▁system , ▁using ▁double ▁side band / un supp ress ed ▁car rier ▁operation ▁which ▁didn ' t ▁need ▁network ▁time base ▁synchron ization ▁to ▁maintain ▁frequency ▁accuracy . ▁ ▁N - 2 ▁was ▁similar ▁in ▁heter od yn ing ▁scheme , ▁but ▁in ▁discrete ▁trans istor ized ▁" pl ug - in ▁unit " ▁architecture , ▁while ▁N 3 ▁used ▁the ▁same ▁frequency ▁plan ▁but ▁a ▁scheme ▁of ▁using ▁single ▁side band ▁with ▁a ▁different ▁voice ▁channel ▁on ▁each ▁side ▁of ▁the ▁car rier , ▁a ▁technique ▁first ▁seen ▁on ▁the ▁ 1 6 ▁channel ▁Type ▁" O " ▁open ▁wire ▁short ▁ha ul ▁car rier ▁of ▁the ▁ 1 9 5 0 s . ▁ ▁This ▁double d ▁the ▁capacity ▁to ▁ 2 4 ▁channels , ▁the ▁same ▁as ▁a ▁basic ▁digital ▁Type ▁T ▁PC M ▁car rier ▁introduced ▁in ▁the ▁late ▁ 1 9 5 0 s , ▁which ▁became ▁the ▁now - ub iqu it ous ▁" T - 1 " ▁of ▁the ▁digital ▁world . ▁ ▁Re pe aters ▁were ▁spac ed ▁approximately ▁ 6 ▁miles ▁( 1 0   km ) ▁apart , ▁depending ▁on ▁wire ▁gauge . ▁ ▁With ▁few ▁exceptions , ▁N - car riers ▁used ▁ 1 9 ▁g age ▁un loaded ▁to ll ▁pairs ▁in ▁two - wire ▁operation . ▁ ▁Each ▁repe ater ▁either ▁received ▁from ▁both ▁directions ▁at ▁a ▁low ▁frequency ▁band ▁and ▁sent ▁in ▁both ▁directions ▁at ▁a ▁higher ▁band , ▁or ▁vice ▁vers a . ▁ ▁This ▁frequency ▁fro