Na usoro mgbasa ozi dijitalụ, enwere usoro transceiver ngwa anya dijitalụ abụọ, nke a na-akpọ "Plesiochronous Digital Hierarchy" (Plesiochronous Digital Hierarchy), nke a na-akpọ PDH transceiver optical; nke ọzọ ka a na-akpọ "Sychronous Digital Hierarchy" (Synchronous Digital Hierarchy), nke a na-akpọ SDH transceiver optical.

Sistemu na-eji usoro dijitalụ plesiochronous (PDH) bụ ịtọ elekere nkenke dị elu n'ọnụ ọnụ nke netwọkụ nkwukọrịta dijitalụ, akara elekere ndị a niile nwere ọnụego ọkọlọtọ edo edo. Ọ bụ ezie na izi ezi nke elekere ọ bụla dị oke elu, a na-enwekarị obere ọdịiche. Iji hụ na nzikọrịta ozi dị mma, ọdịiche dị n'etiti elekere ndị a agaghị agafe oke akọwapụtara. Ya mere, usoro mmekọrịta a abụghị ezigbo mmekọrịta, ya mere a na-akpọ ya "quasi-synchronous."

N'ime netwọk mgbasa ozi n'oge gara aga, a na-eji ọtụtụ n'ime ngwa transceiver anya PDH, usoro a nwere ezigbo mgbanwe maka nkwurịta okwu ọdịnala. Site na mmepe ngwa ngwa nke nkwurịta okwu dijitalụ, ọ na-adị ntakịrị ma na-adị ntakịrị ma ọ bụrụ na ọ na-esi na ya pụta. , na ọtụtụ mgbasa ozi dijitalụ ga-ebufe ya, ya mere, usoro PDH enweghị ike izute mkpa nke mmepe ọrụ mgbasa ozi ọgbara ọhụrụ na njikwa netwọk mgbasa ozi ọgbara ọhụrụ. SDH bụ usoro nnyefe nke pụtara iji gboo mkpa ọhụrụ a.