In Bidir 1+1 Sig APS switching mode the Tx data is sent on the active link only (it is not bridged to both links simultaneously). 1+1 signaling, however, is used for full interoperability with signaling-compliant 1+1 architectures.
In the ingress direction (Rx), the decision to accept data from either the working or protection circuit is based on both locally detected failures/degradation and on what circuit the far-end is listening on (as indicated in the K bytes). If the far-end indicates that it has switched its active receiver, then the local node also switches its receiver (and Tx) to match the far-end. If the local Rx changes from one circuit to another it notifies the far end using the K bytes.
In the egress direction (Tx), the data is only transmitted on the active circuit. If the active Rx changes, then Tx also changes to the same circuit.
Bidirectional 1+1 Signaling APS ensures that both directions of active data flow (including both Rx) are using the same link/circuit (using the two directions of the same fiber pair) as required by the APS standards. If one end of the APS group changes the active receiver, it signals the far end using the K bytes. The far end then also changes its receiver to listen on the same circuit.
Because the router transmits on active circuits only and keeps active TX and RX on the same port, both local and remote switches are required to restore the service.
The APS channel (bytes K1 and K2 in the SONET header – K bytes) exchanges requests and acknowledgments for protection switch actions. In Bidirectional 1+1 Signaling APS switching mode, the router sends correct status on the K bytes and requires the far-end to also correctly update/send the K-bytes to ensure that data is transmitted on the circuit on which the far-end has selected as its active receiver.
Line alarms are processed and generated independently on each physical circuit.
In Bidirectional 1+1 Signaling APS mode, the highest priority local request is compared to the remote request (received from the far end node using an APS command in the K bytes), and whichever has the greater priority is selected. The relative priority of all events that affect APS 1+1 protection is listed in the Table: K1 byte, bits 1 to 4: type of request in descending order. The requests can be automatically initiated (such as signal failure or signal degrade), external (such as lockout, forced switch, request switch), and state requests (such as revert-time timers, and so on).