Muamua lava, masini PCM ma meafaigaluega PDH o ni masini eseese.O le PCM o lo'o tu'ufa'atasia meafaigaluega avanoa mo auaunaga, ma mea faigaluega PDH o masini fa'asalalau fa'apitoa.

O le faʻailoga numera e gaosia e ala i le faʻataʻitaʻiina, faʻatusatusaina ma le faʻailogaina o le faʻaogaina pea o le faʻailoga analog, lea e taʻua o le PCM (pulse code modulation), o lona uiga, pulse code modulation.O lenei ituaiga o faʻailoga eletise eletise e taʻua o le numera baseband, lea e gaosia. e se PCM eletise eletise.O faiga fa'akomepiuta o lo'o i ai nei e fa'aogaina uma le fa'aogaina o le pulse-code modulation (Pulse-code modulation).E le'i fa'aaogaina muamua le PCM e fa'asalalau ai fa'amatalaga komepiuta, ae ia i ai se laina ogalaau i le va o ki nai lo le na'o le lafoina o se fa'ailo telefoni.

PDH masini feavea'i opitika, i le faiga fa'akomepiuta feso'ota'iga, o fa'ailo fa'asalalauina o fa'asologa fa'afuainumera uma.Pe a fa'asalalau atu nei fa'ailoga fa'afuainumera i le va o masini fesuia'i numera, o latou fua faatatau e tatau ona matua'i atoatoa ina ia mautinoa le sa'o o fa'amatalaga fa'asalalau.E ta'ua lea o le “synchronization”.I totonu o le telefoni feaveaʻi, e lua faʻasologa faʻasalalau numera, tasi e taʻua o le "Plesiochronous Digital Hierarchy" (Plesiochronous Digital Hierarchy), faʻapuʻupuʻu o le PDH;o le isi e taʻua o le "Synchronous Digital Hierarchy" (Synchronous Digital Hierarchy), faapuupuuina o le SDH.

Faatasi ai ma le televave o le atinaʻeina o fesoʻotaʻiga numera, o loʻo i ai le itiiti ifo ma le itiiti ifo o faʻasalalauga tuusaʻo, ma o le tele o fesoʻotaʻiga numera e tatau ona fesuiaʻi.O le mea lea, e le mafai e le PDH ona faʻafetaui manaʻoga o le atinaʻeina o pisinisi tau fesoʻotaʻiga faʻaonapo nei ma manaʻoga o le pulega o fesoʻotaiga faʻaonaponei..O le SDH ose faiga fa'asalalau ua tula'i mai e fa'afetauia ai lea mana'oga fou.