▁message ▁back ▁to ▁the ▁host . ▁This ▁causes ▁the ▁local ▁computer ▁to ▁immediately ▁send ▁another ▁packet , ▁avoid ing ▁the ▁laten cy ▁of ▁waiting ▁for ▁an ▁A CK ▁from ▁the ▁remote ▁machine . ▁The ▁data ▁for ▁multiple ▁packets ▁is ▁held ▁in ▁an ▁internal ▁buffer ▁while ▁the ▁mode m ▁is ▁sending ▁it ▁to ▁the ▁remote ▁machine . ▁This ▁allows ▁the ▁packets ▁to ▁be ▁sent ▁contin ually , ▁greatly ▁impro ving ▁channel ▁efficiency . ▁However , ▁this ▁also ▁requires ▁the ▁link ▁between ▁the ▁two ▁systems ▁to ▁be ▁error - free , ▁as ▁the ▁mode m ▁has ▁already ▁A CK ed ▁the ▁packets ▁even ▁before ▁they ▁have ▁been ▁sent . ▁This ▁was ▁normally ▁addressed ▁by ▁using ▁a ▁mode m - level ▁error ▁correction ▁protocol , ▁like ▁Micro com ▁Network ing ▁Pro tocol s . ▁ ▁Pro tocol ▁spo of ing ▁was ▁also ▁widely ▁used ▁with ▁another ▁feature ▁of ▁earlier ▁high - speed ▁mode ms . ▁Before ▁the ▁introduction ▁of ▁echo ▁can cellation ▁in ▁v . 3 2 ▁and ▁later ▁protocol s , ▁high - speed ▁mode ms ▁typically ▁had ▁a ▁very ▁slow ▁" back channel " ▁for ▁sending ▁things ▁like ▁these ▁A CK s ▁back ▁to ▁the ▁sender . ▁On ▁the ▁~ 1 8 , 5 0 0 ▁bit / s ▁Tra il B la zer , ▁for ▁instance , ▁the ▁mode m ▁could ▁send ▁as ▁many ▁as ▁ 3 5 ▁U U CP ▁packets ▁a ▁second ▁to ▁the ▁receiver , ▁but ▁the ▁back channel ▁offered ▁only ▁ 7 5 ▁bit / s , ▁not ▁nearly