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Modem networked embedded systems represent a growing market segment in which security is becoming an essential requirement. The Advanced Encryption Standard (AES) specification is becoming the default choice for such type of systems; however, a proper software implementation of AES is of fundamental importance in order to achieve significant performance. Current implementations presented in the literature differ in terms of the amount of look-up tables used for precomputing the functions of the encryption/decryption phase. This raises some questions regarding which AES implementation is optimal for a specific system configuration that, up to now, has been only empirically solved. In this work, we present an analytical model to study and evaluate the performance of the possible AES implementations in the early phases of system development. We then show that the proposed high-level timing model captures, with significant accuracy, the actual performance of current AES applications and thus it can be used for the early evaluation of optimal AES implementations and to support the design space exploration phase. Validating experiments have been carried out on the Lx architecture, a scalable and customizable VLIW architecture developed by STMicroelectronics and HP Labs. Some final considerations are eventually reported about the relevant characteristics of the analyzed implementations and the role of the cache memory.