▁and ▁management ▁infrastr ucture ▁( Sw MI ) ▁made ▁of ▁T ET RA ▁base ▁stations ▁( T BS ). ▁As ▁well ▁as ▁allowing ▁direct ▁communic ations ▁in ▁situations ▁where ▁network ▁coverage ▁is ▁not ▁available , ▁D MO ▁also ▁includes ▁the ▁possibility ▁of ▁using ▁a ▁sequence ▁of ▁one ▁or ▁more ▁T ET RA ▁terminal s ▁as ▁rel ays . ▁This ▁functionality ▁is ▁called ▁D MO ▁gateway ▁( from ▁D MO ▁to ▁T MO ) ▁or ▁D MO ▁repe ater ▁( from ▁D MO ▁to ▁D MO ). ▁In ▁emer gency ▁situations ▁this ▁feature ▁allows ▁direct ▁communic ations ▁under ground ▁or ▁in ▁areas ▁of ▁bad ▁coverage . ▁ ▁In ▁addition ▁to ▁voice ▁and ▁dispatch ▁services , ▁the ▁T ET RA ▁system ▁supports ▁several ▁types ▁of ▁data ▁communication . ▁Status ▁messages ▁and ▁short ▁data ▁services ▁( SD S ) ▁are ▁provided ▁over ▁the ▁system ' s ▁main ▁control ▁channel , ▁while ▁packet - switch ed ▁data ▁or ▁circuit - switch ed ▁data ▁communication ▁uses ▁specifically ▁assigned ▁channels . ▁ ▁T ET RA ▁provides ▁for ▁authentication ▁of ▁terminal s ▁towards ▁infrastr ucture ▁and ▁vice ▁vers a . ▁For ▁protection ▁against ▁e aves dro pping , ▁air ▁interface ▁encryption ▁and ▁end - to - end ▁encryption ▁is ▁available . ▁ ▁The ▁common ▁mode ▁of ▁operation ▁is ▁in ▁a ▁group ▁calling ▁mode ▁in ▁which ▁a ▁single ▁button ▁push ▁will ▁connect ▁the ▁user ▁to ▁the ▁users ▁in ▁a ▁selected ▁call ▁group ▁and / or ▁a ▁dispatch er . ▁It ▁is ▁also ▁possible ▁for ▁the ▁terminal ▁to ▁act ▁as ▁a ▁one