TIA-102.BBAC-1:2013

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.