[OZAPRS] APRS RF Bandwidth @ 1200 baud

[Richard Hoskin]

Gday,

 

Here is an article I wrote some time ago to help people understand the
limitations of APRS and packet radio.

I hope it is educational, answers a few people's questions and assists in
your understanding of the impact beacon rates have on the RF network.

 

APRS RF Bandwidth @ 1200 baud

Here is an explanation of APRS and the effects of channel traffic on it's
performance. (Bandwidth)

In this example a mobile stations transmission rate is set to 30 sec.

(Not recommended in practice)

Packets are transmitted at a maximum speed of 1200 bits per second.

So assuming an APRS posit (position packet) is 50 bytes in size from a non
compressed tracker or IGate (IGates decompress compressed packets before
retransmissions) the packet will consists of 400 bits of data plus (from
memory) 296 bits for ax.25 protocol overheads.

Total number of packet bits 696

The time taken to transmit a packet is DataTime + TXDelay +TXTail In this
case 0.58 + 0.35 +0.04 = 0.97 Sec ~ 1 Second per packet.

This packet is transmitted 3 times assuming the standard mobile path of
RELAY,WIDE. So in effect this one packet is taking up 3 seconds of RF time
or available bandwidth.

Therefore you could theoretically have a maximum of 20 (60sec / 3) mobile
stations transmitting one posit every minute or 10 mobile stations
transmitting one posit every 30 seconds before all available bandwidth
would
be used. ( No transmissions from home stations or objects.)

In practice and due to the nature of ax.25 (as a CSMA protocol with hidden
transmitters etc.) the best bandwidth you could expect for maximum
efficiency (90% of txed data delivered from point A to point B) is 30% to
40% of the total bandwidth. So in reality you could have 4 mobile stations
transmitting posits through 2 hops (digipeaters) every 30 seconds before
you
start creating packet collisions and loosing data on the network. Or you
could have 24 stations transmitting through 2 hops every 3 minutes. This
again does not include home stations or objects.

This is how we get the figure of 100 packets (of 80 bytes) per 10 min as
an
average for the VK3 network. (equals 33% of the bandwidth)

As you can see packet radio is not all that efficient especially with a
large number of station or digis on a network. As APRS uses a the
non-connected (IU) feature of ax.25 it is possible to get away with using
bit more of the bandwidth (approx. 50%) with reasonable efficiency (maybe
70%). This would be considered the Peek Traffic Load of the RF network. If
the load went to around 60% of the bandwidth you would find that only one
in
5 or 6 of the txed packets would get through one hop. (In our mobile
example, txing one posit every 30 sec in this network would mean that a
posit would be rxed by the receiving station one digipeater away on
average
every 2.5 to 3 minutes. A receiving station two digipeaters away would rx
one posit every 5 to 6 minutes)

Note that all these figures are approximations to give you an
understanding
of how a packet network could perform. So much of the practical side of
the
systems is dependent on local conditions.

A few more thoughts:

At a tx rate of 30 sec you will get an updated position every 0.83 Kms
when
traveling at 100 kph, at 3min its 5kms per update. What is the resolution
and accuracy of the maps and equipment you are using? I Km per pixel, 500
mtrs per pixel or 1 mtr per pixel.

What are you using the APRS system for?

Search and rescue, Asset management, Race times and competitor locations,
Tracking friends and family for fun.

You need to take this in to consideration when you set your Beacon Rate.

Richard Hoskin

VK3JFK

  

 

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