User's Manual

XTend™RFModule‐ProductManual
©2013DigiInternatonal,Inc. 52
Acknowledged Communications
Acknowledged Mode
Attributes:Reliable delivery through positive acknowledgements for each packet
Throughput, latency and jitter vary depending on the quality of the channel and 
the strength of the signal.
Required Parameter Values (TX module): RR (Retries) >= 1
Related Commands: Networking (DT, MK, RR), Serial Interfacing (PK, RN, RO, RB, TT)
Recommended Use: Use for applications that require Reliable Delivery. If messages are smaller
than 256 bytes, use RB and RO commands to align RF packets to application packets.
Connection Sequence
Figure4‐07. AcknowledgedModeState
Diagram(TXmodule)
After sending a packet while in
Acknowledged Mode, the TX
(transmitting) module listens for an
ACK (acknowledgement). If it
receives the ACK, it will either move
on to sending a subsequent packet
(if more transmit data is pending) or
will wait for exactly RN random delay
slots before allowing another
transmission (if no more data is
pending to be transmitted).
If the TX module does not receive
the ACK within the allotted time, it
will retransmit 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 module, there are
never any back-off delays between
retransmissions. Note that during
back-off delays, the TX module will
go into Idle Mode and may receive
RF data. This can have the effect of
increasing the back-off delay, as the
module cannot return to Transmit (or
retransmit) Mode as long as it is receiving RF data.
After receiving and acknowledging a packet, the RX (receiving) module will move to the next
frequency and listen for either a retransmission or new data for a specific period of time. Even if
the TX module has indicated 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 module will always detect the immediate retransmission, which will hold off the
communications channel and thereby reduce collisions. RX modules acknowledge each
retransmission they receive, but they only pass the first copy of a packet they receive out the
UART.
RB and RO parameters are not applied to subsequent packets, meaning that once transmission
has begun, it will continue uninterrupted until the DI buffer is empty or the streaming limit (TT
parameter) has been reached. As with the first packet, the payload of each subsequent packet
includes up to the maximum packet size (PK parameter), and the TX module checks for more
pending data near the end of each packet.