Alcatel-Lucent 7750 SR OS Interface Configuration Manual page 68

Ports
Bidirectional 1+1 Signalling APS
In Bidir 1+1 Sig APS switching mode the data path supports 1:1 data path architecture (data is sent
only on the active link); 1+1 signalling, however, is used for full interoperability with signalling-
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 SR-OS node will also switch 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 will also change to the same circuit.
Bidirectional 1+1 Signalling APS ensures that both directions of active data flow (including both
Rx) are using the same link/circuit (e.g. 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 will signal the far
end using the K bytes. The far end will then also change 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) is used to exchange requests
and acknowledgments for protection switch actions. In Bidirectional 1+1 Signalling 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 Signalling 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
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, etc.).
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Table 16 on page 71 in descending order. The requests can be
7750 SR OS Interface Configuration Guide