Muri sisitemu yo kohereza itumanaho rya sisitemu, hariho ibice bibiri bya sisitemu ya optique ya transceiver, imwe yitwa "Plesiochronous Digital Hierarchy" (Plesiochronous Digital Hierarchy), bita PDH optique transceiver; ikindi cyitwa "Synchronous Digital Hierarchy" (Synchronous Digital Hierarchy), bita SDH optique transceiver.

Sisitemu ikoresha urukurikirane rwa sisitemu ya plesiochronous (PDH) nugushiraho isaha ihanitse kuri buri node y'urusobe rw'itumanaho rya digitale, ibimenyetso by'aya masaha byose bifite igipimo kimwe. Nubwo ubunyangamugayo bwa buri saha buri hejuru cyane, burigihe hariho itandukaniro rito. Kugirango tumenye neza itumanaho, itandukaniro riri hagati yaya masaha ntirishobora kurenga urwego rwagenwe. Kubwibyo, ubu buryo bwo guhuza ntabwo aribwo buryo bwo guhuza ukuri, kubwibyo byitwa "quasi-synchronous."

Mu miyoboro y'itumanaho ryashize, ibyinshi mu bikoresho bya PDH optique ya transceiver byakoreshejwe, uru ruhererekane rufite uburyo bwiza bwo guhuza n'imihindagurikire y'itumanaho gakondo. , kandi ibyinshi mubitumanaho bigomba kwimurwa, kubwibyo, urukurikirane rwa PDH ntirushobora guhaza ibikenewe mu iterambere rya serivise yitumanaho rigezweho no gucunga imiyoboro igezweho. SDH ni uburyo bwo kohereza bwagaragaye kugirango buhuze ibyo bikenewe.