Hauv kev sib txuas lus digital, muaj ob lub digital optical transceiver series, ib qho hu ua "Plesiochronous Digital Hierarchy" (Plesiochronous Digital Hierarchy), hu ua PDH optical transceiver; lwm qhov yog hu ua "Synchronous Digital Hierarchy" (Synchronous Digital Hierarchy), hu ua SDH optical transceiver.

Lub kaw lus siv plesiochronous digital series (PDH) yog los teeb tsa lub moos ua haujlwm siab ntawm txhua qhov ntawm cov kev sib txuas lus digital, cov cim ntawm cov moos no txhua tus muaj tus qauv zoo ib yam. Txawm hais tias qhov tseeb ntawm txhua lub moos yog siab heev, ib txwm muaj qee qhov sib txawv me ntsis. Txhawm rau kom ntseeg tau qhov kev sib txuas lus zoo, qhov sib txawv ntawm cov moos no yuav tsum tsis txhob dhau qhov kev teev tseg. Yog li ntawd, txoj kev synchronization no tsis yog qhov tseeb synchronization, yog li nws hu ua "quasi-synchronous."

Hauv kev sib txuas lus yav dhau los, feem ntau ntawm PDH optical transceiver cov cuab yeej siv, cov koob no muaj kev hloov tau zoo rau kev sib txuas lus ib txwm muaj. , thiab feem ntau cov kev xa tawm digital yuav tsum tau xa mus, yog li, PDH series tsis tuaj yeem ua tau raws li cov kev xav tau ntawm kev tsim kho kev sib txuas lus niaj hnub thiab kev tswj xyuas kev sib txuas lus niaj hnub. SDH yog ib qho kev sib kis uas tau tshwm sim los ua kom tau raws li qhov xav tau tshiab no.