Information regarding DC wander
Visualisation aid: 


"All models are wrong.
Some models are useful" ... George Box.


Imagine feeding a signal through a capacitor.

If you send AC (sine or square waves etc) with a mean DC level of zero, then the signal passes through it unimpeded. 

If you apply DC to a capacitor it will charge exponentially to the driving voltage and current flow will fall exponentially to zero -  a capacitor does not pass DC..

A comms channel of the sort you are dealing with is effectively an AC coupled circuit. You must maintain the AC nature of the signal so that it does not "appear to be DC".