Abstract: Wireless stations (108-1-108-N) contending for exclusive access for a predetermined period of time (S232) to transmit on a communication medium (112) are regulated by a common external controller (104) using a single timing parameter (S204). Attempts at transmission access are preceded by respective delays that are expired simultaneously and at a common rate by the stations at times when the stations sense the medium to be idle (S224). The delays are pseudo-randomly selected to avoid collisions among stations in their respective access attempts (S208). Default, initial values of the respective delays (S204) applied to the stations, when multiplied by the respective expected number of transmission attempts by the stations over the long term over periods of time when no transmission attempt is unsuccessful, yield respective products equal to a constant value common to all stations (S312). The default values therefore serve as a knob by which the controller regulates air time opportunity.

Abstract: A method, system and computer readable medium for the authenticating of an audio-visual signal, such as digital images or video comprising the generation of a robust image signature with variable size. In a preferred embodiment DC-values of blocks of a digital image are calculated and areas with similar DC-values are merged into regions. The signature is based on said regions and of variable length, depending on the desired localisation ability or an allowable signature length. The resulting signature bits are robust to compression and other allowable image operations. The hierarchical solution provides both robustness and tampering localisation.