Chekutanga pane zvese, PCM midziyo nePDH michina yakasiyana zvachose.PCM inosanganiswa sevhisi yekuwana michina, uye PDH midziyo ndeye optical kutapurirana michina.

Chiratidzo chedhijitari chinogadzirwa nesampling, quantizing uye encoding inoenderera ichichinja analog chiratidzo, iyo inonzi PCM (pulse code modulation), kureva, pulse code modulation. nePCM magetsi terminal.Azvino madhijitari ekufambisa masisitimu ese anoshandisa pulse-code modulation (Pulse-code modulation) system.PCM yakanga isati yashandiswa kuendesa data yekombuta, asi kuve ne trunk mutsara pakati pekuchinja pane kungoendesa chiratidzo chefoni.

PDH optical transmission equipment, mudhijitari yekukurukurirana system, masaini anotumirwa ese ari digitized pulse sequences.Kana aya madhijitari masaini hova achifambiswa pakati pedhijitari switching madivayiri, iwo mareti anofanirwa kunyatsoenderana kuti ave nechokwadi chechokwadi chekufambisa kwemashoko.Izvi zvinonzi "synchronization."Mudhijitari yekufambisa dhijitari, kune maviri edhijitari ekutumira akateedzana, imwe inonzi "Plesiochronous Digital Hierarchy" (Plesiochronous Digital Hierarchy), yakapfupikiswa sePDH;imwe yacho inonzi "Synchronous Digital Hierarchy" (Synchronous Digital Hierarchy), yakapfupikiswa seSDH.

Nekukurumidza kukura kwekufambiswa kwemashoko edhijitari, kune mashoma uye mashoma anonongedzera-ku-kunongedzo akananga, uye mazhinji edhijitari anofanirwa kuchinjirwa.Naizvozvo, iyo PDH yakatevedzana haigone kusangana nezvinodiwa zvemazuva ano ekufambiswa kwebhizinesi rekufambiswa kwemashoko uye zvinodikanwa zvemazuva ano telecommunications network manejimendi..SDH inzira yekufambisa iyo yakabuda kuti isangane nechido chitsva ichi.