[OZAPRS] Igate to RF

[Richard Hoskin]

The AIM of APRS is to have one National 2mtr frequency that will allow a
mobile station when ever it is in range to access the network.

The operator of the station does not need or want to constantly change
frequencies to 'get in' when travelling out of his / her local area. This is
why 145.175 Mhz was established as the primary ARPS channel.

The UHF APRS frequency of 439.100 MHz was established to provide an APRS RF
channel for Home stations with additional services that are not suitable for
the VHF channel due to band wide limitations. This will leave the VHF
channel available for Mobile & Country station.

To accommodate the requirements of all and fulfil the primary objective of
the National 2mtr frequency for APRS the following strategies where adopted
by VK3

Victorian APRS Network Design

I thought this may be helpful for others who are building up APRS networks.

Here is what we did to plan the APRS network here in VK3.

First asked ourselves the following questions and came up with the listed
answers;

1) Q: What APRS stations are most important for a local station to see on
his map?

    A: Local stations. They have more meaning for us than stations in other
states, Europe or the USA.

2) Q: What APRS stations are next in importance for a local station to see
on his map?

    A: Neighbouring local stations, other Australian stations followed by
New Zealand station followed by the rest of the world.

3) Q: What is the biggest single limitation in an APRS network?

    A: RF Bandwidth.

4) Q: What volume of traffic do the local station generate on RF over a 10
minute period?

    A: In VK3 over 10 minutes it is measured at approx. 30 to 40 packets.

5) Q: What is a reasonable volume of traffic that will not overload the RF
channel and allow mobile stations access to the RF network when required.

    A: Approx 100 packets per 10 minutes on average. Exceeding this average
greatly reduces the chances of a mobile station from accessing the RF
network especially if it is in a poor coverage area.

6) Q: How much additional traffic can we let onto the RF network?

    A: Approx. 60 packets per 10 min.

7) Q: Where do we get these packets from?

    A: The next priority the local stations want to see.

We found that the total volume of traffic that came through the IGate from
500km radius of Seymour was about right to keep the local channel running on
average at 100 packets per 10 min. If the traffic volume on the RF network
increases we can reduce the amount of traffic coming through the IGate by
reducing this radius.

The rate of 100 packets per min assumes that each packet is approx. 80 bytes
in length with a TXDelay of the digipeaters set to 35 mS and a TXTail of
4mS. If the size of the average packet increases then you will need to
decrease your networks packet rate. If the packet size decreases (enable
compression in UI-View, reduce status text etc.) you can increase your
packet rate.

 One of the biggest generators of traffic on a local RF network is
digipeatered packets. Each time a packet is digipeated it reduced the
available RF bandwidth by half. If an IGate is sending 5 packets per minute
on to the RF network through 2 digipeaters the IGate is in effect creating a
traffic level of 15 packets per minute. (IGateTx + Digi1Tx + Digi2Tx) * 5

 In VK3 the APRS network has been designed with the following
considerations.

1) All Home station will be a maximum of 1 digipeater hop from the centre of
the RF network.

2) The TX/TX IGate be placed as close to the centre of the network as
practical.

3) All home station will have a maximum of 2 digipeater hops to access an RX
IGate.

4) Home stations will have a maximum of 3 digipeater hops to get from one
side of the RF network to the other.

5) If greater RF coverage is required a new RF network is to be used with a
new RX IGate.

6) Beacon transmission times: Home, Stationary stations & Objects 30 min,
Weather stations 15 min, mobile station 2 min.