primarily testing bi {HEAD ~ directional communicative connectivity}, via the LAN, between a primary and {TAIL ~ a backup redundant network access point} of the set of network access points 
between
primarily testing bi { HEAD ~ directional communicative connectivity } , via the LAN , between a primary and { TAIL ~ a backup redundant network access point } of the set of network access points
OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH REL OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH