  In this paper we discuss the asymptotic spectrum of the spin chain
description of planar N=4 SUSY Yang-Mills. The states appearing in the spectrum
belong to irreducible representations of the unbroken supersymmetry SU(2|2) x
SU(2|2) with non-trivial extra central extensions. The elementary magnon
corresponds to the bifundamental representation while boundstates of Q magnons
form a certain short representation of dimension 16Q^{2}. Generalising the
Beisert's analysis of the Q=1 case, we derive the exact dispersion relation for
these states by purely group theoretic means.
