CP ▁" g " ▁protocol ▁and ▁K erm it ▁both ▁use ▁ 6 4 - byte ▁packets , ▁which ▁on ▁a ▁ 9 6 0 0 ▁bit / s ▁link ▁takes ▁about ▁ ▁of ▁a ▁second ▁to ▁send . ▁X MODE M ▁used ▁a ▁slightly ▁larger ▁ 1 2 8 - byte ▁packet , ▁which ▁takes ▁about ▁ ▁of ▁a ▁second ▁to ▁send . ▁ ▁The ▁next ▁packet ▁of ▁data ▁cannot ▁be ▁sent ▁until ▁the ▁A CK ▁for ▁the ▁previous ▁packet ▁is ▁received . ▁In ▁the ▁case ▁of ▁X MODE M , ▁for ▁instance , ▁that ▁means ▁it ▁takes ▁a ▁minimum ▁of ▁ ▁of ▁a ▁second ▁for ▁the ▁entire ▁cycle ▁to ▁complete ▁for ▁a ▁single ▁packet . ▁This ▁means ▁that ▁the ▁overall ▁speed ▁is ▁only ▁half ▁the ▁theoretical ▁maximum , ▁a ▁ 5 0 % ▁channel ▁efficiency . ▁ ▁Pro tocol ▁spo of ing ▁addresses ▁this ▁problem ▁by ▁having ▁the ▁local ▁mode m ▁recognize ▁that ▁a ▁data ▁transfer ▁is ▁under way , ▁often ▁by ▁looking ▁for ▁packet ▁headers . ▁When ▁these ▁are ▁seen , ▁the ▁mode m ▁then ▁looks ▁for ▁the ▁end ▁of ▁the ▁packet , ▁normally ▁by ▁knowing ▁the ▁number ▁of ▁bytes ▁in ▁a ▁single ▁packet . ▁X MODE M , ▁for ▁instance , ▁has ▁ 1 3 2 ▁bytes ▁in ▁a ▁packet ▁due ▁to ▁the ▁header ▁and ▁check sum ▁being ▁added ▁to ▁the ▁ 1 2 8 ▁bytes ▁of ▁actual ▁data . ▁When ▁the ▁mode m ▁sees ▁the ▁packet ▁has ▁ended , ▁it ▁immediately ▁sends ▁of ▁spo of ed ▁A CK