This may be appropriate when {HEAD ~ the availability information} depends on {TAIL ~ the specific communication unit} which placed the request or when such requests are relatively infrequent 
depends on
This may be appropriate when { HEAD ~ the availability information } depends on { TAIL ~ the specific communication unit } which placed the request or when such requests are relatively infrequent
OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH