If a robust communications channel is present, {HEAD ~ the two modems} can initiate exchange of {TAIL ~ the diagnostic and test information} 
If a robust communications channel is present , { HEAD ~ the two modems } can initiate exchange of { TAIL ~ the diagnostic and test information }
OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL