metrotek Bercut-ET, Руководство по эксплуатации

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70 Analysis. Typical tasks and solutions
5.15 BERT
BERT (Bit Error Rate Test) — this test allows to measure bit error rate ,
the ratio of error bits to the total number of bits transmitted. To perform the
test, a specific binary pattern (known both to the source and destination) is
encapsulated into an Ethernet frame which is to be transmitted.
At the destination, the sent pattern is compared to the source one and
then the bit error rate is counted. To connect to the TDM-network, the
interface converter is used to convert packet network (Ethernet) traffic into
the TDM-traffic format.
Testing can be performed at four layers of the OSI model.
• At the Physical layer test data is send piecemeal with specified inter-
frame gap (IFG). In this case the test is performed from the port A(B)
to the port B(A) (see fig. 5.83) or using the Loopback function (see
fig. 5.84).
Figure 5.72. Physical layer frame
•
At the Data Link layer , all data is encapsulated into an Ethernet
frame thus allowing to transmit test packets through a network with the
OSI’s second-layer equipment (for example, network switch). Possible
connection schemes are shown on fig. 5.85, 5.86, and 5.87.
Figure 5.73. Data link layer frame
•
At the Network layer , all data is encapsulated into IP packet, and then
— into Ethernet-frame thus allowing to transmit test packets through a
network with both OSI’s second- and third-layer equipment (for exam-
ple, network switch, network router). Possible connection schemes are
shown on fig. 5.85, 5.86, and 5.87.
Figure 5.74. Network layer frame
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