 vulnerability to single-point failure and eliminating the bandwidth penalty of maintaining central control. Prioritised access A TDMA system allows devices either to reserve mechanisms guaranteed medium access slots, or contend for access during a prioritised contention period. Network management The bandwidth overhead associated with the MAC protocol efficiency scales with the number of devices, assuring a low overhead for networks with few devices. Wireless PAN Standards 65 mS Super frame N-1 Super frame N Super frame N+1 Media access slots (MAS) Beacon period 256 us Figure 10-5: WiMedia MAC Superframe Structure Media access slots that are not reserved are available for use by any device, based on prioritised contention access (PCA). Here the prioritisation mechanism ensures that asynchronous but time-sensitive transmissions, such as those to and from a user interface device, will get priority over other non-time-sensitive devices. The contention access period is also used to increase the robustness of isochronous connections by providing an opportunity for the MAC layer to retransmit any failed packets from the DRP period. The Wireless USB Channel The wireless USB specification defines the way in which a wireless USB channel is established within the superframe structure and associated MAS reservation and control mechanisms. A host creates the wireless USB channel using DRP to reserve media access time slots that will be used for communication by all devices in the cluster. The host controls the channel using a sequence of control packets called micro-scheduled management commands (MMCs), which are transmitted during the reserved media access slots. These commands are used to dynamically schedule and control channel time for communication between the host and devices in the cluster. The MMC is a broadcast packet that contains a cluster ID to enable devices to identify control packets for their cluster. Each MMC specifies the breakdown into micro-scheduled channel time allocations (MS-CTAs) of reserved time un