forwarding unit of the first working equipment; 
     the first forwarding unit of the m-th (m is an integer varying from 1 to (n−1)) working equipment being connected to the first forwarding unit of the (m+1)-th working equipment; and 
     the first forwarding units of the working equipments and the second forwarding unit of the protection equipment forming a transmission system which forwards high speed signals received by the high speed transmission line interface unit of the protection high speed transmission lines from the protection high speed transmission lines, to a multiplex converting unit of any working equipment successively, as high speed signals to be objects of demultiplexing in the multiplex converting unit in question instead of high speed signals received by the high speed transmission line interface unit, and forwarding (sending) high speed signals multiplexed by the multiplex converting unit of any working equipment to the high speed transmission line interface unit of the protection equipment as high speed signals to be sent from the protection high speed transmission lines. 
     According to a terminal multiplexer constructed as such a terminal multiplexer, by forwarding high speed signals which should be sent to or received from protection high speed transmission lines, successively between forwarding units of the working equipments and protection equipment, it is possible to extend the protection high speed transmission lines as substitutes of a faulty working high speed transmission line up to a multiplex converting unit of a working equipment connected to the faulty working high speed transmission line. Thus, capacity corresponding to the transmission capacity of the protection high speed transmission line is sufficient as the capacity of signal line needed for connection between the working equipments and the protection equipment. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a view showing construction of a rack used for a terminal multiplexer; 
     FIG. 2A is a block diagram showing construction of LTE in 1:1 configuration; 
     FIG. 2B is a view showing a state in which units of the LTE of FIG. 2A are mounted in a rack; 
     FIG. 3A is a block diagram showing construction of ADM applied for 2-Fiber UPSR/BLSR; 
     FIG. 3B is a view showing a state in which units of the ADM of FIG. 3A are mounted in a rack; 
     FIG. 4A is a block diagram showing construction of an LTE in 1:n configuration; 
     FIG. 4B is a view showing a state in which units of the LTE of FIG. 4A are mounted in racks; 
     FIG. 5A is a block diagram showing another construction of an LTE in 1:n configuration; 
     FIG. 5B is a view showing a state in which units of the LTE of FIG. 5A are mounted in racks; 
     FIG. 6A is a block diagram showing construction of an ADM applied for 4-Fiber BLSR; 
     FIG. 6B is a view showing a state in which units of the ADM of FIG. 6A are mounted in racks; 
     FIG. 7 is a block diagram showing construction of an ADM in 1:1 linear configuration; 
     FIG. 8 is a block diagram showing construction of an ADM in 1:n linear configuration; 
     FIG. 9A is a block diagram showing construction of an SELH unit; 
     FIG. 9B is a block diagram showing construction of an SELH(P) unit; 
     FIG. 10 is a schematic view showing construction of a circuit which performs switching of time slots using memory; 
     Fig. 11 is a schematic view showing construction of a delay circuit; 
     FIG. 12 is a block diagram showing a unit which can be used commonly as an SELH unit and an SELH(P) unit; 
     FIG. 13A is a view showing basic construction of a transmission system using an LTE; 
     FIG. 13B is a view illustrating switch from working lines to protection lines; 
     FIG. 13C is a view illustrating another example of switching from working lines to protection lines; 
     FIG. 14A is a view showing construction of a transmission system using an ADM; 
     FIG. 14B is a view showing construction of a transmission system in which a plurality of ADMs are connected linearly; 
     FIG. 14C is a view showing construction of a transmission system in which a plurality of ADMs are connected in a ring shape; 
     FIG. 15A is a view illustrating switching from working high speed transmission lines to protection high speed transmission lines by LTEs in 1:1 configuration; 
     FIG. 15B is a view illustrating switching from working high speed transmission lines to protection high speed transmission lines by LTEs in 1:n configuration; 
     FIG. 16A is a view showing basic construction of a 4-Fiber BLSR transmission system; 
     FIG. 16B is a view illustrating switching from working high speed transmission lines to protection high speed transmission lines in 4-Fiber BLSR; 
     FIG. 16C is a view illustrating another example of switching from working high speed transmission lines to protection high speed transmission lines in 4-Fiber BLSR; 
     FIG. 17A is a view showing basic construction of a transmission system of 2-Fiber UPSR or 2-Fiber BLSR; 
     FIG. 17B is a view illustrating switching from working slots to protection slots on high speed transmission lines in 2-Fiber UPSR or 2-Fiber BLSR; and 
     FIG. 17C is a view illustrating another example of switching from working high speed transmission lines to protection high speed transmission lines in 2-Fiber UPSR or 2-Fiber BLSR. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Now, an embodiment of the present invention will be described. 
     In the present embodiment, there are provided 8 kinds of unit: a 10G interface which sends and receives signals to or from an optical fiber transmission line for optical/electrical and electrical/optical conversion, for example; SOH for overhead processing of signals; SELH, a selector on the side of the high speed signals; SELH(P), a selector on the side of the high speed signals; SWH, a switch on the side of the high speed signals; SWL, a switch on the side of the low speed signals; PINF for sending and receiving signals between racks; and SELL, a selector on the side of the low speed signals for selection of signals between low speed transmission lines, and one or more kinds of LINF responsible for interface with low speed transmission lines. By combining these units, terminal multiplexers such as LTE or ADM adapted for various configurations of transmission system are constructed. Each unit comprises one or more electronic boards mounted in a rack. 
     Further, 10G interface and SOH together make a high speed interface undertaking interface with the optical fiber transmission line. Accordingly, 10G interface and SOH may be treated as one high speed interface unit. 
     Further, as shown in FIG. 1, in the present embodiment, the terminal multiplexer is constructed by using a rack having 5 slots. When electric boards are mounted into the rack, they are connected by wiring provided in the rack. In the following description of the present embodiment, respective slots are referred to as COM, HS 1 , HS 2 , LS 1  and LS 2 . 
     Into the slot COM, there is mounted an electronic board bearing various control systems which perform clock generation, power control, system switching control, maintenance control, and the like. 
     First, FIGS. 2A and 2B show LTEs connected in point-to-point manner in a 1:1 configuration. 
     As shown in FIG. 2A, in this case, the LTE comprises two 10G interface units  10   a ,  10   b , two SOH units  20   a ,  20   b , two duplicated SELH units  30   a ,  30   b , plural sets of duplicated SELL units  40   a ,  40   b , and plural sets of duplicated LIF units  50   a ,  50   b . Further, FIG. 2B shows slots of a rack into which respective units are mounted. 
     In an LTE having such construction, at the time of normal operation, optical signals are received by working (W) 10G interface  10   a  from working optical fiber transmission line  100   a  which transmits signals at a transmission speed of 10G, and, after being converted into electrical signals, they are sent to working SOH  20   a , subjected to given overhead processing, and sent to working SELH  30   a . These signals are sent through working SELH  30   a  to working SELL  40   a , and demultiplexed there. Then, signals in respective time slots are sent to respective working LIF  50   a  in accordance with the destinations of the signals. Each LIF  50   a  converts the received signals to low speed signals and sends them to low speed transmission line  300 . 
     Conversely, signals received by respective working LIFs  50   a  from low speed transmission lines  300  are subject to time division multiplexing at working SELL  40   a , sent to working SELH  30   a  and then to both the working SOH  20   a  and the protection SOH  20   b , and there subjected to the overhead processing. Then, the signals are sent to the working 10G interface unit  10   a  and the protection 10G interface unit  10   b , converted to optical signals, and sent respectively to working optical fiber transmission line  100   b  and protection optical fiber transmission line  200   b  on the sending side. Namely, the working and protection optical fiber transmission lines  100   b ,  200   b  on the sending side are sent the same signals which are converted from the signals received from the low speed transmission lines by time division multiplexing. Thus, in fact, even at the time of normal operation, each LTE receives the same signals from the working optical fiber transmission line  100   a  and the protection optical fiber transmission line  200   a . Further, the signals are subject to the overhead processing at protection SOH  20   b , and inputted into working and protection SELHs. 
     Now, in such a construction, when a problem arises in some working optical fiber transmission line, the LTE receiving signals from the optical fiber transmission line in question performs the following switching processing. 
     Namely, substituting for signals received from working SOH  20   a , working SELH  30   a  selects signals received from protection SOH  20   b , and sends them to working SELL  40   a . Protection SELH  30   b  performs the same operation. As described above, the same signals are received from the working optical fiber transmission line  100   a  and from the protection optical fiber transmission line  200   a , and the above operation completes switching from the working lines to the protection lines as described above referring to FIG.  13 B. 
     Next, there will be described upgrading of such an LTE in a 1;1 configuration to ADM of 2-Fiber BLSR or ADM of 2-Fiber UPSR. 
     Here, ADM of 2-Fiber BLSR and ADM of 2-Fiber UPSR can be realized with the same construction. FIG. 3A shows the construction of ADM, and FIG. 3B shows a rack into which units are mounted. 
     As will be understood from FIGS. 3A and 3B, upgrading to ADM in this case can be realized by changing the two duplicated SELHs  30   a ,  30   b  in the construction of LTE shown in FIG. 2A into duplicated two SWHs  60   a ,  60   b , and by changing two duplicated SELLs  40   a ,  40   b  in the construction of LTE into two duplicated SWLs  70   a ,  70   b . Here, SELH and SWH are made to be common in the interface of signal lines with the rack, and, accordingly, they can be exchanged by removing an electronic board constituting SELH from the rack and mounting an electronic board constituting SWH into the rack. 
     Now, operation of ADM of 2-Fiber UPSR will be described. 
     It is assumed that an optical fiber transmission line  110   a  on the West side and an optical fiber transmission line  120   a  on the East side are working, and an optical fiber transmission line  110   b  on the West side and an optical fiber transmission line  120   b  on the East side are protection. At the time of normal operation, signals in respective time slots inputted from the optical fiber transmission line  110   a  on the West side are inputted through 10G interface  10   a  and SOH  20   a  to SWH  60   a , and switched there to SWL  70   a  and SOH  20   b  in accordance with destinations of signals of respective time slots. Signals sent to SWL  70   a  are switched to respective LIFs  50   a . Signals from LIFs  50   a  are inputted through SWL  70   a  to SWH  60   a , and switched there to SOHs  20   a ,  20   b . Further, to SWH  60   a , there are inputted signals from the optical fiber transmission line  120   b  through 10G interface  10   b  and SOH  20   b , and SWH  60   a  switches signals in respective time slots to SOH  20   b  in accordance with their destinations. The signals switched from SWH  60   a  to SOHs  20   a ,  20   b , are sent to 10G interfaces  10   a ,  10   b , and to the optical fiber transmission lines  110   b ,  120   a , respectively. 
     In such a construction, there may arise such a problem that, for example, signals from ADM located on the upstream side of the faulty point with respect to signal flow on a ring formed by working optical fiber transmission lines can not be received from the optical fiber transmission line  110   a  on the West side. In that case, as for signals from ADM located upstream from the faulty point, SWH  60   a  of ADM switches signals received from SOH  20   b  to SWL, instead of signals received from SOH  20   a.    
     When each ADM performs such an operation, the automatic protection switching shown in FIG. 17C can be realized. 
     Next, there will be described operation of ADM having the construction of FIG. 3A in the case of 2-Fiber BLSR. 
     To clarify signal flows between transmission lines, description is focused on the relation of operation of SWH  60   a  with signal flows between transmission lines, without referring to operations of other parts. At the time of normal operation, in ADM, signals inputted from the optical fiber transmission lines  110   a ,  120   b  on the receiving side on both West and East sides are inputted to SWH  60   a . Among the received signals, signals in the working time slots are switched by SWH  60   a  to working time slots of the low speed transmission lines or of the optical fiber transmission lines  110   b ,  120   a . Signals inputted from the low speed transmission lines are switched by SWH  60   a  to the working time slots of the optical fiber transmission lines  110   b ,  120   a . Further, signals in protection time slots inputted from the optical fiber transmission lines on the receiving side on both West and East sides are transited (forwarded) by SWH  60   a  to protection time slots of the optical fiber transmission lines  110   b ,  120   a ,  120   b ,  110   a  on the other side. 
     On the other hand, at the time of automatic protection switching, in two ADMs adjacent to a faulty working optical fiber transmission line, SWH  60   a  stops transiting (forwarding) signals between the protection time slots, and switches signals which have been switched to working time slots of the faulty working optical fiber transmission line, to protection time slots of an optical fiber transmission line transmitting signals from the ADM in question in the opposite direction to the troubled working optical fiber transmission line. Further, instead of the signals received from the working time slots of the faulty working optical fiber transmission line, signals received from protection time slots of an optical fiber transmission line transmitting signals to the ADM in question in the opposite direction to the faulty working optical fiber transmission line, become objects of switching to the low speed transmission lines and the working time slots of the optical fiber transmission line transmitting signals from the ADM in question in the opposite direction to the faulty working optical transmission line. 
     The above-described operation realizes the switching operation at the time of the automatic protection switching shown in FIG.  17 B. 
     Next, there will be described upgrading of LTE in 1:1 configuration shown in FIG. 2A to LTE in 1:n configuration. 
     FIG. 4A shows construction of LTE in 1:n configuration, and FIG. 4B shows racks into which units are mounted. 
     As shown in the figures, in this case, LTE comprises one rack for protection processing, connected with protection optical fiber transmission lines  200   a  and  200   b , and n racks for working processing, connected with working optical fiber transmission lines  100   a ,  100   b ,  110   a ,  110   b , . . . , lnO a , lnO b  respectively. 
     Each rack constitutes a bay as a functional unit of LTE in 1:n configuration. Each of n bays for working processing  310 ,  320 , . . . has such a construction that, in LTE in 1:1 configuration shown in FIG. 1, the protection 10G interface  10   b  has been removed, and PINF  80  has been substituted for the protection SOH  20   b . The bay  300  for protection processing has such a construction that, in the rack for working processing, two duplicated SELH have been replaced by two duplicated SELH(P)  90 . 
     Here, an electronic board constituting each SOH  20  and an electronic board constituting each PIN  80  are common in their signal interface with a rack, and thus, an electronic board constituting SOH  20  and an electronic board constituting PINF  80  can be exchanged by removing the former from the rack and mounting the latter into the rack. Further, an electronic board constituting each SELH  30  and an electronic board constituting each SELH(P)  90  are common in their signal interface with a rack, and thus, an electronic board constituting SELH  30  and an electronic board constituting SELH(P)  90  can be exchanged by removing the former from the rack and mounting the latter into the rack. In the present embodiment, LTE in 1:1 configuration can be upgraded to LET in 1:n configuration, by upgrading the LTE in 1:1 configuration to one of bays for working processing, and newly introducing the other bays constituting LTE in 1:n configuration. 
     At the time of normal operation, each bay for working processing in LTE of FIG. 4A operates similarly to the above-described operation of LTE in 1:1 configuration, each performing multiplex conversion between working optical fiber transmission lines connected thereto and low speed transmission lines connected thereto, However, in LTE in 1:n configuration, at the time of normal operation, signals are not sent to the protection optical transmission lines  200   a ,  200   b.    
     Next, there will be described such a case that, where a problem has arisen, for example, in an optical transmission line  110   a  connected to a working bay  320  on the receiving side. In this case, SELH(P)  90  of the bay  300  for protection processing sends signals received from a protection optical fiber transmission line  200   b  through 10G interface  10  and SOH  20  to PINF  80 . Those signals are converted to low speed signals, for example, each having a rate of 150M, and thereafter are sent, through optical interconnect connecting racks, to PINF  80  of a bay  310 . PINF  80  of the bay  310  transits the signals received from the bay  300  to PINF  80  of the bay  320 . PINF  80  of the bay  320  sends the signals received from the bay  310  to SELH  30 . SELH  30  of the bay  320  selects the signals received from PINF  80  instead of the signals from SOH  20 , and sends the selected signals to SELL  40 . 
     Conversely, when a problem arises in an optical fiber transmission line  120   b  connected to the working bay  320  on the sending side, SELH  30  of the bay  330  sends signals to PINF  80  instead of SOH  20 . On receiving the signals, PINF  80  sends them to PINF  80  of the bay  310 . PINF  80  of the bay  310  transits the signals received from the bay  320  to the bay  300 . On receiving the signals, PINF  80  of the bay  300  converts them to serial signals, and sends them SELH(P)  90 . SELH(P)  90  sends the signals received from PINF  80  to SOH  20 . The signals are sent through the protection optical fiber transmission line  200   b.    
     Thus, in this construction, PINF  80  extends the Protection optical transmission line to the working bay connected to the faulty working optical fiber transmission line. 
     The above-described operation is performed by LTEs at both ends of the faulty working optical fiber transmission line, thus realizing the uni-directional switching of FIG.  13 B. 
     Further, the bi-directional switching of FIG. 18C is realized by the same operation as the uni-directional switching in the case of both sending and receiving optical fiber transmission lines being faulty. 
     Alternatively, the bi-directional switching may be realized by connecting respective PINFs  80  of the bays in a ring shape as shown in FIGS. 5A and 5B. 
     In this construction, extension by PINFs  80  of a protection optical fiber transmission line on the receiving side to a working bay connected to a faulty working optical transmission line, and extension by PINFs  80  of a protection optical fiber transmission line on the sending side to the working bay connected to the faulty working optical transmission line are realized through different routes. For example, as shown in the figures, extension of a protection optical fiber transmission line  200   a  on the receiving side to the bay  320  is attained through PINFs  80  of the bays  300 ,  310 , while extension of a protection optical fiber transmission line  200   b  on the sending side to the bay  320  is attained through PINFs  80  of the bays  320 ,  330 , . . . ,  3 n 0 , and  300 . 
     An advantage of employing this ring-shape construction is that the amount of hardware can be kept to a low level because for example, the number of signal lines connecting racks is small. 
     Next, there will be described upgrading ADM of 2-Fiber BLSR and ADM of 2-Fiber UPSR shown in FIGS. 3A and 3B to ADM of 4-Fiber BLSR. 
     FIG. 6A shows construction of ADM in this case, and FIG. 6B shows racks into which units are mounted. 
     As shown in the figures, ADM in this case comprises a rack for working processing, connected with working optical fiber transmission lines  110   a ,  110   b ,  120   a  and  120   b , and a rack for protection processing, connected with protection optical fiber transmission lines  200   a ,  200   b ,  210   a  and  210   b . Bays corresponding to respective racks have the same construction as ADM of 2-Fiber BLSR and ADM of 2-Fiber UPSR shown in FIG.  3 A. Accordingly, in this case, upgrading can be performed by letting the existing ADM of 2-Fiber BLSR or 2-Fiber UPSR be one bay of ADM of 4-Fiber BLSR, introducing a new bay having the same construction, and connecting SWHs  60   a ,  60   b  of both ADM with optical interconnects. As LTE in 1:1 configuration can be upgraded to ADM of 2-Fiber BLSR or 2-Fiber UPSR, LTE in 1:1 configuration, can of course also be upgrade to ADM 4-Fiber BLSR. 
     Now, operation of ADM of FIGS. 6A and 6B will be described. 
     To clarify signal flows between transmission lines, description is focused on the relation of operation of SWH  60   a  with signal flows between transmission lines, without referring to operations of other parts. At the time of normal operation, in a working bay  400 , signals received from optical fiber transmission lines on the receiving side on the West and East sides are inputted into SWH  60   a . SWH  60   a  switches the received signals in respective time slots to low speed transmission lines or to time slots of optical fiber transmission lines on the other side  110   b ,  120   a . Further, signals received from the low speed transmission lines are switched by SWH  60   a  to time slots of the optical fiber transmission lines  110   b ,  120   a.    
     Further, SWH  60   a  of a bay  410  or protection processing forwards signals received from an optical fiber transmission line  200   a  to an optical fiber transmission line  210   a  as they are, and forwards signals received from an optical fiber transmission line  210   b  to an optical fiber transmission line  200   b  as they are. 
     At the time of automatic protection switching, when, for example, a problem arises in the working optical fiber transmission lines  120   a ,  120   b  on the East side, SWH  60   a  of the bay  400  for working processing sends signals which has been switched to the working optical fiber transmission line  120   a , to SWH  60   a  of the bay  410  for protection processing. Further, instead of signals which have been received from the working optical transmission line  120   b , signals received from the bay  410  for protection processing is made an object of switching. On the other hand, when, SWH  60   a  of the bay  410  for protection processing stops the forwarding operation between the above-described protection optical fiber transmission lines and performs the switching operation shown in FIG. 16B, it switches the signals received from the bay  400  to the optical fiber transmission line  200   b , and sends signals received from the optical fiber transmission line  200   a  to SWH  60  of the bay  400 . Further, when SWH  60   a  of the bay  410  for protection processing stops the forwarding operation between the above-described protection optical fiber transmission lines and performs the switching operation shown in FIG. 16C, it switches the signals received from the bay  400  to the optical fiber transmission line  210   a  and sends signals received from the optical fiber transmission line  210   b  to SWH  60  of the bay  400 . 
     The above described operation is performed by ADMs adjacent to the troubled portion, realizing the automatic protection switching shown in FIGS. 16B,  16 C, 
     Next, there will be described upgrading of ADM of 2-Fiber BLSR or ADM of 2-Fiber UPSR shown in FIG. 8A to ADM in 1:1 linear configuration. 
     FIG. 7 shows construction of ADM in this case. 
     As shown in the figure, ADM in this case has the same construction as ADM of 4-Fiber BLSR shown in FIG.  6 A. Accordingly, upgrading can be performed by exchanging similar units. Further, as described above, the present construction can also be obtained by upgrading LTE in 1:1 configuration. 
     Further, its operation at the time of normal operation is the same as 4-Fiber BLSR. Its operation at the time of the automatic protection switching as shown in FIG. 15B is same as the case of the switching operation shown in FIG.  16 B. 
     Last, there will be described upgrading to ADM in 1:n linear configuration. 
     FIG. 8 shows construction of ADM in this case. 
     As shown, in this case, ADM has such a construction that there are provided (1+n) sets of bays 800-80n, one set as a protection bay set and n sets as working bay sets, with each set comprising East bay  810  and West bay  811 . East bay 810 is obtained, in one of the racks constituting ADM in 1:1 linear configuration shown in FIG. 7, by removing two 10G interfaces  10   a ,  10   b , and replacing two SOHs  20   a ,  20   b  with two PINFs  80   a ,  80 . West bay  811  is the remaining rack of FIG. 7, and PINFs  80   a ,  80   b  of respective bay sets are successively connected in a chain so that protection bay set  800  is located at the end. 
     Accordingly, one of plural bay sets of ADM can be constructed by using ADM in 1:1 linear configuration. Further, as can be seen from FIG. 8, each East bay has such a construction that  10 G interface of each bay of LTE in 1:n configuration is removed, SELH is replaced with SWH, SOH with PINF, and SELL with SWL, and introduced PINFs are connected successively. Accordingly, ADM in 1:n linear configuration can be obtained by upgrading LTE in 1:n configuration. 
     The protection bay set  800  is connected with one sending and one receiving protection optical fiber transmission lines for each of the East and West sides. Each of n working bay sets  801 - 80   n  is connected with one sending and one receiving working optical fiber transmission lines for each of the East and West sides. 
     At the time of normal operation, the West bay of each working bay set  801 - 80   n  operates similarly to the above-described operation of the working bay of ADM in 1:1 linear configuration, realizing transmission between ADMs or between ADM and LTE using n working transmission lines. 
     On the other hand, at the time of automatic protection switching, when, for example, a problem arises in the optical fiber transmission line on the West side of the second working bay set, as in the above-described case of LTE in 1:n configuration, protection optical fiber transmission line on the West side is extended to SWH of the West bay of second working bay set, by transmitting signals through 10G interface, SOH and SWH of the West bay of the protection bay set; SWH and PINF of the East bay of the protection bay set; PINF of the East bay of the first working bay set; working PINF and SWH of the East bay of the second working bay set; and SWH of the West bay of the second working bay set, in this order. That extended protection optical fiber transmission line is used by SWH of the West bay of the second working bay set, instead of the faulty optical fiber transmission line on the West side. 
     Such an operation is performed by ADMs located on both ends of the faulty optical fiber transmission line, realizing switching from the working optical fiber transmission line to the protection optical fiber transmission line. 
     Alternatively, bay sets of Fig,  8  may be connected in a ring shape as in the LTE in 1:n configuration shown in FIG. 5A, and extension of the protection optical fiber transmission line may be realized in such a manner that signals are forwarded only in one rotational direction on the ring. For example, when a problem arises in the optical fiber transmission line on the West side of second working bay set, similarly to the above-described case of LTE in 1:n configuration, the receiving protection optical fiber transmission line on the West side is extended to SWH of West bay of the second working bay set, by forwarding signals through 10G interface, SOH and SWH of the West bay of the protection bay set; SWH and PINF of the East bay of the protection bay set; PINF of the East bay of the first working bay set; working PINF and SWH of the East bay of second working bay set; and SWH of the West bay of second working bay set, in this order. Further, the sending protection optical fiber transmission line on the West side is extended to working SWH of the West bay of the second working bay set, by transiting signals through working SWH of the West bay of the second working bay set; working SWH and PINF of the East bay of the second working bay set; working PINF of the East bay of the third working bay set; . . . ; working PINF of the East bay of the n-th working bay set; PINF and SWH of the East bay of the protection bay set; and SWH, SOH and  10 G interface of the West bay of the protection bay set, in this order. 
     As described above, according to the present invention, it is possible to upgrade a terminal multiplexer, utilizing the constructions of the existing terminal multiplexers. Further, as has been described, each construction of the units of LTE in 1:1 configuration and ADM of 2-Fiber BLSR/2-Fiber UPSR is a duplicated one. Accordingly, at the time of upgrading, necessary units may be exchanged in one of the twin systems at once, and the other system may be used during the exchange, so that the transmission system may be operated continuously. 
     However, if a difference in delay times exists between input and output signals for SELH, SELH(P) and SWH due to processing in SELH, SELH(P) and SWH, problems may be caused such as loss and duplication of signals at the time of switching of the system in operation. 
     Accordingly, in the present embodiment, SELH is constructed as shown in FIG. 9A, and SELH(P) as shown in FIG. 9B so that delay times are made to coincide in these units. 
     Here, switching of signals in time slots as in SWH is performed, as shown in FIG. 10, by means of memory  900 , write circuit  901  which sequentially writes signals in each time slot coming in and out, and read circuit  902  which reads signals in each time slot from the memory in order set by a controller (not shown). Accordingly, a delay time is usually produced which is larger than the time corresponding to the number of time slots. This delay time is generally larger than a delay time in a selector which performs a selection operation, such as SELH and SELH(P). 
     In the present invention, delay circuit  1300  is provided for SELH and SELH(P) for adjusting the delay time of signals. As shown in FIG. 11, the delay circuit  300  comprises a sequential counter  302  which sequentially generates addresses in memory  301  for writing signals in each time slot coming in and out, and a decoder  303  which obtains read address by adding a count value of the sequential counter  302  and an offset corresponding to an adjust time desired. According to such an delay circuit  1300 , time elapsing between writing and reading signals to and from the memory  301  can be adjusted by suitably setting the offset value added by the decoder  303 . 
     In FIG. 9A, the reference numeral  320  denotes a selector which selects signals sent from working SOH or signals sent from protection SOH or PINF, and sends the selected signal to SELL. 
     Further, in FIG. 9B, the reference numeral  320  shows a selector for sending signals sent from PINF to a protection optical fiber transmission line. 
     As SELH and SELH(P) shown in FIGS. 9A and 9B, a unit provided with three selectors  1301 ,  1302 ,  1303 , as shown in FIG. 12, may be used in common. In this case, when it is used as SELH(P), selections in the selectors  1301 ,  1302  are fixed to form the same signal flow as in FIG. 9B, and when used as SELH, selections in the selectors  1302 ,  1303  are fixed to form the same signal flow as in FIG.  9 A. 
     Further, as a construction of an interface frame of an optical fiber transmission line related to the present embodiment, there may be used SONET synchromous Optical network OC-N frame construction having a transmission rate of 51.84 Mb/s or a multiple thereof, or STM-N frame construction in SDH hierarchy regulated in ITU Recommendation having a transmission rate of 155.52 Mb/s or a multiple thereof. 
     As described above, according to the present embodiment, a terminal multiplexer can be upgraded using the existing terminal multiplexer, and construction of a transmission system can be changed by that upgrading. 
     Further, as in the above-described case of LTE in 1:n configuration, a terminal multiplexer can be constituted by a plurality of racks, and, when working and protection systems are switched, the capacity of signals which are required to be sent and received between racks can be made to be lower in level. Thus, the present construction is suitable for constructing a terminal multiplexer using a plurality of racks.