Field Guide · term

Also known as: WACN, wide area communications network

A Wide Area Communications Network (WACN) is a 20-bit code that names a P25 wide-area network — the top level of the P25 identity hierarchy.1 On its own a WACN is not enough to pinpoint a system; combined with the System ID and the RFSS it forms the globally unique address of the trunked system a radio is registered on. The WACN space is administered so that operators receive blocks that do not collide, much as IP address blocks are allocated.

WACN 0xBEE00 (20 bits) System ID 0x2A7 (12 bits) RFSS 1 (8 bits) sites 1…n RFSS 2 (8 bits) sites 1…n
P25 identity nests: a WACN contains System IDs, which contain RFSSs, which contain sites — together a globally unique address.

How it works

The WACN, System ID, and RFSS are broadcast on the control channel in the network-status and identifier messages, so any radio — or monitor — can learn exactly which system it is hearing without prior configuration. A subscriber radio uses this triple during registration and roaming: as it moves between sites it checks whether a candidate site advertises its home WACN and System ID before affiliating, and treats a foreign WACN as another operator’s network.

The 20-bit WACN allows roughly a million distinct wide-area networks. It sits above the 12-bit System ID (which distinguishes systems within one operator’s WACN) and the 8-bit RFSS (which distinguishes subsystems within a system). Note this identity chain is separate from the fast per-frame Network Access Code: the NAC filters individual frames, while the WACN/System ID/RFSS triple is the registered network identity.

In practice

  • The WACN is written as a five-hex-digit value (20 bits), e.g. 0xBEE00; public databases list it together with the System ID as a system’s fingerprint.
  • Some large shared networks — statewide interoperability systems and the like — span many operators under one coordinated WACN, so the WACN can group agencies that share infrastructure.
  • A radio treats a site advertising a foreign WACN as another network and will not register there, which is how roaming stays contained to the home system.

Relevance to SDR

The WACN is one of the most useful things a monitor can extract, because it and the System ID let a scanner recognise a system across sites and match it to public databases of known networks. GopherTrunk decodes the WACN from the P25 control channel and reports it alongside the System ID and RFSS, which is how it labels which network a given control channel belongs to. Like the other identity fields it is descriptive data, not a security mechanism — reading the WACN does not defeat encryption on voice traffic.

Sources

  1. Project 25 — Wikipedia, on the P25 system-identity hierarchy including the WACN. ↩

See also