I think that :
1-SDH stands for Synchronous Digital Hierarchy,it’s a standard of transmission where the unit is
STM-1 ( 155Mb).This standard is built over multimode fiber.It represents the evolution of PDH: plesiochronous…,based on G703 standard with the famous E1=2Mb.
2-Synchronisation, is the first TS 0 in the E1 wich shows to the system that the E1 frame starts from here.