The problem is not the length 2048 but the way you use the fsm constructor
trellis.fsm(2,7,[79,109])
You are trying to create an FSM for a (7,2) CC.
This means you need to provide a vector of 2x7 integers, each
representing the generator polynomial between the i-th input (i=0,1)
to the j-th output (j=0,1,...,6)
However you only provide 2 of them.
Are you sure this is what you want to do?
------------
this is a snippet from fsm.cc
//######################################################################
//# Automatically generate the FSM from the generator matrix
//# of a (n,k) binary convolutional code
//######################################################################
fsm::fsm(int k, int n, const std::vector<int> &G)