SM ). ▁ ▁Spe ech ▁signals ▁in ▁T ET RA ▁are ▁sample d ▁at ▁ 8   k Hz ▁and ▁then ▁com pressed ▁with ▁a ▁voc oder ▁using ▁algebraic ▁code - ex c ited ▁linear ▁prediction ▁( AC EL P ). ▁This ▁creates ▁a ▁data ▁stream ▁of ▁ 4 . 5 6 7   k bit / s . ▁This ▁data ▁stream ▁is ▁error - prote ction ▁encoded ▁before ▁transmission ▁to ▁allow ▁correct ▁dec oding ▁even ▁in ▁no isy ▁( err one ous ) ▁channels . ▁The ▁data ▁rate ▁after ▁coding ▁is ▁ 7 . 2   k bit / s . ▁The ▁capacity ▁of ▁a ▁single ▁traffic ▁slot ▁when ▁used ▁ 1 7 / 1 8 ▁frames . ▁ ▁A ▁single ▁slot ▁consists ▁of ▁ 2 5 5 ▁us able ▁symbols , ▁the ▁remaining ▁time ▁is ▁used ▁up ▁with ▁synchron isation ▁sequences ▁and ▁turning ▁on / off , ▁etc . ▁A ▁single ▁frame ▁consists ▁of ▁ 4 ▁sl ots , ▁and ▁a ▁mult iframe ▁( wh ose ▁duration ▁is ▁ 1 . 0 2 ▁seconds ) ▁consists ▁of ▁ 1 8 ▁frames . ▁Hyper frames ▁also ▁exist , ▁but ▁are ▁mostly ▁used ▁for ▁providing ▁synchron isation ▁to ▁encryption ▁algorithms . ▁ ▁The ▁down link ▁( i . e ., ▁the ▁output ▁of ▁the ▁base ▁station ) ▁is ▁normally ▁a ▁continuous ▁transmission ▁consisting ▁of ▁either ▁specific ▁communic ations ▁with ▁mobile ( s ), ▁synchron isation ▁or ▁other ▁general ▁broadcast s . ▁All ▁sl ots ▁are ▁usually ▁filled ▁with ▁a ▁burst ▁even ▁if ▁idle