Time slots – Codan Radio P25 Training Guide User Manual
Page 101
P25 RADIO SYSTEMS | TRAINING GUIDE
Chapter 7: P25 Phase 2 Page 93
Time Slots
The radio physical channel for P25 two-slot TDMA consists of a 12.5 kHz frequency pair divided into
30 msec time slots. The physical traffi c channel supports two logical voice channels. A P25 Phase 2
superframe consists of twelve sequential 30 ms time slots for a total duration of 360 ms, the same time
interval for 48 Phase 1 micro-slots (a Phase 1 superframe). A P25 Phase 2 ultraframe consists of 4
consecutive superframes. Figure 7-7 shows the superframe structure for two-slot TDMA.
Figure 7-7: Superframe Structure for Two-Slot TDMA
The fi rst 10 slots in a superframe, numbered 0 to 9, can be used for voice or signaling information. If they
are used for signaling (call setup / teardown, and hangtime) they are called a Fast Associated Control
Channel (FACCH). If they are used to transport voice frames, they are called a Voice Transport Channel
(VTCH). The VTCH is used to exchange voice and encryption synchronization signaling (ESS). The
last 2 slots in a superframe, numbered 10 and 11, can only be used for signaling information. They are
called the Slow Associated Control Channel (SACCH) and they are in an inverted position.
The Inter-slot Signaling Channel (ISCH) is the logical channel that is located between 2 consecutive
outbound slots. It is sized to occupy the space reserved in the inbound path for ramping up and down,
pilot sequences (at the beginning and at the end of the burst) and guard time. This logical ISCH is
composed of 40 consecutive bits (comprised of the 20 bits at the end of an outbound slot and the 20
bits at the beginning of the next outbound slot).
The ISCH is used to provide:
a symbol / timeslot synchronization mechanism for receiving subscribers
a means of identifying channel numbering
the current position within the superframe and/or ultraframe
access control for usage of the inbound SACCH
Slow Associated
Control Channel
(SACCH)
Superframe 360 ms 4320 bits
Outbound
Inbound
0
9
8
7
6
5
4
3
2
1
11
10
0
9
8
7
6
5
4
3
2
1
11
10
Voice Transport
Channel
(VTCH)
Fast Associated
Control Channel
(FACCH)
Outbound
Inbound
0
9
8
7
6
5
4
3
2
1
11
10
0
9
8
7
6
5
4
3
2
1
11
10
OR
Inter-slot Signaling Channel (ISCH)
30 ms
360 bits