asterisk users - Feb 2005 - zttool user's manual & HDLC Abort errors

Hello all,

Does anyone know where I can get an explanation of how to use zttool? 
I am running a PRI between to asterisk servers for testing and on one
end of the PRI (the pri_cpe side), I see
Tx/Rx Levels:         0/  3
On all other PRIs, I see 0/0.  Can anyone tell me what this means? 
The real problem is that I get HDLC Abort errors every couple of
minutes on this PRI and I am trying to figure out why.  Any help would
be greatly appreciated.

Thanks,
Patrick