  It is known that the space of momenta of DSR can be identified with de Sitter
space. In this paper, we discuss the relation of the noncanonical phase space
of the Magueijo-Smolin model of DSR with the canonical 5-dimensional phase
space in which the de Sitter space of momenta is embedded. We suggest that in
analogy with the momentum variables, also the position variables should be
constrained to lie on a null hypersurface of the five-dimensional space
