| 摘要 |
<p>Devices (eg Digital cameras) may include short distance (eg. 10s of centimetres) communication systems which may transmit unicast (one to one) or multi/broadcast (one to many) signals. The mode may be indicated by a flag bit in a frame header. The receivers may be synchronised by ensuring that the sampling period is less than the maximum delay of the transmission channel. In both modes there is a single synchronised acknowledgement (ACK) slot. In multicast mode all receiving devices use the same common multicast ACK signal pattern. Due to the receiver synchronisation the combined signals will still be a valid ACK at the transmitter (Fig. 6). In a unicast mode a receiver generates one of multiple unicast ACK signal patterns. If multiple devices receive a unicast frame their ACKs will probably collide and interfere to produce an invalid ACK response (Fig. 7). Hence, devices overhearing unicast transmissions may be detected from the repeated ACK failures. In both modes failed reception can be indicated with a negative ACK (NACK).</p> |
