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CHAPTER 5: Operation
Framer
The framer is used to determine the status of the line. In normal operation, the
framer transmits and expects to receive framed packets from the far end. If the
framer receives framed packets from the far end, CTS and CD will be active. If
framed packets are not received, CTS and CD will be inactive. The restart
procedure uses this information to determine if a valid connection is made (cable
disconnect, poor cable quality, etc). In normal data mode, if the Line Driver
receives four seconds of unframed packets, it will restart and begin trying to re-
establish a connection with the far end. The distinction between framed packets
and unframed packets becomes important when we discuss the pattern generator.
Pattern Gen./Det.
This part of the Processor generates and detects the 511/511E patterns. When
transmitting 511 patterns, the information is unframed (because it originates after
the framer) and is intended to be evaluated only by another processor. If the units
are in data mode and the pattern generator is enabled on one end of the link, the
far end will begin receiving unframed packets and assume that the line has gone
down. During test modes, we force the pattern generator to time out before it can
cause the link to be killed.
Loop Control
This part of the processor is used to control loopbacks. In a local loop, the data is
looped back towards the local DTE. In a remote loop, the data is looped back to
the line, but it is also allowed to pass through to the framer and to the remote
DTE.
5.3.2 R
ESTART
P
ROCEDURE AND
T
IMEOUTS
The restart procedure is in place to allow the units to re-establish a connection
after the framer begins seeing unframed packets. Table 5-1 shows the amount of
time the framer must see consecutive unframed packets before the unit will restart
and try to establish a new line connection. The reason that there are different
restart times will become apparent after reading the rest of the document. The
511/511E Time Out shown refers to the amount of time the 511/511E pattern will
be valid. At the end of this time, the pattern will automatically turn itself off and
the normal data path will be re-established. The ER LED will flash, indicating to
the user that the test has timed out. The ER LED will stop flashing once the
511/511E switch is placed into the normal position.
