Mbere ya byose, ibikoresho bya PCM nibikoresho bya PDH nibikoresho bitandukanye rwose.PCM ihuriweho nibikoresho bya serivisi, kandi ibikoresho bya PDH nibikoresho byohereza optique.

Ikimenyetso cya digitale gikozwe no gutoranya, kubara no gushushanya ibimenyetso bihora bihinduranya ibimenyetso, byitwa PCM (pulse code modulasiyo), ni ukuvuga modula code modulasi. Ubu bwoko bwikimenyetso cyamashanyarazi bwitwa signal ya baseband signal, ikorwa na PCM y'amashanyarazi.Sisitemu yohereza ibyuma bya none byose ikoresha pulse-code modulation (Pulse-code modulation) sisitemu.PCM ntabwo yari isanzwe ikoreshwa mu kohereza amakuru ya mudasobwa, ahubwo yari ifite umurongo wimbere hagati ya switch aho kohereza gusa ibimenyetso bya terefone.

Ibikoresho byohereza PDH optique, muri sisitemu yitumanaho rya sisitemu, ibimenyetso byoherejwe byose ni digitale ya pulse ikurikirana.Iyo ibyo bimenyetso bya digitale byanyujijwe hagati yibikoresho byo guhinduranya ibyuma, igipimo cyacyo kigomba kuba gihamye rwose kugirango amakuru atangwe neza.Ibi byitwa "guhuza."Muri sisitemu yo kohereza hakoreshejwe uburyo bwa digitale, hariho ibice bibiri byohereza amakuru, imwe yitwa "Plesiochronous Digital Hierarchy" (Plesiochronous Digital Hierarchy), mu magambo ahinnye yitwa PDH;ikindi cyitwa "Synchronous Digital Hierarchy" (Synchronous Digital Hierarchy), mu magambo ahinnye nka SDH.

Hamwe niterambere ryihuse ryitumanaho rya digitale, haribintu bike kandi bike byerekanwa-byerekanwa bitaziguye, kandi ibyinshi mubitumanaho bigomba guhinduka.Kubwibyo, urukurikirane rwa PDH ntirushobora guhaza ibikenerwa mu iterambere ry’itumanaho rya kijyambere hamwe n’ibikenerwa mu micungire y’itumanaho rya kijyambere..SDH ni uburyo bwo kohereza bwagaragaye kugirango buhuze ibyo bikenewe.