why pre-post amble is required?
DQS is a bidirectional, 3-state signal that is also a primary timing reference for its corresponding data lane. It needs that extra preamble time to establish its phase alignment for the DLL (that is, ‘train’ the DLL) so that the data following can be sampled properly with the best timing margin.
The postamble is used to ensure a smooth hand-off between read and write transactions, that is, to allow turn-around time so the host and DDR device don’t clash with each other in a way that generates a false edge on DQS.
Micron application note TN 46-05 explains in more detail.