The access mechanism for devices to transmit during a DNTS is Slotted Aloha. A device that has a device notification to transmit selects a message slot in a DNTS using a uniformly distributed random integer value, in the range {0, N-1} where N is the number of message slots in the DNTS instance. A device will begin transmitting the device notification packet at the point it determines the start of the message slot, measured from the beginning of the previous MMC packet, based on its local clock. Individual time slots within a DNTS have

a fixed duration of tNOTIFICATIONSLOT. A WDNTS must be scheduled to occur within 25ms of its associated MMC packet.

Table 5-8. WDNTSCTA Block Format

5.3Transaction Group Timing Constraints

The host must order individual transactions within transaction groups to minimize the number of ‘bus turns’ (e.g. turning the hosts’ radio from Transmit to Receive or Receive to Transmit). This means that a transaction group must be constructed to have all the host Transmit protocol time slots immediately after the MMC, followed by a ‘bus turn’ and then all Device Transmit protocol time slots. Note there is another ‘bus turn’ between the last Device Transmit time slot and the next scheduled MMC packet (see Figure 5-6).

Figure 5-6. Example Wireless USB Transaction Group Organization

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An inter-slot time is the time between the end of the last packet transmission of one protocol time slot to the start of the next protocol time slot (or transmission of an MMC packet). In general, the intent of the inter-slot time is to ensure that transmissions between protocol time slots do not overlap. Inter-slot times must be long enough in duration to guard against the maximum clock drift between a device’s local clock and the ideal clock, see Figure 5-7 (which also provides general the method for calculating a guard time (tGUARDTIME).)

Figure 5-7. TDMA Slot Guard Time Reference

Note that the ppm (parts per million) term depends on the clock rate of the PHY and the maximum drift is a function of the elapsed time since the last synchronization event (i.e. the interval). In order to minimize the effects of Guard Time on the available bandwidth, Wireless USB uses MMCs as clock synchronization reference points.

The following discussion applies to determining the minimum timing constraints for allowable inter-slot time. Inter-slot times are, as noted above, a timing component used only by the host to calculate individual time slot durations. Actual inter-slot times are host-implementation dependent, but must meet the minimum requirements described below.

The first two protocol time slots in Figure 5-6 indicate OUT (host to device) transmissions. Protocol time slots in a transaction group must be ordered OUT then INs, so the host is the transmitter of all the packets beginning from an MMC until the first IN protocol time slot. This looks in many respects like a burst transfer (supported in many PHY standards), although the recipient devices in adjacent protocol slots may be different for this

application. For adjacent OUT protocol time slots, the minimum inter-slot time must be tINTERSLOTTIME . There is no need to add guard times between these consecutive OUT transactions (or between the MMC and the first

OUT,) since the host is the transmitter of all these packets. The receiving device, however, must start listening at least a calculated guard time (tGUARDTIME) before the anticipated packet start time. The Wireless USB standard inter-slot time (tINTERSLOTTIME) is a PHY-related timing parameter, see Table 5-11.

During device to host (IN) transactions, the minimum inter-slot time between successive IN transactions must

be tINTERSLOTTIME plus tGUARDTIME since the IN time slots could potentially be used by different transmitters with drifting clocks.

When an MMC or OUT protocol time slot is followed by an IN protocol time slot, or an IN protocol time slot is

followed by an MMC, then the minimum inter-slot time must be equal to the calculated guard time (tGUARDTIME) plus the host’s bus switch time (tBUSTURNTIME) which is a PHY-related timing parameter, see Table 5-11. The sum of these is the bus turn inter-slot time (tBUSTURNINTERSLOTTIME). Figure 5-6 illustrates these inter-slot idle times.

The final components in calculating protocol slot time durations are the inter-packet gaps and size of preambles between packets in slots where multiple packets are transmitted (e.g. burst-mode packet transmissions). PHY standards may (or may not) define a minimum and maximum value for burst-mode inter-packet gaps and the use of streaming preambles (which can be shorter than standard preambles). When necessary, Wireless USB does define a maximum requirement for the streaming mode inter-packet gaps, see Table 5-11. The availability of streaming preambles is also a PHY-specific parameter. Section 5.3.1 summarizes the streaming-mode timing constraints for the PHY. These rules and parameters allow a host implementation to calculate protocol slot time durations with only the transmit rate, number and size of the data packets as variables in the calculation, all other slot time terms are constants.

Figure 5-8 summarizes component parts the host takes into consideration when calculating the durations of data phase time slots when the burst-mode size is greater than one. Regardless of the direction of transmit (OUT or IN), the first packet in a protocol time slot is required to have a standard preamble. Between each data packet is

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an allowance for a streaming-mode inter-packet gap (tSTREAMIPG) and either a streaming-mode or standard preamble. At the end of the time slot is an allowance for the inter-slot idle time (see above discussion around

Figure 5-6).

Figure 5-8. Example Wireless USB Burst Data Phase Time Slot Layout

Figure 5-8 also illustrates the timing constraints for protocol time slots immediately following an MMC packet. When the protocol time slot following an MMC is an OUT the MMC and the first data packet in the OUT time

slot must be separate by tINTERSLOTTIME. When the protocol time slot following an MMC is an IN, the MMC and the first data packet being transmitted by the device must be separated by tBUSTURNINTERSLOTTIME.

5.3.1 Streaming-Mode Inter-packet Constraints for the PHY

The PHY standard defines a strict set of rules for implementing streaming-mode data transmissions. A summary of the rules are repeated below. The final standard authority on differences between this specification and the PHY standard is the PHY standard.

The first data packet in a Wireless USB data burst (i.e. multi-data packet data phase) must always use a standard

(length) preamble. All packets in a Wireless USB data burst must be separated by tSTREAMIPG. For data rates of 200Mb/s and lower, all data packets of the burst must use a standard preamble. For data rates greater than

200Mb/s, the host may use streaming mode preambles for OUT data phase bursts and will instruct via the bmTXAttributes.Data Burst Preamble Policy WDTCTA parameter the pattern of streaming and standard preambles to use for the device’s burst transmission during the associated protocol time slot.

5.3.2 Protocol Synchronization

All Wireless USB Protocol timings are specified relative to the beginning of the preamble for the MMC packets. Figure 5-9 illustrates Wireless USB protocol synchronization and relative reference points.

Figure 5-9. Protocol Timing Relative to MMC

Devices reset their protocol clocks to zero at the beginning of an MMC preamble. All channel time offsets (nextMMC and WXCTA time slot allocations) in the MMC are specified by the host relative to the start of the preamble for the current MMC.

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