Consider classic error-correcting problem:

there is finite set $A$ and string $a_1...a_n$, $a_i \in A$ in the begin.

in the end we have $b_1...b_n$. Set places of errors  $E = \{i| a_i\not= b_i \}$, $t = |E|$

We want to find algorithm that coding some string $c_1...c_k \to a_1...a_n$ and $b_1...b_n \to c_1...c_k$. We want $k/n, t/n \to$ min (I simplified some). 

In classic case we don't know $E$ and I offer to consider  error-correcting problem when we know it. 

Has it been considered? What is relations between this problem and classic?