Murata Electronics North America 910M Benutzerhandbuch

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 7.04 ms overhead for 
tuning, the synchronization signal and parameter updating, as well as 1.11 ms overhead 
for each remote.  Thus the amount of time allocated per remote slot is roughly: 
 

hop duration – base slot – 7.04ms - ( # of registered remotes)·1.11ms 

( # of registered remotes) 

 
Take for example a network comprised of a base station and 5 remotes.  A hop duration 
of 25 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  19.2 kbps asynchronous).  
Counting the 7.04 ms overhead for the base packet and making use of the fact that our RF 
rate is 172.8 kbps, we determine that the base slot requires approximately: 

 
 
 

32·8 

172.8kbps 

+ 7.04 ms  =  8.52 ms 

 

Each remote time slot will be: 
 
    25 

ms 

– 

8.52 

ms 

– 

(5)·1.11 

ms 

   = 2.18 ms 

5 

 
From our RF data rate of 172.8kbps we see that it takes 46.3 

µs to send a byte of data, so 

each remote will be able to send up to    
 
 

 

 

 

 

 =  47 bytes of data per hop. 

2.18 ms 

46.3

µs us

 
However, the WIT910 sends data in groups of 4 bytes. Thus, each remote will be able to 
send 44 bytes of data. Note that the 44 bytes is the actual number of data bytes that can be 
sent.  If the WIT910 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: 
 

44 bytes 

25 ms

 

=  14.08 kbps 

 
It is also useful to remember that the asynchronous data input to the WIT910 is stripped 
of its start and stop bits during transmission by the radio, yielding a "bonus" of 10/8 or 
25% in additional capacity. Thus, 1.25 x 14.08 kbps = 17.6 kbps asynchronous. In actual 
deployments, some allowance must be made for retransmissions of data, yielding a 
throughput somewhat less than the calculated value.   
 
The above calculations are provided as a means of estimating the capacity of a multipoint 
WIT910 network.  To determine the precise amount of capacity, you can actually set up 

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