Acknowledged Mode Connection Sequence - Digi XTend User Manual

Acknowledged communications: Acknowledged mode

Acknowledged mode connection sequence

After sending a packet while in Acknowledged mode, the TX (transmitting) device listens for an acknowledgment
(ACK). If it receives the ACK, it either moves on to sending a subsequent packet if more transmit data is pending or
waits for exactly RN random delay slots before allowing another transmission if no more data is pending transmit.
If the TX device does not receive the ACK within the allotted time, it retransmits the packet with a new RF initializer
following the ACK slot. There is no delay between the first ACK slot and the first retransmission. Subsequent
retransmissions incur a delay of a random number of delay slots, between 0 and RN. If RN is set to 0 on the TX device,
there are never any back-off delays between retransmissions. During back-off delays, the TX device goes into Idle
Mode and may receive RF data. This can have the effect of increasing the back-off delay, as the device cannot return
to Transmit (or retransmit) mode as long as it is receiving RF data.
After receiving and acknowledging a packet, the RX (receiving) device moves to the next frequency and listens for
either a retransmission or new data for a specific period of time. Even if the TX device indicates that it has no more
pending transmit data, it may not have received the previous ACK, and so may retransmit the packet, possibly with no
delay after the ACK slot. In this case, the RX device always detects the immediate retransmission, which holds off the
communications channel and reduces collisions. RX devices acknowledge each retransmission they receive, but they
only pass the first copy of a packet they receive out the UART.
The device does not apply the RB and RO parameters to subsequent packets, meaning that once transmission begins,
it continues uninterrupted until the DIN buffer is empty or it reaches the streaming limit (TT parameter. As with the
first packet, the payload of each subsequent packet includes up to the maximum packet size (PK parameter), and the
TX device checks for more pending data near the end of each packet.
The TT parameter specifies the maximum number of bytes that the TX device sends in one transmission event, which
may consist of many packets and retries. If a device reaches the TT parameter limit, the TX device forces a random
delay of 1 to RN delay slots (exactly 1 delay slot if RN is zero). Each packet counts only once toward TT, no matter how
many times the packet is retransmitted.
Subsequent packets in Acknowledged mode are similar to those in Streaming mode, with the addition of an ACK
between each packet, and the possibility of retransmissions. The device sends subsequent packets without an RF
initializer, as the RX devices are already synchronized to the TX device from the preceding packet(s) and they remain
synchronized for the duration of the transmission event. Each packet retransmission includes an RF initializer.
Once the TX device sends all pending data or reaches the TT limit, the acknowledged transmission event is complete.
The TX device does not transmit again for exactly RN delay slots, if the local RN parameter is set to a non-zero value.
The RX device does not transmit for a random number of delay slots between 0 and (RN-1), if the local RN parameter is
set to a non-zero value. The intent of these delays is to lessen congestion following long bursts of packets from a
single TX device, during which several RX devices may have themselves become ready to transmit.
XTend RF Module User Guide 73