In digital transmission system, there are two kinds of digital transmission line, called "quasi-synchronous digital hierarchy" referred PDH;
Another called "synchronous digital hierarchy" referred SDH. PDH (quasi-synchronous digital hierarchy) is a small-capacity Optical Optical, usually in pairs application, called point applications.
Capacity is generally 4 * E1,8 * E1,16 * E1. SDH (Synchronous Digital Hierarchy) Optical larger capacity, generally 16E1 to 4032E1.
SDH can replace PDH, SDH but not replace PDH.