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APRS paths in practice

This page explains APRS paths from the protocol point of view: how to read notation such as WIDE2-1, what the difference is between traced and non-traced paths, and how one-part and two-part paths are actually processed.

1. What an APRS path is

Section titled “1. What an APRS path is”

An APRS path is a list of hops or aliases that tells the RF network how a frame may be repeated further.

Examples:

WIDE2-1
SP2-2
WIDE1-1,WIDE2-1
SP1-1,SP2-2
WIDE2-1,RFONLY

Key rules:

  • path elements are separated with commas,
  • the path is processed from left to right,
  • the next element is not touched until the first still-unconsumed element is handled,
  • RFONLY and NOGATE are not repeat hops, but markers that restrict gating to APRS-IS.

In practice, local transmission without digipeaters means no path at all. Many programs and radios also use the convenient label DIRECT, but at protocol level this simply means transmission without digi hops.

2. How to read N-N

Section titled “2. How to read N-N”

In paths such as WIDEn-N or SPn-N, the number on the left identifies the alias family and its initial scope, while the number on the right acts as the remaining repeat counter.

Example:

WIDE2-2

means that the frame may still be repeated two times by successive digipeaters. After the first repeat, the counter is reduced:

WIDE2-2 -> WIDE2-1

After the second repeat, the path is consumed:

WIDE2-1 -> WIDE2*

The asterisk * marks an element that has already been used and should not be used again for further repeats.

Similarly:

SP2-2 -> SP2-1 -> SP2*

3. Traced and non-traced paths

Section titled “3. Traced and non-traced paths”

The main difference is whether the digipeater inserts its own callsign into the path.

Traced path

Section titled “Traced path”

With a traced path, each digipeater that repeats the frame leaves a visible trace. That makes it possible to see how the packet travelled through RF.

Example:

SP8XYZ-9>APRS,WIDE2-2:...

After the first digi:

SP8XYZ-9>APRS,SR5AAA*,WIDE2-1:...

After the second digi:

SP8XYZ-9>APRS,SR5AAA*,SR5BBB*:...

Advantages:

  • packet routing is easier to inspect,
  • network behaviour is easier to diagnose.

Disadvantage:

  • the frame grows with every repeat because additional digi callsigns are inserted.

Non-traced path

Section titled “Non-traced path”

With a non-traced path, the digipeater does not insert its own callsign. It only reduces the counter or marks the element as consumed.

Example:

SP8XYZ-9>APRS,SP2-2:...

After the first digi:

SP8XYZ-9>APRS,SP2-1:...

After the second digi:

SP8XYZ-9>APRS,SP2*:...

Advantages:

  • shorter frame,
  • less packet growth across repeated hops.

Disadvantage:

  • you cannot see which exact digipeaters forwarded the frame.

In practice, WIDE aliases are usually traced, while national or regional aliases such as SP or WM are sometimes configured as non-traced. That is not a magical property of the alias word itself, but a result of local digi network configuration.

4. One-part path

Section titled “4. One-part path”

A one-part path has one element, but that does not necessarily mean only one repeat.

Examples:

WIDE2-1
WIDE2-2
SP2-1
SP2-2

Interpretation:

  • WIDE2-1 is one path element and one repeat,
  • WIDE2-2 is one path element and two repeats,
  • SP2-1 is one path element and one repeat,
  • SP2-2 is one path element and two repeats.

This is a common trap: the number of path elements is not the same thing as the number of repeats.

Traced one-part example:

WIDE2-2
-> DIGI1*,WIDE2-1
-> DIGI1*,DIGI2*

Non-traced one-part example:

SP2-2
-> SP2-1
-> SP2*

5. Two-part path

Section titled “5. Two-part path”

A two-part path has two elements separated by a comma and is always processed from left to right.

Example:

WIDE1-1,WIDE2-1

The first element must be consumed first:

WIDE1-1,WIDE2-1
-> DIGI1*,WIDE2-1
-> DIGI1*,DIGI2*

There is one important practical detail here: a first element such as WIDE1-1 or SP1-1 usually has the special role of a helper segment for fill-in digi stations.

As a rule, such a digi should consume only that first element and should not continue repeating the rest of the path. For a handheld or mobile station, WIDE1-1 is mainly there so that nearby home stations or other local helper digis can bridge the local coverage hole, but stop after handling only WIDE1-1.

Another example:

SP1-1,SP2-2

Processing:

SP1-1,SP2-2
-> SP1*,SP2-2
-> SP1*,SP2-1
-> SP1*,SP2*

Important conclusions:

  • WIDE1-1,WIDE2-1 is two path elements and two repeats in total,
  • WIDE1-1,WIDE2-2 is two path elements, but three repeats in total,
  • SP1-1,SP2-2 is also two path elements and three repeats in total.
  • a first element such as WIDE1-1 or SP1-1 usually makes sense as the helper segment for a digi that stops after consuming only that single hop.

A traced two-part path grows faster than a non-traced path because each hop may add another digi callsign.

6. RFONLY and NOGATE

Section titled “6. RFONLY and NOGATE”

At the end of a path you may also see extra markers:

WIDE2-1,RFONLY
WIDE1-1,WIDE2-1,NOGATE

In practice they serve a similar purpose: they are used to block gating of RF traffic to APRS-IS.

These elements:

  • do not add repeat hops,
  • do not extend RF range,
  • only limit movement of traffic toward the internet side.

7. Practical conclusions

Section titled “7. Practical conclusions”
  • no path means local transmission without digi repeats,
  • a one-part path is often enough where many repeats are not needed,
  • a two-part path gives more reach, but quickly increases the number of copies on channel,
  • traced paths are better for diagnostics,
  • non-traced paths usually keep the frame shorter,
  • combinations starting with WIDE1-1 make sense only if they fit local practice and local network topology,
  • it is always best to follow the recommendations used in your region, because the local digi network decides which aliases actually work and how they are processed.