RFC 1483 (rfc1483) - Page 3 of 16


Multiprotocol Encapsulation over ATM Adaptation Layer 5



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RFC 1483                Multiprotocol over AAL5                July 1993


                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.

4.  LLC Encapsulation

   LLC Encapsulation is needed when several protocols are carried over
   the same VC.  In order to allow the receiver to properly process the
   incoming AAL5 CPCS-PDU, the Payload Field must contain information



Heinanen


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