Field Guide · term

Also known as: system id, sysid, system identity

A System ID (often written SysID) is a 12-bit code that identifies a specific trunked system within a WACN, the middle level of the P25 identity hierarchy.1 It is not globally unique on its own — a given SysID value can appear under many different WACNs — but the pair WACN + System ID names one operator’s system unambiguously, and adding the RFSS number pins down a specific subsystem within it.

WACN — 20 bits0xBEE00 System ID — 12 bits0x2A7 RFSS — 8b0x01 WACN + System ID + RFSS = one system's globally unique address
The System ID is the 12-bit middle field; only together with the WACN and RFSS does it uniquely name a system.

How it works

The System ID is broadcast continuously on the control channel in the network-status messages, so radios and monitors learn it without configuration. A subscriber radio compares an advertised WACN and System ID against its home network when deciding whether to affiliate: a match means “this is my system, register here,” a mismatch means “this is someone else’s network, keep scanning.” This is the mechanism behind roaming across sites of the same system versus rejecting a foreign one.

Because the 12-bit field allows only 4096 values, the same System ID is reused freely across the world; it is meaningful only inside its parent WACN. That is why database listings quote the WACN and System ID together as the canonical identifier of a P25 network. The System ID is distinct from the per-frame Network Access Code, which is a fast squelch filter rather than a registered identity.

In practice

  • The System ID is written as a three-hex-digit value (12 bits), e.g. 0x2A7, and is quoted with the WACN as the canonical identifier of a P25 network.
  • Because values repeat across WACNs, a System ID quoted on its own is ambiguous — a common source of confusion when comparing systems from different operators.
  • A subscriber radio’s home configuration includes its WACN and System ID; matching both is the precondition for affiliating to any site it hears.

Relevance to SDR

For a monitor the System ID, read together with the WACN, is the key that matches a control channel to a named system in reference databases and distinguishes it from neighbouring networks. GopherTrunk decodes the System ID from the P25 control channel and reports it next to the WACN and RFSS, giving each tracked control channel a concrete network identity. It is descriptive metadata — knowing the System ID does not unlock encrypted voice.

Sources

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

See also