9XTend-PKG-U™ USB RF Modem User’s Guide
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4.3. Acknowledged Communications
4.3.1. 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 modem): 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 modem)
After sending a packet while in
Acknowledged Mode, the TX (trans-
mitting) modem 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 trans-
mission (if no more data is pending
to be transmitted).
If the TX modem 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 modem, there are
never any back-off delays between
retransmissions. Note that during
back-off delays, the TX modem will
go into Idle Mode and may receive
RF data. This can have the effect of
increasing the back-off delay, as the
modem cannot return to Transmit (or
retransmit) Mode as long as it is receiving RF data.
After receiving and acknowledging a packet, the RX (receiving) modem will move to the next fre-
quency and listen for either a retransmission or new data for a specific period of time. Even if the
TX modem 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 modem will always detect the immediate retransmission, which will hold off the com-
munications channel and thereby reduce collisions. RX modems 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 modem checks for more
pending data near the end of each packet.
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