Protection in SONET

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

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 2

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 3

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 4

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 5

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 6

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 7

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 8

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 9

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 10

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 11

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 12

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 13