Thanks to CovaDSL$
for providing this info:
When an ADSL circuit is provisioned, the DSLAM is automatically set for the best performance, and in most cases, no adjustment is necessary.
However, there are times when an unstable line requires adjustments to be made in the DSLAM. The advantage of using SAFE MODE is that provisioning parameters can be adjusted and the end user does not have to cancel or downgrade their service, in most cases.
SAFE MODE changes the port provisioning from Fast Path or Interleave. This process lowers the maximum line rate and makes the CPE train with a stronger signal.
Changing provisioning from Fast Path to Interleave is a common troubleshooting technique that aids in stabilizing marginal lines by providing error checking. ** The disadvantage of "manually" provisioning from Fast Path to Interleave is that it adds an additional 32ms of ping time (64ms total), increasing latency. ** When using SAFE MODE, the Interleave delay is 8ms, instead of 32ms.**
Interleave availability is determined by the type of line card (DSLAM card) that is being used for a specific service.
Safe mode removes the minimum downstream rate. This allows the circuit to train at very low rates. In safe mode, these are the adjusted speeds:*
6016k downstream has a maximum downstream speed of 4000k.*
3084k downstream has a maximum downstream speed of 2048k.*
1536k downstream service (varying packages) has a maximum downstream speed of 928 kbps and a minimum speed of 0 kbps.*
TeleSurfer (no longer offered 'new'), in safe mode, has a maximum downstream speed of 384 kbps and a minimum speed of 0.
Circuits are provisioned using safe mode as manual changes are not permanent, and are lost when a port is reprovisioned due to scheduled maintenance in the DSLAM.
Hope this explanation of safe mode helps.A big thanks to Hall , Raybro & Doctor Olds for providing updates to this post.