Though both Synchronous Digital Hierarchy (SDH) and Plesiochronous Digital Hierarchy (PDH) are related to transmission of large amounts of data for telecommunication systems, they accomplish this through different means. PDH transmits voice and large amounts of data using digital transport equipment, such as fiber optics and radio systems, according to Wikipedia. SDH transmits data synchronously using lasers on fiber optics or highly coherent light from LEDs.
SDH makes use of atomic clocks, which enables transmitting of data at a tightly synchronized rate and allows inter-country networks to run synchronously. This synchronization feature in SDH also reduces the rate of buffering needed in telecommunication networks. PDH is less synchronized than SDH. Because of this difference SDH is replacing the PDH system.
Another difference is that SDH can support higher data rates of transmissions than to PDH, according to Christiealwis.com. To function, the SDH method requires higher-capacity equipment, network management features, and transmission media. It also requires more software control, and there is less interaction between the equipment and the user. Although PDH has low data rates and more synchronization problems, the method is easier to operate, and because the system lacks high network management features, it can be controlled and managed manually.