# Patent

## Title
Switching of PVC connection

## Abstract

The invention provides a PVC switching control method for an ATM communication network which allows high speed changeover of a connection upon occurrence of/release from a trouble and is superior in reliability and maintenance facility and simple in control. A master PVC connection and an OAM connection are set between two ATM exchanges, and a bypassing PVC connection and an OAM connection prepared in advance for bypassing are set between the two ATM exchanges. If occurrence of/release from a trouble with and of the master PVC connection is recognized by the ATM exchanges using an OAM function, then the operative PVC connection is switched between the master PVC connection and the bypassing PVC connection.

## Background
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a network management technique, and more particularly to an ATM (Asynchronous Transfer Mode) exchange system and a switching control method for a permanent virtual connection (hereinafter referred to as “PVC connection”) for an ATM network.
2. Description of the Related Art
As diversification, increase in speed and increase in capacity of services in an ATM network in recent years proceed, it becomes a significant subject to improve an ATM exchange system in terms of the performance, function and reliability. As a necessary function, it is demanded particularly to augment the reliability of a PVC connection. Specifically, when a trouble occurs with a circuit, it is significant to continue communication of a PVC connection with a minimum cell loss.
As a method of preventing disconnection of a PVC connection by a trouble of a circuit or an apparatus, it is a possible idea to employ a redundant configuration (dual configuration, n+1 multiple configuration) in hardware for a circuit itself. This method, however, is disadvantageous in that it requires complicated maintenance.
As another method, it is known to use a network management system (NMS) to automatically or manually switch, when a trouble occurs with a PVC connection, the PVC connection to another PVC connection of another route (refer to, for example, Japanese Patent Laid-Open No. 2000-59374). With the method, however, when a trouble occurs or upon release from a trouble, it is necessary for the network management system to notify each ATM exchange of setting/deletion of a PVC connection. Therefore, the method has a problem in that considerable time is required for the exchanging process. Further, if a trouble occurs with the network management system, then this makes setting itself of a PVC connection impossible.
Thus, a method has been proposed wherein network management is performed autonomously and discretely without depending upon a network management system. For example, Japanese Patent Laid-Open No. 266480/1997 discloses a method wherein, when a trouble occurs with a circuit, a routing function of a switched virtual connection (SVC) is used to set a bypassing SVC connection.
Meanwhile, Japanese Patent Laid-Open No. 215148/1999 discloses another method wherein both of a PVC connection and an SVC connection are used such that, if a trouble occurs with the PVC, a call setting procedure by the SVC is executed to set an optimum SVC connection as a bypass link.
However, with the conventional autonomous PVC bypassing method, however, when a trouble occurs with a currently used PVC connection, a bypassing SVC connection is searched for and set. Therefore, rapid switching cannot be anticipated. Further, although a bypassing connection is set and switchably used when a trouble occurs with a currently used connection and then the bypassing connection is disconnected and the currently used connection is switchably used upon release of the currently used connection, the conventional autonomous PVC bypassing method does not propose any autonomous countermeasure of software against occurrence of/release from a trouble which may occur with the currently used or bypassing connection.
SUMMARY OF THE INVENTION
An aspect of the present invention is directed to providing a PVC switching control method which allows high speed changeover of a connection upon occurrence of/release from a trouble and is superior in reliability and maintenance facility and simple in control.
According to an aspect of the present invention, there is provided a PVC switching control method for controlling a PVC connection in an ATM communication network, comprising the steps of setting a plurality of PVC connections and individually corresponding controlling connections between two ATM exchanges of the ATM communication network, and detecting, by each of the ATM exchanges, occurrence of and release from a trouble with and of a PVC connection corresponding to any of the controlling connections and switching an operative PVC connection to another one of the PVC connections in response to a result of the detection.
The PVC switching control method may be configured such that, if, while one of the PVC connections is used as the currently used PVC connection, it is detected from the corresponding controlling connection that a trouble has occurred with the PVC connection, then each of the ATM exchanges switches the operative PVC connection to another one of the PVC connections as a bypassing PVC connection. Further, the PVC switching control method may be configured such that, if, while the bypassing PVC connection is used, it is detected that the currently used PVC connection has been released through the corresponding controlling connection, then each of the ATM exchanges switches the operative PVC connection to the currently used PVC connection.
The controlling connections may be set by an operation administration and maintenance (OAM) function. Further, each of the ATM exchanges may detect a trouble through the fact that an alarm indication signal cell of the operation administration and maintenance function is inputted thereto or through the fact that a continuity check cell of the operation administration and maintenance function is not inputted thereto.
According to another aspect of the present invention, there is provided a PVC switching control method for controlling a PVC connection in an ATM communication network, comprising the steps of setting a master PVC connection and a master side operation administration and maintenance connection corresponding to the master PVC connection between a first ATM exchange and a second ATM exchange, setting a bypassing PVC connection prepared in advance for bypassing of the master PVC connection and a bypassing side operation administration and maintenance connection corresponding to the bypassing PVC connection between the first and second ATM exchanges, and switching, if both of the first and second ATM exchanges detect a trouble of the master PVC connection through the master side operation administration and maintenance connection, the PVC connection to the bypassing PVC connection by means of the first and second ATM exchanges.
The PVC switching control method may be configured such that, if, while the first and second ATM exchanges use the bypassing PVC connection, the first and second ATM exchanges detect a release of the master PVC connection through the master side OAM connection, each of the first and second ATM exchanges switches the PVC connection to the master PVC connection.
The PVC switching control method may be configured such that a plurality of repeating ATM exchanges are connected on a route of the bypassing PVC connection and a connection for forming the bypassing PVC connection is set in each of the repeating ATM exchanges. In this instance, each of the first and second ATM exchanges may designate the connection set in advance and signal an ATM cell to a neighboring one of the plurality of repeating ATM exchanges through the designated connection.
In summary, according to the PVC switching control methods for an ATM communication network, a master PVC connection and an operation administration and maintenance connection corresponding to the PVC connection are set between two ATM exchanges, and a bypassing PVC connection and an operation administration and maintenance connection prepared in advance for bypassing are set between the two ATM exchanges. Occurrence of and release from a trouble with and of the master PVC connection can be detected through the operation administration and maintenance connection, and autonomous and high speed switching between the master PVC connection and the bypassing PVC connection can be achieved. Since such detection and switching as described above do not rely upon control of a network management system, the switching speed of the PVC connection can be augmented, and the cell loss is reduced and the reliability of the PVC connection is augmented.
Also where three or more PVC connections are set between two stations of an ATM communication network, switching between PVC connections and switching back between the PVC connections can be executed by control for a single apparatus. Therefore, the control is simplified. Further, when a trouble occurs with and/or is eliminated from a circuit, a network manager need not perform PVC connection setting again. Therefore, maintenance is facilitated.
The above and other aspects, features and advantages of the present invention will become apparent from the following description and the appended claims, taken in conjunction with the accompanying drawings in which like parts or elements are denoted by like reference symbols.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1a is a block diagram showing an ATM network configuration to which a PVC switching control method according to the present invention is applied;
FIG. 1b is a schematic view of an ATM exchange showing a master and bypassing connection group (PVC group) in an ATM exchange at a terminating portion of a PVC connection;
FIG. 2 is a block diagram of an ATM network showing a flow of an ATM cell during normal operation of the ATM network shown in FIG. 1a; 
FIG. 3 is a block diagram of the ATM network configuration of FIG. 1a showing a flow of an ATM cell when a trouble occurs with a master PVC connection;
FIG. 4 is a flow diagram illustrating a status transition of a PVC switching function of the ATM network configuration of FIG. 1a; 
FIG. 5 is a block diagram of another ATM network showing a flow of an ATM cell during normal operation; and
FIG. 6 is a similar view but illustrating a flow of an ATM cell when a trouble occurs with a master PVC connection in the ATM network of FIG. 5.



DESCRIPTION OF THE PREFERRED EMBODIMENT
FIG. 1a shows an ATM network configuration to which a PVC switching control method according to the present invention is applied, and FIG. 1b shows an ATM exchange and illustrates a master and bypassing connection group (PVC group) in an ATM exchange at a terminating portion of a PVC connection. The PVC group includes a master PVC connection, a master side OAM connection, a bypassing side PVC connection and a bypassing side OAM connection.
In the present embodiment, in order to simplify description, it is assumed that the ATM network includes ATM exchanges 1 to 6 and currently used and bypassing PVC connections are set between the ATM exchange 1 and the ATM exchange 4. Further, each of the ATM exchanges 1 to 6 is provided with an OAM function and can supervise an AIS (Alarm Indication Signal) cell and/or a CC (Continuity Check) cell.
In particular, a master connection group 101 including a PVC connection 103 and an OAM connection 104 are set bidirectionally in a route from the ATM exchange 1 to the ATM exchange 4 through the ATM exchange 2 and the ATM exchange 3. Further, a bypassing connection group 102 including a PVC connection 105 and an OAM connection 106 is set in another route from the ATM exchange 1 to the ATM exchange 4 through the ATM exchange 5 and the ATM exchange 6 thereby to form a bypass path to be used when a trouble occurs with the master connection.
Referring to FIG. 1b, in each of the ATM exchanges 1 and 4, traffic and VPI/VCI corresponding to a PVC set are reserved in advance. Consequently, when occurrence of/release from a trouble is detected by an OAM connection, PVC switching can be performed immediately.
Each of the ATM exchanges 1 and 4 which are terminations of the master connection group 101 and the bypassing connection group 102, respectively, can utilize an OAM function to detect occurrence of a trouble with a corresponding PVC connection and switch, when a trouble occurs, the PVC connection at a high speed to the other PVC connection. A trouble occurring with a PVC connection can be detected by monitoring an AIS cell and/or a CC cell by the OAM function by means of the ATM exchanges 1 and 4.
In this manner, in the ATM network according to the present embodiment, the connections 104 and 106 for a controlling OAM are extended for the master PVC connection 103 and the bypassing PVC connection 105, respectively. In the following, operation of the ATM network is described.
(Operation)
FIG. 2 illustrates a flow of an ATM cell in the ATM network of FIG. 1 when the ATM network operates normally. Referring to FIG. 2, while the network operates normally, the master PVC connection group 101 is used for communication, and the ATM exchange 1 and the ATM exchange 4 supervise to discriminate whether or not an AIS cell is inputted to normally confirm whether or not the master PVC connection 103 is normal. However, it is otherwise possible to supervise for a CC cell of the OAM function to confirm that the PVC connection 103 is normal.
FIG. 3 shows a flow of an ATM cell in the ATM network when a trouble occurs with the master PVC connection. Referring to FIG. 3, if a trouble 301 occurs with a transmission line of the ATM exchanges 2 and 3 on the route of the master PVC connection, then the ATM exchange 1 and the ATM exchange 4 recognize the occurrence of the trouble 301 from an AIS/CC cell of the OAM function and change over the PVC connection from the master PVC connection 103 to the bypass PVC connection 105.
In this manner, even if a trouble occurs with the route of the master PVC connection 103, the PVC communication can be continued without interruption through the bypass PVC connection 105 which bypasses the trouble 301. In this instance, since the OAM connection 106 is extended to the bypass PVC connection 105, similar detection of occurrence of/release from a trouble can be performed.
If the trouble of the master PVC connection is eliminated after the switching to the PVC connection 105, then since the ATM exchange 1 and the ATM exchange 4 can recognize this using the OAM function thereof, the PVC connection can be switched from the bypass PVC connection 105 to the master PVC connection 103 automatically.
FIG. 4 illustrates a status transition of the PVC switching function of the ATM network. Referring to FIG. 4, reference numerals (1) to (6) represent occasions at which a status transition occurs, and (1) represents a trouble with the master connection, (2) a trouble with the bypassing connection, (3) a release of the master connection, (4) a release of the bypassing connection, (5) manual switching, and (6) manual release.
A [status S1] represents an ordinary state wherein the network is operating normally and the master and bypassing connections are normal while the master connection is currently used. In this state, both of manual switching and automatic switching upon occurrence of a trouble with the master connection are possible.
A [status S2] represents a state wherein both of the master and bypassing connections are normal and the bypassing connection is currently used. In this state, both of manual release and automatic switching upon occurrence of a trouble with the master connection are possible.
A [status S3] represents a state wherein a trouble has occurred with the master connection and the bypassing connection is currently used. Both of manual release and automatic switching upon occurrence of a trouble with the bypassing connection are impossible.
A [status S4] represents a state wherein a trouble has occurred with the bypassing connection and the master connection is currently used. Both of manual switching and automatic switching upon occurrence of a trouble with the master connection are impossible.
A [status S5] represents a state wherein a trouble has occurred also with the master connection which was normal and both of the master and bypassing connections are in trouble. In this state, manual switching is impossible. If the trouble of the bypassing connection is eliminated (occasion (4)), then automatic switching is performed to establish the status S3 wherein the bypassing connection is used. If the master connection is released (occasion (3)), then automatic switching is performed to establish the status S4 wherein the master connection is used.
A [status S6] represents a state wherein a trouble has occurred also with the bypassing connection which was normal and both of the master and bypassing connections are in trouble. In this state, manual release can not be performed. If the master connection is released (occasion (3)), then automatic switching is performed to establish the status S4 wherein the master connection is used. Also when the trouble of the bypassing connection is eliminated (occasion (4)), automatic switching is performed to establish the status S3 wherein the bypassing connection is used.
In this manner, in the ATM network described above, augmentation of a function of a PVC connection provided by an ATM exchange by software can be anticipated. The following three advantages are anticipated with the present embodiment. (1) Since an NMS need not be used to notify each ATM exchange of setting/deletion of a PVC connection, the changeover speed of a PVC connection is high, and the reliability of the PVC connection is improved. (2) Even where a plurality of PVC connections are set between two ATM exchanges, processes for switching and switching back between PVC connections can be performed by control for a single apparatus and therefore are simplified. (3) Upon occurrence of/release from a trouble with a circuit, the network manager need not perform setting for a PVC connection again.
FIG. 5 shows a configuration of an ATM network and illustrating a flow of an ATM cell in normal operation according to another embodiment of the present invention. FIG. 6 illustrates a flow of an ATM cell when a trouble occurs with a master PVC connection in the ATM network.
In the present embodiment, in order to simplify description, it is assumed that the ATM network includes ATM exchanges 10 to 60 and currently used and bypassing PVC connections are set between the ATM exchange 10 and the ATM exchange 40. Further, similarly as in the first embodiment described hereinabove, each of the ATM exchanges 10 to 60 is provided with an OAM function and can supervise an AIS (Alarm Indication Signal) cell and/or a CC (Continuity Check) cell.
In a normal operation state of the ATM network, bypassing connections 501 and 502 are extended between the ATM exchanges 50 and 60 in the bypass route. More particularly, traffic and VPI/VCI corresponding to the PVC group to be protected are reserved for the bypassing connections 501 and 502 in the ATM exchanges 50 and 60 as seen in FIG. 1b. 
If a trouble occurs with the master PVC connection, then the ATM exchanges 10 and 40 switch the PVC connection from the master PVC connection to the bypass PVC connection as described above. In this instance, however, the ATM exchange 10 designates the bypassing connection 501 and transmits an ATM cell to the neighboring ATM exchange 50 through the bypassing connection 501. Similarly, also the ATM exchange 40 designates the bypassing connection 502 and transmits an ATM cell to the neighboring ATM exchange 60 through the bypassing connection 502. Consequently, further rapid PVC switching control can be performed.
It is to be noted that, while, in the embodiments described above, two repeating exchanges are involved, the foregoing description similarly applies also where three or more repeating exchanges are involved.
While preferred embodiments of the present invention have been described using specific terms, such description is for illustrative purposes only, and it is to be understood that changes and variations may be made without departing from the spirit or scope of the following claims.

## Claims

1. A PVC switching control method for controlling a PVC connection in a communication network, comprising:
setting a plurality of PVC connections and individually corresponding controlling connections between two exchanges of the communication network;
detecting, by each of the exchanges, occurrence of or release from trouble with a PVC connection through the corresponding controlling connection; and
switching an operative PVC connection to another one of the PVC connections in response to a result of the detection.

2. The PVC switching control method as claimed in claim 1, wherein, if, while one of the PVC connections is used as a currently used PVC connection, it is detected from the corresponding controlling connection that trouble has occurred with the currently used PVC connection, then each of the exchanges switches the operative PVC connection to another one of the PVC connections as a bypassing PVC connection.

3. The PVC switching control method as claimed in claim 2, wherein, if, while the bypassing PVC connection is used, it is detected that the currently used PVC connection has been released through the corresponding controlling connection, then each of the exchanges switches the operative PVC connection to the currently used PVC connection.

4. The PVC switching control method as claimed in claim 1, wherein the controlling connections are set by an operation administration and maintenance function.

5. The PVC switching control method as claimed in claim 4, wherein each of the exchanges detects trouble through receipt of an alarm indication signal cell from the operation administration and maintenance function over one of the controlling connections.

6. The PVC switching control method as claimed in claim 4, wherein each of the exchanges detects trouble through failure to receive a continuity check cell from the operation administration and maintenance function over one of the controlling connections.

7. A PVC switching control method for controlling a PVC connection in a communication network, comprising:
setting a master PVC connection and a master side operation administration and maintenance (OAM) connection corresponding to the master PVC connection between a first exchange and a second exchange;
setting a bypassing PVC connection prepared in advance for bypassing of the master PVC connection and a bypassing side OAM connection corresponding to the bypassing PVC connection between the first and second exchanges; and
switching, if both of the first and second exchanges detect trouble of the master PVC connection through the master side OAM connection, the master PVC connection to the bypassing PVC connection at the first and second exchanges.

8. The PVC switching control method as claimed in claim 7, wherein, if, while the first and second exchanges use the bypassing PVC connection, the first and second exchanges detect a release of the master PVC connection through the master side OAM connection, each of the first and second exchanges switches the PVC connection to the master PVC connection.

9. The PVC switching control method as claimed in claim 7, wherein a plurality of repeating exchanges are connected on a route of the bypassing PVC connection and a connection for forming the bypassing PVC connection is set in each of the repeating exchanges.

10. The PVC switching control method as claimed in claim 9, wherein each of the first and second exchanges designates a connection set in advance and signals a cell to a neighboring one of the plurality of repeating exchanges through the designated connection.

11. A PVC switching control system for controlling a PVC connection in a communication network, comprising:
means for setting a plurality of PVC connections and corresponding controlling connections between first and second exchanges of the communication network;
means for detecting, by each of the first and second exchanges, occurrence of or release from trouble with a PVC connection based on information from the corresponding controlling connection; and
means for switching an operative PVC connection to another one of the PVC connections in response to a result of the detection.

12. The PVC switching control system of claim 11, wherein the controlling connections include operation administration and maintenance (OAM) connections to provide one of an alarm signal or a continuity check signal.

13. The PVC switching control system of claim 11, wherein, if, while one of the PVC connections is used as a currently used PVC connection, it is detected from the corresponding controlling connection that trouble has occurred with the currently used PVC connection, then each of the first and second exchanges switches the operative PVC connection to another one of the PVC connections as a bypassing PVC connection.

14. The PVC switching control system of claim 13, wherein, if, while the bypassing PVC connection is used, it is detected that the currently used PVC connection has been released through the corresponding controlling connection, then each of the first and second exchanges switches the operative PVC connection to the currently used PVC connection.

15. The PVC switching control system of claim 11, wherein the controlling connections are set by an operation administration and maintenance (OAM) function.

16. The PVC switching control system of claim 15, wherein each of the first and second exchanges detects trouble via receipt of an alarm indication signal cell from the OAM function over one of the controlling connections.

17. The PVC switching control system of claim 15, wherein each of the first and second exchanges detects trouble via failure to receive a continuity check cell from the OAM function over one of the controlling connections.

18. A first exchange in a network, comprising:
means for setting a master PVC connection and a master side operation administration and maintenance (OAM) connection corresponding to the master PVC connection between the first exchange and a second exchange;
means for setting a bypassing PVC connection prepared in advance for bypassing of the master PVC connection and a bypassing side OAM connection corresponding to the bypassing PVC connection between the first and second exchanges;
means for detecting trouble of the master PVC connection through the master side OAM connection; and
means for switching the master PVC connection to the bypassing PVC connection if trouble of the master PVC connection is detected.