A Glossary of P25 Terminology — Part 3 (L to R)

APCO P25 Logo

In this part of the P25 radio terminology, we cover multicast IP address, P25 gateways, QoS and other important P25 terms.

Previous parts of the P25 glossary: Part 1 (A to C) and Part 2 (D to K).


Local Area Network

Link Data Unit. Voice calls are sent over the CAI as a series of LDUs.


Multicast IP address
An IP address that refers to a group of hosts rather than a single host. These hosts will all accept packets with this IP address.


Network Access Code. The 12 most significant bits of the network identifier information that precedes every packet sent on the CAI. The NAC identifies which network the data belongs to, allowing base stations and mobiles to ignore packets belonging to interfering networks.

Network Element
A network element is any device that is network-connected. A Tait digital network consists of a number of network elements. The base station, P25 CG console gateway and P25 TAG trunked analog gateway are network elements designed and manufactured by Tait.

Normal Squelch
A type of squelch operation in which the receiver unmutes on any signal with the correct NAC (digital P25) or subaudible signaling (analog FM).


A set of 8 bits.


Project 25. A suite of standards and requirements endorsed by the TIA and intended for digital public safety radio communications systems.

P25 CG console gateway
A Tait network element that acts as a gateway between an analog dispatch console and a channel group.

P25 TAG trunked analog gateway
A Tait network element that acts as a gateway between an analog dispatch console and an RFSS controller in a Tait P25 Trunked Network.

The PA (power amplifier) is a base station module that boosts the exciter output to the required transmit level.

The PMU (power management unit) is a module in the base station that provides power to the subrack and monitors power conditions. Gateways can also have a PMU.

A well-defined signal that is transmitted at the beginning of digital P25 calls to facilitate downlink voting and to allow the transmit buffer to fill.

Project 25
A project set up by APCO (the Association of Public Safety Communications Officials International), together with other US governmental organizations, to develop standards for interoperable digital radios to meet the needs of public safety users.

Push To Talk. The button on a SU that keys the transmitter.


Quality Of Service. A router feature that gives real-time data such as voice calls priority over other data.


The reciter is a module of a Tait base station that provides both receiver and exciter functionality.

Repeater Talkaround
Repeater talkaround allows the SU user to bypass repeater operation and communicate directly with other SUs. While repeater talkaround is active, all transmissions are made on the receive frequency programmed for the channel.

Watch out for the final part of the glossary next week.

We have recently released a set of guides that cover P25 Best Practice for specifying, procuring and implementing a P25 radio system. You can either browse the P25 Best Practice website or download the P25 guides in PDF format for free.

Leave a Reply

Your email address will not be published. Required fields are marked *