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Introduction 44
Model 3088A Series Getting Started Guide 4 • Operation
Note
Local loopback causes bit errors on the remote end if the link is up.
.
Figure 14. Local Analog Loopback diagram
The data received from the serial interface is looped back before going out on the DSL line. Note that this
loopback occurs after the pattern generator/BER meter. This means that running a 511 pattern in conjunction
with an LAL should result in no error detected by the meter.
The data path for the RDL is shown in figure 15.
Figure 15. Remote Digital Loopback diagram
The RDL causes the remote unit to loop the data received from the DSL line back tow the DSL line.
Patterns
The 3088A can generate and detect 511 patterns. These can be initiated either by the middle DIP switch or by
the console command dsl set pattern <off|511>. When the pattern is started, the DSL framer uses its internal
511 pattern generator for its DSL TX data instead of the data received from the serial interface. Also, the
framer’s internal BER Meter tries to detect a 511 pattern in the DSL RX Data.
Because the BER Meter always runs when the pattern generator runs, the meter will detect errors if either the
pattern is not either looped back or the remote unit is not transmitting a 511 pattern.
DSL
Framer
511 Pattern
Generator
511 BER
Meter
DSL
Framer
511 Pattern
Generator
511 BER
Meter
Line
DSL
Framer
511 Pattern
Generator
511 BER
Meter
DSL
Framer
511 Pattern
Generator
511 BER
Meter
Line