Codan Radio P25 Training Guide - Manual (Page 12)

TRAINING GUIDE | P25 RADIO SYSTEMS
Chapter 1: Introduction To P25 Page 4
HOW DOES PHASE 1 P25 WORK?
Phase 1 P25 radios operate in a similar manner to conventional analog FM radios. P25 radios will
operate in conventional analog mode, making them backwards compatible with existing analog radio
systems. When the P25 radio operates in digital mode, the carrier is moved to four specifi c frequency
offsets that represent four different two-bit combinations. This is a modifi ed 4 level FSK used in analog
radio systems. In analog mode, the P25 radio will operate exactly the same as conventional analog
systems, with the capability for CTCSS, DCS, pre-emphasis and de-emphasis, wideband or narrowband
operation and other standard analog features.
In P25 digital mode, the P25 transmitter will convert all analog audio to packets of digital information by
using an IMBE™ vocoder, then de-vocode the digital information back to analog audio in the receiver.
Error correction coding is added to the digital voice information as well as other digital information.
Analog CTCSS and DCS are replaced by digital NAC codes (as well as TGID, Source and Destination
codes for selective calling). Encryption information can be added to protect the voice information, and
other digital information can also be transmitted such as a user defi ned low speed data word or an
emergency bit.
Figure 1-1: P25 Radio System Operation
Figure 1-1 shows the different operational modes of P25 Radio Systems in digital and analog modes.
P25 systems use the Common Air Interface (CAI). This interface standard specifi es the type and
content of signals transmitted by P25 compliant radios. A P25 radio using the CAI should be able to
communicate with any other P25 radio using the CAI, regardless of manufacturer.
Phase P25 1 radios are designed for 12.5 kHz channel bandwidths. Phase 1 P25 radios must also be
able to operate in analog mode on 25 kHz or 12.5 kHz channels. This backward compatibility allows
P25 users to gradually transition to digital while continuing to use analog equipment.
In Phase 2, P25 radios will use a 12.5 kHz channel bandwidth but will be divided into two time slots,
effectively giving 6.25 kHz of bandwidth per voice channel. Phase 2 radios must also be able to
operate in Phase 1 mode for backwards compatability with Phase 1 radio systems.
Analog
Mode
Digital
Mode
Narrowband Wideband
CTCSS DCS Carrier
Voice
Message
Data
Packet
Secure
(Encrypted)
Clear
(Not Encrypted)
Conventional Trunked
Confirmed Unconfirmed
P25 Radio
System
Announcement
Group Call
(Trunked)
Unit to PSTN
Call
(Trunked)
Individual
(Unit to Unit)
Call
Routine
Group Call
Emergency
Group Call
TSBK