For optical routes, {HEAD ~ the accumulated data} includes signal impairment information, aggregated from the impairments allocated to {TAIL ~ each link}, along the paths being considered during routing 
For optical routes , { HEAD ~ the accumulated data } includes signal impairment information , aggregated from the impairments allocated to { TAIL ~ each link } , along the paths being considered during routing
OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH