1. Network Survivability

2. Network Survivability
Want high availability for connections
A common requirement: % availability
Network links, nodes, individual channels can fail
Need survivable network that can continue providing service in the presence of failures
Survivability can be provided by protection switching
Provide some redundant capacity in the network
Automatically reroute around failure using the redundant capacity

3. Protection
Usually implemented in a distributed manner to ensure fast restoration of service after a failure
In most cases, protection schemes are engineered to protect against a single failure event
How to deal with more than one concurrent failure?
Divide the network into smaller subnetworks and restrict the protection scheme to within a subnetwork
Ensure that the mean time to repair a failure is much smaller than the mean time between failures

4. Basic Concepts Working path: carry traffic under normal operation
Protection path: an alternate path to carry the traffic in case of failures
Working and protection paths are diversely routed so that both paths are not lost in case of a single failure

5. Dedicated v.s Shared Protection
Dedicated protection: each working connection is assigned its own dedicated protection bandwidth
Shared protection: if a set of working connections will not fail simultaneously, they can share protection bandwidth
Reduce bandwidth needed for protection
Protection bandwidth can be used to carry low-priority traffic under normal conditions

6. Nonrevertive v.s Revertive Protection
Traffic is switched from the working path to the protection path when a failure occurs
Nonrevertive protection: traffic remains on the protection path until it is manually switched back onto the working path
Revertive protection: once the working path is repaired, traffic is automatically switched back from the protection path onto the working path
Shared protection schemes are usually revertive

7. Unidirectional v.s Bidirectional Protection Switching
Unidirectional protection switching
When a fiber is cut, only the affected direction of traffic is switched over to the protection fiber
Used in conjunction with dedicated protection schemes
Traffic transmitted simultaneously on the working and protection paths
The receiver at the end of the paths simply selects the better of the two arriving signals
Not require a signaling protocol between the receiver and the transmitter

8. Unidirectional v.s Bidirectional Protection Switching
When a fiber is cut, both directions of traffic are switched over to the protection fibers
The receiver needs to inform the transmitter of the cut  require a signaling protocol called automatic protection switching (APS) protocol
How does an APS protocol work?
If a receiver in a node detects a fiber cut, it turns off its transmitter on the working fiber and then switches over to the protection fiber to transmit traffic
The receiver at the other node detects the loss of signal on the working fiber and then switches its traffic over to the protection fiber

9. Path/Span/Ring Switching
Path switching: the connection is rerouted end to end on an alternate path
Span switching: the connection is rerouted on a spare link between the nodes adjacent to the failure
Ring switching: the connection is rerouted on a ring between the nodes adjacent to the failure