The explanation is simple,
there is direct connection between time and frequency.
for example if
tf=0.020; i.e. time frame is 20 ms then symbol-rate is 1/tf
1/20 ms = >> 1/tf
then You can count the number of elements in the frame, i.e nf=fix(tf/ts); i.e.
or in a frame of 20ms You will have 144 discrete elements of the signal.