Verilink 1558D (34-00255) Product Manual - Manual (Page 18)

Operation 3-2 1558D APS CSU/DSU
1
5
5
8
CO #1 CO #2
1
5
5
7 1
5
5
7 1
5
5
8
DTE
T1, A
IXC Network
Master Slave Slave Master
T1, B
T1, A
T1, B
IXC Local Loop APS application, both ends
DTE
Telco and/or IXC Network
1
5
5
8 1
5
5
8
T1, A
T1, B
Customer Provided end-to-end APS Application
Master
Slave
DTE DTE
T1, A
T1, B
Figure 3-1 Typical APS Applications
• If the CSU Line Loop or Payload Loop activate signal is
received by both paths simultaneously, no change of LB state
shall occur, regardless of their current states.
The “one-way” protection characteristic of the APS can
result in split-service operation. That is, service from the
1558D near APS to the 1558D far APS via one access path
while the 1558D far APS to the 1558D near APS service is
on the other access path. This is desirable to minimize ser-
vice disruptions. However, normal maintenance and fault
isolation techniques can’t be carried out while service is
being delivered in the split mode. Service to and from the
APS must be forced to a common access, either manually or
via maintenance message, before the standby path LB can
be activated.
Forced/Locked Capability
A manual path select control is provided on the APS which
selectively forces service to either Path A or Path B and
effectively inhibits the other path from accepting service.
The manual service select control supersedes all automatic
transfers and forces the path not inhibited to carry the ser-
vice, regardless of its status or condition.
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 1558D 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 second
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.