[OZAPRS] Analysis of VK1 APRS traffic.

[Carl Makin]

Hi Bob,

On 16/02/2007, at 12:15 PM, Robert Bruninga wrote:

>> 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...
>
> Not meaning to nit-pick, but...

Ahh, I wondered if that would elicit comments. <grin>

> I'm not sure about the details of your network, but APRS is
> generally not a CSMA network to the Users.

Yes, I agree.  The more I thought about it (after I sent the email  
<sigh>)  I realised it only approached that ideal on a network where  
everyone can hear each other. The hidden node problem upsets the  
apple cart considerably.  I suspect it performs slightly better than  
pure ALOHA since there is, as you said, a small amount of carrier  
sense happening and the p-persistance setup probably punts it a  
little towards "slotted ALOHA" in performance.  Even so I wouldn't  
expect it to reach much higher.

> Further, in a large area with lots of surrounding digis (We have
> on the order of 50 in the surrounding 200 miles, then 95% or

We're a *lot* more sparsely populated that your area!  I'd guess we  
would have less than 10 in the surrounding 200 miles, and they are  
mainly spread in a north<->south line.

> So APRS is more like a NON-CSMA ALOHA network.  Hence, the
> throughput is about 18%, which is the theoretical for an ALOHA
> channel.

Interesting then that our utilisation is around 18% eh?

> Again, I am completely blind to your area, so ignore this next
> comment if it does not apply.  But if I was in an area with a
> very tall mountain top digi that covered a well populated area
> very well, and yet that digi was seeing too much from out of
> area (it had constant QRM on its input that none of the local
> users can hear), one thing to drastically give priority to local
> users is to put an input on a LOCAL frequency that digipeats to
> the MAIN frequency.  This way, the local users get priority
> every time.

It is one of the highest digis in the country actually and it has  
excellent coverage, but I think we have the reverse problem, in that  
the main source of frames is local and very strong to the digipeater  
making it very hard for more distant stations to get in.  However I  
like the idea of segregating the IGate traffic.  That will definitely  
require more thought.

Thanks!


Carl.

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