Certified Wireless USB - Data Transport Architecture

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. The extent to which this practice is applied can be adjusted, resulting in a varying degree of equity between competing devices.

The specification defines four particular data transfer types; their identifying features are summarized here.

• Bulk transfers tap the channel as bandwidth is available. Delivery is guaranteed, but neither transfer rate nor latency are, though the host can attempt to leverage pending transfers or endpoints. They are used for high-volume transfers exhibiting a sharp time-varying behavior. They use unidirectional pipes.
• Interrupt transfers serve short transactions which demand high reliability and low latency. Maximum service period is guaranteed, as are a number of retries during said period.
• Isochronous transfers provide guaranteed transfer rates and bounded latency for transmission attempts, as well as on-average constant data rate (although dependent on the medium, usually comparable to the rates achievable by wired USB). There is also at least one guaranteed retry during the service period, and it supports additional reliability against error bursts by adding delay to the stream according to buffering capacity; payload sizes can be adjusted. Still, it may eventually be necessary to discard the oldest data in the buffers (the receiver can be informed of the amount of information discarded while the channel is not usable). Hosts will only discard data if the presentation time for a packet expires.
• Control transfers are the same as in USB 2.0. The system uses a best-effort policy, but software may restrict channel access and available bandwidth for devices.

Power management can also affect data transport, since devices may control their power use at their discretion. The fact that the communications protocol is based on TDMA means that both host and devices know exactly when their presence is not required, and can use this to enter power saving modes. Devices may turn off their radios transparently to the host while maintaining their connections. They can also turn off over extended periods of time if they previously notify the host, as they will ignore all communications from said host. Eventually, the device will trigger the wakeup procedure and check for pending work.

In turn, the host will usually turn its radio off when it is not needed. If it decides to stop the channel, be in temporarily or to enter hibernation or shutdown states, it must notify the devices before it can do so.