This clause defines the scope of this standard and provides related information including normative and informative references and a glossary of terms.

Scope of the Standard

The reference point for the P25 two-slot TDMA CAI is U_m2, and this is shown in Figure 1-1 for orientation to the other parts of the air interface protocols. Document [R6] explains that the U_m2 interface supports trunked voice and control services between the Radio Frequency Sub-System (RFSS) and the subscribers in the system.¹ In addition to the U_m2 interface for the P25 Phase 2 two-slot TDMA CAI, a P25 Phase 2 system also supports a Um interface for the P25 Phase 1 FDMA CAI, and this is also shown in Figure 1-1. While U_m and U_m2 both support trunked voice and control, the U_m2 interface has twice the spectrum efficiency of the U_m interface when used as a VCH.

The two-slot TDMA CAI is divided into a physical layer and a media access control layer, and these are abbreviated as the PHY and the MAC respectively. The PHY defines the modulation and other functions (see [R3]) used to transport information through the U_m2 interface. The MAC has functions defined in this standard to convey trunked voice and associated control information through the PHY, and subsequently through the U_m2 interface. The PHY and MAC are shown in the diagram in Figure 1-1. The TDMA PHY and MAC protocols are complemented by their FDMA Phase 1 equivalent protocol that is indicated in Figure 1-1 as the FDMA CAI.

Above the FDMA CAI and two-slot TDMA CAI are common functions that can operate using either air interface. The common functions shown in Figure 1-1 include P25 digital voice, encryption, and trunking control. The P25 half-rate vocoder used in TDMA is defined in [R4] and the full rate vocoder used in FDMA is defined in [R4]. The full-rate vocoder operates on the FDMA CAI through the Um reference point. The half-rate vocoder operates on the two-slot TDMA CAI via the MAC and PHY. If the voice is encrypted, then the P25 block encryption protocol (see [R5]) is applied to the relevant voice mode to provide encrypted voice service through either the FDMA CAI or the two-slot TDMA CAI. The interconnection of the vocoder and encryption functions to both FDMA CAI and two-slot TDMA CAI indicates the interoperability of those functions with both channels. The voice information, either encrypted or unencrypted, is conveyed through the TDMA CAI on a logical channel known as the two-slot TDMA Traffic Channel (TCH). The remainder of this standard abbreviates this term to TDMA TCH. The voice and encryption functions for the TCH are typically present at the end points of a call, as represented by the protocol stacks in Figure 1-1 with Subscriber labels.

Another common function shown in Figure 1-1 is the trunking control. Trunking control functions in the FDMA CAI are conveyed through a Phase 1 FDMA control channel or through link control words embedded in voice messages. The FDMA control channel messages are defined in [R2] and the procedural protocol is defined in [R1]. The Phase 1 FDMA control channel is abbreviated in many instances in this standard to FDMA CCH. All of the messages in [R2] that pertain to two-slot TDMA voice services in the MAC are functionally defined in this standard as MAC Protocol Data Units (PDUs) in Clause 8. The procedural protocol for voice messages is defined in Clause 7 with supporting examples provided in Annex C and Annex D. The trunking control MAC PDUs are conveyed through the two-slot TDMA CAI in a logical channel associated with voice that is either a Slow Associated Control Channel (SACCH) or a Fast Associated Control Channel (FACCH) defined in this standard. The interconnection of the trunking control function to both the FDMA CAI and twoslot TDMA CAI indicates the interoperability of trunking control with both channels. The SACCH and FACCH permit the subscribers on a call to operate a trunking control protocol with the RFSS. This is why the subscriber stacks and the RFSS stack in Figure 1-1 contain a trunking control function block.

¹ Future support by the P25 Phase 2 Two-Slot TDMA CAI for P25 packet data service is expected to involve extensions to this standard.