System and method for implementing fast layer 2 protection in passive optical networks | Patent Publication Number 20070097859
US 20070097859 A1System and method for providing fast protection in a passive optical network (“PON†) including two central hubs and a plurality of network nodes, wherein the network nodes are connected to a first of the central hubs by a first optical path and to a second of the central hubs by a second optical path. The method comprises setting the first and second central hubs to master mode and slave mode, respectively, such that only the first central hub transmits traffic to and from the network nodes; detecting a failure in the first optical path; exchanging status information between the first and second central hubs; and if no failure has occurred in the second optical path, setting the first and second central hubs to slave mode and master mode, respectively, such that only the second central hub transmits traffic to and from the network nodes.
- 1. A method for providing fast protection in a passive optical network (“PON†) including two central hubs and a plurality of network nodes connected to a first one of the central hubs by a first optical path and to a second one of the central hubs by a second optical path, the method comprising: nsetting the first and second central hubs to master mode and slave mode, respectively, such that only the first central hub transmits traffic to and from the network nodes; detecting a failure in the first optical path; exchanging status information between the first and second central hubs; and if no failure has occurred in the second optical path, setting the first and second central hubs to slave mode and master mode, respectively, such that only the second central hub transmits traffic to and from the network nodes.
- 8. A system for implementing a fast protection scheme in a passive optical network (“PON†) including two central hubs and a plurality of network nodes, wherein the network nodes are connected to a first of the central hubs by a first optical ring and connected to a second of the central hubs by a second optical ring, the system comprising: nmeans for setting the first central hub to master mode and the second central hub to slave mode such that only the first central hub transmits traffic to and from the network nodes via the first optical ring; means for detecting a failure in the first optical ring; means for exchanging control messages between the first and second central hubs regarding the ring status of the first and second optical rings; and means for setting the second central hub to master mode and the first central hub to slave mode such that only the second central hub transmits traffic to and from the network nodes via the second optical ring if no failure has occurred in the second optical ring.
- 14. A passive optical network (“PON†) for implementing a fast protection scheme, the PON comprising: na first central hub connected to a plurality of network nodes via a first optical ring; a second central hub connected to the network nodes via second optical ring, the second central hub connected to the first central hub via a connection for enabling periodic exchange of ring status information between the hubs; wherein the first central hub is set to master mode and the second central hub is set to slave mode such that only the first central hub transmits traffic to and from the network nodes; and wherein responsive to detection of a failure in the first optical ring, if no failure has occurred in the second optical ring, setting the first and second central hubs to slave mode and master mode, respectively, such that only the second central hub transmits traffic to and from the network nodes.
1. Technical Field of the Invention
The present invention generally relates to passive optical networks (“PONs†). More particularly, and not by way of any limitation, the present invention is directed to system and method for implementing fast layer 2 (“L2†) protection in PONs.
2. Description of Related Art
Wavelength Division Multiplex Passive Optical Networks (“WPONs†) rely on shared optical wavelength and unidirectional connections. Ethernet is used as the L2 technology in WPON. To prevent single point of failure, WPON employs dual-hub architecture; as a result, it is necessary to maintain a loop-free topology. However, the standard spanning tree cannot correctly function in such a network.
Currently, the solution to the foregoing problem is to utilize spanning tree protocols (“STP†) for providing both a loop-free network and protection. STP and the more recently developed rapid STP (“RSTP†) and multiple STP (“MSTP†) work efficiently in local area networks (LANS) and switched Ethernet networks for offering network protection at L2. This solution is deficient, however, in that bridges need bi-directional ports to exchange bridge protocol data units (“BPDUs†). Point-to-point connections are required to fully enable fast convergence. WPON is based on broadcast and shared optical bandwidth. Additionally, WPON nodes use asymmetric ports, i.e., some ports have only receivers, for efficient and cost effective data distribution. As a result, the broadcast and asymmetrical nature of the network prevents standard L2 protection schemes from functioning properly.
Therefore, what is needed is a system and method to provide L2 protection for networks that employ broadcast media and asymmetric traffic distribution.
One embodiment is a method for providing fast protection in a passive optical network (“PON†) including two central hubs and a plurality of network nodes connected to a first one of the central hubs by a first optical path and to a second one of the central hubs by a second optical path. The method comprises setting the first and second central hubs to master mode and slave mode, respectively, such that only the first central hub transmits traffic to and from the network nodes; detecting a failure in the first optical path; exchanging status information between the first and second central hubs; and if no failure has occurred in the second optical path, setting the first and second central hubs to slave mode and master mode, respectively, such that only the second central hub transmits traffic to and from the network nodes.
Another embodiment is a system for implementing a fast protection scheme in a passive optical network (“PON†) including two central hubs and a plurality of network nodes, wherein the network nodes are connected to a first of the central hubs by a first optical ring and connected to a second of the central hubs by a second optical ring. The system comprises means for setting the first central hub to master mode and the second central hub to slave mode such that only the first central hub transmits traffic to and from the network nodes via the first optical ring; means for detecting a failure in the first optical ring; means for exchanging control messages between the first and second central hubs regarding the ring status of the first and second optical rings; means for setting the second central hub to master mode and the first central hub to slave mode such that only the second central hub transmits traffic to and from the network nodes via the second optical ring if no failure has occurred in the second optical ring.
Another embodiment is a passive optical network (“PON†) for implementing a fast protection scheme. The PON comprises a first central hub connected to a plurality of network nodes via a first optical ring; a second central hub connected to the network nodes via second optical ring, the second central hub connected to the first central hub via a connection for enabling periodic exchange of ring status information between the hubs; wherein the first central hub is set to master mode and the second central hub is set to slave mode such that only the first central hub transmits traffic to and from the network nodes; wherein responsive to detection of a failure in the first optical ring, if no failure has occurred in the second optical ring, setting the first and second central hubs to slave mode and master mode, respectively, such that only the second central hub transmits traffic to and from the network nodes.
A more complete understanding of the present invention may be had by reference to the following Detailed Description when taken in conjunction with the accompanying drawings wherein:
Embodiments of the invention will now be described with reference to various examples of how the invention can best be made and used. Like reference numerals are used throughout the description and several views of the drawings to indicate like or corresponding parts, wherein the various elements are not necessarily drawn to scale.
In general, a fast L2 protection scheme for a PON is proposed herein. Using a dual hub WPON as an example, in this approach, one of the hubs is in master mode, while the other is in slave mode. Two hubs using the proposed L2 protocol exchange underlying WPON topology status. During normal operation, only the master hub forwards traffic to and from the network nodes. When a failure occurs in a working path, or ring, of the network, the master hub detects the failure and immediately informs the slave hub to take over the traffic on the working links. At the same time, the master node stops forwarding the corresponding traffic to/from the network nodes to avoid duplication.
Referring now to
Gigabit Ethernet (“GE†) ports (not shown) are installed at the hubs 102, 104, and nodes D1-D3 to send and receive Ethernet frames. Thus, a loop-free active topology is required in the WPON 100. However, as previously indicated, WPON uses, either fully or partially, shared wavelength for downstream traffic (i.e., from hub to node) and a dedicated wavelength for upstream traffic (i.e., from node to hub). Besides the BTV traffic, as previously indicated, the WPON 100 utilizes the asymmetric and unidirectional GE ports present at both the hubs 102, 104, and nodes D1-D3. As a result, spanning tree protocols, which require bidirectional links and supporting ports, cannot operate correctly on a network such as the WPON 100.
In accordance with features of one embodiment, a fast L2 protection scheme is proposed in which each hub 102, 104, has a detection mechanism for detecting possible failures on one or both of the rings 106, 108. One of the hubs, e.g., the hub 102, is in master mode and the other hub, e.g., the hub 104, is in slave mode. By definition herein, the ring connected to the master hub is designated the working ring and the ring connected to the slave hub is designated the protection ring. The hubs 102, 104, periodically exchange underlying WPON topology status information using L2 control protocol via a connection 109. In particular, the hubs 102, 104, exchange status information regarding the status of the rings 106, 108. Traffic from a provider network 110 is sent to both hubs 102, 104, using a 1+1 protection scheme. Each node D1-D3 also sends the upstream traffic to both hubs 102, 104, using the 1+1 protection scheme. During normal operation, only the master hub 102 forwards the traffic to/from the nodes D1-D3 via the working ring 106. In contrast, the slave hub 104 drops, or filters out, the traffic that reaches its GE ports.
As will be described in detail below, each of the master and slave hubs 102, 104, determines whether to forward traffic on its corresponding ring 106, 108, respectively, based on the status of the rings. As previously noted, ring status information is periodically exchanged between the hubs 102, 104. The L2 protection method uses 1+1 for redundancy and blocks the slave hub port (i.e., filtering the traffic to and from the WPON 100) to keep the topology loop-free.
When a failure occurs in the working ring 106, the master hub 102 detects the failure based on loss of the optical signal on both upstream and downstream links at the optical layer. A failure in the protection ring 108 is similarly detected by the slave hub 104. Thus, the failure detection and location are fast. In response to detection of such a failure, the hubs 102, 104, exchange ring status information via L2 protocol control messages on the connection 109 as previously described and the protection process is initiated as will be described in greater detail below. In one embodiment, the L2 protocol control message comprises Ethernet/L2 frames. It should be noted that there is no timer involved in this process; rather, convergence time will typically depend primarily on failure detection and the control message processing time at the hubs 102, 104. Accordingly, the embodiments enable the performance of fast protection using the L2 protocol.
To better understand the operation of the proposed method, examples are presented in
Referring now to
If failures are detected in both the working ring (block 404) and the protection ring (block 406) (i.e., if the cable is cut), the master hub is partially blocked such that only traffic to and from nodes still connected to the master hub is transmitted via the working ring (block 412). Similarly, the slave hub is partially blocked such that only traffic to and from nodes still connected to the slave hub is transmitted via the slave ring (block 414).
It should be noted that, although the embodiments described herein are described with reference to optical rings, other types of optical path configurations that are known or may become known may also be employed.
An advantage of the embodiments described herein is that they provide L2 protections for both link failure and hub node failure. Moreover, they provide instant switchover to the protection node/ring upon detection of link failure; no timer is required during failure handling.
Another advantage of the embodiments described herein is that control information is only exchanged between hubs. Light weight local operation imposes very light overhead on the network. Additionally, no new hardware or software is required at the nodes; the protocol runs only on the hubs.
It is believed that the operation and construction of the present invention will be apparent from the Detailed Description set forth above. While the exemplary embodiments of the invention shown and described have been characterized as being preferred, it should be readily understood that various changes and modifications could be made therein without departing from the scope of the present invention as set forth in the following claims.