Verilink APS 2000 T1 Line Protection (880-502411-001) Product Manual - Manual (Page 11)

Overview
Verilink APS 2000 User Manual 1-3
Errors and Alarms The errors and alarms which activate switching are loss of frame
(LOF), Loss of Signal (LOS), high Bit Error Rate (BER), and high
Bipolar Violations (BPV)s.
Abnormal Network
Codes
In addition to errors or degraded/failed T1 signals, APS 2000
specifically recognizes other service-affecting conditions which
appear as clean T1 signals. These include loopbacks and injected
test signals.
A loopback condition in the middle or far end of a circuit would
appear to the near end as a clean signal. However, service is
interrupted when loopbacks are present. Similarly, if a clean signal
(from a test set, for example) was injected into one or both
directions of transmission from the middle of a circuit, no alarms
would be generated.
APS 2000 transparently sends a unique “network code” from each
end of the system. The near-end and far-end codes are different
and are automatically selected when the APS is in operation. If
either end receives its own network code (indicating a loopback is
present) or no network code (indicating “foreign signal” injection),
the system will switch as if errors had been received.
Switch Code All 1-for-n automatic protection switching systems must switch
both ends of a circuit when a failure occurs in one or both
directions of transmission. If the failure is in one direction only,
the failed end must communicate with the remote end to tell it to
switch.
The APS 2000 system provides for two different selectable methods
of communicating this “switch code” (and the unique Network
Codes discussed above) to the far end to ensure end-to-end
operability. This end-to-end operability is assured regardless of
network topology or carrier-based transmission equipment. The
methods are ESF Facility Data Link (FDL) and density bit. ESF FDL is
the default and density bit is an option.
ESF Facility Data Link (FDL)
ESF FDL signalling is the normal (default) method of
communicating switch code.
It sends a priority bit-oriented protocol (BOP) message (as defined
in ANSI T1.403) to inform the far end when to switch and when to
restore.
Using the FDL offers a number of advantages:
• The messages are reserved for APS usage so the detection time
may be kept short.
• Payload loopback may be asserted without risk of single-ended
restoration (“half switch”) due to a switched unit receiving its
own clean signal.