CORE·LAB — Phase 1 vs Phase 2

One channel, two conversations.

P25 lives in a 12.5 kHz channel. Phase 1 puts one voice path in it. Phase 2 splits that same channel in time into two slots — doubling capacity on spectrum you already license. Toggle between them, watch the channel divide, then see what happens when a legacy radio joins the call.

Parameters below are the verified TIA‑102 values (modulation, symbol/bit rates, vocoder, slot timing), framed for a modern ASTRO 25 environment (System Release 2024.1) built on software‑defined GTR 8000 base radios and GCM 8000 comparators. Phase 2 TDMA is trunked‑only; Phase 1 FDMA remains load‑bearing for talk‑around, mutual‑aid interop, and the control channel. This teaches the standard, not a specific system's config.

12.5 kHz channel · time-domain view Phase 1 · FDMA

Modulation

C4FM

Symbol / bit rate

4800 baud · 9600 bps

Vocoder

IMBE 7.2 kb/s

Voice paths / channel

Physical layer

Interoperability scenario

A Phase 1-only portable joins a Phase 2 talkgroup. Watch what the Zone Controller has to do to include it.

Why it matters

Spectrum is licensed and finite. Phase 2's TDMA gives an agency two simultaneous talk paths in the same 12.5 kHz they already hold — capacity without new spectrum. But Phase 1 isn't going anywhere: on a modern system it's still the load-bearing floor for direct/talk-around, mutual-aid interoperability with legacy agencies, and the 9600 bps control channel itself. The catch you'll meet in the field: the moment a Phase 1-only radio joins a talkgroup, that channel drops back to a single FDMA path. Interoperability has a spectral-efficiency price.