Since 1990, the IEEE 802.11 working group is preparing a standard for wireless local area networks. The current version of the standard includes two radio spread spectrum physical layers and one infrared physical layer, all sharing a common medium access control layer (MAC). This paper describes some of the work performed during the specification of the infrared (IR) physical layer (PHY) that resulted in many contributions to the definition of the standard. Among the many technical solutions that have impact in the performance of the physical layer, the choice of a modulation method and the frame format are the most important. For the modulation method, Pulse Position Modulation (PPM) was adopted because of its high power efficiency. This paper describes the work performed regarding the definition of the frame format that better matches the particular characteristics of the indoor wireless infrared channel. The proposed frame format is presented and its performance is evaluated through analytical calculation of the frame error rate. For each field of the frame there is an explanation of the proposed format based on the probabilities of imitation, non-detection and/or error. The frame format here described was proposed to the IEEE 802.11 working group and was adopted and included in the current version of the standard.