Okokuqala, izixhobo zePCM kunye nezixhobo zePDH zizixhobo ezahlukeneyo ngokupheleleyo.I-PCM iyisixhobo esihlanganisiweyo sokufikelela kwinkonzo, kwaye izixhobo zePDH zizixhobo zothumelo lokukhanya.

Isiginali yedijithali iveliswa ngesampulu, ukulinganisa kunye nokufaka ikhowudi ye-analog eguqukayo ngokuqhubekayo, ebizwa ngokuba yi-PCM (i-pulse code modulation), oko kukuthi, i-pulse code modulation. ngetheminali yombane yePCM.Iinkqubo zangoku zothumelo lwedijithali zonke zisebenzisa inkqubo ye-pulse-code modulation (I-Pulse-code modulation) inkqubo.I-PCM ayizange isetyenziswe ekuqaleni ukuhambisa idatha yekhompyutheni, kodwa ukuba ibe nomgca we-trunk phakathi kokutshintsha endaweni yokuhambisa isignali yefowuni kuphela.

Izixhobo zokuhambisa i-PDH optical transmission, kwinkqubo yonxibelelwano lwedijithali, iimpawu ezithunyelwayo zonke zilandelelwaniso lwe-pulse lwedijithali.Xa le mijelo yesignali yedijithali ihanjiswa phakathi kwezixhobo zokutshintsha idijithali, iireyithi zabo kufuneka zihambelane ngokupheleleyo ukuqinisekisa ukuchaneka kokuhanjiswa kolwazi.Oku kubizwa ngokuba "yi-synchronization."Kwinkqubo yokuhanjiswa kwedijithali, kukho iindidi ezimbini zokuhanjiswa kwedijithali, enye ibizwa ngokuba yi "Plesiochronous Digital Hierarchy" (Plesiochronous Digital Hierarchy), ifinyeziwe njengePDH;enye ibizwa ngokuba yi “Synchronous Digital Hierarchy” (i-Synchronous Digital Hierarchy), ifinyeziwe njenge-SDH.

Ngophuhliso olukhawulezayo lonxibelelwano lwedijithali, kukho ukuhanjiswa okuthe ngqo kokuncinci kunye nokuncinci, kwaye uninzi lothumelo lwedijithali kufuneka lutshintshwe.Ngoko ke, uchungechunge lwePDH alukwazi ukuhlangabezana neemfuno zophuhliso lwamashishini onxibelelwano lwanamhlanje kunye neemfuno zolawulo lwenethiwekhi yonxibelelwano lwangoku..I-SDH yinkqubo yothumelo eye yavela ukuhlangabezana nale mfuno intsha.