Abstract:
The present disclosure provides a passive optical network (PON) system and a method for protecting the service of the system for service recovery and fault locating in case of a failure of the network, wherein the PON system comprises an optical line terminal (OLT), an optical distribution network (ODN) and an optical network terminal (ONT) equipment protection group comprising a plurality of ONT equipment groups, each of which is connected to at least one of other ONT equipment groups within the ONT equipment protection group for the mutual protection relationship. The PON system of the present disclosure does not require equipment and link redundancy for backup, contributes to reduced cost and improved utilization of resources, and provides a means for diagnosing any faults of the links and equipment in the network.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is claiming priority of Chinese Application No. 200610034310.0 filed Mar. 15, 2006, entitled “Passive Optical Network System and Method for Protecting the Services of the Same” which application is incorporated by reference herein in its entirety. 
     BACKGROUND OF THE DISCLOSURE 
     The present disclosure relates generally to the optical network field, and more particularly to a PON system and method for protecting its services. 
     A Passive Optical Network, or PON, is an optical fiber cabling network accessed by a user in which the central part contains no active electronic devices and all of the user signals converge going upstream and are broadcast going downstream using only passive components, such as optical dividers. Passive optical network technology is a broadband access technology that has great competitive advantages and is an important technical means that offers user access over the “the last mile”. By using passive optical components in its optical distribution network (ODN) and optical fiber as the medium for transmission, the technology has the advantages of high bandwidth, high immunity to electromagnetic interference, low loss, low cost, and simple design. In recent years, various PON technologies have gradually become commercially available. 
     Referring to  FIG. 1 , most existing optical network systems adopt a tree topology. User services access the network through an optical network unit (ONU) or an optical network terminal (ONT), are pooled on the ODN, and are ultimately sent to an optical line terminal (OLT). However, a pure tree topology does not have measures for failure protection and recovery or means for locating a failure. If an equipment or link fault is not corrected for a long time, the user services may be greatly affected and security may be seriously impaired. It is also very difficult to locate the fault. Therefore, the security of the PON is an issue of great importance and the PON must be capable of protecting its services in the event that it has a fault. 
     The existing PON systems achieve service protection by means of fully-protected optical fiber switchover, as shown in  FIG. 2 . In such a system, there are two links between the OLT and the ODN, and between the ONU and each ONU/ONT, where one link serves as redundancy backup to ensure a quick recovery of services in case of a fault. 
     However, the existing failure protection measures achieve unobstructed network traffic and normal services by adding a large number of redundant equipment and have the following defects:
         1. High construction cost. Each ONU requires two sets of identical transmission equipment requires an additional optical interface, which results in a dramatic increase in the ONU cost.   2. Low bandwidth utilization. Only one of the two work channels is working while the other one is always idle, resulting in an increased waste of resources.   3. Inability to locate a fault and indicate fault type.       

     SUMMARY 
     The embodiment of the present disclosure provides a PON system and a service protection method that is capable of service protection and fault diagnosis when a link or equipment fault occurs in the PON system. The PON system and service protection method have reduced construction cost and improved bandwidth utilization. 
     According to an embodiment of the present disclosure, a PON system is provided, which comprises an OLT connected to an ODN for processing user service data and for managing and controlling the ODN and the ONT equipment groups; an ODN, which is connected to the OLT equipment group and equipment within the ONT equipment groups, for collecting user service data received from the ONT equipment groups and for transmitting user services data to the OLT equipment group. The PON system further comprises an ONT equipment protection group connected to the ODN for protecting the services of the ONT equipment group within the ONT equipment protection group and transmitting the received user service data to the ODN. 
     According to an embodiment of the present disclosure, a PON system service protection method is provided, which comprises the steps of: 
     determining an optical network terminal (ONT) equipment protection group that comprises a plurality of ONT equipment groups, each of which is connected to another ONT equipment group within the ONT equipment protection group via a backup link for the mutual protection relationship; 
     sending a fault alert by the OLT, upon detecting a failed ONT equipment group, to the ONT equipment group that has the mutual protection relationship with the failed ONT equipment group; and 
     receiving services for the failed ONT equipment group by the ONT equipment that has the mutual protection relationship with the failed ONT equipment group upon receiving the fault alert. 
     Compared with the prior art, the embodiments of the present disclosure have the following effects:
         1. One backup link is added between any two equipment without the need to add redundant equipment, thereby improving network security while greatly reducing the construction cost.   2. A backup link is added between any two equipment without an additional redundant service channel, thereby avoiding resource waste and improving bandwidth utilization.   3. An embodiment of the present disclosure also provides a means for diagnosing link faults and equipment faults in the PON.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a drawing illustrating one embodiment of the tree topology architecture of the PON. 
         FIG. 2  is a schematic drawing illustrating one embodiment of how the fully-protected optical fiber switchover is performed in the PON. 
         FIG. 3  is a schematic drawing illustrating the networking of the PON system of an embodiment of the present disclosure. 
         FIG. 4  is a schematic drawing illustrating the PON system service protection strategy of an embodiment of the present disclosure. 
         FIG. 5  is a block diagram illustrating the service port switching module of an embodiment of the present disclosure in which the backup link between ONUs is an optical fiber link. 
         FIG. 6  is a block diagram illustrating the service port switching module of an embodiment of the present disclosure in which the backup link between the ONUs is an Ethernet link. 
     
    
    
     DETAILED DESCRIPTION 
     The preferred embodiments of the present disclosure are described in details below in conjunction with drawings. 
       FIG. 3  is a schematic drawing showing the networking of the PON system of an embodiment of the present disclosure, in which the ONU/ONT equipment is connected to the ODN equipment via an optical fiber and the ODN equipment is connected to the OLT equipment via an optical fiber. The user service data is connected via the ONU/ONT, gathered on the ODN, and is then sent to the OLT for further processing. The ONU/ONT equipment that needs protection is grouped into a protection group, and the pieces of equipment within this group are interconnected to each other via a backup link. In this embodiment, two pieces of equipment are grouped into a protection group, and the two pieces of equipment within the protection group are interconnected via a backup link to form a mutual protection relationship, where the protection group details are recorded in the OLT. 
     The following describes how the device of the present disclosure achieves service protection and fault locating in the event of a failure of the PON. 
     As shown in  FIG. 4 , ONU 1  and ONU 2  periodically check each other&#39;s state via the backup link and send the state report to the OLT after the check. When the optical fiber linking ONU 1  and the ODN is broken, or when the PON interface module connecting ONU 1  and the ODN fails, ONU 1  detects a disruption in its connection with the OLT, initiates the fault recovery mechanism, communicates its failure to ONU 2  through the regular checking mechanism, and requests ONU 2  to receive its data transmission work. At the same time, the OLT also detects the disconnection with ONU 1  and alerts ONU 1 &#39;s backup partner, ONU 2 , to receive the data transmission work for ONU 1 . Upon receiving the alert from the OLT, ONU 2  agrees to receive the work for ONU 1 , and ONU 1  sends the service data via the backup link to ONU 2 , which in turn sends the data to the ODN. In this manner, the services of ONU 1  are resumed. 
     When there is a power outage fault in ONU 1 , a disruption in the backup link, or a system failure, the OLT detects the disconnection of ONU 1  and immediately alerts ONU 2  to give ONU 1  a fault diagnosis via the backup link. ONU 2  first determines whether the backup link is normal, and if not, ONU 2  determines that the cause of the fault is an ONU 1  power outage or a backup link disruption. If the backup link is normal and ONU 2  is unable to receive the regular query message from ONU 1 , it is determined that the fault is an ONU 1  system fault, which requires diagnosis and troubleshooting from operators. 
     The method for implementing service protection when a fiber disconnection occurs between ONU 1  and the ODN or a failure of the PON interface module connecting ONU 1  and the ODN occurs may vary depending on the backup link: 
     When the backup link is an optical fiber link: 
     In this case, the ONU internally adopts a service port switching module, as shown in  FIG. 5 . The circular ring containing a triangle as shown is preliminarily called an optical divider. The optical divider has the following function: any incoming optical signal to any one of the three interfaces A, B and C, is allowed to exit from the other two interfaces and remains undistorted for the life of the effective wavelength window except for a slight loss of optical power. The module may be an integrated optical component or may be formed by combining three 1:2 optical dividers. When each of the three 1:2 optical dividers is interconnected with the other two in the pattern of the triangle contained in the circle shown, an incoming optical signal to interface A may be divided into two signals and exits from interfaces B and C. Similarly, an optical signal coming into interfaces B or C may exit from interfaces A and C or A and B. 
     The module further includes an optical switch K 1 /K 2 , which controls the connection/disconnection of the optical path based on the electric signal applied. When there is a disruption in the branch fiber M 1  connecting ONU 1  and the OLT, ONU 1  detects the disrupted connection with the OLT and initiates the fault recovery mechanism, wherein ONU 1  closes the optical switch K 1  and the optical signal sent from the optical module is transferred to ONU 2  via the backup link N. Upon detecting the disconnection of ONU 1  and determining that the ONU 1 &#39;s backup partner is ONU 2  by looking this information up in its own database, the OLT alerts ONU 2  to initiate the fault recovery mechanism, and ONU 2  closes its own optical switch K 2  upon receiving the alert. When both ONU 1  and ONU 2  close their respective optical switches K 1  and K 2 , ONU 1  will be able to log onto the PON again via the ONU 2 &#39;s optical fiber M 2  and thereby resume its services. 
     Referring to  FIG. 5 , now ONU 1  and ONU 2  can be seen as two PON terminals connected to a 1:2 optical divider and the physical link from ONU 1  to the OLT is basically the same as the original one. Therefore, fault recovery is achieved without requiring ONU 1  and ONU 2  to do anything other than the above. 
     When the backup link is an Ethernet link: 
     In this case, the ONU internally adopts another service port switching module, as shown in  FIG. 6 . The module includes a protocol conversion module and an Ethernet switch. The protocol conversion module is located between the Ethernet switch and the optical network interface and converts between a PON protocol and an Ethernet protocol. The Ethernet switch is connected to a user service processing module, the protocol conversion module, and a backup Ethernet port, and is used to establish the connection between the user service processing module, the protocol conversion module, and the backup Ethernet port through the division of a virtual local area network (VLAN). 
     When there is a disruption in the branch fiber M 1  linking ONU 1  and the ODN, ONU 1  detects the disrupted link with the OLT, initiates the fault recovery mechanism, and communicates its failure and the need for ONU 2  to receive ONU 1 &#39;s data transmission work through the periodical query mechanism. Meanwhile, the OLT also detects the disconnection of ONU 1  and alerts ONU 2  to work as a backup partner to receive the data transmission work for ONU 1 . Upon receiving the alert from the OLT, ONU 2  responds to ONU 1  by agreeing to receive the data transmission work for ONU 1 . On the Ethernet switch, ONU 2  connects port A to port B by dividing the VLAN and establishes a connection between the backup Ethernet port E 2  and the PON upstream port to enable data transmission from E 2  to the PON upstream port, and reports the connection to the OLT. Upon receiving the response message from ONU 2 , ONU 1  switches the upstream data flow from the PON port to the backup Ethernet port E 1 , that is, switches from port A on the Ethernet switch to port B by reconfiguring the VLAN. Upon receiving the ONU 1  failure alert and alerting ONU 2  to receive the data transmission work of ONU 1 , the OLT adds the bandwidth parameters, which generally include the minimum bandwidth, the maximum bandwidth, and so forth, assigned to ONU 1  to those of ONU 2  such that the transmission of ONU 1  data over ONU 2  will not affect ONU 2 &#39;s own services. 
     In another embodiment, ONU 2  may separately assign a logic path over the PON to ONU 1  and report the assignment to the OLT. The OLT gives ONU 1 &#39;s original bandwidth parameter values directly to the backup logic path such that the data and bandwidth parameters for ONU 1  and ONU 2  will not be intermixed or affect each other. 
     The above embodiments permit ONU 2  to transfer the OLT 1 &#39;s data to the OLT so that the services of ONU 1  are not disrupted in the event that ONU 1  has a disconnection fault or its PON interface module has a failure, without affecting ONU 2 &#39;s own service. 
     While the present disclosure has been described with reference to preferred embodiments thereof, it will be understood by those ordinary skilled in the art that various modifications and improvements can be made therein without departing from the principles thereof, and all such modifications and improvements should be deemed encompassed within the scope thereof.