[OZAPRS] Analysis of VK1 APRS traffic.

[Carl Makin]

Evening All,

Prompted by Ian's tale of problems getting into Mt Ginini while  
mobile I wrote a small bit of perl to connect to the TNC port of my  
IGate and collect some simple channel stats.

Over a 46 hour period I got the following statistics;

Sampling interval: 60 seconds.

Max frames per interval: 		53
Min frames per interval: 		0
Average frames per interval: 	13
Total Frames: 				38,476.00

Max bytes per interval: 		4,642.00
Min bytes per interval:			0
Average bytes per interval:	1,359.46
Total Bytes: 					3,786,085.00

Max stations per interval:		6
Min stations per interval:		0
Average stations per interval:	2
(no Total as I didn't collect that data)

Interestingly the Average and Median stats were pretty much identical.

Here is where I'm starting to assume things.  I think 1200 baud gives  
us about 120 bytes per second, or 7200 bytes per 60 second interval  
maximum.  Based on that I worked out;

Min channel utilisation: 		0
Max channel utilisation:		65%
Average channel utilisation:	18.88%
Median channel utilisation:	18.64%
then I rounded all the individual utilisation figures to whole  
numbers to get the mode, which was 19.

Graphing the data shows no major deviations from the above values.

So, what does this tell us?  I'm not sure. :(  The 65% utilisation  
figure was a anomaly reached on only 1 interval and would seem to set  
a maximum utilisation rate for the channel.  There are 2 digipeaters  
within the coverage of my IGate, however only one of them, Mt Ginini  
is used by the two IGates.  The vast majority of the frames  
originated at VK1NPW, which is the primary IGate and were then  
digipeated by VK1RGI (Mt Ginini).  In fact it's pretty safe to say  
that nearly all frames transmitted in the area are repeated by VK1RGI.

Hunting on the net it looks like 50% to 65% is a reasonable maximum  
utilisation rate for a p-persistence CSMA network like our APRS  
network however other sites indicate you should divide the maximum  
throughput for the channel by the number of users. If that's the case  
then our channel utilisation averages 39% which would certainly make  
it hard for mobile stations to get into Mt Ginini from outside the  
ACT (ie weaker signals) especially as both Peter, VK1NPW and I are  
quite strong into Mt Ginini.

Comments?


Carl,
vk1kcm.

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