24
2-WIRE SHORT-RANGE DSL LINE DRIVER (HDSL WITH V.35 OR X.21)
Local Loop with 511/511E
When the unit is placed into a mode 1 local loop and the 511/511E pattern
generator is activated, the local pattern generator begins sending out a 511/511E
pattern to the loop control block. The loop control block will loop this data back
to the 511/511E pattern detector block, which will evaluate the data for errors.
Because the 511/511E pattern generator is contained within the processor, the
data is unframed so the framer will begin seeing unframed packets. The framer
receives this unframed data and cannot distinguish this information from a line
disconnection (this would cause the unit’s restart procedure to start). What we
have done to allow this mode to work is to add timeouts for the pattern generators.
When the 511/511E is initiated, the line restart procedure is changed to one
minute. The 511/511E pattern will timeout after 45 seconds. So if the 511/511E is
turned on during a local loop, the restart procedure is set to one minute, but the
511/511E pattern will time out after 45 seconds, allowing the framer to begin
seeing framed packets (and not restart the box).
After the 511/511E pattern times out, the ER LED will begin flashing. It will
remain this way until the pattern-generator switch is turned off. Note that the data
at the local DTE and the remote DTE are not valid. Because the data is unframed
there is no way for the framer to send this data out to the DTE. This is an
important distinction because other units will send out the 511 pattern.
Figure 5-5. Local loop mode 1 with 511/511E.
When the unit is placed into a mode 2 local loop, the 511/511E pattern generator
on the local unit is unavailable for transmission. As can be seen from Figure 5-6,
the 511/511E pattern generator has no data path connections available. The
511/511E pattern generator is still available on the remote unit. For more
information on the proper operation of this pattern generator, please refer to the
Remote Digital Loop with 511/511E
section.
Framer
Processor
Processor
Framer
Pattern
Gen./Det.
Pattern
Gen./Det.
Loop
Control
Loop
Control
Line
X
X
X
X
X
X
