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CHAPTER 5: Operation
5.2 Central Office Loops
The Micro T1 CSU/DSU also responds to central-office-initiated loop commands.
When in D4 framing mode, the Micro T1 CSU/DSU will implement the “loop up”
command when it recognizes the pattern “10000” in the data stream for a
minimum of 5 seconds. The “loop down” command is implemented by the pattern
“100” in the data stream for a minimum of 5 seconds.
When operating in ESF framing mode, loopback commands are issued via the
Facility Data Link (FDL). The line loop message will cause a loopback before data
enters the framer portion of the CSU. The payload loop message will cause the
Micro T1 CSU/DSU to loop data after the framer portion of the CSU.
The Micro T1 CSU/DSU will respond to Universal Loopback Deactivate to clear all
central-office loops.
