NWAPRS serves: Alberta, British Columbia, Northwest Territories,
Washington, Oregon, Idaho, and Montana

BACKGROUND AND INFO ON APRS DIGIPEATING AND PATH SELECTION

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 overwhemingly used between fixed locations, typically with better antennas and more power.]

*** WHY DIGIPEATING IS MORE ESSENTIAL ***

Signal levels that you may consider adequate on voice WON'T BE 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-hard-quieted, operation so many people inflict on voice repeaters, especially with underpowered handhelds, JUST WON'T 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. The receiving station just ignores the packet if it is defective in anyway. [ This is the price you pay for the one-to-many broadcast nature of APRS, compared to the one-on-one nature of traditional connected packet. ]

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. [ Note that the instruction manual for the Kenwood D700 acknowledges this fact by stating that you can't expect reliable packet operation until the S-meter reads full scale. ]

{{ FUNDAMENTAL UNAVOIDABLE FACT OF PACKET LIFE: Roughly speaking, a given antenna installation and transmitter power will produce about 1/2 to 1/3 the RELIABLE range on APRS packet that it produces on FM voice. }}

*** APRS DIGIPEATER USAGE ***

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

1) "Public" 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 callsign of "WIDEn-N" .

2) Personal home stations typically running digipeater duty alongside other activities (such as being an APRS client and/or Internet gateway). This type responds to the alias callsign "WIDE1-1".

The assumption is that there are far more home stations then wide-area digis. Therefore a mobile is far more likely to be heard by a nearby home station than by a much more distant digipeater.

On the other hand, even a minimal home station is likely to have a better and higher antenna system than anything on a mobile. Thus the home station's likleyhood of successfully transmitting to even a fairly distant digipeater is much greater than a mobile's. This is especially true for an area situated in a geographic low spot, or "urban canyons" where mobiles have trouble "getting out".

*** 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" be either other stations listening on RF, or a fixed station that will receive your packet and transfer it into the Internet and then onward to findu.com.

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. [ Normally, digipeaters will also respond to the alias "WIDE1-1" so if a digi hears you directly, it can also serve as the first hop. ]

If you want multiple digipeat hops, you specify something like "WIDE1-1,WIDE2-1". As in conventional packet, each digipeater in the chain "crosses off" the callsign 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:

WIDE1-1,WIDE2-1 {as sent by the user}

*WIDE2-1 {after home station first digipeat}

*WIDE2 {after first WIDEn-N digipeat}

or

WIDE1-1,WIDE2-1 {as sent by the user}

WIDE2-1 {after first WIDEn-n digipeat}

WIDE2 {after second WIDEn-n digipeat}

The path is now "used up" and no further digis will repeat this packet. This kind of path will work with any kind of TNC pressed into duty as digipeaters.

Because all APRS digipeaters use the same generic callsigns, the re-transmission process can happen in several geographic directions simultaneously if several digis 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-FLOOD", is sharply different from the directed linear sequence of digis, each identified by a unique callsign, used in traditional connected packet.

If you know them, you CAN use explicit callsigns in APRS paths instead of the generic WIDEn-N. This is one approach to reducing unnecessary retransmissions, especially in your home territory where you likely will know the actual callsigns of local digis.

In order to shorten the path strings to allow more packets per minute , APRS introduced a new convention. You add a fake "SSID" to the WIDEn-N "callsign" in the path, indicating the number of hops desired. Each digipeater that processes the packet decrements the value of the "SSID" but doesn't cross it off as "used up". When the SSID decrements to zero, further digipeating stops. This type of "WIDEn-N" digipeating path looks like this for the example above:

(Some work needed here still)

[ Actually I have simplified the path examples for purposes of discussion. In reality, the very first WIDEn-N digi (but no subsequent ones) is supposed to insert it's own real callsign (marked as used) into the path in front of the WIDEn-N phrase, before forwarding it. This allows users many WIDEn-N digi hops away to determine the general location the packet originated from. ]

Note that these advanced paths require that the "callsign" actually be changed by each digi that processes it. This process of "callsign substitution" is unique to APRS and requires special APRS awareness in TNCs. Currently only the Kantronics KPC3+/KPC9612+, and TNC2 clones with UI-DIGI firmware, can do this "standalone" without a computer attached).

Some other considerations:

1) Normally, you would never put more than one "WIDE1-1" in a path.

2) --NEVER-- put WIDE1-1 in a path after WIDEn-N. If you do this, dozens (or hundreds) of home stations within earshot of one or more digis will needlessly clog the channel retransmitting the WIDEs packets for no reason.

3) Paths longer than about WIDE3-3 are almost useless. The probability of success goes down exponentially as the path lengthens. On the other hand, you can create literally thousands of useless packets for every transmission, as the UI-FLOOD spreads outward over hundreds of miles. Indeed, in some areas, intelligent digipeaters are automatically reformatting excessively long paths to something more reasonable such as WIDE2-2 or WIDE3-3.