Nke mbụ, akụrụngwa PCM na akụrụngwa PDH bụ ngwaọrụ dị iche iche.PCM bụ akụrụngwa nnweta ọrụ agbakwunyere, yana akụrụngwa PDH bụ ngwa nnyefe ngwa anya.

A na-emepụta akara ngosi dijitalụ site n'inyocha, ịkọwapụta na ntinye akara ngosi analog na-agbanwe na-aga n'ihu, nke a na-akpọ PCM (modulation pulse code), ya bụ, pulse code modulation. A na-akpọ ụdị akara ngosi eletriki eletriki akara ngosi baseband dijitalụ, nke a na-emepụta. site na PCM eletriki ọnụ.Sistemụ nnyefe dijitalụ dị ugbu a niile na-eji sistemu pulse-koodu modulation (Mkpụrụedemede-koodu).Ejighị PCM na mbụ bufee data kọmputa, kama inwe eriri akpati n'etiti switches kama ibunye naanị akara ekwentị.

Ngwa nnyefe ngwa anya PDH, n'ime usoro nzikọrịta ozi dijitalụ, akara ndị a na-ebunye bụ usoro mkpụrụ akụ digitized.Mgbe a na-ebufe iyi iyi egwu dijitalụ n'etiti ngwaọrụ mgbanwe dijitalụ, ọnụego ha ga-adịrịrị kpamkpam iji hụ na izi ezi nke nnyefe ozi.A na-akpọ nke a “mmekọrịta.”N'ime usoro mgbasa ozi dijitalụ, enwere usoro nnyefe dijitalụ abụọ, nke a na-akpọ "Plesiochronous Digital Hierarchy" (Plesiochronous Digital Hierarchy), nke akpọrọ PDH;nke ọzọ ka a na-akpọ “Sychronous Digital Hierarchy” (Synchronous Digital Hierarchy), nke akpọrọ SDH.

Site na mmepe ngwa ngwa nke nzikọrịta ozi dijitalụ, a na-enwe ntakịrị ma na-adị ntakịrị ma na-ebufe ihe na-aga n'ihu, na ọtụtụ mgbasa ozi dijitalụ ga-agbanyerịrị.Ya mere, usoro PDH enweghị ike gboo mkpa nke mmepe azụmahịa nke nkwukọrịta ọgbara ọhụrụ yana mkpa njikwa netwọkụ nkwukọrịta ọgbara ọhụrụ..SDH bụ usoro nnyefe nke pụtara iji gboo mkpa ọhụrụ a.