The shape represents the spectral signature of the physical layer and modulation type supported by a wireless network. A “parabolic” shape is used to denote HR-DSSS BPSK (High-Rate Direct Sequence Spread Spectrum with Binary Phase-Shift Keying), this is, wireless networks with supported data rates up to 11 Mbps (e.g. 802.11b when operating at 1, 2, 5.5 and 11 Mbps data rates). The “trapezoidal” shape corresponds to physical layer and modulation types that make possible wireless networks with supported data rates greater than 11 Mbps. Find out more about modulation types here.