1+1 APS
In the 1+1 architecture, there is one working interface (circuit) and one protection interface, and the same payload from the transmitting end is sent to both the receiving ends. The receiving end decides which interface to use. The line overhead (LOH) bytes (K1 and K2) in the SONET frame indicate both status and action.
The protection interfaces need to be configured with an IP address of the chassis that has the working interface, using APS commands. The APS Protect Group Protocol, which runs on top of UDP, provides communication between the process controlling the working interface and the process controlling the protection interface. Using this protocol, interfaces can be switched because of a chassis failure, degradation or loss of channel signal, or manual intervention. In bidirectional mode, the receive and transmit channels are switched as a pair.
Two SONET connections are required to support APS.
- Revertive option — For any failure on working line, the software switches to protection line and when the working line recovers, it waits based on the revertive timer and reverts back to working line as active link.
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Non-revertive option — When the signal fails, the software switches to the protection line and does not automatically revert back to the working line. This is the default option.
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SONET PW (SAToP or CEP)
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SONET local connect
Benefits of APS
The following lists the benefits of APS:
- APS performs switchovers with minimal loss of data and time-consuming reroutes are avoided.
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There is no visibility that a failure has occurred beyond the network element in which it is residing; other nodes are not affected by the failure.
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Implementation of APS guards a network against complex restarts and resynchronizations since failures are isolated to a local device.
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With APS, the effect of a failure is greatly minimized and a fast switchover guarantees minimal effect on the network.
APS 1+1 for SONET Layer 1 traffic
SONET linear APS 1+1 provides protection against both fiber cuts and front card or back card failures. APS 1+1 requires a redundant protection line for every working line. The traffic is simultaneously carried by the working and the protection lines. Hence, the receiver that terminates the APS 1+1 should select the traffic from one of the lines and continue to forward the traffic. APS 1+1 provides protection in unidirectional and bi-directional modes:
- Uni-directional Protection: The receiving end can switch from working to protection line without any coordination at the transmit end since both lines transmit the same information.
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Bi-directional Protection: The receiving end switches from working to protection line by coordinating at the transmit end.
Scenario for Bidirectional APS 1+1
In the above figure, two devices NCS 4200 chassis are connected to provide APS 1+1 bi-directional protection. The highlighted one is the working line and the other is the protection line. The traffic is transmitted on both working and protection lines and received only on one line.
In a scenario where you encounter a fiber cut,
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There is a cable cut in the working line. So, the device 2 NCS 4200 chassis 2 receives a Loss of Signal (LOS) on working line.
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Device 2 NCS 4200 chassis 2 starts generating K2 byte and sends it to the Device 1 NCS 4200 chassis 1 over the protection line.
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Device 1 NCS 4200 chassis 1 receives the K2 byte and reacts on the receiving K2 byte.
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Device 1 NCS 4200 chassis 1 starts sending K1 byte to the Device 2 NCS 4200 chassis 2 on the protection line.
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Device 1 NCS 4200 chassis 1 starts sending K2 byte to Device 2 NCS 4200 chassis 2 on the protection line.
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Device 2 NCS 4200 chassis 2 receives the K1/K2 byte and starts receiving the data from protection line. The protection line now acts as the active line.
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Device 2 NCS 4200 chassis 2 sends K2 byte over the new active line to Device 1 NCS 4200 chassis 1. Device 1 NCS 4200 chassis 1 receives this signal and starts accepting the data from this new active line.
Scenario for Unidirectional APS 1+1
In the above figure, two devices NCS 4200 chassis are connected to provide APS 1+1 unidirectional protection. The figure shows a working line and a protection line. The traffic is transmitted on both working and protection line and received only on one line.
In a scenario where you encounter a fiber cut,
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Device 1 NCS 4200 chassis 1 receives a LOS on RX working line.
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Device 2 NCS 4200 chassis 2 detects LOS and starts receiving the data from the protection line. The protection line now becomes the active line.
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Device 1 NCS 4200 chassis 1 receives the K2 byte and knows about switching event on device 2.
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