Communication services are surrounding our daily life; internet, mail, cellular phones... These services are managed by several protocols. In this paper we propose a generic model for the Ethernet protocol based on time Petri nets. A complete modular service-protocol model can help in better managing services and protocols and to easily change a system element without affecting the other elements. Also it can help us to study different properties of the system such as performance, functionality, validation and any qualitative or quantitative property.