ion and TS 16 for signaling. The E1 frame is illustrated in Figure 20.18. The sequences of bits from bit 9 to bit 128 and from bit 137 to bit 256 may be used for ATM cell mapping. ATM cells can also be directly mapped into special E3 and E4 frames. The first has 530 octets available for cells (i.e., exactly 10 cells) and the second has 2160 octets (not evenly divisible). 20.12.4 Mapping ATM Cells into SDH 20.12.4.1 At the STM-1 Rate (155.520 Mbps). SDH was described in Section 19.2. Figure 20.19 illustrates the mapping procedure. The ATM cell stream is first mapped into the C-4, which, in turn, is mapped into the VC-4 container along with VC-4 path overhead. The ATM cell boundaries are aligned with STM octet boundaries. Since the C-4 capacity (2340 octets) is not an integer multiple of the cell length (53 octets), a cell may cross a C-4 boundaries. The AU-4 pointer (octets H1 and H2 in the SOH) is used for finding the first octet in the VC-4. 20.12.4.2 At the STM-4 Rate (622.080 Mbps). As shown in Figure 20.20, the ATM cell stream is first mapped into the C-4-4c and then packed into the VC-4-4c container along with the VC-4-4c overhead. The ATM cell boundaries are aligned with the STM-4 octet boundaries. The C-4-4c capacity (9360 octets) is not an integer multiple of the cell length (53 octets); thus a cell may cross a C-4-4c boundary. The AU pointers are used for finding the first octet of the VC-4-4c. 9 4 bytes 291 x 4 bytes VC-4-4C ATM payload: 599,040 kbps AU pointer STM-4 SOH structure AU pointers ATM cell 53 octets Figure 20.20 Mapping ATM cells into the STM-4 (655.080 Mbps rate) frame structure for the SDH-based UNI. [From Figure 10/1.432, page 15, ITU-T Rec. 1.432 (Ref. 4).] REVIEW EXERCISES Payload Payload Path overhead Active overhead bytes at the UNI Undefined overhead bytes at the UNI Figure 20.21 Mapping ATM cells directly into a SONET STS-1 frame. (From Ref. 1, courtesy of Hewlett-Packard.) 20.12.5 Mapping ATM Cells into SONET ATM cells are mapped directly into the SONET payload (49.54 Mbps). As wit