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 Europe or the USA.

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

    A: 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 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 VK* and ZL* 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.

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 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 the 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 IGate.

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

From these considerations we came up with a star configured network that roughly a 200km radius depending on local terrain.

The design we used for VK3 may not work in other locations as the local terrain has a large part to play in the RF network design. It takes a lot of hard work and a bit of luck to get access to the Wide digipeater sites that your network will benefit most from.

Richard Hoskin

VK3JFK