
Preliminary Information
MT9071
43
Figure 9 - PCM30 Link Frame Format
PCM30 timeslot 0 is reserved for basic frame alignment, CRC-4 multiframe alignment and the communication
of maintenance information (data link). In most configurations, timeslot 16 is reserved for either CAS or CCS.
For V5.2 applications, timeslots 15, 16 and 31 may be used for CCS. The remaining timeslots (referred to as
channels) carry either PCM encoded voice signals or digital data. Channel alignment and bit numbering is
consistent with timeslot alignment and bit numbering. However, channels are numbered 1 to 30 and do not
directly relate to timeslots. Also see Table 28 - T1 DS1 & ST-BUS DSTi/DSTo Timeslot Relationship.
5.1.1
Two encoding options are available in the transmit and receive directions. These encoding options include High
Density Bipolar 3 (HDB3) encoding and no encoding.
E1 Encoding and Decoding Options
These options are selectable with control register bit THDB3 and RHDB3 (see Table 83 - E1 Interrupts and I/O
Control - R/W Address Y02).
In all cases, Alternate Mark Inversion (AMI) is used in both transmit and receive directions.
5.2
E1 Frame Alignment
Timeslot 0 of every 125us frame is reserved for basic frame alignment and contains a Frame Alignment Signal
(FAS) or a Non-Frame Alignment Signal (NFAS). FAS and NFAS occur in consecutive basic frames (see Table
8 - E1 CRC-4 FAS and NFAS Structure).
Bit one of the FAS can be either a CRC-4 remainder bit or an international usage bit.
Bit one of the NFAS can be either a CRC-4 multiframe alignment signal, an E-bit or an international usage bit.
Refer to an approvals laboratory and national standards bodies for specific requirements.
Bit two of the FAS and NFAS is used to distinguish between FAS (bit two = 0) and NFAS (bit two = 1) frames.
Bits three to eight of the FAS are used for basic frame alignment. Basic frame alignment is initiated by a search
for the bit sequence 0011011 which appears in the last seven bit positions of the FAS, see Section 5.5 E1
Framing Algorithms.
PCM30 TIMESLOT (8/2.048)us
TIMESLOT
0
BIT 1
BIT 2
BIT 3
BIT 4
BIT 5
BIT 6
BIT 7
BIT 8
TIMESLOT
31
TIMESLOT
30
TIMESLOT
0
TIMESLOT
31
PCM30 BASIC FRAME - 125us
FRAME
0
FRAME
15
FRAME
14
FRAME
0
FRAME
15
PCM30 MULTIFRAME FRAME - 2ms