WIT2410
©
2000- 2005 Murata
Inc
9
M-2410-0000 Rev G
When setting up a 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. (Note that this is different than the
208 byte maximum for the base station.) The base station requires 1.7 ms overhead for
tuning, the synchronization signal and parameter updating, as well as a guard time of
500
s 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)∙500
s
( # 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 WIT2410
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
WIT2410 is stripped of its start and stop bits during transmission by the radio, yielding a
"bonus" of 10/8 or 25% in additional capacity.
The above calculations are provided as a means of estimating the capacity of a multipoint
WIT2410 network. To determine the precise amount of capacity, you can actually set up
= 0.32 ms
0.32 ms
17.36
s
us
32∙8
460.8kbps
+ 1.7 ms = 2.3 ms
18 bytes
10 ms
= 18 kbps
