Digipeaters
  1. What is a Digipeater
  2. Advanced Features
  3. Setting up UI-View Digipeater
  4. Setting up Stand-Alone Digipeater

What is a Digipeater ?

 In order to extend the range of transmission, APRS and UI-View use a facility known as digipeating ('digitally repeating') - this facility operates like a voice repeater albeit digitally. Any packet that is received by a station which contains either its own callsign, or an alternative callsign (known as an 'alias') within a part of the packet known as the 'Digipeater Path' is automatically retransmitted.

Each packet has the provision for up to seven digipeater callsigns within the 'Digipeater Path' and therefore can be repeated up to a maximum of seven times.

RELAY and WIDE ?

APRS and UI-View make use of generic aliases of 'RELAY' and 'WIDE'. The reason for using generic aliases is so that the network is then accessible to any station within range of any digipeater - Using aliases other than'RELAY' and 'WIDE' are pointless, and will reduce the effectiveness of the network.

RELAY digipeaters are intended to increase the range of low-power mobile stations, and should be closely spaced, therefore every station should enable the alias 'RELAY'.

WIDE digipeaters, as the name suggests, are intended to cover a large areas and ideally are placed on high ground to provide maximum coverage - in a similar manner to existing voice repeaters - typically 50 miles apart. The wide area digipeater should have the generic alias 'WIDE' for the same reason as described above.

For example a mobile station using a TH-D7 hand-held sends a message using with the following 'Digipeater Path':

G4EJP-9>APK001,RELAY,WIDE

The nearest station will digipeat the packet because it contains 'RELAY' (The '*' below shows this packet has been digipeated).

G4EJP-9>APK001,RELAY*,WIDE

This message is received by the nearest 'WIDE' station, which digipeats it.

G4EJP-9>APK001,RELAY*,WIDE*

Thus with only three packets, a message from a station using a hand-held transceiver is delivered to all the users within a 50 mile radius!

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Advanced Features

Duplicate Removal

The range of transmission could be extended even further by amending the 'Digipeater Path' to 'RELAY,WIDE,WIDE,WIDE' - problems may occur when a packet is repeated several times back and forth between adjacent digipeaters (this is known as a 'ping-pong'). Worse still, this packet may be propagated out onto the network resulting in unwanted traffic.

Another possible cause of the ping-pong problem is an error in the firmware of some TNCs. Instead of simply adding the '*' to the last alias as the packet is digipeated, they remove all the preceding '*'s - potentially allowing the packet to be digipeated back along the path it came.

Eliminating duplicate packets can prevent this problem. The system looks for packets with identical contents which occur within a predetermined time-frame and selectively ignore the packets that occur within it - however this must be short enough to allow for retries.

Callsign Substitution

Another means of resolving this problem is to replace the generic callsigns 'RELAY' and 'WIDE' with the callsign of the digipeater as the packet is repeated, i.e.

G4EJP-9>APK001,RELAY,WIDE

The nearest station will digipeat the packet because it contains 'RELAY' (This is replaced with its own callsign 'G0VRM*').

G4EJP-9>APK001,G0VRM*,WIDE

This message is received by the nearest 'WIDE' station, which again digipeats it in the same manner.

G4EJP-9>APK001,G0VRM*,M1BLR*


WIDEn-n

Another solution to the problem described above is WIDEn-n digipeating. A packet enters the network with the 'Digipeater Path' of 'WIDE7-7'. Each time the packet is repeated, the SSID (the number after the '-') is decremented until it reaches '0', then the packet is ignored.

G0VRM>CQ,WIDE7-7
G0VRM>CQ,WIDE7-6
G0VRM>CQ,WIDE7-5

etc, until finally...

G0VRM>CQ,WIDE7

The problem with this approach is that the path taken to get to the destination is lost, the only way that a station can acknowledge message is to use 'WIDE7-7' and hope that the message eventually gets there - in addition to going everywhere else!

TRACEn-n

As the name suggests, this feature allows the route to be traced. For example, a packet entering the network with the 'Digipeater Path' of 'TRACE7-7' would be propagated as:

G0VRM>CQ,TRACE7-7
G0VRM>CQ,G4EJP*,TRACE7-6
G0VRM>CQ,G4EJP*,M0ABC*,TRACE7-5

until finally...

G0VRM>CQ,G4EJP*,M0ABC*,M1DEF*,G0GHI*,G6JKL*,G8MNO*,G6XYZ*

The disadvantage of this method is that packets get progressively longer as they are repeated, making this less ideal for DX (The packet is longer, hence there is more chance of errors or collisions occurring during the time taken to transmit it).

UI-View reduces unwanted traffic, as acknowledgements are sent using UI-View by reversing the order of the received callsigns

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Setting up the UI-View Digipeater

When running in KISS mode, UI-View supports a fully featured APRS digipeater. The following screen allows you to set the following parameters. The example settings shown below are for my station, which were copied from the accepted recommended settings.

  • Enable Digi - Enable Digipeater.
  • UI-Only - Only Digipeat UI Frames - YES.
  • Alias Substitution - Enable Alias Substitution.
  • WIDEn-n - Enable 'WIDEn-n' Digipeating.
  • TRACEn-n - Enable 'TRACEn-n' Digipeating.
  • Alias(es) - Digipeat on the following Callsigns.
  • Sub Alias - Substituting this callsign.
  • Dupe secs - Time frame for duplicates.
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Setting up a stand-alone Digipeater

Enabling Digipeater

There may be instances when you may wish to allow your station to operate as a stand-alone 'RELAY' digipeater, without monopolising your computer - for example when running a program that requires all of your CPU time.

Most TNCs provide the facility to operate as a simple digipeater. However, for this to work, it must be enabled - to setup a TINY-2 use the following commands:
(you will probably have to refer to the manual for other TNCs)

cmd: MYcall G0VRM (Substituting your own callsign!).
cmd: MYAlias RELAY
cmd: DIGipeat ON (Enable the Digipeater)
Now your TNC will digipeat any packet it hears with either your callsign, or the generic alias of 'RELAY' in the 'Digipeater Path'.

Setting Beacon Text

In addition to providing a stand-alone digipeater, you may wish to also make the location of your digipeater available to other APRS users. The simplest method is to simply surround your locator square with square brackets, then add the remainder of your beacon text - this has the advantage that your beacon is also readable by human users on conventional packet!
[IO93RS]Andy's RELAY Digipeater in Hessle.
The disadvantage of this type of beacon is that the location is only accurate to within one locator square and its format is incompatible with the Kenwood TH-D7 hand-held transceiver (showing as ??) on the display.

Alternatively, if you know your own latitude and longitude in Degrees, Minutes and Seconds from a map or a GPS Display. You can use a beacon text of the form:

!ddmm.mmN/dddmm.mmW#Andy's RELAY Digipeater in Hessle.
You will first need to convert the seconds into decimal fractions of a minute. Simply dividing by 60 does this:

The GPS displays your latitude as 53 Degrees, 43 Minutes and 59.9 Seconds North, this becomes 5343.99N in the beacon text - similarly, your longitude is displayed as 0 Degrees, 25 Minutes and 38.3 Seconds West, and becomes 00025.64W. Your text therefore becomes:

!5343.99N/00025.64W#Andy's Relay Digipeater in Hessle.
This format is compatible with all APRS systems

Enabling Beacon

Once you have defined your beacon text, you can then set your TNC using the following commands.

cmd: UNproto CQ,WIDE (Propagate to nearest 'WIDE').
cmd: BText <Your Beacon Text> (Described Above).
cmd: Beacon E 90
The position of your digipeater is now automatically transmitted once every 15 minutes to all users within range of the local 'WIDE' digipeater. This will appear on their screens as either compass bearing and distance, or as a location/waypoint on their map (depending upon the hardware/software used).

It is not necessary to propagate this information any further than the local 'WIDE' as it is of no interest to users beyond this range.

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A.D.Russell G0VRM/P.R.Sheppard G4EJP