HI Ayman,
As far as i know , each equipment in the n/w has to be sync’d to teh master clk . The Master clk can be derived from PSTN PCM streams then MSC -XCDR- BSC-BTS . The interconnecting backbone equipments also has to derive the clk from the E1s which interconnect the above elements .
In case on Siemens STM equipments the E1s are selected and priority is fixed to sync the STM nodes . as the STM packed structure requires clk for exactly reproducing the STM packets .
secondly the delay , i think it should be propogation delay , when a signal passes from one end of the node to the other node . the signal has to travel a distance , which induces a time delay . further in case of Repeater stations the signals are first captured then reshaped – amplified and reproduced , for these process ,time is required . hence time delay .