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Unidirectional 1+1 Signalling APS
In Uni 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 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). Although it is not required in the APS standards, the
SR-OS implementation of Unidirectional 1+1 Signalling APS uses standards based signaling to
keep both the Rx and Tx on the same circuit / port. 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. f the active Rx
changes, then Tx will also change to 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. For a single failure a data
outage is limited to a maximum of 100 milliseconds.
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 Unidirectional 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 Unidirectional 1+1 Signalling APS switching mode:
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7710 SR OS Interface Configuration Guide
K-bytes are generated/transmitted based on local request/condition only (as required by
the APS signalling).
Local request priority is compliant to 1+1 U-APS specification.
RX and TX are always forced on to the same (active) circuit (bi-directional). This has the
following caveats:
If an APS switch is performed due to a local condition, then the TX direction will be
moved as well to the newly selected RX circuit (old inactive). The router will send
LAIS on the old active TX circuit to force the remote end to APS switch to the newly
active circuit. Note that some local request may not cause an APS switch when a
remote condition prevents both RX and TX direction to be on the same circuit (for
example an SD detected locally on a working circuit will not cause a switch if the
protection circuit is locked out by the remote end).
Interface Configuration
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