Designing an architecture that supports pervasive computing usually addresses two issues: the context within interact the system components and the services which are available in the environment. This paper proposes a semantic approach to design an architecture for pervasive systems focusing on these two issues. The ambient is viewed as a set of context elements named CoxEl. Physically a CoxEl is a (mobile) device presents in the environment. The core of a CoxEl is an ontology that represents the context, the services, the resources and the neighbourhood of every CoxEl. This ontology is also used for semantic interaction between the CoxEls. This paper illustrates the different semantic interactions that may occurs among the CoxEls. It also describes the roles that play the CoxEls within the ambient.