Operation
3-3
1558A APS
transfers and forces the path not inhibited to carry the ser-
vice, regardless of its status or condition.
3.2.7
APS Switching Time
Two times are of significance in specification of the APS
protection switching function. The first is the latency period
between the point when the switching criteria is satisfied on
the active access path and the point at which restoration
occurs on the standby access path. This time is less than 50
milliseconds.
The second is the duration of the switching transient event
associated with physically moving service from the active to
the standby access path. The 1558A APS can complete the
transition from active to standby path within 1 bit time.
The decision to transfer service from the active path to the
standby path, and vice-versa, is based on a priority system,
where the highest priority condition that satisfies a transfer
controls, regardless of the conditions in lower priorities.
• Priority 1 – LB Status:
If the standby path’s LB is activated, then service shall
remain on the active path.
• Priority 2 – Failed Status:
If the standby path is in a failed state then service shall
remain on the active path. A failed state could be a result
of excess ES, CSES, LOS, or LOF.
• Priority 3 – Error Event Threshold Criteria:
This level of transfer is based on the occurrence of 1 sec-
ond events within the current 15 minute interval as
described in the TR 54017 Addendum.
Each error event requires storage for the following values:
threshold setting, current count, and a service transfer flag.
The threshold setting for the events (ES and CSES) may
range from 1 to 900 seconds. Transfer of service is based
only on these events.
If the threshold of an event is 0, that event is inactive and is
not a criterion for transfer of service. The current count tal-
lies the total number of error event seconds within the cur-
rent 15 minute interval. This count is reset at the beginning
of a new 15 minute interval.
The service transfer flag is set for the event that causes a
transfer of service. It allows the cause of the service transfer
to be determined even after the current count has been reset
at the beginning of a new interval. This flag is reset when a
service transfer acknowledgment message is received, which
is described in the next section.
3.2.8
APS Switching Parameters
The 1558A unit switches from the active line to the standby
line based upon user definable alarm parameters. The alarm
performance parameters that can configured by the user are
Errored Seconds, Consecutively Severely Seconds, Loss of
Signal, and Loss of Framing. These alarm parameters and
their definition are described below.
Errored Seconds (ES) - The user can define that the 1558A
switches from the active line to the standby line when a cer-
tain number of errored seconds have been detected during
the current 15 minute performance interval (900 seconds).
The ES threshold can be set from 0 (setting this parameter to
a zero value disables ES switching) to 900 seconds. The
1558A is configured at the factory with 20 errored seconds
as the default errored second value. The definition of an ES
is a second containing one or more CRC errors.
Consecutive Severely Errored Seconds (CSES) - The user
can define that the 1558A switches from the active line to
the standby line when a certain number of Consecutive
Severely Errored Seconds have been detected during the
current 15 minute performance interval (900 seconds). The
CSES threshold can be set from 0 (setting this parameter to
a zero value disables CSES switching) to 900 seconds. The
1558A is configured at the factory with 2 consecutive
severely errored seconds as the default CSES value. The
definition of a CSES is when two or more consecutive SES
have been detected (note that CSES are transparent to the
boundary between 15 minute intervals). A severely errored
second is defined as a second containing 320 or more ESF
errors.
Loss of Signal (LOS) - The user can define that the 1558A
switches from the active line to the standby line when a loss
of signal state has been detected. The 1558A is shipped
from the factory with loss of signal parameter enabled. The
definition of a loss of signal is when the receive signal from
the network contains
≥
175 consecutive bit intervals contain-
ing all zeros (no pulses). The condition will be cleared when
one or more intervals contain a one.
Loss of Frame (LOF) - The user can define that the 1558A
switches from the active line to the standby line when a loss
of frame error state has been detected. The 1558A is shipped
from the factory with the loss of frame parameter enabled.
An LOF alarm condition occurs when 32 or more consecu-
tive ESF frames contain frame bit errors.
When the current count of either ES or CSES is greater than
or equal to the threshold setting or when a LOS or LOF (if
enabled) is detected, the service shall be transferred from
the active path to the standby path as long as the standby
path is available (not in an alarm state or in a looped state).
A requirement for the standby path to be available is that the
timer circuitry, described below, is not active and a LOS or
LOF condition does not exist.
