Murata Electronics Corporation 7 6/2/2021
more is necessary. An added benefit of using the power save mode to set a duty cycle is
improved average current consumption efficiency. Refer to the Status Commands section for
details of this command.
When setting up a TDMA network, keep in mind that time slot length, maximum packet size
and hop duration are all interrelated. The hop duration parameter will determine the time
slot size and the maximum amount of data that can be transmitted per hop by the remotes.
There is a hard limit of the absolute maximum amount of data that can be sent on any given
hop of 212 bytes regardless of any parameters. The base station requires 1.7 ms overhead for
tuning, the synchronization signal and parameter updating, as well as a guard time of 500s
between each remote slot. Thus the amount of time allocated per remote slot is roughly:
hop duration – base slot – 1.7ms - ( # of registered remotes-1)∙500s
( # of registered remotes)
Take for example a network comprised of a base station and 10 remotes. A hop duration of
10 ms is chosen. We decide that the base station needs to be able to send up to 32 bytes each
hop (equivalent to a capacity for the base of ~ 32 kbps). Counting the 1.7 ms overhead for
the base packet and making use of the fact that our RF rate is 460.8 kbps, we determine that
the base slot requires approximately:
Each remote time slot will be:
10 ms – 2.3 ms – (9)∙0.5 ms
10
From our RF data rate of 460.8kbps we see that it takes 17.36 s to send a byte of data, so
each remote will be able to send up to
= 18 bytes of data per hop.
Note that the 18 bytes is the actual number of data bytes that can be sent. If the WIT2420 is
using a protocol mode, the packet overhead does not need to be considered. So in this
example, the total capacity per remote would be:
If we figure a minimum margin of safety for lost packets and retransmissions of about 20%,
we see that this would be more than sufficient to support 14.4 kbps of continuous data per
remote. It is also useful to remember that the asynchronous data input to the WIT2420 is
stripped of its start and stop bits during transmission by the radio, yielding a "bonus" of 10/8
or 25% in additional capacity.
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