Lenaneong la phetisetso ea puisano ea dijithale, ho na le letoto la li-transceiver tse peli tsa digital optical, e 'ngoe e bitsoa "Plesiochronous Digital Hierarchy" (Plesiochronous Digital Hierarchy), e bitsoang PDH optical transceiver; e 'ngoe e bitsoa "Synchronous Digital Hierarchy" (Synchronous Digital Hierarchy), e bitsoang SDH optical transceiver.

Sistimi e sebelisang letoto la dijithale la plesiochronous (PDH) ke ho beha oache e nepahetseng hantle sebakeng se seng le se seng sa marang-rang a puisano ea dijithale, matšoao a lioache tsena kaofela a na le sekhahla se lekanang. Le hoja ho nepahala ha oache ka ’ngoe ho phahame haholo, ho lula ho e-na le liphapang tse fokolang. E le ho netefatsa boleng ba puisano, phapang pakeng tsa lioache tsena ha ea lokela ho feta moeli o boletsoeng. Ka hona, mokhoa ona oa ho hokahanya ha se oa 'nete, kahoo o bitsoa "quasi-synchronous."

Nakong e fetileng ea marang-rang a lithelefono, lisebelisoa tse ngata tsa PDH optical transceiver li ne li sebelisoa, letoto lena le na le mokhoa o motle oa ho ikamahanya le mekhoa e tloaelehileng ea puisano ea ntlha-to-point. , 'me boholo ba li-transmission tsa dijithale li tlameha ho fetisoa, ka hona, letoto la PDH le ke ke la finyella litlhoko tsa tsoelo-pele ea litšebeletso tsa mehala ea kajeno le tsamaiso ea morao-rao ea marang-rang. SDH ke mokhoa oa phetisetso o hlahileng ho fihlela tlhoko ena e ncha.