3Com SuperStack II PathBuilder S330 Reference Manual page 247

These frames are then followed by the ABCD bits of each active DS0. Two DS0's
ABCD bits are provided in each byte after the last DS0 group. Figure 174 shows an
example of three DS0s and their ABCD bits sent in a structured encapsulation.
Figure 174 Multiframe Structure for 3x64kbit/s DS1 with CAS
AAL1 Pointer
ABCD for
First
Transported
Timeslot
ABCD for
Third
Transported
Timeslot
Figure 175 illustrates AAL1 structured DS1.
Figure 175 AAL1 Structured DS1
FEC for SN/CSI
Lost Cell
Detection
Clock Recovery
ATM Header
AAL 1
VCC
5 Bytes
CSI - Convergence Sublayer Indicator
FEC - Forward Error Correction
SNP - Sequence Number Protection
SN - Sequence Number
SAR - SDU - Segmentation and Reassembly Service Data Unit
VCC - V irtual Channel Connection
. . .
ABCD for Second
Transported Timeslot
CSI SN SNP
1 Bit 3 Bits 4 Bits
AAL 1 Data
48 Bytes
CBR Module
First Transported Octet of Multiframe
Second Transported Octet of Multiframe
First 125 µsec Frame of Multiframe for
DS1
Second 125 µsec Frame of Multiframe for
DS1
Last 125 µsec Frame of Multiframe for
DS1
Signaling
Substructure
SAR-SDU
AAL Layer
47 Bytes
ATM Layer
235
SAR