headers, not payloads. The MAC layer is particularly relevant to W-USB. It uses superframes divided in 256 time slots, the first of which are dedicated to the transfer of beaconing information. Slots can further be allocated to meet the necessities of clusters of devices, also identified by MMC's (see below). A host maintains one or more W-USB communication channels and is fully aware of the MAC layer, whereas a device only needs to use the defined W-USB interface to communicate through existing channels. There are three degrees of MAC consciousness in devices. The highest of these corresponds to a self-beaconing device, which is able to perform beaconing on its own. The following degree represents directed-beaconing devices, which are unaware of MAC frames and have limited beaconing capabilities, depending on the host to detect and beacon for nearby devices. Lastly there are non-beaconing devices, which have a very limited ability to transmit and receive; on the other hand, devices which are undetectable by the host can not be affected by these devices, nor can affect them. Thus, non-beaconing devices can only operate in very close vicinity to the host. Directed- and self-beaconing devices must be able to identify their hidden neighbors, which they do by emitting beacons. On their end, hosts manage global timers with the precision the physical medium requires (20 ppm). Channel time is sent within MMC's, and it is used for slot allocation, so it is important that hosts perform accurate beaconing. Devices may as well beacon reservation declarations. The superframe includes device notification time slots for asynchronous transfers initiated by the devices (which do not use pipes, but instead tap the bus layer directly); the host dynamically assigns slots as needed. Besides these, W-USB transactions between the host and endpoints are carried out as in USB. Data transport architecture Transactions use TDMA microscheduling while adhering to USB semantics. A split-transaction protocol is used to allow multiple transactions to be carried out simultaneously. This is related to the transaction group concept, which consists of a microscheduled management command (MMC) and allocated time slots for the execution of its associated workload. Wireless data transfers tend to incur in very significant overheads; to mitigate this W-USB replaces these with the burst mode data phase, which groups one or more data packets which reducing packet delimiters and separation gaps, in contrast with the USB rule of one data packet per transaction.