The goal of this paper is to show how formal specification can be applied to a full-fledged, real-world protocol while maintaining, or even enhancing, readability. The system we formally specify is Ethernet as it appears in IEEE 802.3. We focus on the specification of the Medium Access Control (MAC) layer—the part of the Data Link Layer that implements a 1-persistent CSMA/CD protocol — and its interfaces with adjacent layers. The specification method is based on one of Henzinger's real-time models. We believe that the readability of our specification is due to the graphical presentation using transition graphs of real-time programs.