Operation
3-1
1558A APS
Operation
3.0
Introduction
The 1558A provides the capability to automatically switch
from a “defective” T1 service to a standby facility. This
capability minimizes any actual service outage to the cus-
tomer. Typically, switching from the active to the standby
path is transparent to the user (voice “hitless” with minimal
impact on data services). The 1558A Automatic Protection
Switch (APS) equipment is compatible with AT&T TR
54017, Addendum, February, 1991. When the 1558A is con-
figured for internal ESF CSU operation, it is fully compati-
ble with AT&T TR 54016 ESF CSU requirements.
3.1
Applications
APS T1 protection service (per TR 54017) is either provided
by a Common Carrier (AT&T or MCI, etc.) or by the cus-
tomer. When a Common Carrier provides the service, APS
equipment is installed in the central office and at the cus-
tomer premise location(s). In the case of customer provided
APS, the customer installs a 1558A at both ends of the mis-
sion-critical service.
Figure 3-1 on page 3-2
depicts some
of the more common APS applications.
TxPORT APS equipment must be installed in pairs (one
TxPORT unit at one end of the service and another unit at
the other end of the service). One unit of the pair must be
configured for ‘Master’ mode and the other as Slave mode.
3.2
General Operation
The 1558A constantly monitors the status and quality of the
path signal (A and B) received on both the active access
path and standby access path. In general, the status and qual-
ity of the signal is based on the ESF parameters described in
Section 3.1 of TR 54016 and additional parameters
described in this document and in TR 54017, 1991. If the
performance of the active path is determined to be impaired,
the protection switching equipment will automatically select
the standby access path if that path is ready for service (not
in an alarm or a maintenance state). The level of impairment
at which a transfer is accomplished is dependent upon the
alarm threshold values set in the equipment.
Following a switch, the former standby access path becomes
the active path. The 1558A continues to monitor both of the
incoming data streams after a switch has occurred. If both
active and standby access paths exceed their specified
thresholds, service will be maintained on the currently
active access path.
The switch from the active to the standby path is completely
independent of any external equipment. Switching from an
active path to a standby path only involves the receive paths
(Path A and Path B), not the transmit paths.
3.2.1
Revertive and Non-Revertive Switching
The default configuration of the 1558A APS provides non-
revertive protection. Specifically, this means that service,
when transferred to the standby path, remains on the new
path until its performance degrades past the threshold set-
tings. This is in contrast to the optional revertive mode
which returns service to the default path when it has
returned to a minimal level of error free performance.
3.2.2
Default Power-Up Path
The desire for geographically diverse routing may also
result in diversity in the medium of transmission (copper,
fiber, microwave, etc.). Certain media have higher intrinsic
performance criteria and are therefore the service of prefer-
ence. To accommodate this preference, on power up, the
1558A APS unit forces the Path A as the active path.
3.2.3
Loss of Signal / Loss of Frame
When a loss of signal is detected from the DTE side, the
1558A units generates an AIS signal towards the facility
paths (Paths A and B).
When a LOS (Loss Of Signal) is detected on the active path
from the facility side of the APS, the 1558A will immedi-
ately switch the service to the standby facility (assuming
that the standby path is not in an alarm or a maintenance
state and the LOS alarm is enabled). If the standby path is in
an alarm state, the service will remain on the failed path
until the alarm condition clears on the standby path.
Optionally, in the event that there is a signal present but it is
so impaired that it results in a LOF (Loss Of Frame), the
1558A shall immediately switch after detecting 32 consecu-
tive framing errors (approximately 93 milliseconds) to the
standby path (assuming that it is not in an alarm or a mainte-
nance state and the LOF alarm is enabled).
3.2.4
Bipolar Violations
The 1558A APS units will not allow BPV
(Bipolar Violations) in the DS-1 data stream unless they are
used for B8ZS (Bipolar 8 Zero Substitution) encoding for
clear channel capability. Where used, the B8ZS code will be
employed to prevent the generation of an all zero octet.
