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RS‐232/485 RF Modem Product Manual – MAN0782‐02
Acknowledged Mode
Acknowledged Mode Connection Sequence
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.
Events up through the
'Transmit Packet' process are
common to all three RF
Communication Options. Refer
to the Transmit Mode section
[p 18] for more information.
After sending a packet while in
acknowledged mode, the
transmitting modem listens for
an 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 transmitting 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 transmitting modem,
there are never any back-off
delays between retransmissions.
Note that during back-off delays, the transmitting modem will go into Idle Mode and may receive
RF data. This can have the effect of increasing the back-off delay, as the radio cannot return to
RF transmit (or retransmit) mode as long as it is receiving RF data.
After receiving and acknowledging a packet, the receiving modem will move to the next
frequency and listen for either a retransmission or new data, for a specific period of time. Even if
the transmitting modem has indicated that it has no more pending transmit data, it may have not
received the previous ACK, and so may retransmit the packet, possibly with no delay after the
ACK slot. In this case, the receiving modem will always detect the immediate retransmission,
which will hold off the communications channel and thereby reduce collisions. Receiving modems
acknowledge each retransmission they receive, but they only pass the first copy of a packet they
receive out the UART.
Figure 3.13. Acknowledged Mode State Diagram
Page 25
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