▁the ▁standard , ▁so - called ▁subsets , ▁make ▁practical ▁implementations ▁possible ▁on ▁such ▁platforms . ▁Rem ark ably , ▁even ▁some ▁of ▁the ▁most ▁modern ▁platforms ▁need ▁minor ▁accommod ations ▁and ▁therefore ▁support ▁for ▁these ▁subsets ▁is ▁still ▁needed . ▁ ▁Of ▁these ▁protocol s ▁EC M ▁is ▁by ▁far ▁the ▁simplest — frames ▁are ▁simply ▁sent ▁and ▁received ▁without ▁modification ▁one ▁at ▁a ▁time . ▁This ▁was ▁a ▁good ▁strategy ▁for ▁USB ▁ 1 . 1 ▁systems ▁( current ▁when ▁the ▁protocol ▁was ▁issued ) ▁with ▁ 6 4 ▁byte ▁packets ▁but ▁not ▁for ▁USB ▁ 2 . 0 ▁systems ▁which ▁use ▁ 5 1 2 ▁byte ▁packets . ▁ ▁The ▁problem ▁is ▁that ▁E ther net ▁frames ▁are ▁about ▁ 1 5 0 0 ▁bytes ▁in ▁size — about ▁ 3 ▁USB ▁ 2 . 0 ▁packets , ▁and ▁ 2 3 ▁USB ▁ 1 . 1 ▁packets . ▁According ▁how ▁the ▁USB ▁system ▁works , ▁each ▁packet ▁is ▁sent ▁as ▁a ▁transfer , ▁a ▁series ▁of ▁maximum - length ▁packets ▁terminated ▁by ▁a ▁short ▁packet ▁or ▁a ▁special ▁Z LP ▁( zero - length ▁packet ). ▁After ▁this , ▁there ▁is ▁bus ▁laten cy , ▁where ▁nothing ▁is ▁sent ▁until ▁another ▁transfer ▁can ▁be ▁initi ated . ▁This ▁reduces ▁bus ▁occup ancy , ▁meaning ▁that ▁nothing ▁is ▁sent ▁for ▁considerable ▁fra ctions ▁of ▁bus ▁time . ▁A ▁gap ▁every ▁ 2 3 ▁frames ▁is ▁not ▁notice able , ▁but ▁a ▁gap ▁every ▁three ▁frames ▁can ▁be ▁very ▁cost ly