the structure of "TCH + SACCH" is depend on timeslot?

[Chengcheng Gou]

Hi,

I am studying the TCH channel, and read the BB source. I find that
the structure of  "TCH + SACCH" is depend on ts.that is when the timeslot
is even, the 12 of a TCH multiframes is SACCH,and the 25 is idle;and When
the timeslot is odd, the 25 of a TCH multiframes is SACCH,and the12 is idle.

is that right? if that`s right, why ? I can`t find any Material about
it. Thanks!