Verilink APS 2000 T1 Line Protection (880-502411-001) Product Manual - Manual (Page 12)

Overview
1-4 Verilink APS 2000 User Manual
APS uses a standards-based (non-proprietary) method of
controlling switching events.
The potential disadvantage is that some network-based DACS
systems do not provide end-to-end connectivity of the FDL. If the
carrier’s network includes a DACS, the DACS must either be
optioned for “through” FDL usage or the T1 must be routed around
the DACS. If these options are not available, another method must
be selected to communicate switch code.
56kb Density Bit signalling
This method relies on the fact that if one or more of the DS0
channels in a T1 system contain data at 56kbs, the eighth bit
of the DS0 byte may be used as a communications channel.
Normally, the eighth bit is forced to a “1” to ensure proper
ones-density on the T1 signal, but this requirement may be
waived under either of the following scenarios:
• If the network T1 is optioned for B8ZS, density is assured.
• If the network cannot be B8ZS for some reason, and only a
single DS0 is used for the 56kbs + signalling bit, density
can still be maintained by using the bit during frames
where the frame bit is a “1.” The extra “1” in the frame bit
allows one DS0 to be all zeros without violating the density
rules as defined by ATT PUB 62411 and ECSA T1.403.
The ESF FDL signalling method is the preferred method of
communicating “switch code” and is set by default. Density bit
signalling may be selected if the system either does not use ESF
lines, or if an older DACS system prevents ESF FDL continuity.
CSU Removal APS 2000 does not use, nor require, any form of “common control.”
Each APS/CSU module is independent of other modules and its
actions are independent of the actions of other modules (within the
scope of the priority system discussed above).
Even physical removal of a module does not affect the switching
and restoral process. In the unlikely event of a failed module, the
module may be removed and replaced without disrupting service
to the effected DTE. If the module to be removed is associated with
a working line, a normal switch to the protection line will occur at
both ends. Once the module is reinstalled, a normal reset is
allowed.
NOTE: The far end module should not be in the INHIBIT control
mode when removing a near-end module. When a module is
removed at either end, the empty slot appears to the priority
system as a failed line requesting to switch to the spare and
is still part of the priority system. Make sure the spare is not
being used by a higher priority line (or INHIBIT higher
priority lines) before removing a module.