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If you are expecting the T3SU to do the PPP encapsulation, then you are using the wrong device. For IP traffic, generally there is a router with a HSSI or V.35 interface connected to the T3SU to do the IP routing and the L2 encapsulation. The T3SU is a layer 1 (CSU/DSU) device for terminating an un-channelized DS3. Any layer 2 encapsulation has to be done by other equipment.
If this is the wrong hardware and you need help determining what equipment will work in your application, you can call ADTRAN's pre-sales Applications Engineering at 888-423-8726.
Hope this helps,
Since there has been no further discussion on this topic I will mark it as "assumed answered". If you have any additional questions feel free to continue the discussion.