Protection in SONET
• Path layer protection scheme: operate on
individual connections
• Line layer protection scheme: operate on
the entire set of connections at once
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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
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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
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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
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Self-Healing Rings
• A ring is the simplest topology that is 2connected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
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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-byconnection basis
• Essentially path layer 1+1 protection
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Unidirectional Path-Switched
Rings (UPSR)
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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
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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
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– Employ ring switching, not span switching
Bidirectional Line-Switched
Rings (BLSR)
• Operate at the line layer
• Protection bandwidth can be used to carry lowpriority 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
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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
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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
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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
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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
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