US Patent 7948961 Wireless proximity estimation

A wireless content protection system includes a coordinator, a source device, and a sink device. The coordinator includes a scheduler. The source and sink devices each have a content protection manager. One of the content protection manager allocates and releases bandwidth via the coordinator for a test message and a response message for a round-trip time computation. The test message is transmitted in a source-to-sink channel time block. The response message is transmitted in a sink-to-source channel time block. The round-trip time is the period between the end of the source-to-sink channel time block and the beginning of the sink-to-source channel time block.