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A Digipeater is a station that does digital repeating. In classic packet radio each such hop had to be explicitly defined by the sender. In APRS all digipeaters use generic aliases. The sender does not have to know the callsign of the digipeater but just indicate the number of hops he wants to go.



The Digipeating Path and Development of WIDEn-N

If there are multiple aliases in the PATH list, these fields are used only in the order left-to-right. An asterix * is used to show that a element in the path is no longer active. In the path ABCD*,EFGH,IJKL only the EFGH path element is active because the previous path element ABCD* is "used up". And the IJKL is not yet active because it is not the left-most active field.



Local and Wide Digipeaters

As with voice VHF repeaters, digipeaters are installed in places of prominence to allow for extended range. If a small station can "hit" the digipeater, his packet will go much farther then his own station can reach. Local digipeaters (called **FILL-IN** digipeaters) are usually located in blind spots where mobiles are unable to be heard by the main digipeaters. THese **FILL-IN** digis only respond to the **WIDE1-1** path. This keeps them from needlessly repeating all other traffic from the high digis.

Digipeating example


A nearby station EFGH is configured to respond to WIDE1-1, so he does so. He decrements the second number (the SSID) making -1 into -0 and retransmits the packet with path EFGH,WIDE1*, WIDE2-2. The convention is that -0 is not displayed and when a path element is used up then a * is appended. A WIDEn-N digipeater will also insert its own callsign into the path before the WIDEn-N path element that it acted on so that the path is traceable through all the digipeaters that relayed it. In this example the packet now includes EFGH showing it passed through station EFGH.

Another station IJKL (configured to respond to WIDEn-N) picks it up and sends it out with EFGH,WIDE1*,IJKL,WIDE2-1. The destination WIDE2-1 still has one hop so it is not yet marked with a * and this packet is available for further digipeating.

Then station MNOP (configured to respond to WIDEn-N) hears it. Because WIDE2-1 is still active, he repeats it back out as EFGH,WIDE1*,IJKL,MNOP,WIDE2* and that's it. Since there are no more active path elements any digipeating station receiving that packet will not repeat it.


Dupe Checking

You might be thinking, "Hey, what's to keep //EFGH// from hearing that packet that //IJKL// sent out? //EFGH// would send it out again, //IJKL// would pick it up and send it out again. This thing could just loop around and not really get anywhere." To solve this, all modern digipeaters have //dupe-checking// algorithms.

    • WIDEn-N** Digipeaters have some **memory** of what has gone through them recently. This is known as dupe checking. Otherwise they would spend a lot of their time just booming packets back and forth amongst each other and the little pip-squeak tracker stations would never be heard at all. The dupe checking algorithm in the digipeater will act to suppress re-transmission of packets that have recently been transmitted. The main paramenter of this **memory** is in the form of a time specification i.e. the number of seconds that received packets are kept around for comparing to incoming packets.

Other details of the //New-N Paradigm// include settings to TRAP abusive user paths with large values of **N**. Usually only **WIDE2-2** is recommended in most high density areas and surrounding territority.

Suggested Setup for a Mobile/Fixed APRS Station

Please read suggested setups for Paths.

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