14 Operation on an ISDN Access
250
ARGUS 145
PLUS
14.4 Bit error rate test
The bit error rate test (BERT = Bit Error Rate Test) is used to check the transmission quality
of the access circuit.
As a rule, the network operator will guarantee an average error rate of 1 x 10
-7
, in other
words in long-term operation 1 bit error in 10 million transmitted bits. A higher bit error rate
will be especially noticeable in transmitting data.
The application program detects the errors in the data blocks transmitted and requests that
the remote partner send them again, which reduces the effective throughput of the ISDN
connection.
In the bit error rate test, the tester establishes an ISDN connection to a remote tester or
calls itself, sends a standardized (quasi-) random number string and compares the
received data with that which was sent. The individual bit errors are summed and
depending on the test procedure and equipment evaluated in accordance with the ITU
Guideline G.821.
During the test, the ARGUS counts the bit errors and after the test is done it calculates the
bit error rate and other parameters in accordance with G.821. Aself/ local test requires both
B channels since the bit error rate test checks the B-channels in both directions
simultaneously.
As a rule, the quality of the network operator’s access circuits is quite good. Therefore, no
bit errors should occur in a one-minute test. However, if an error occurs, the test should be
repeated with a measurement time of 15 minutes to achieve higher statistical precision.
The access circuit is heavily distorted, if more than 10 bit errors occur within a test period of
15 minutes.
Contact the network operator or the supplier of the PBX equipment and ask them to test
your access circuit.
The BERT can be performed in three different ways:
1.
BERT in an extended call to oneself
A remote number is not needed, since the ARGUS sets up the ISDN connection to
itself. In this case, the ARGUS requires two B-channels for the test.
2.
BERT with a loopbox
A loopbox (e.g., another member of the ARGUS family of testers at the remote end)
is required. The test uses one B-channel.
