Protection in SONET

1. Protection in SONET • Path layer protection scheme: operate on individual connections • Line layer protection scheme: operate on the entire set of connections at once

2. Protection in SONET • Point-to-Point Links • 1+1 protection • 1:1 protection • 1:N protection • Self-healing rings • Unidirectional path-switched rings (UPSR) • Four-fiber bidirectional line-switched rings (BLSR/4) • Two-fiber bidirectional line-switched rings (BLSR/2) • Ring interconnection • Dual homing

3. Point-to-Point Links • 1+1 protection • Traffic transmitted simultaneously on two separate fibers from the source to the destination • Destination simply selects one of the two fibers for reception • Fast protection • Require no signaling protocol between the two ends

4. Point-to-Point Links • 1:1 protection • Traffic transmitted over only the working fiber • If the working fiber is cut, the source and destination both switch over to the protection fiber • An APS protocol is required • Not as fast as 1+1 protection • Advantages over 1+1 protection • Under normal operation, the protection fiber can be used to transmit lower-priority traffic • Can extend to 1: N protection where N working fibers share a single protection fiber

5. Self-Healing Rings • A ring is the simplest topology that is 2-connectedresilient to failures • Much of the carrier infrastructure today uses SONET rings called self-healing rings • Can automatically detect failures and reroute traffic away from the failed links/nodes rapidly (within 60ms) • Three ring architectures: UPSR, BLSR/4, BLSR/2 • Unidirectional ring: carry working traffic in only one direction of the ring • Bidirectional ring: carry working traffic in both directions

6. Unidirectional Path-Switched Rings (UPSR) • Two fibers: one working and one protection • Traffic transmitted simultaneously on the working fiber in the clockwise direction and on the protection fiber in the counter clockwise direction • For each connection, destination node monitors both the working path and the protection path and selects the better signal • Traffic switch-over done on a connection-by-connection basis • Essentially path layer 1+1 protection

7. Unidirectional Path-Switched Rings (UPSR) • Simple to implement • Require no communication between the nodes • Protection capacity = working capacity • No sharing of the protection bandwidth between connections • Each bidirectional connection uses up capacity on every link in the ring and has dedicated protection bandwidth • Popular in access networks

8. Bidirectional Line-Switched Rings (BLSR) • Working traffic carried on both directions • Four-fiber BLSR (BLSR/4) • Two working fibers and two protection fibers • Employ both span switching and ring switching • Span switching: when a transmitter or receiver on a working fiber fails, the traffic is routed onto the protection fiber between the two nodes on the same link • Ring switching: in case of a fiber cut, the traffic is rerouted around the ring on the protection fibers • Two-fiber BLSR (BLSR/2) • Two fibers, both carry working traffic, half the capacity on each fiber reserved for protection purposes • Employ ring switching, not span switching

9. Bidirectional Line-Switched Rings (BLSR) • Operate at the line layer • Protection bandwidth can be used to carry low-priority traffic during normal operation • Allow protection bandwidth to be shared between non-overlapping connections • Widely deployed in long-haul and interoffice networks • Most metro carriers have deployed BLSR/2s • Many long-haul carriers have deployed BLSR/4s

10. Bidirectional Line-Switched Rings (BLSR) • Handling node failures • The failure of a node is seen by all its adjacent nodes as link failures • Errors can occur if each of the adjacent nodes performs restoration assuming single link failure • Solution: • Nodes adjacent to the failed node exchange messages to determine if they have both recorded link failures • If so, don’t restore any traffic that originates or terminates at the failed node (squelching) • Slower restoration time

11. Ring Interconnection • Simple way: connect the drop sides of two ADMs on different rings back to back • The interconnection is broken if one of the ADMs fails or the link between the two ADMs fails • Solution: dual homing • Use two hub nodes to perform interconnection • Connections are setup between the originating node and both the hub nodes using the drop-and-continue feature in the ADMs

12. Protection in IP Networks • Slow failure detection • Adjacent routers send hello packets every 10 sec and declare the link down if miss 3 successive hello packets • Slow failure restoration • IP uses dynamic, hop-by-hop routing of packets based on the routing table maintained by the routers • When a failure occurs, the routing protocol updates the routing table at each router in a distributed manner • Routing table convergence time: several seconds • During convergence, packets could be lost or loop within the network

13. Protection in IP Networks • Use Multi-protocol label switching (MPLS) to achieve faster restoration • MPLS allows label-switched paths (LSPs) to be set up between nodes • Packets can be sent along the LSPs • Protection schemes can be implemented within the MPLS layer • E.g., precompute a backup LSP for each working LSP, reroute packets onto the backup LSP when a working LSP fails