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.
Digipeating is defined in AX.25 specification document. An initiating station can include in each packet a list of intermediate stations which are supposed to digipeat the packet, passing it on toward the desired destination. The list of intermediate stations is called the path. Stations receiving packets examine the path of received packets. Using it's configuration information a receiving station can decide if a packet is a candidate for digipeating. When this is true the packet addressing information is modified slightly and the modified packet is transmitted.
The idea is to get packets to propagate outward from the initiating station... but not so far as to be an unnecessary burden on the system and surrounding users. The practical range of a VHF 1200 baud APRS channel is estimated to include the nearest 60 stations (approximately), called the Aloha Circle.
The Digipeating Path and Development of WIDEn-N
If there are multiple aliases in the PATH they are active 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 An asterix * is used to show that a element in the path is no longer active.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.
When APRS was first invented the generic calls of RELAY, WIDE and TRACE were used for all digipeaters, but there was no duplicaton suppression mechanism. Paths beyond 2 hops could fold back ending up with dozens of unwanted duplicate packets. By 1994 a WIDEn-N and TRACEn-N scheme was proposed by WB4APR. It was finally implemented in 1998 by Kantronics, providing perfect dupe suppression and becoming the standard and most efficient way of operating. When a digipeater sees an "n-N" packet, it digipeats it and decrements the "N" by 1. This continues until the "N" is zero.
Up until 2004 APRS in the USA was using a mixture of the old generic paths and the new n-N type paths and the presence of the old paths was causing terrible QRM. TO combat these inefficiencies, WB4APR launched the New-N Paradigm to phase out all the old legacy paths and greatly simplify paths to simply WIDEn-N and to make these paths 100% traceable. For current recommended paths, see the Paths page.
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.
Station ABCD sends a packet with path WIDE1-1, WIDE2-2.
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.
Again, the list of destinations are consumed in left-to-right order. Path tracing allows digipeaters to insert their callsigns prior to the active path as a way of tracking the path the packet actually took through the system. The numbers after the dash in the WIDEn-N format are decremented until they reach -0 and disappear altogether. Destinations that are all used-up are marked with a *.
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 parameter of this memory 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 territory.
Suggested Setup for a Mobile/Fixed APRS Station
Please read suggested setups for Paths.