AAL5 Frame Format

No matter which multiplexing method is selected, routed and bridged. PDUs shall be encapsulated within the Payload field of AAL5 CPCS-PDU.

The format of the AAL5 CPCS-PDU is given below:

AAL5 CPCS-PDU Format

+-------------------------------+

| . |

| CPCS-PDU Payload |

| up to 2^16 - 1 octets) |

| . |

+-------------------------------+

| PAD ( 0 - 47 octets) |

+-------------------------------+ -------

| CPCS-UU (1 octet ) |

+-------------------------------+

| CPI (1 octet ) |

+-------------------------------+CPCS-PDU Trailer

| Length (2 octets) |

+-------------------------------|

| CRC (4 octets) |

+-------------------------------+ -------

The Payload field contains user information up to 2^16 - 1 octets.

The PAD field pads the CPCS-PDU to fit exactly into the ATM cells . such that the last 48 octet cell payload created by the SAR sublayer will have the CPCS-PDU Trailer right justified in the cell.

The CPCS-UU (User-to-User indication) field is used to transparently transfer CPCS user to user information. The field has no function under the multiprotocol ATM encapsulation described in this memo and can be set to any value.

The CPI (Common Part Indicator) field alings the CPCS-PDU trailer to 64 bits. Possible additional functions are for further study in CCITT. When only the 64 bit alignment function is used, this field shall be codes as 0x00.

The Length field indicates the length, in octets, of the Payload field. The maximum value for the Length field is 65535 octets. A Length field coded as 0x00 is used for the abort function.

The CRC field protects the entire CPCS-PDU except the CRC field itself.

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