[OZAPRS] FW: [aprssig] APRS Channel Capacity...

[Richard Hoskin]

The email from Bob Bruninga below is a good estimate of what the average
APRS RF channel can handle before it is considered to be overloaded. As
you
will read it does not take a lot to overload the channel if APRS Station
Operators do not understand the system.

As a general rule a channel is considered at maximum capacity when it
averages 10 packets per minute over the period of an hour. (This
measurement
must be taken from the main digi (VK3CV-1 in Victoria's case)to get an
accurate count.)

Cheers
Richard.

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--

Here is another stab at defining channel capacity on APRS and showing how
totally overloaded the channel is:.

Lets say we have about 50 stations per digi transmitting at typical rates.
This should result in only about 11 packets per minute.  Easily handled
by an APRS digi...

  30 Home stations at 1 pkt/30 mins   =  1
   6 WX stns at 1 every 12 mins       = .5
   6 local mobiles at 1 every 2 mins  =  3
   6 mobiles at 1 every minute        =  6
                                        11 packets per minute

But wait, what about the same density in the three surrounding digis
with each of them having another 45 users:

  30 Home stations at 1 pkt/30 mins   = 1        1
  30 homes out of area with 2 hops    = 1 x 3 =  3
   6 WX stns at 1 every 12 mins       =.5        0.5
   6 WX stns adjacent with 2 hops     =.5 x 3 =  1.5
   6 local mobiles at 1 every 2 mins  = 3        3
   6 adjacent mobiles at 1 every 2    = 3 x 3 =  9
   6 mobiles at 1 every minute        = 6        6
   6 1 min mobiles at 2 hops          = 6 x 3 = 18
                                                42 packets per min

But wait, most people like to run 3 hops!  So sorrounding each of
those digipeaters there are at least two other digis beyond that with each
one of them supporting their 50 users as well.  This is the result:

  30 Home stations at 1 pkt/30 mins   = 1     =  1
  30 homes out of area with 2 hops    = 1 x 3 =  3
  30 homes out of area with 3 hops    = 1 x3x2=  6
   6 WX stns at 1 every 12 mins       =.5        0.5
   6 WX stns adjacent with 2 hops     =.5 x 3 =  1.5
   6 Wx stns adjacent with 3 hops     =.5 x3x2=  3
   6 local mobiles at 1 every 2 mins  = 3        3
   6 adjacent mobiles at 1 every 2    = 3 x 3 =  9
   6 adjacent 3 hop mobiles 1 evy 2   = 3 x3x2= 18
   6 mobiles at 1 every minute        = 6
   6 1 min mobiles at 2 hops          = 6 x3  = 18
   6 1 min mobiles at 3 hops          = 6 x3x2= 36
                                              -----
                                    TOTAL     =100 packets give or take

And given that every packet has to be digipeated, it takes about 1.5
seconds for every packet. For a total channel capacity of about 40 packets
per minute.  GIven that the stated aloha channel capacity
for lets say about 90% reliability has been shown to be about 18% of
channel capacity, this means, a channel is operating at optimum when there
are about 8 packets per minute.

Notice that this example results in an order of magnitude overloading of
the channel.  100 packets trying to get through when only about 8 or 10
can do so reliably.  Now even if lets say that every single packet somehow
is perfeclty timed so as to minimize any possible collisions so that you
can ignore the 18% ALOHA channel capactiiy.  The result is then 40 packets
per minute, and still there is 2.5 times that much traffic, and this is
only to support 30 home stations and 12 mobiles per digi!

THink about it.  ALso, think about those folks blasting out WIDE4-4 or
WIDE5-5 packets...(in populated areas)...

I should re-do this again, since there are not that many mobiles.
Probably droping them by a factor of 3 is about right and then the channel
would only be about overloaded by a factor of two or so.

Notice that this says NOTHING about the area of coverage.  It doesnt
matter.  No matter how big or how high the digi is, it can still only
siupport these numbers of users reliably.  If it has any more active
stations in its coverage circle, then you have collisions and you are not
hearing everythign that is out there...

So I still feel that the channel capacity per digi for "typical" aprs
operations certainly cannot be more than about 36 or so... and still
expect any kind of reliability.  Notice also, that the above analysis
included NO MESSAGES and NO ACKS...

...
de WB4APR at amsat.org, Bob


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