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Duplicate Repeater IDs

As a regions mesh network grows, the likelihood of two repeaters sharing the same short identifier increases. MeshMapper has a robust system for detecting, handling, and resolving these "collisions" to ensure data integrity.

The "2-Char" Limitation

MeshCore repeaters are identified by a Short ID consisting of the first two characters of its Public ID (e.g., A1, 4F, 09). As a message travels across different repeaters in a mesh to its destination, these repeaters append their 2 character ID to the message as a "hop". MeshMapper operates using these 2 character ID's to associate coverage information with which repeaters were involved in that ping.

Since this Short ID is only two hexadecimal digits long, there are only 254 possible combinations (01 to FE...00 and FF are reserved in the MeshCore firmware). As the number of repeaters in a region increases, it becomes statistically inevitable that two completely different devices will end up generating with the same Short ID.

When this happens, it is called a Collision.

How MeshMapper Handles Collisions

MeshMapper prioritizes data accuracy. If two repeaters share the ID A1, and a message repeat is heard by or is received via A1, the system has no way of knowing which physical repeater was actually involved. To prevent potentially inaccurate data from being represented on the map, MeshMapper enters a "Quarantine" mode for the affected devices.

1. Detection & Quarantine

When a new repeater appears on the network with a Short ID that is already in use by a different device: - Both repeaters (the existing one and the new one) are immediately flagged as Excluded. - The system assigns the new repeater a temporary internal ID (e.g., X1) to differentiate it in the database and map, but it remains linked to the collision. - Both repeaters are excluded from regional or global leaderboards and statistics (like maximum distance reached)

2. Map Appearance

  • Red Icons: Both repeaters will appear on the map with Red icons (instead of the standard Orange or Grey).
  • Popups: Clicking on the repeater may show its status as "Excluded" or "Duplicate" and will list the repeaters that are in collision.
  • Pings: Clicking a grid square on the map that has a repeater involved in an active collision will draw red dashed lines to each repeater in the collision group (and list distance in KM).
  • Leaderboards: Both repeaters are immediately removed from all Leaderboards (Best Repeaters, Max Range, etc.) both on the local region and globally to prevent skewed statistics.

3. Impact on Collected Data

  • Ambiguity: If a ping is ingested containing a repeater that has a collision (e.g., via A1), MeshMapper cannot attribute that hop to a specific location with certainty.
  • Data Integrity: This data is forever flagged as being ambiguous, and will never associate with a repeater (even after the collision is resolved).

Resolution

Collisions are typically resolved in one of two ways:

Automatic Cleanup (Stale Data)

Often, a collision occurs because an old, offline repeater is still in the database when a new one comes online. - MeshMapper runs a cleanup routine (typically every few days). - If one of the colliding repeaters has not been heard from in 3 days, it is considered "Stale" and is automatically deleted. - The Survivor: Once the stale duplicate is removed, the remaining active repeater is automatically restored to Active status. Its icon will turn back to normal, it will reappear on Leaderboards, and new incoming pings will properly associate themselves with this repeater again.

Manual Resolution

If both repeaters are active and legitimate (a true collision between two live devices): - Region administrators receive an alert regarding the collision. If able, it is suggested that the administrator resolve the issue by asking the owner of the new repeater to generate a new ID. - If the administrator knows the the collision has been cleared, they may manually reinstate the remaining repeater

Summary Table

State Indicator Meaning
Active Green / Grey Icon Normal operation. Unique ID.
New Orange Icon Recently discovered (less than 14 days old).
Excluded Red Icon Duplicate ID Detected. Data from this repeater is currently untrusted.

Permanent Solution

MeshCore does not have to use 2-character IDs. This is a limitation that can be, albeit with a lot of work and coordination from the MeshCore developers, resolved by implementing 4-character (or more) IDs.

Please post your concerns on the discussion here: https://github.com/meshcore-dev/MeshCore/issues/1083

With 4-character HEX IDs, a region can have up to 65,536 different repeaters without ID collisions.