Pele ho tsohle, lisebelisoa tsa PCM le lisebelisoa tsa PDH ke lisebelisoa tse fapaneng ka ho felletseng.PCM ke lisebelisoa tse kopaneng tsa phihlello ea lits'ebeletso, 'me lisebelisoa tsa PDH ke lisebelisoa tsa phetiso ea optical.

Letšoao la dijithale le hlahisoa ka ho etsa lisampole, ho lekanya le ho kopanya lets'oao la analog le ntseng le fetoha, le bitsoang PCM (pulse code modulation), ke hore, pulse code modulation. ka setsi sa motlakase sa PCM.Sistimi ea hajoale ea phetisetso ea dijithale kaofela e sebelisa mokhoa oa ho feto-fetoha ha pulse-code (Pulse-code modulation).PCM e ne e sa sebelisoe qalong ho fetisetsa data ea k'homphieutha, empa ho ba le mola oa kutu pakeng tsa li-switches ho e-na le ho fetisa letšoao la mohala feela.

Lisebelisoa tsa phetisetso ea optical PDH, tsamaisong ea puisano ea dijithale, matšoao a fetisoang kaofela ke tatelano ea pulse ea digitized.Ha melapo ena ea matšoao a dijithale e fetisoa lipakeng tsa lisebelisoa tsa ho fetola dijithale, litefiso tsa tsona li tlameha ho lumellana ka botlalo ho netefatsa ho nepahala ha phetiso ea tlhahisoleseling.Sena se bitsoa "synchronization."Lenaneong la phetisetso ea dijithale, ho na le lihlopha tse peli tsa phetisetso ea dijithale, e 'ngoe e bitsoa "Plesiochronous Digital Hierarchy" (Plesiochronous Digital Hierarchy), e khutsufalitsoeng e le PDH;e 'ngoe e bitsoa "Synchronous Digital Hierarchy" (Synchronous Digital Hierarchy), e khutsufalitsoeng e le SDH.

Ka nts'etsopele e potlakileng ea likhokahano tsa dijithale, ho na le li-transmission tse tobileng tse fokolang le tse fokolang, 'me li-transmission tse ngata tsa digital li tlameha ho fetoloa.Ka hona, letoto la PDH ha le khone ho fihlela litlhoko tsa nts'etsopele ea khoebo ea sejoale-joale ea mehala le litlhoko tsa tsamaiso ea sejoale-joale ea likhokahano tsa mehala..SDH ke mokhoa oa phetisetso o hlahileng ho fihlela tlhoko ena e ncha.