▁( continu ous ▁mode ). ▁Although ▁the ▁system ▁uses ▁ 1 8 ▁frames ▁per ▁second ▁only ▁ 1 7 ▁of ▁these ▁are ▁used ▁for ▁traffic ▁channels , ▁with ▁the ▁ 1 8 th ▁frame ▁reserved ▁for ▁sign alling , ▁Short ▁Data ▁Service ▁messages ▁( like ▁S MS ▁in ▁G SM ) ▁or ▁synchron isation . ▁The ▁frame ▁structure ▁in ▁T ET RA ▁( 1 7 . 6 5 ▁frames ▁per ▁second ), ▁consists ▁of ▁ 1 8 , 0 0 0 ▁symbols / s ; ▁ 2 5 5 ▁symbols / sl ot ; ▁ 4 ▁sl ots / frame , ▁and ▁is ▁the ▁cause ▁of ▁the ▁perce ived ▁" am plit ude ▁mod ulation " ▁at ▁ 1 7   Hz ▁and ▁is ▁especially ▁apparent ▁in ▁mob iles / port ables ▁which ▁only ▁transmit ▁on ▁one ▁slot / 4 . ▁They ▁use ▁the ▁remaining ▁three ▁sl ots ▁to ▁switch ▁frequency ▁to ▁receive ▁a ▁burst ▁from ▁the ▁base ▁station ▁two ▁sl ots ▁later ▁and ▁then ▁return ▁to ▁their ▁transmit ▁frequency ▁( TD MA ). ▁ ▁Radio ▁frequencies ▁ ▁Air ▁interface ▁encryption ▁ ▁To ▁provide ▁confident ial ity ▁the ▁T ET RA ▁air ▁interface ▁is ▁encrypted ▁using ▁one ▁of ▁the ▁T ET RA ▁Enc ryption ▁Algorithm ▁( TE A ) ▁ci ph ers . ▁The ▁encryption ▁provides ▁confident ial ity ▁( prote ct ▁against ▁e aves dro pping ) ▁as ▁well ▁as ▁protection ▁of ▁sign alling . ▁ ▁Currently ▁ 4 ▁different ▁ci ph ers ▁are ▁defined . ▁These ▁TE A ▁ci ph ers