preferred embodiments of the present invention will be described below with reference to the accompanying drawings . fig1 is the conceptual view of an orderwire controller according to the present invention . this orderwire controller 10 is implemented in a network element ( ne ) to support voice communication between maintenance people over an orderwire circuit . the orderwire controller 10 comprises a plurality of mixers 11 to 1 - n , combination controllers 2 , and digital code converters 3 . each mixer 1 - 1 to 1 - n is designed to add up , in a digital fashion , the orderwire signals supplied from a plurality of ring networks to which the ne belongs . those mixers 1 - 1 to 1 - n are coupled to appropriate ring networks through the combination controllers 2 and digital code converters 3 ( described later ). the ring networks may be sdh networks that transport signals in the form of synchronous transfer module level four ( stm - 4 ), for example , but the invention should not be limited to any specific signal format . in the illustrated system , there are a plurality of rings a to f , each of which comprises any number of member nes . the nes may each have a local telephone set to allow maintenance people to talk to each other , if necessary . while fig1 depicts such specifics only for the ring a , the other rings also have similar structure . the mixers 1 - 1 to 1 - n are properly allocated , so that they serve the rings a to f in the following way : mixer 1 - 1 serves the rings a and d , mixer 1 - 2 serves the rings b and c , mixer 1 - 3 serves the ring e , and . mixer 1 - n serves the ring f . the entire system comprising the proposed orderwire controller 10 and ring networks is referred to herein as the orderwire control system . the combination controllers 2 comprise a matrix switch which provides flexible connections among the rings a to n and the mixers 1 - 1 to 1 - n . that is , the combination controllers 2 can accept the orderwire signals from the rings a to f and direct them to any of the mixers 1 - 1 to 1 - n , thereby providing a variety of mixing patterns as desired . with this mechanism , two or more rings can form a consolidated domain of orderwire communication . in the system of fig1 , the rings a and d share a single orderwire circuit , as do the rings b and c . on the other hand , when a mixer is assigned to a single ring , the orderwire signal will be confined within that ring , meaning that a fully functional orderwire circuit can be formed . the rings e and f in the system of fig1 are an example of this single ring arrangement . the most extreme case is when every ring needs an isolated orderwire circuit , in which case the orderwire controller 10 has to provide as many mixers as the number of rings . the digital code converters 3 each convert an a law or mu - law coded orderwire signal into a digital voice signal with unified quantizer step sizes . the resultant linear coded signals are supplied to the mixers 1 - 1 to 1 - n through the combination controllers 2 . since the orderwire communication is bidirectional , the digital code converters 3 also convert a linear coded digital voice signal into either an a - law or mu - law coded signal , depending the coding method used in a destination ring . while fig1 shows specific combinations of ring networks , the present invention is not restricted to those combinations . fig2 shows a variation of the combination control , in which the combination controller 2 is programmed in such a way that all orderwire signals of the rings a to f will be concentrated to a single mixer 1 - 1 . this arrangement permits all the nes on the rings a to f to communicate with each other over a shared orderwire circuit . referring next to fig3 to 6 , the following section will present a more specific system where the proposed orderwire controller 10 is placed at the junction point of two ring networks . what has been described above as the combination controller 2 is now called a “ matrix switch .” fig3 shows a two - ring system according to the present invention . although it is not explicitly shown in fig3 , the proposed orderwire controller 10 is implemented in a network element 10 a which provides junction functions . the illustrated system comprises two rings , a and b . more specifically , the ring a is formed by four nes 10 a , 21 , 22 , and 23 , and the ring b is formed by four nes boa , 31 , 32 , and 33 . the two rings share the ne 10 a as the junction node located therebetween . all those nes are equipped with a local telephone set for maintenance purposes . the proposed orderwire controller 10 in the ne 10 a can be configured as a master or slave station independently for each ring . specifically , in the illustrated system of fig3 , the ne 10 a serves as a slave station at both sides , a and b . each ne 21 to 23 on the ring a has a mixer for combining orderwire signals , as do the nes 31 to 33 on the ring b . the ne 10 a has two mixers 1 - 1 and 1 - 2 to support the rings a and b , respectively . accordingly , both rings a and b have a fully functional orderwire circuit with ring protection and restoration functions . the ne 21 serves as the mater station on the ring a , as does the ne 31 on the ring b . both master nes 21 and 31 are configured to terminate their respective orderwire circuits at their west port . fig4 shows the internal structure of the ne 10 a . overhead demultiplexers 4 - 1 to 7 - 1 terminate the overhead of incoming sdh / sonet frames . to this end , the overhead demultiplexers 4 - 1 to 7 - 1 comprise failure detectors 4 a to 7 a and e1 / e2 setting units 4 b to 7 b , respectively . the failure detectors 4 a to 7 a detect link failure . the e1 / e2 setting units 4 b to 7 b determine which to use e1 byte or e2 byte for orderwire signaling , and whether to enable or disable the orderwire function itself . overhead multiplexers 4 - 2 to 7 - 2 insert overhead information to each sdh / sonet frame to be transmitted . to this end , the overhead multiplexers 4 - 2 to 7 - 2 comprise alarm insertion units 4 c to 7 c and e1 / e2 setting units 4 d to 7 d , respectively . the alarm insertion units 4 c to 7 c insert a link failure alarm when a link failure is detected . the e1 / e2 setting units 4 d to 7 d determine which to use e1 byte or e2 byte for orderwire signaling , and whether to enable or disable the orderwire function itself . digital code converters 3 a , 3 c , 3 f , and 3 h receives orderwire signals from the overhead demultiplexers 4 - 1 to 7 - 1 and convert them into linear coded digital voice signals with unified quantizer step sizes . the resultant signals are supplied to matrix switches 2 a and 2 b , which feed the signals to the mixers 1 - 1 and 1 - 2 , appropriately controlling the cross - connections between their inputs and outputs . the matrix switches 2 a and 2 b also accept digital voice signals from the mixers 1 - 1 and 1 - 2 and redirect them to digital code converters 3 b , 3 d , 3 e , and 3 g . those digital code converters 3 b , 3 d , 3 e , and 3 g each convert a given linear coded signal into an a - law or mu - law coded orderwire signal . the resultant signals are supplied to their associated overhead multiplexers 4 - 2 to 7 - 2 . a telephone set interface 8 , coupled to the mixers 1 - 1 and 1 - 2 , controls connection to an external telephone set 10 a - l . fig5 is a diagram showing the internal structure of the mixers 1 - 1 and 1 - 2 and the telephone set interface 8 . the mixer 1 - 1 is composed of digital adders 1 a to 1 e . the mixer 1 - 2 is composed of digital adders 1 a to 1 j . numerals in parentheses indicate the sources and destinations of orderwire signals . signal ( 1 ), for example , is supplied from the matrix switch 2 a and distributed to the digital adders 1 a , 1 c , 1 d , and 1 e in the mixer 1 - 1 . other signals ( 2 ) to ( 4 ) are distributed similarly . the transmit voice signal produced in the telephone set 10 a - 1 is delivered to the ring b through the east and west ports of the ne 10 a . the receive voice signals come in from the ring b through the east and west ports , which are supplied to the telephone set 10 a - 1 via a telephone set interface 8 . this telephone set interface 8 comprises the following components : a transformer 8 a , an a / d converter bb , a d / a converter 8 c , digital code converters 3 i and 3 j , and a selector 8 d . the transformer 8 a is a hybrid circuit which is used to convert from 2 - wire interface to 4 - wire interface and vice versa , besides supplying electric power . the a / d converter 8 b receives an analog signal from the transformer 8 a and converts it into digital form . the digital code converter 3 i then converts that digital signal to a linear coded signal and sends it to the selector 8 d . the selector 8 d selects the destination of the signal , either the mixer 1 - 1 or the mixer 1 - 2 . the selector 8 d also receives digital signals from the mixers 1 - 1 and 1 - 2 . it selects either one of those signals and supplies the selected signal to the digital code converter 3 j . the digital code converter 3 j then converts the digital signal to an a - law coded or mu - law coded signal and passes it to the d / a converter 8 c . the d / a converter 8 c converts the received digital signal into an analog signal , which is supplied to the transformer 8 a . when no link failure is assumed , the network system of fig3 will operate as follows . in the illustrated situation , the orderwire circuit is closed within each ring . the ne 21 serves as the master station on the ring a , disabling the use of e1 / e2 bytes at its west port to prevent howls from happening . similarly , the ne 31 serves as the master station on the ring b , disabling the use of e1 / e2 bytes at its west port . the ne 10 a is configured not to allow the two rings a and b to exchange their orderwire signals ; its internal matrix switches 2 a and 2 b provide straight connections between the ring a and the mixer 1 - 1 , and between the ring b and the other mixer 1 - 2 . for illustrative purposes , a specific orderwire link between the ne 21 and ne 23 will be discussed below . it is also assumed that the ring a transports stm - 4 frames ; i . e ., four stm - 1 frames # 1 to # 4 are time - division multiplexed . at the ne 21 , the voice signal originating from the telephone set 21 - 1 is processed in the mixer of the ne 21 and then inserted to the e1 # 1 byte of st - 4 traffic . this orderwire signal reaches the ne 23 via the ne 22 . the ne 23 terminates the e1 # 1 byte of each stm - 4 frame and sends out the received orderwire signal to its local telephone set 23 - 1 through its internal mixer . in the opposite direction , the orderwire signal is sent from the ne 23 to the ne 21 via the ne 22 , being carried on the e1 # 1 byte . the ne 21 terminates the e1 # 1 byte to extract an incoming orderwire signal . in this way , the orderwire communication is accomplished between the two nes 21 and 23 . now consider the orderwire link between the ne 10 a and ne 33 on the ring b , assuming that the ring b transports stm - 4 frames . at the ne 10 a , the voice signal originating from the telephone set 10 a - 1 passes through the mixer 1 - 2 in the ne 10 a and is then inserted to the e1 # l byte of stm - 4 traffic . this orderwire signal propagates over the ring b , finally reaching the destination ne 33 . the ne 33 terminates the e1 # 1 byte of each stm - 4 frame and sends out the received orderwire signal to its local telephone set 33 - 1 through the mixer . similarly the orderwire signal is sent in the opposite direction , from the source ne 33 to the destination ne 10 a . the ne 10 a terminates the e1 # 1 byte to extract an incoming orderwire signal . in this way , the orderwire communication is accomplished between the two nes 10 a and 33 . while the normal operations of the system have been explained above , the following section will discuss the behavior in a fault situation . suppose that the proposed two - ring system has encountered a link failure at the point pa as indicated by the dotted circle in fig6 . this link failure causes the nearest ne 22 to alarm the master ne 21 of the ring a about the link failure . to resolve this problem situation , the master ne 21 triggers its ring restoration function , which enables the use of e1 / e2 bytes at its own west port , as well as making the ne 22 terminate the orderwire circuit at the east port . now that the orderwire circuit has been restored , the voice signal originating from the telephone set 21 - 1 passes through the mixer in the ne 21 and then is inserted to , for example , the e1 # 1 byte of stm - 4 frames . this orderwire signal propagates toward the ne 23 over the ring a . the ne 23 terminates the e1 # 1 byte of each stm - 4 frame and sends the received orderwire signal to the telephone set 23 - 1 through its own internal mixer . likewise , the voice signal originating from the ne 23 reaches the destination ne 21 via the ne 10 a , tracking the same path in the reverse direction . in this way the orderwire communication channel between the two nes 21 and 23 can be maintained , despite of the link failure . the same mechanism would work in the ring b ; the explanation will not be repeated here . referring next to fig7 , the following section will describe how the rings a and b share the same orderwire circuit . for illustrative purposes , a particular group of nodes ( nes 21 , 23 , 10 a and 33 ) will be considered . in the system of fig7 , the nes 21 and 31 serve as the master station on the rings a and b , respectively , disabling the use of e1 / e2 bytes at their west port to prevent howls from happening . to enable the two rings a and b to exchange their orderwire signals , the ne 10 a is configured so that the internal matrix switches 2 a and 2 b will connect both rings to the mixer 1 - 1 . the signal of the telephone set 10 a - 1 is also directed to the mixer 1 - 1 via the selector 8 d ( fig5 ). the voice signal originating from the telephone set 21 - 1 is fed to a mixer in the ne 21 and inserted to , for example , the e1 # 1 byte of stm - 4 frames . this orderwire signal reaches the ne 23 via the ne 22 . terminating the e1 # 1 of incoming stm - 4 frames , the ne 23 supplies its local telephone set 23 - 1 with the received orderwire signal through the internal mixer . the ne 23 also forwards the orderwire signal to the next ne 10 a through its east port , mixing with its local voice signal and inserting that data to the e1 # 1 byte of outgoing stm - 4 frames . the ne 10 a receives the stm - 4 frames at its west port , terminating their e1 # 1 byte . the received orderwire signal is supplied to the telephone set 10 a - l through the mixer 1 - 1 . the ne 10 a also sends that orderwire signal to the ne 33 through its east port on the ring b , mixing with its local voice signal and inserting that data to the e1 # 1 byte of outgoing stm - 4 frames . the ne 33 receives those stm - 4 frames at its west port , terminating their e1 # 1 byte . the received orderwire signal is supplied to the telephone set 33 - 1 through a mixer in the ne 33 . in this way , the orderwire voice signals propagate from the ne 21 to the ne 33 . the orderwire circuit also carries voice signals in the opposite direction ( i . e ., ne 33 —& gt ; ne 10 a —& gt ; ne 23 —& gt ; ne 22 ) in a similar way , although it will not be explained here . referring next to fig8 , the following section will now describe a system where ring networks using the a - law coding method are linked with other ring networks using the mu - law coding method . in the illustrated system , the mixer 1 - 1 serves the rings a and d , and the mixer 1 - 2 serves the rings b , c , e , and f , under the cross connection control of the matrix switches 2 a and 2 b . with this setup , the two rings a and d share a common domain of orderwire signaling , as do the other rings b , c , e , and f . note that the rings a , b , and e use a - law coding , while the rings c , d , f use mu - law coding . the digital code converters 3 convert a - law or mu - law coded orderwire signals into linear coded signals before supplying them to the mixers 1 - 1 to 1 - n . this signal conversion permits the mixers 1 - 1 and 1 - 2 to perform digital summation directly , assuming that all the input signals are linear coded . it is therefore possible for the nes on different rings to communicate with each other , no matter which coding method they actually use , a - law or mu - law . fig9 shows the structure of the digital code converter 3 . the digital code converter 3 comprises an alaw / linear converter 30 a , a mu - law / linear converter 30 b , and a switch 30 c . the a - law / linear converter 30 a converts a - law coded signals into linear coded signals and vice versa . likewise , the mu - law / linear converter 30 b converts mu - law coded signals into linear coded signals and vice versa . the switch 30 c selects either one of the a - law / linear converter 30 a and mu - law / linear converter 30 b on demand . the above conversion mechanism is used as follows . when an a - law coded signal sa is given , the switch 30 c selects the output of the a - law / linear converter 31 , thereby providing a linear coded signal sb . when a linear coded signal sc is given , and an a - law coded output is needed , the switch 30 c chooses the a - law / linear converter 30 a . the a - law / linear converter 30 a then converts the given signal sc to an a - law coded signal sd . in this way , the digital code converter 3 converts an a - law or mu - law coded orderwire signal into a linear coded signal , or vise versa . this enables the orderwire facilities to function correctly even when the network system involves both a - law based rings ( sdh ) and mu - law based rings ( sonet ). the above discussion will now be summarized as follows . according to the present invention , the orderwire controller 10 comprises a plurality of mixers 1 - 1 to 1 - n which add up orderwire signals together , and a combination controller 2 which controls the combinations of the orderwire signals to be mixed by providing appropriate cross connections . in multi - ring network systems , the proposed orderwire controller 10 enables each ring to have a fully functional orderwire circuit that is tolerant to link failures , thus providing improved usability and maintainability . further , according to the present invention , the mixers 1 - 1 to 1 - n are implemented with digital adders , which can be integrated into a small chip at lower costs . because of their high noise immunity , the digital mixers can provide more channels for simultaneous orderwire communication . the foregoing is considered as illustrative only of the principles of the present invention . further , since numerous modifications and changes will readily occur to those skilled in the art , it is not desired to limit the invention to the exact construction and applications shown and described , and accordingly , all suitable modifications and equivalents may be regarded as falling within the scope of the invention in the appended claims and their equivalents .