WHY DIGIPEATING IS REQUIRED

"Digipeater" is short for "Digital Repeater"; a repeater for packet data rather than voice. Unlike the standard voice repeater that receives on one frequency and retransmits what it hears simultaneously on another frequency, the usual digipeater is a single frequency device. It receives a packet of data, stores it in internal memory and then a moment later retransmits it on the SAME frequency.

Digpeating is much more critical to APRS than to conventional packet because APRS heavily involves packet data transmission to and from moving vehicles. Traditional packet was overwhelmingly used between fixed locations, typically with better antennas and more power.

Signal levels that you may consider adequate on voice probably WON'T be adequate on packet, because data transmission is an all-or-nothing proposition. ALL of a packet has to be received PERFECTLY to recover ANY data from it. The kind of noisy, scratchy, not-completely-noise-free, operation so many people inflict on voice repeaters, especially with underpowered handhelds, JUST WONT WORK on data transmissions. A pop, a momentary burst of white noise, flutter, or multipath-induced phase distortion that you don't even notice on voice WILL be fatal to a packet transmission.

With APRS, the problem is even worse than with conventional (connected) packet because it operates in a non-connected mode. With traditional packet, a station receiving a defective packet will automatically send a request for retransmission to the sending station, or the sending station will automatically retry if the receiving station doesn't acknowledge in a reasonable time. With APRS there is no ACK/NAK (Negative Acknowledgement) handshaking process. The sending station just blasts out packets at intervals and "hopes" the receiving station(s) get them error-free. The receiving station just ignores the packet if it is defective in anyway.

Signals to/from mobile units can and do fluctuate in strength by 15-20 dB as the mobile moves over even a short distance. For reliable data transmission, you must have massively excess signal strength over the intended path. Enough excess signal that even with a 20dB drop, the signal will remain noiseless and hard quieted.

APRS DIGIPEATER USAGE

To increase the reliability of transmission from mobiles (i.e. likelihood that a packet will "get through"), APRS uses two categories of digipeaters:

• "WIDE Area" digipeaters in high locations (typically hilltops, the tallest building in town, water towers, etc); i.e. similar to the placement one would choose for a voice repeater. This type responds to the alias call sign of "WIDEn-N".
  
• "Fill-In" digipeaters in areas where mobile stations have poor access to any WIDE Area digipeaters (typically home stations running an APRS client with digipeating enabled). This type traditionally responded to the alias call sign "RELAY" although this has been phased out and the alias is now "WIDE1-1".

HOW APRS PATHS ARE USED

PATH settings determine what kind and how many digipeaters will be used to deliver your packets to their destination. Typically the "destination" will be either other stations listening on RF, or an I-Gate that will receive your packet on RF and transfer it into the Internet.

A transmission path of "WIDE1-1, WIDE2-1" is requesting the helping hand of nearby cooperating home stations as the first step into the APRS network.

As in conventional packet, each digipeater in the chain "crosses off" the call sign it responded to. This example shows results as a user tries to use three wide area digipeaters in succession. The path string will change like this as the packet propagates from digi to digi. Note that these advanced paths require that the "call sign" actually be changed by each digi that processes it. This process of "call sign substitution" is unique to APRS and requires special APRS awareness in TNCs.

By placing two WIDEn-N statements in series in the path, you allow a simple home station "Fill-In digipeater" to "relay" the first hop while leaving the second n-N hop(s) for "real" WIDEn-N digis to properly process and decrement.
The example below shows the life of a packet that has been digipeated in this way:

• In areas without home station fill-in digipeaters, a "real" WIDEn-N digi will act on the first hop and decrement it to zero (WIDE1-0) which shows on-the-air as " WIDE1* " . By contrast a "non APRS Aware" home station will retransmit the packet as " WIDE1-1* "; i.e. not N-n decremented but still marked as used.
• The next digi to hear the packet will act on the second hop WIDE2-2 and transmit it decremented to WIDE2-1.
• The third digi, if any, will transmit the packet decremented to to WIDE2-0 . (actually shows as "WIDE2*" ).
  No further digipeating will occur.

Click here for an animated GIF demonstrating this process

Because all APRS digipeaters use the same generic call signs, the re-transmission process can happen in several geographic directions simultaneously if several more digipeaters are within range of the one transmitting. A widening circle of digipeats involving more and more digis on each hop will spread outward from the user in all directions. This phenomenon, known as UI flooding, is sharply different from the directed linear sequence of digis, each identified by a unique call sign, used in traditional connected packet.

Today's recommended universal path settings under the "New Paradigm" are:

* WIDE1-1, WIDE2-2 (Will produce three hops and will take advantage of home fill-in digis. Use in rural areas with low APRS activity only.)
* WIDE1-1, WIDE2-1 (Will produce two hops and will take advantage of home fill-in digis. Use in busy urban and suburban areas.)