Chapter 6. Diagnostics61200659L1-1 MX2800 STS-1 User Manual 6-5CSU LoopbackA CSU L OOPBACK enables the MX2800 STS-1 to generate a CSUloop-up pattern (001001…) towards the T1 CSU attached to theselected T1 line for six seconds. After six seconds have elapsed, thepattern will cease and incoming network traffic will be passedthrough to the CSU device. If the CSU device responded to theCSU loop-up pattern, it will return all data back towards thenetwork. A loop-down pattern (0000100001….) will be generatedtowards the CSU for six seconds when D ATA MODE is selected.CSU Loopback w/BERTA CSU L OOPBACK W /BERT enables the MX2800 STS-1 to test thelocal T1 loop to the CSU using the standard QRSS pseudo-randombit sequence. When CSU L OOPBACK W /BERT is selected, theMX2800 STS-1 will initiate a CSU loopback towards the CSUattached to the selected T1 line similar to the CSU LOOPBACK testabove. Six seconds after starting the CSU loop-up pattern, theMX2800 STS-1 will cease sending the CSU loop-up pattern andbegin sending an unframed 511 pattern towards the CSU. If theCSU device responded to the CSU loop-up pattern, theMX2800 STS-1 will check the incoming pattern for errors.Additional menu items will appear to show the state of patternsynchronization, error count, and a clear error count option (seeFigure 6-7 on page 6-6). Selecting D ATA M ODE will cease thetransmission of the QRSS pattern and start transmission of a loop-down pattern as previously described.When in CSU L OOPBACK, only the D ATA MODE for the T1 under testmay be selected. Selecting any other option will result in an errormessage being displayed.