Intuition
v(r) is the intrinsic load on router r
each (1-B(j)) is a “thinning” of the load, so load on trunk k is just the “thinned” sum of the loads on all routes that share trunk k
B(k) depends on some other B(j)’s through the B()=E() equation and eqn.
So, each B(k) is implicitly defined by the others, forming the Erlang map