Abstract:
A redundancy control method is disclosed that controls a first redundancy function that switches between a working line and a protection line in response to a detection of a line error and a second redundancy function that performs a path switching in response to a detection of a path error in a ring network operating at a path rate lower than a line rate thereof. The method comprises a step of masking the path error detection within a period from a time of the detection of the line error to an expected time of the detection of the path error.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to a method for controlling redundancy and a transmission device, and particularly relates to a method for controlling redundancy and a transmission device for use in a ring network operating at a path rate lower than a line rate thereof. 
     2. Description of the Related Art 
     In recent years, there has been a growing demand for IP networks with greater reliability. RPR (Resilient Packet Ring) having a ring switching function for LAN (the term “LAN” as used herein includes WAN and MAN as well as LAN) is one promising technology to improve the reliability of the IP networks. In RPR networks, Ethernet™ frames are encapsulated in RPR frames. The RPR redundancy employs a steering method and a wrapping method, which are standardized in IEEE 802.17. Path failure information is transmitted between stations using control frames, thereby achieving reliability close to that of synchronous transmission systems such as SONET (Synchronous Optical Network) and SDH (Synchronous Digital Hierarchy). It is understood that the term “SONET” is used hereinafter as short for “SONET/SDH”. 
     On the other hand, SONET communications networks provide SONET redundancy, in which a working (W) line is switched to a protection (P) line when a failure occurs on the working line, and the protection line is switched back to the working line when the failure is recovered from. 
     The existing infrastructure for the SONET communications networks is huge. 
     Currently, it is believed that using the existing SONET infrastructure is an effective way to readily and efficiently deploy the above-described RPR technology. However, if the RPR ring network is formed with use of the existing SONET infrastructure as a physical layer, both the SONET redundancy function and the RPR redundancy function are activated in the event of failure, resulting in a conflict. 
     In view of this problem, Patent Document 1 discloses a method for controlling the conflict between the SONET redundancy function and the RPR redundancy function by statically switching two modes with use of SONET overhead. In one mode, the RPR redundancy function is disabled while a BLSR (Bi-directional Line-Switched Ring) redundancy function, which is one of the SONET redundancy functions, is enabled. In the other mode, the RPR redundancy function is enabled while the BLSR redundancy function is disabled.
     [Patent Document 1] Japanese Patent Laid Open Publication No. 2004-23480   

       FIGS. 1A-1C  are block diagrams illustrating a related-art RPR over SONET network in which the existing SONET infrastructure is used as a physical layer. Referring to  FIG. 1A , stations  1 ,  2 , and  3  form a ring network through working (W) lines and protection (P) lines. The station  1  sends a data item α to the station  3 , and a data item β to the station  2 . 
     If a failure occurs on the working (W) line between the stations  1  and  2  as shown in  FIG. 1B , the following operations are performed. With reference to  FIGS. 1B and 2A , in the station  2 , an optical interface section  4  detects a line error on the working (W) line and reports the line error to a CPU section  6 . The CPU section  6  activates a SONET redundancy function such that connection to an RPR section  8  is switched by a switch section  7  from the optical interface section  4  for the working (W) line to an optical interface section  5  for the protection (P). 
     Once a SONET redundancy operation is completed by the SONET redundancy function in this way, a path of the RPR is restored. However, the RPR section  8  of the station  2  detects a path error before the completion of the SONET redundancy operation, so that an RPR redundancy function is activated although not needed. 
     When the RPR redundancy function is activated, a transmission route for the data item β is switched to a route of the station  1 —the station  3 —the station  2  as shown in  FIG. 1C . Therefore, the available bandwidth between the station  1  and  3  decreases. Moreover, the unwanted activation of the RPR redundancy function causes a temporary signal interruption. 
     When the failure is recovered from, the following operations are performed. Referring to  FIG. 2B , when a command is input or when a WTR (Wait To Restore) state is over, the CPU section  6  sends a signal such that the switch section  7  switches back connection from the optical interface section  5  for the protection (P) line to the optical interface section  4  for the working (W) line. Because a pointer is relocated during the switchback from the optical interface section  5  to the optical interface section  4 , the RPR section  8  detects a path error due to the pointer relocation. As a result, the RPR redundancy function is activated although not needed. 
     As described above, although a path failure is recovered from by the SONET redundancy operation, the unwanted activation of the RPR redundancy function causes reduction of available bandwidth and temporary interruption of signals. 
     It may be a solution to this problem to increase the delay time of the activation of the RPR redundancy function with respect to the detection of the path failure using an RPR hold off timer; With this method, however, activation of the RPR redundancy function with the increased delay is applied even when RPR redundancy is really needed. 
     Turning back to the method disclosed in Patent Document 1, if both the working line and the protection line fail, i.e., if a double failure occurs in the mode where the RPR redundancy function is disabled, the RPR redundancy function cannot be activated. 
     SUMMARY OF THE INVENTION 
     The present invention solves at least one problem described above. 
     According to one aspect of the present invention, there is provided a redundancy controlling method and a transmission device using the same, capable of preventing reduction of available bandwidth and temporary signal interruption due to unwanted activation of an RPR redundancy function. 
     According to another aspect of the present invention, there is provided a redundancy controlling method for controlling a first redundancy function that switches between a working line and a protection line in response to a detection of a line error and a second redundancy function that performs a path switching in response to a detection of a path error in a ring network operating at a path rate lower than a line rate thereof, the method comprising a step of masking the path error detection within a period from a time of the detection of the line error to an expected time of the detection of the path error. This method makes it possible to prevent reduction of available bandwidth and temporary signal interruption due to unwanted activation of the second redundancy function. 
     It is preferable that the redundancy controlling method described above further comprise a step of stopping masking the path error detection so as to enable the second redundancy function if a line error is detected on the protection line after the switching by the first redundancy function from the working line to the protection line. This method makes it possible to handle the double errors on the working line and the protection line, i.e., a double failure, using the second redundancy function. 
     According to still another aspect of the present invention, there is provided a transmission device for use in a ring network operating at a path rate lower than a line rate thereof, having a first redundancy function that switches between a working line and a protection line in response to a detection of a line error, and a second redundancy function that performs a path switching in response to a detection of path error. The transmission device comprises a line error detecting unit that detects the line error on the working line and on the protection line, a switching unit that enables the first redundancy function so as to switch from the working line to the protection line in response to the detection of the line error on the working line, a path error detecting unit that detects a path error in a signal output from the switching unit, and a masking unit that masks the path error detection by the path error detecting unit within a period from a time of the detection of the line error on the working line by the line error detecting unit to an expected time of the detection of the path error by the path error detecting unit. This transmission device can prevent reduction of available bandwidth and temporary signal interruption due to unwanted activation of the second redundancy function. 
     In the above transmission device, it is preferable that the masking unit mask the path error detection by the path error detecting unit within a period from a start time of the switching to an expected time of the detection of the path error by the path error detecting unit when the switching unit switches back from the protection line to the working line. With this configuration, the transmission device can prevent reduction of available bandwidth and temporary signal interruption due to unwanted activation of the second redundancy function during the switchback. 
     In the above transmission device, it is also preferable that the masking unit&#39; stop masking the path error detection such that the second redundancy function is enabled if the line error detecting unit detects the line error on the protection line after the switching by the switching unit from the working line to the protection line. This transmission device can handle the double failure on the working line and the protection line by using the second redundancy function. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A-1C  are block diagrams illustrating redundancy operations of a related-art RPR over SONET network; 
         FIGS. 2A and 2B  are block diagrams illustrating operations in the related-art RPR over SONET network upon occurrence and recovery of a failure; 
         FIG. 3  is a block diagram illustrating an RPR over SONET network according to an embodiment of the present invention; 
         FIG. 4  is a block diagram illustrating operations performed upon occurrence of a failure in accordance with a redundancy controlling method of an embodiment of the present invention; 
         FIG. 5  is a signal timing chart for illustrating operations performed upon occurrence of a failure; 
         FIG. 6A  illustrates a SONET synchronous transport module STS-1 format; 
         FIG. 6B  illustrates an STS-Nc format; 
         FIG. 6C  illustrates a POH format; 
         FIGS. 7A-7C  are block diagrams illustrating redundancy operations performed in an RPR over SONET network according to an embodiment of the present invention; 
         FIG. 8  is a block diagram illustrating operations performed upon recovery of a failure in accordance with a redundancy controlling method of an embodiment of the present invention; 
         FIG. 9  is a block diagram illustrating operations performed upon occurrence of a double failure in accordance with a redundancy controlling method of an embodiment of the present invention; 
         FIG. 10  is a signal timing chart for illustrating operations performed upon occurrence of a double failure; and 
         FIG. 11  is a block diagram illustrating another operations set performed upon occurrence of a failure in accordance with a redundancy controlling method of an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The following description provides exemplary embodiments of the present invention with reference to the accompanying drawings. 
     &lt;Configuration of RPR Over SONET Network&gt; 
       FIG. 3  illustrates a configuration of an RPR over SONET network according to an embodiment of the present invention. Stations  10 ,  20 , and  30  forming the network are SONET devices. The stations  10  and  20  are interconnected through a working (W) line  40  and a protection (P) line  41 . The stations  20  and  30  are interconnected through a working (W) line  42  and a protection (P) line  43 . The stations  30  and  10  are interconnected through a working (W) line  44  and a protection (P) line  45 . The working (W) lines  40 ,  42 , and  44  and the protection (P) lines  41 ,  43 , and  45  are made of optical fibers. 
     The stations  10 ,  20 , and  30  have the same configuration. The station  10  comprises optical interface sections  11 - 14 , a switch section  15 , an RPR section  16 , and a CPU section  17 . 
     The optical interface sections  11  and  12  are connected to the working line  40  and the protection line  41 , respectively, and adapted to transmit and receive optical signals. The optical interface sections  13  and  14  are connected to the working line  44  and the protection line  45 , respectively, and adapted to transmit and receive SONET signals. The switch section  15  is adapted to cross-connect the signals sent from the optical interface sections  11 - 14  and the RPR section  16 . 
     The RPR section  16  is connected to an outside LAN. The RPR section  16  is adapted to convert LAN format signals sent from the LAN to SONET format signals to send the converted signals to the switch section  15 , and convert SONET format signals sent from the switch section  15  to LAN format signals to send the converted signals to the LAN. The CPU section  17  controls the entire operations of the stations  10 ,  20 , and  30 . 
     &lt;Operations Upon Occurrence of Failure&gt; 
     In an RPR over SONET network operating at a path rate lower than a line rate thereof, the rate at which an interface section detects a path error (path error detection rate) is slower than the rate at which an optical interface detects a line error (line error detection rate) upon occurrence of a failure on a line. Accordingly, a path error is detected with a delay relative to detection of a line error. In an embodiment of the present invention, taking advantage of this detection time difference, a CPU section sends an RPR redundancy masking signal within the time period between the line error detection and the path error detection. 
       FIG. 4  is a block diagram illustrating operations performed upon occurrence of a failure in accordance with a redundancy controlling method of an embodiment of the present invention. In  FIG. 4 , components identical to those in  FIG. 3  bear the same reference numerals.  FIG. 5  is a signal timing chart for illustrating operations performed upon occurrence of a failure, wherein a line rate is OC-48 (about 2.5 Gbps), a path rate is STS-12C (about 600 Mbps), and a BER is 1×10 −5 . Operations utilizing an RPR redundancy masking signal in accordance with an embodiment of the present invention are described with reference to (C)-(F) of  FIG. 5 , while operations utilizing no RPR redundancy masking signal are described with reference to (A) and (B) of  FIG. 5  for ease of understanding. 
     Referring to  FIGS. 4 and 5 , a failure occurs on a working line  40  at time t 0 . About 8 ms later, at time t 1 , an optical interface section  11  detects a line error, such as light interruption, and switches a line error detection signal ((A) and (B) of  FIG. 5 ) from low level to high level. In response to the line error detection signal, the optical interface section  11  switches an interruption signal ((D) of  FIG. 5 ) to high level and sends the interruption signal to a CPU section  17  at time t 2 . 
     In response to the interruption signal, the CPU section  17  switches a line switching signal and an RPR redundancy masking signal ((E) of  FIG. 5 ) to high level and sends the line switching signal to a switch section  15  and the RPR redundancy masking signal to an RPR section  16  at time t 3 . The switch section  15  enables a SONET redundancy function such that the connection to the RPR section  16  through the switch section  15  is switched from a working line  40  to a protection line  41 . This switching operation takes about 20 ms, which is the time length between time t 3  and time t 5 . 
     The RPR section  16  is also adapted to terminate SONET format signals to perform path error detection using the B3 byte of POH (Path Overhead).  FIGS. 6A ,  6 B, and  6 C illustrate a SONET synchronous transport module STS-1 format, an STS-Nc format, and a POH format, respectively. An error detecting code BIP-8 (BIT Interleaved Parity-level  8 ) calculated over a payload area of a synchronous transport module of a previous frame is placed in the B3 byte of the POH. The RPR section  16  compares the calculated error detecting code BIP-8 with an error detecting code BIP-8 placed in the B3 byte of a synchronous transport module of the current frame to check for a difference, which indicates that a path error has occurred. In this way, the RPR section  16  detects the path error. 
     If no RPR redundancy signal is provided, unlike the illustrated embodiment of the present invention, the RPR section  16  detects a path error and switches a path error detection signal ((B) of  FIG. 5 ) from low level to high level at time t 4  about 25 ms after time t 0 . That is, the time difference between the path error detection and the line error detection is about 17 ms. However, in the illustrated embodiment of the present invention, since the CPU section  17  sends the RPR redundancy masking signal at time t 3 , the path error detection signal ((F) of  FIG. 5 ) remains low level. Accordingly, the RPR redundancy function is not activated, and an RPR path is restored by the operations of the SONET redundancy function. 
     Referring to  FIG. 7A , stations  10 ,  20 , and  30  form a ring network through working (W) lines and protection (P) lines. The station  10  sends a data item α to the station  30 , and a data item β to the station  20 . In  FIGS. 7A-7C , components identical to those in  FIG. 3  bear the same reference numerals. 
     If a failure occurs on the working line  40  between the stations  10  and  20  as shown in  FIG. 7B , the following operations are performed. In the station  20 , an optical interface section  11  detects a line error on the working line  40  and reports the line error to a CPU section  17 . The CPU section  17  activates a SONET redundancy function to send a line switching signal for switching connection of a switch section  15  from the optical interface section  11  for the working (W) line to an optical interface section  12  for the protection (P) line, and also send an RPR redundancy masking signal to an RPR section  16 . 
     As a result, in the station  20 , the SONET redundancy function is activated while an RPR redundancy function is not activated. The ring network therefore operates in a state shown in  FIG. 7C , thereby preventing reduction of available bandwidth between the stations  10  and  30  and temporary signal interruption due to unwanted activation of the RPR redundancy function. 
     &lt;Operations Upon Recovery from Failure&gt; 
       FIG. 8  is a block diagram illustrating operations performed upon recovery from a failure in accordance with a redundancy controlling method of an embodiment of the present invention. In  FIG. 8 , components identical to those in  FIG. 3  bear the same reference numerals. With reference to  FIG. 8 , when a command is input or when a WTR (Wait To Restore) state is over, a CPU section  17  generates a line switching signal that causes a switch section  15  to switch back the connection from an optical interface section  12  for a protection (P) line  41  to an optical interface section  11  for a working (W) line  40  and an RPR redundancy masking signal. The CPU section  17  sends the line switching signal to the switch section  15  and the RPR redundancy section to the RPR section  16 . 
     In response to the line switching signal, the connection to the RPR section  16  is switched back by the switch section  15  from the optical interface section  12  for the protection (P) line  41  to the optical interface section  11  for the working (W) line  40 . This switchback operation takes about 20 ms. 
     The RPR section  16  relocates the pointer contained in a SOH (Section OverHead) ( FIG. 6A ) that points to a start position of an STS-1 during the switchback operation from the optical interface section  12  to the optical interface section  11 . The start position of the POH of the STS-1 SPE is changed by the relocation of the pointer, resulting in a difference between a calculated error detecting code BIP-8 for the current frame and an error detecting code BIP-8 of the following frame. This difference is detected as a path error. 
     The path error is detected about a dozen ms after the start of the switchback. Since the RPR redundancy masking signal is sent from the CPU section  17  to the RPR section  16  before the detection of the path error, a path error detection signal is not output. Accordingly, an RPR redundancy function is not activated. 
     &lt;Operations Upon Occurrence of Double Failure&gt; 
       FIG. 9  is a block diagram illustrating operations performed upon occurrence of a double failure in accordance with a redundancy controlling method of an embodiment of the present invention. In  FIG. 9 , components identical to those in  FIG. 3  bear the same reference numerals.  FIG. 10  is a signal timing chart for illustrating operations performed upon occurrence of the double failure, wherein a line rate is OC-48 (about 2.5 Gbps), a path rate is STS-12C (about 600 Mbps), and a BER is 1×10 −5 . 
     With reference to  FIG. 9 , after a switching operation from a working line  40  having a failure to a protection line  41 , a failure occurs on the protection line  41  at time t 10 . About 8 ms later, at time t 11 , an optical interface section  12  detects a line error indicating the failure on the protection line  41 , such as light interruption, and switches a line error detection signal ((A) of  FIG. 10 ) from low level to high level. In response to the line error detection signal, the optical interface section  12  switches an interruption signal ((B) of  FIG. 10 ) to high level and sends the interruption signal to a CPU section  17  at time t 12 . 
     Since the CPU section  17  has already received an interruption signal from an optical interface section  11 , the CPU section  17  recognizes occurrence of double failure upon receiving the interruption signal from the optical interface section  12 . The CPU section  17  maintains a line switching signal and an RPR redundancy masking signal ((C) of  FIG. 10 ) at low level, and stops outputting the line switching signal and the RPR redundancy masking signal. 
     As a result, the connection to an RPR section  16  is not switched by a switch section  15  from the optical interface section  12  for the protection line  41  to the optical interface section  11  for the working line  40 . In other words, a SONET redundancy function is not activated. 
     Accordingly, the RPR section  16  detects a path error and switches a path error detection signal ((D) of  FIG. 10 ) from low level to high level at time t 13  about 25 ms after time t 10 . For example, if the working line  40  between the stations  10  and  20  fails as shown in  FIG. 7B  and the protection line  41  also fails in the state shown in  FIG. 7C  before the working line  40  is restored, the above-described RPR redundancy function switches a transmission route for the data item β to a route of the station  10 —the station  30 —the station  20 . In the meantime, line and path recovery operations are performed. 
     &lt;Another Operations Set Upon Occurrence of Failure&gt; 
       FIG. 11  is a block diagram illustrating another operations set performed upon occurrence of a failure in accordance with a redundancy controlling method of an embodiment of the present invention. In  FIG. 11 , components identical to those in  FIG. 3  bear the same reference numerals. With reference to  FIG. 11 , when a failure occurs on a working line  40 , an optical interface section  11  detects a line error, such as light interruption, and switches a line error detection signal from low level to high level. The optical interface section  11  generates an interruption signal in accordance with the line error detection signal, and sends the interruption signal to a CPU section  17 . 
     The CPU section  17  generates a line switching signal and a B3 rewrite signal, and sends the line switching signal and the B3 rewrite signal to a switch section  15 . Then, the switch section  15  enables a SONET redundancy function such that connection to an RPR section  16  is switched from the optical interface section  11  for the working line  40  to an optical interface section  12  for a protection line  41 . Further, the switch section  15  calculates an error detecting code BIP-8 over a payload area of a received synchronous transport module, and rewrites a B3 byte of a synchronous transport module of the following frame based on the calculated error detecting code BIP-8. 
     The RPR section  16  compares a calculated error detecting code BIP-8 with an error detecting code BIP-8 of the following frame, and determines that there is no difference. Therefore, the RPR section  16  detects no path error and outputs no path error detection signal. Accordingly, an RPR redundancy function is not activated, and an RPR path is restored by the SONET redundancy function. 
     RPR over SONET ring networks are often operated at a path rate lower than a line rate thereof, for example, at a line rate of OC (Optical Carrier)-192, 48, and a path rate of STS-1, 3, 12. In an aspect of the present invention, since unwanted activation of the RPR redundancy function in RPR over SONET networks does not occur after switching operations due to a line failure and after switchback operations due to recovery of a line, efficient use of the bandwidth in the RPR and prevention of unnecessary signal interruption can be achieved. On the other hand, the RPR redundancy function activates in the event of a double failure, thereby ensuring network reliability. 
     While 1+1 SONET redundancy functions are used in the illustrated embodiments, the same effects are achieved in BLSR (Bi-directional Line Switched Ring) because line switching operations in BLSR are performed in the same manner. It is understood that the present invention is not limited to the above embodiment. 
     The above embodiments employ the optical interface sections  11 - 14  as components corresponding to a line error detecting unit in the appended claims, the switch section  15  as a component corresponding to a switching unit and as a component corresponding to a data rewrite unit, the RPR section  16  as a component corresponding to a path error detecting unit, and the CPU section  17  as a component corresponding to a masking unit. 
     The present application is based on Japanese Priority Application No. 2005-285421 filed on Sep. 29, 2005, with the Japanese Patent Office, the entire contents of which are hereby incorporated by reference.