Kuhlelo lokudlulisa ukuxhumana kwedijithali, kukhona uchungechunge olubili lwe-digital optical transceiver, olulodwa lubizwa ngokuthi “Plesiochronous Digital Hierarchy” (Plesiochronous Digital Hierarchy), okubizwa ngokuthi i-PDH optical transceiver; enye ibizwa ngokuthi “I-Synchronous Digital Hierarchy” (I-Synchronous Digital Hierarchy), ebizwa ngokuthi i-SDH optical transceiver.

Uhlelo olusebenzisa uchungechunge lwedijithali lwe-plesiochronous (PDH) luwukusetha iwashi elinemba kakhulu endaweni ngayinye yenethiwekhi yokuxhumana yedijithali, amasignali alawa mawashi anesilinganiso esijwayelekile esifanayo. Nakuba ukunemba kwewashi ngalinye kuphakeme kakhulu, kuhlale kunomehluko omncane. Ukuze kuqinisekiswe ikhwalithi yokuxhumana, umehluko phakathi kwalawashi akumele udlule ububanzi obushiwo. Ngakho-ke, le ndlela yokuvumelanisa ayikona ukuvumelanisa okuyiqiniso, ngakho ibizwa ngokuthi "quasi-synchronous."

Esikhathini esidlule samanethiwekhi okuxhumana, iningi lemishini ye-PDH ye-transceiver optical yasetshenziswa, lolu chungechunge lukwazi ukuguquguquka okuhle ekukhulumisaneni kwendabuko kwephuzu kuya kwephuzu.Ngokuthuthukiswa okusheshayo kokuxhumana kwedijithali, kukhona ukudluliselwa okuqondile okumbalwa nokuncane. , futhi ukuthunyelwa okuningi kwedijithali kufanele kudluliswe, ngakho-ke, uchungechunge lwe-PDH alukwazi ukuhlangabezana nezidingo zokuthuthukiswa kwesevisi yesimanje yezokuxhumana kanye nokuphathwa kwenethiwekhi yesimanje yezokuxhumana. I-SDH iwuhlelo lokudlulisa oluvelile ukuhlangabezana nalesi sidingo esisha.