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Basic Troubleshooting and Repair
The S1 switch settings on the PRI Card shown in the following illustration
inform the T1 module of the location of the clock source and must be set
as indicated.
Loopback
Loopback is used to verify and locate the source of either a cable or a
synchronization problem through the devices in a T1 transmission path.
Devices in the T1 transmission path (PBX, CSU, Channel bank, smart
jacks, etc.) will generally have some type of loopback mode. Loopback
modes will typically have the effect of taking the input T1 stream and
transmitting the stream back to the source; the nodes thereby verify that
portion of the link.
In the absence of a loopback mode, the T1 transmits and receives pairs
that can be directly connected together to physically create a loopback
condition.