Why PPM is used ?

Hi,

According to ISO 15693, 1 out of 256 or 1 out of 4 Pulse position coding is used for communication between reader and tag.
Firstly, I didn't understand the concept of 1 out of 256 and 1 out of 4.
Secondly, in the case of PPM, synchronization is required between Tx and Rx though the circuitary is simple.
So, I don't understand the need of PPM in communication from reader to tag using RFIds.


Thanks