Trunked Radio Systems
Before cell phones, there was trunked radio—a system where thousands of users shared a pool of frequencies automatically. Public safety agencies (police, fire, EMS) use trunked radio to coordinate without interference.
What is Trunking?
Trunked radio solves the problem of limited radio frequencies. Instead of each agency having dedicated channels, trunking systems pool frequencies and automatically assign them as needed. A computer controller manages talk groups, channel assignment, and priority levels.
How Trunking Works
When a user presses the push-to-talk button, the radio sends a request to the trunking controller. The controller assigns an available channel and sends a "grant" message to all radios in that talk group. When the user releases the button, the channel returns to the pool for others.
Talk Groups
Talk groups are virtual channels—users only hear transmissions from their own group. A police department might have talk groups for dispatch, tactical operations, detective bureau, and emergency alerts. Interoperability talk groups allow communication between agencies during emergencies.
Project 25 (P25)
Project 25 is the digital standard for public safety radio in North America:
- P25 Phase I: FDMA (2 slots per channel), 12.5 kHz
- P25 Phase II: TDMA (2 voice channels per channel)
- Codec: IMBE (4.4 kbps) later VSELP
- Encryption: AES-256 for secure communications
Motorola Systems
Motorola developed many trunking standards:
- Type I: 5-tone signaling, 5-bit user IDs
- Type II: 16-bit IDs, more talk groups
- Type II SmartZone: Multi-site roaming
- ASTRO: Motorola's P25 platform