  The discovery of alkaline iron selenide $A$Fe$_{1.6+x}$Se$_2$ ($A=$ K, Rb,
Cs) superconductors has generated considerable excitement in the condensed
matter physics community because superconductivity in these materials may have
a different origin from the sign reversed s-wave electron pairing mechanism, a
leading candidate proposed for all other Fe-based superconductors. Although
$A$Fe$_{1.6+x}$Se$_2$ are isostructural with the metallic antiferromagnetic
(AF) iron pnictides such as (Ba,Ca,Sr)Fe$_2$As$_2$, they are insulators near
$x=0$ and form a $\sqrt{5}\times\sqrt{5}$ blocked AF structure (Fig. 1a)
completely different from the iron pnictides. If magnetism is responsible for
superconductivity of all iron-based materials, it is important to determine
their common magnetic features. Here we use neutron scattering to map out spin
waves in the AF insulating Rb$_{0.89}$Fe$_{1.58}$Se$_2$. We find that although
Rb$_{0.89}$Fe$_{1.58}$Se$_2$ has a N$\rm \acute{e}$el temperature ($T_N=475$ K)
much higher than that of the iron pnictides ($T_N\leq 220$ K), spin waves for
both classes of materials have similar zone boundary energies. A comparison of
the fitted effective exchange couplings using a local moment Heisenberg
Hamiltonian in Rb$_{0.89}$Fe$_{1.58}$Se$_2$, (Ba,Ca,Sr)Fe$_2$As$_2$, and iron
chalcogenide Fe$_{1.05}$Te reveals that their next nearest neighbor (NNN)
exchange couplings are similar. Therefore, superconductivity in all Fe-based
materials may have a common magnetic origin that is intimately associated with
the NNN magnetic exchange interactions, even though they have metallic or
insulating ground states, different AF orders and electronic band structures.
