For every < 1 we establish the existence of a separable Banach space whose Szlenk index is w+1 and which is universal for all separable Banach spaces whose Szlenk index does not exceed . In order to prove that result we provide an intrinsic characterization of which Banach spaces embed into a space admitting an FDD with Tsirelson type upper estimates. Instytut Matematyczny PAN, 2009.