Abstract:
A local network node, connected to a remote network node by an active transmission system and a standby transmission system, includes an active receiver for receiving communication signals from the active transmission system and detecting an operating state of the active transmission system. An active transmitter transmits communication signals over the active transmission system to the remote network node. A standby receiver receives, from the standby transmission system, a status signal indicating an operating state of the active transmission system detected by the remote network node. A standby transmitter transmits a status signal indicating the operating state detected by the active receive circuitry over the standby transmission system to the remote network node. A control circuit is responsive to an operating state detected by the active receiver and an operating state indicated by the status signal of the standby receiver for performing a protection switching between the active and standby transmission systems.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to protection switching between active and standby systems, and more specifically to protection switching of a communication network where transmission lines are duplicated for backup purposes. 
     2. Description of the Related Art 
     Japanese Laid-Open Patent Specification 2-260729 discloses a duplicated network of active and standby transmission systems between local and remote control units such as time-division multiplexers. Each of the active and standby transmission systems includes a local line termination unit and a remote line termination unit interconnected by high-seed digital dedicated lines. Each of the local and remote control units transmits signals in the coded mark inversion (CMI) format, As a means for establishing frame synchronization, the rule of violation is utilized. If a failure occurs in the active system, the local line termination unit loses track of a transmitted frame and sends a unit-not-ready signal to the local control unit, recognizing that a failure has occurred. The local control unit then stops sending the frame synchronization signal to the remote line termination unit. The latter responds to the lost of a transmitted frame by sending a unit-not-Receive signal to the associated control unit, signalling the occurrence of a failure in the active system. Protection switching from the failed active system to the standby system is then performed by the local and remote control units. 
     However, in transmission systems where the rule of violation is not used as a means of establishing frame synchronization or where the type of units that produce a unit-not-ready signal in the event of a failure is not used, it is impossible to signal the occurrence of a system failure to the remote communication unit. In addition, since the loss of frame synchronization does not necessarily indicate the actual occurrence of a failure, protection switching may take place when the system loses synchronization due to a cause other than a system failure, resulting in a difficulty to distinguish between a system failure and a loss of synchronization. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a communication network capable of protection switching of duplicated transmission systems of any type of line codes while eliminating the need to provide distinction between actual failures and simulated failures. 
     According to a first aspect of the present invention, there is provided a network node connected to a remote network node by an active transmission system and a standby transmission system, comprising active receive circuitry for receiving communication signals from the active transmission system and detecting an operating state of the active transmission system, active transmit circuitry for transmitting communication signals over the active transmission system to the remote network node, standby receive circuitry for receiving, from the standby transmission system, a status signal indicating an operating state of the active transmission system detected by the remote network node from the transmitted communication signals, standby transmit circuitry for transmitting a status signal indicating the operating state detected by the active receive circuitry over the standby transmission system to the remote network node, and control circuitry responsive to an operating state detected by the active receive circuitry and an operating state indicated by the status signal of the standby receive circuitry for performing a protection switching between the active and standby transmission systems. 
     According to a second aspect, the present invention provides a protection switching method for a local network node and a remote network node interconnected by an active transmission system and a standby transmission system, comprising the steps of (a) receiving, at the local network node, communication signals from the remote network node over the active transmission system, and detecting an operating state of the active transmission system, (b) transmitting, from the local network node, a first status signal indicating the detected operating state over the standby transmission system to the remote network node, (c) receiving, at the remote network node, communication signals from the local network node over the active transmission system, and detecting an operating state of the active transmission system, (d) transmitting, from the remote network node, a second status signal indicating the detected operating state over the standby transmission system to the local network node, (e) receiving the first status signal of the local network node at the remote network node over the standby transmission system, (f) receiving the second status signal of the remote network node at the local network node over the standby transmission system, and (g) performing a protection switching at the local and remote network nodes between the active and standby transmission systems in accordance with the first and second status signals. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be described in further detail with reference to the accompanying drawings, in which: 
     FIGS. 1A and 1B a block diagram of a duplicated communication network according to the present invention; 
     FIG. 2 is a timing diagram of a selector-and-timing controller of each line transmitter of FIG. 1; 
     FIG. 3A is a block diagram of the network for describing its operation when the active system is operating normally; 
     FIG. 3B is a block diagram of the network for describing its operation when a failure occurs in the active system; and 
     FIG. 3C is a block diagram of the network after a protection switching occurs between the failed active system to the standby system. 
    
    
     DETAILED DESCRIPTION 
     Referring now to FIGS. 1A and 1B, there is shown a communication network according to the present invention. As one example, the network is comprised of two switching nodes such as electronic private branch exchanges  1  and  2  respectively shown in FIGS. 1A and 1B. Switching nodes  1  and  2  are interconnected by a first pair of transmission lines  13 A and  13 B and a second pair of transmission lines  14 A and  14 B, one of the transmission line pairs being active while the other being inactive as a backup for the active system. 
     In FIG. 1A, the switching node  1  includes a line interface unit  5  connected between a switch  3  and the transmission lines  13 A,  13 B and a line interface unit  9  connected between the switch  3  and the transmission lines  14 A,  14 B. Switch  3  and the line interface units  5  and  6  are controlled by a central processor  4 . 
     Similarly, the switching node  2 , shown in FIG. 1B, includes a line interface unit  17  connected between a switch  15  and the transmission lines  13 A,  13 B and a line interface unit  21  connected between the switch  15  and the transmission lines  14 A,  14 B. Switch  15  and the line interface units  17  and  21  are controlled by a central processor  16 . 
     All line interface units are identically comprised of a line transmitter, a line receiver and a line controller. In each switching node, each line controller is connected to the central processor, which serves as a communication path between the line controllers. 
     Line interface unit  17  includes a line transmitter  18  connected via the transmission line  13 A to a line receiver  6  of the line interface unit  5  whose line transmitter  7  is connected via the transmission line  13 B to the line receiver  19  of line interface unit  17 . Similarly, the line interface unit  9  includes a line transmitter  11  connected via the transmission line  14 B to a line receiver  23  of the line interface unit  21  whose line transmitter  22  is connected via the transmission line  14 A to the line receiver  10  of line interface unit  9 . 
     For simplicity, only the line transmitter  18  of interface unit  17  is shown in FIG.  1 B and the line receiver  6  of line interface  5  are shown in detail in FIG.  1 A. 
     Each line transmitter, as represented by the transmitter  18  in FIG. 1B, consists of a memory  30 , a selector  31  and a selector-and-timing controller (STC)  32 . Node status data such as normal indication, alarm indication and response indication are stored in the memory  30  to selectively supply one of the stored node status data to the selector  31  in accordance with a control signal from the line controller  20 . Selector-and-timing controller  32  receives an active/standby (ACT/STBY) signal from the line controller  20 . The active/standby signal is  0  when the line transmitter is in active mode and is  1  when it is in standby mode. Selector-and-timing controller  32  controls the selector  31  to be positioned to the upper terminal when the ACT/STBY logic is 0. Otherwise, it causes the selector  31  to be positioned in the lower terminal and provides a slot position signal to enable the output of selector  30  to be forwarded onto the transmission line  13 A during a specified timeslot. 
     As illustrated in FIG. 2, when the line transmitter is active, the active/standby signal is at logic 0 and the selector-and-timing controller  32  supplies a zero-output signal to the selector  31 . In the presence of this zero-level signal, the selector  31  selects transmit data (TX Data) from the switch  15  and forwards it onto the transmission line  13 A. The transmit data is of a framed structure containing a series of time-division digital signals on timeslots TS 0  to TS N . 
     On the other hand, when the line transmitter is in standby mode, the active/standby signal is at logic 1 and the selector-and-timing controller  32  is enabled to supply the timeslot signal as shown in FIG. 2 to the selector  31 . In response to the timeslot signal, the selector  31  is moved to the lower position to selects the output of the memory  30  and inserts the signal selected by the memory  30  into a timeslot TS X  of each frame that is specified by the timeslot signal. 
     Each line receiver, as represented by the receiver  6 , consists of a fault detector  40 , a line monitor  41  and a timing circuit  42 . If the line receiver  6  is in active mode, the fault detector  40  receives the transmitted data of the switching node  2  and examines its framing bits and alarm bits to detect a failure in the transmission line  13 A and forwards the receive data to the switch  3 . Line monitor  41 , which is also connected to the transmission line  13 A, is enabled when the line receiver  6  is inactive. Line monitor  41  examines node status data inserted on a timeslot of the standby transmission line  13 A specified by the timing circuit  42  and supplies the inserted status indication to the line controller  8 . Thus, the line status indication from the transmitter  18  represents a status of the active system as detected by the switching node  2 . The line status indication received by the line monitor  41  is communicated via the line controller  8  to the central processor  4 . 
     Each of the central processors  4  and  16  exchanges data with the associated active line controller to determine if the line transmitter and receiver of the active line interface unit are functioning properly. If a failure is detected in the active line interface of a switching node, the central processor of the node performs protection switching to the standby system in the same manner as it does when a failure is detected by the fault detector  40 . 
     The operation of the network will be described below with the aid of FIGS. 3A,  3 B and  3 C by assuming that lines  13 A and  13 B and their associated circuitry are active and lines  14 A and  14 B and their associated circuitry are set idle in the standby mode. 
     In FIG. 3A, if the active transmission line  13 A and its associated circuitry are functioning properly, the fault detector  40  of line receiver  6  produces no alarm signal and the line controller  8  sends “normal” indication to the central processor  4 . Central processor  4  knows that the active line  13 A and the associated receiver  6  are normally operating and commands the line controller  12  to read “normal” indication from the memory  30  of the standby transmitter  11  for transmission on the timeslot TS X  over the standby line  14 B. At the switching node  2 , the normal indication signal is received by the line monitor  41  of receiver  23  and communicated to the central processor  16  via the line controller  24 . The communication paths of the normal indication of the active line  13 A are indicated by solid thick lines. 
     If the active transmission line  13 B and its associated circuitry are also functioning properly, the fault detector  40  of line receiver  19  produces no alarm signal and the line controller  20  sends “normal” indication to the central processor  16 . Central processor  16  knows that the active line  13 B and the associated receiver  19  are normally operating and commands the line controller  24  to read “normal” indication from the memory  30  of the standby transmitter  22  for transmission on the timeslot TS X  over the standby line  14 A. At the switching node  1 , the normal indication signal is received by the line monitor  41  of receiver  10  and communicated to the central processor  4  via the line controller  12 . The communication paths of the normal indication of the active line  13 B are indicated by dotted thick lines. 
     The normal indication may be transmitted between the switching nodes at periodic intervals over the standby transmission lines as long as the active system is working properly. 
     If a failure occurs in the transmission line  13 A, as indicated by a symbol “X” in FIG. 3B, the fault detector  40  of line receiver  6  supplies an alarm signal to the active line controller  8 . This alarm signal is communicated to the central processor  4 . In response, the central processor  4  commands the standby line controller  12  to read alarm indication from the status data memory  30  of transmitter  11  and transmit it over the standby transmission line  14 B. Line receiver  23  of the switching node  2  alarms the line controller  24 , which communicates the alarm to the central processor  16 . Central processor  16  recognizes that a failure has occurred in the transmission line  13 A and commands the line controller  24  to return a response indication to the switching node  1 . Line controller  24  thus reads response indication from the memory  30  of transmitter  22  for transmission over the standby line  14 A. In the switching node  1 , the line monitor  41  of receiver  10  receives the response indication and communicates this signal to the central processor  4  via the line controller  12 . Central processor  4  knows that the transmitted alarm signal is received by the remote switching node  2 . The communication paths of the alarm indication of the active line  13 A are indicated by dotted thick lines in FIG.  3 B. 
     Central processors  4  and  16  now perform protection switching from the current active system to the current standby system by controlling the associated switches  3  and  15  so that transmit data from the node  2  is supplied to the now active line transmitter  22  and transmit data from the node  1  is supplied to the now active line transmitter  11 , as illustrated in FIG.  3 C. In the transmitters  11  and  22 , transmit data from the associated switches is selected for full-frame transmission over lines  14 B and  14 A. In FIG. 3C, the line monitor  41  of the receiver  6  is connected to the line  13 A to monitor transmitted normal indication signal in order to detect the recovery of the line from failure.