[aprssig] alt input digipeater implementation (brain storm)
aprs at kd4rdb.com
Thu Mar 31 09:41:03 CST 2005
Just to get people thinking... here are a 7 ways to implement an
alternate input frequency digipeater. It can be extremely simple or
very tightly coordinated. In the end, it is up to the owner of the
digipeaters to weigh the pros and cons of each method. This gets to be
like cars.... Speed (network reliability) is money, how fast do you want
The whole point to the alt input digi is to give the locals a clear shot
into the local digipeater... a priority "back door" into their local
digi with less congestion and less of a chance of colliding with another
In any case, try to use a digipeater that is capable of callsign
substitution so you can easily identity the entry point in the network.
We'll start with the cheap-charlie method. This will work areas that
have little locally generated traffic, and lots of DX coming in.
1)Take a digi in your area with reasonable coverage, and simply program
the radio to listen on 144.99 with a -600 offset. This method will
stomp over any traffic on 144.39 when it transmits, but if the users in
the area decide that they can live with the loss of the occasional DX
2)Take a digi in your area and program it as above 144.99/-600. Place a
2nd reciever on site that can hear 144.39 and use it's squlech
indication hooked to the XCD pin on the digi's TNC. Disadvantage: your
TNC can't hear any packets on 144.99 that come in while it's digipeating
a packet out on 144.39.
3)Hook a TNC up to a receiver on 144.99, and a transceiver on 144.39.
Create a Y cable for the TNC to allow it to connect to two radios... one
to listen, one to TX. Use the 144.39 radio squelch indication hooked to
the tnc's XCD pin. Use 2m 600khz cavities just like voice repeaters
do. Disadvantage: duplexer cavities are expensive. Advantage: the TNC
can contine to receive packets while it's digiepeating them on 144.39.
4)Use 2m 600khz duplexer cavities to allow simultaneous operation on
144.39 and 144.99. Use a multiport digi program such as digined or
ui-view to collect data from a TNC on 144.99, and also provide full
serivce digi on 144.39. Disadvantage: duplexers are expensive.
Advantage: everything is at one site.
5)Create a cross band digi. TNC listens on 144.99, transmits on
440mhz. Add a 440mhz receiver to your local 144.39 digipeater site
(which is a few miles away). Use multiport digipeating program such as
digined at the main digipeater site. Disadvantage: must make changes to
the local wide digi.
6)The cross band digi can be extended to other satellite recieving sites
on 144.99 all digipeating data on 440mhz uplink to the local digi. If
all 440mhz stations can hear each other, they can avoid keying over one
another. Advantage to this system is that if two cars on opposite sides
of town TX on 144.99 at the same time, both packets get delivered to the
digi via 440mhz. Since the two alt input sites can hear each other on
440mhz, the first one to digipeat will cause the 2nd to hold off. This
is expandable until the remote stations stop being able to hear each other.
7) If we extend idea #6 to the point that all stations on 440mhz can't
hear each other (let's say they are so far from the main digi that they
have to use a beam to get to it, and thus can't hear other stations off
to the side of their beam), then we need to implement some sort of
slotting system. This could be down by using a synched clock source
such as GPS, or clock derived from a local "good time" FM radio station,
and using a (ahem) basic stamp to assert XCD on the TNC and release it
when that TNC's coordinated "slot time" comes around. For example, if
you have 5 remote reciever sites, you'd program each one to transmit
every 5th second (assuming packets are one second long, tops). A DAMA
variantion would also work here... the central digi could poll all the
remote stations for traffic instead of allowing them to depend on clock
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