This paper presents a new steganographic method called WiPad (Wireless Padding). It is based on the insertion of hidden data into the padding of frames at the physical layer of WLANs (Wireless Local Area Networks). A performance analysis based on a Markov model, previously introduced and validated by the authors in [10], is provided for the method in relation to the IEEE 802.11 a/g standards. Its results prove that maximum steganographic bandwidth for WiPad is as high as 1.1 Mbit/s for data frames and 0.44 Mbit/s for acknowledgment (ACK) frames. To the authors' best knowledge this is the most capacious of all the known steganographic network channels.