Verilink APS 2000 T1 Line Protection (880-502411-001) Product Manual - Manual (Page 60)

Performance Monitoring
4-8 Verilink APS 2000 User Manual
Error Event
Monitoring
The CSU circuits monitor the incoming signals from both directions
for the error events described below.
CRC-6 Error Event In ESF framing, the CSU uses the Cyclic Redundancy Check (CRC)
error-checking algorithm for real-time logic error checking. The
transmitting CSU calculates a six-bit CRC code for the entire ESF
frame (4,632 bits) and obtains a six-bit (CRC-6) coded signature for
those data bits. It then transmits this code to the far-end CSU in the
overhead bits of the next ESF frame. The receiving CSU recalculates
the CRC of the received ESF and compares it with the CRC sent over
the network. If these codes do not match, one or more errors have
occurred in data transmission. The receiving CSU will declare a
CRC-6 error event and the sending CSU will retransmit the frame. If
CRC regeneration is disabled, the transmitting CSU does not insert
the code into a new CRC signal onto the network.
Out-of-Frame
(OOF) Event
An OOF event occurs when any two of four consecutive frame bits
of the incoming signal are incorrect.
Bipolar Violation
(BPV)
The CSU circuits detect and remove unintentional (non-B8ZS) BPVs
in both directions of transmission in all framed modes. In the
unframed transparent mode, BPVs are not corrected in the
equipment-to-network direction. If the CSU is configured to receive
a B8ZS-coded signal, it detects intentional B8ZS BPVs but does not
indicate that these BPVs have been received. B8ZS bipolar
violations are either passed through or decoded, depending on the
option setting of the CSU.
Frame Bit Error
(FE) Event
An FE is a framing bit error that occurs in the received signal from
either direction of transmission.
Severely Errored
Framing (SEF)
Event
The CSU declares an SEF event when two or more FEs occur within a
3-millisecond period. For ESF framing, this interval may or may not
coincide with an ESF frame (6 framing bits).
Low Density State A low-density state occurs when the incoming signal from the
equipment violates the ONEs density requirement for which the
CSU is configured. Whenever the CSU inserts a ONE into the
incoming signal from the equipment to maintain the minimum
pulse density, it generates and records an equipment low-density
second (DTED).
Loss of Signal
(LOS) State
A loss of signal occurs when the network signal is absent for more
than approximately 175 milliseconds of zeros (
± 75 zeros) and is
also treated as an OOF. In the transmit direction toward the
network, an LOS causes the CSU to send the selected keep-alive
signal to the network.