m ▁or ▁router ▁sim ul ates ▁(" sp o of s ") ▁the ▁remote ▁endpoint ▁of ▁a ▁connection ▁to ▁a ▁locally ▁attached ▁host , ▁while ▁using ▁a ▁more ▁appropriate ▁protocol ▁to ▁communicate ▁with ▁a ▁compatible ▁remote ▁device ▁that ▁performs ▁the ▁equivalent ▁spo of ▁at ▁the ▁other ▁end ▁of ▁the ▁communic ations ▁link . ▁ ▁File ▁transfer ▁spo of ing ▁Error ▁correction ▁and ▁file ▁transfer ▁protocol s ▁typically ▁work ▁by ▁calculating ▁a ▁check sum ▁or ▁C RC ▁for ▁a ▁block ▁of ▁data ▁known ▁as ▁a ▁packet , ▁and ▁transm itting ▁the ▁resulting ▁number ▁at ▁the ▁end ▁of ▁the ▁packet . ▁At ▁the ▁other ▁end ▁of ▁the ▁connection , ▁the ▁receiver ▁re - calcul ates ▁the ▁number ▁based ▁on ▁the ▁data ▁it ▁received ▁and ▁comp ares ▁that ▁result ▁to ▁what ▁was ▁sent ▁from ▁the ▁remote ▁machine . ▁If ▁the ▁two ▁match ▁the ▁packet ▁was ▁transm itted ▁correctly , ▁and ▁the ▁receiver ▁sends ▁an ▁A CK ▁to ▁signal ▁that ▁it ' s ▁ready ▁to ▁receive ▁the ▁next ▁packet . ▁ ▁The ▁time ▁to ▁transmit ▁the ▁A CK ▁back ▁to ▁the ▁sender ▁is ▁a ▁function ▁of ▁the ▁phone ▁lines , ▁as ▁opposed ▁to ▁the ▁mode m ' s ▁speed , ▁and ▁is ▁typically ▁about ▁ ▁of ▁a ▁second ▁on ▁short ▁links ▁and ▁may ▁be ▁much ▁longer ▁on ▁long - distance ▁links ▁or ▁data ▁networks ▁like ▁X . 2 5 . ▁For ▁a ▁protocol ▁using ▁small ▁packets , ▁this ▁delay ▁can ▁be ▁larger ▁than ▁the ▁time ▁needed ▁to ▁send ▁a ▁packet . ▁For ▁instance , ▁the ▁U U