1. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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