36
MULTIPORT CSU/DSU
2. Perform a bit error rate test (BERT) using the internal V.52 generator (as
described in
Section 5.4
), or using a separate BER Tester. If the BER test
indicates a fault, and the Local Line Loopback test was successful for both
CSU/DSUs, you may have a problem with the twisted-pair line between the
two units. Check the twisted-pair line for proper connections and continuity.
5.3.3 T
ELCO
T
ESTING
The digital service provider’s central office can perform CSU loop and DSU
diagnostic testing. These diagnostics allow the central office to evaluate the circuit
operation without making visits to the customer’s premises.
CSU Loop
The CSU loop is activated when the central office reverses the DC sealing current
that flows between the Transmit (TX) and Receive (RX) pairs. The CSU/DSU
recognizes this and loops the signals on the RX pairs back to the central office on
the TX pair. While the CSU loop is activated, the TM indicator lights.
Figure 5-4. CSU Loop.
5.4 Bit Error Rate (V.52) Diagnostics
The CSU/DSU offers two V.52 Bit Error Rate (BER) test patterns. You can invoke
them along with the LAL and RDL tests to evaluate the CSU/DSU(s) and the
communication links.
When a 511 or 2047 test is invoked, the CSU/DSU generates a pseudo-random
pattern of 511 bits (or 2047 bits) using a mathematical polynomial. The receiving
CSU/DSU then decodes the received bits using the same polynomial. If the
received bits match the agreed upon pseudo-random pattern, then the CSU/DSUs
and the communication links are functioning properly.
•
511
: Initiates a built-in 511 bit pseudo-random pattern generator and detector.
DSU
CSU
Digital Network
