Linx Technologies TRM-915-R250 - Manual (Page 13)

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Extended Preamble
As the receivers scan the hop sequence channels they look for the
preamble from a transmitter. When they detect this preamble, they stop
scanning and wait for a packet header. From the packet header they are
able to lock on to the transmitter and synchronize the timing.
It is an advantage in some applications to keep the receivers asleep for
long periods of time and wake them up only periodically to look for data. In
this scenario it is quite possible for the receivers to miss the preamble from
the transmitter and go to sleep without finding the transmission.
To address this issue, the modules can send an extended preamble. The
extended preamble is much longer than the normal preamble and gives
modules a larger window to detect and lock on to the transmitter.
There are two types of packets sent by the transmitter: synchronizing
packets and data packets. The receivers use the synchronizing packets
to lock on to the transmitters and follow them through the next hop. This
packet is the first packet sent after every hop. The packets sent after the
synchronizing packet and before the next hop are data packets. Receivers
can catch and process data packets, but do not lock on and follow the
transmitter unless they process the synchronizing packet.
It is important to note that an extended preamble packet may not be
a synchronizing packet. If it is a data packet the receiver processes
the packet and begins scanning again. It is important for the external
processor or application to keep modules awake long enough to catch the
synchronizing packet on the next hop. This ensures that the modules lock
on to the transmitting module and receive all of the data.
The dwell time on each channel is approximately 395ms plus the times per
baud rate shown in Figure 39. This can be shorter if the host processor
determines that the necessary information has been received. Additionally
the PR_PKT line can be used to determine that the module is processing a
packet and keep it awake.
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