Different architectures for federated database systems have been proposed. We present three federation rules, analogous to the rules for fragmentation in distributed databases, and show that the reference architecture by Sheth and Larson satisfies them. We then analyze three shortcomings of the reference architecture: databases components of several federations, external schemata in user models different from the canonical model, and multiple semantics at the federated schema level, and we provide better solutions to each of them. With these solutions a new 8-level schema architecture framework is developed.