Radio labelings of cycles

A radio labeling of a connected graph G is an assignment of distinct positive integers to the vertices of G, with x is an element of V(G) labeled c(x), such that d(u, v) + \c(u) - c(v)\ greater than or equal to 1 + diam G for every two distinct vertices u, v of G, where diam G is the diameter of G. The radio number rn(c) of a radio labeling c of G is the maximum label assigned to a vertex of G. The radio number rn(G) of G is min{rn(c)} over all radio labelings c of G. Radio numbers of cycles are discussed and upper and lower bounds are presented.