RFC 1483 (rfc1483) - Page 2 of 16


Multiprotocol Encapsulation over ATM Adaptation Layer 5



Alternative Format: Original Text Document



RFC 1483                Multiprotocol over AAL5                July 1993


   Relay Service Specific Convergence Sublayer (FR-SSCS), as defined in
   I.36x.1 [3], is used over the CPCS of AAL5, then routed and bridged
   PDUs are carried using the NLPID multiplexing method described in RFC
   1294 [4].  Appendix A (which is for information only) shows the
   format of the FR-SSCS-PDU as well as how IP and CLNP PDUs are
   encapsulated over FR-SSCS according to RFC 1294.

2.  Selection of the Multiplexing Method

   It is envisioned that VC Based Multiplexing will be dominant in
   environments where dynamic creation of large numbers of ATM VCs is
   fast and economical.  These conditions are likely to first prevail in
   private ATM networks.  LLC Encapsulation, on the other hand, may be
   desirable when it is not practical for one reason or another to have
   a separate VC for each carried protocol.  This is the case, for
   example, if the ATM network only supports (semi) Permanent Virtual
   Circuits (PVCs) or if charging depends heavily on the number of
   simultaneous VCs.

   When two ATM stations wish to exchange connectionless network
   interconnect traffic, selection of the multiplexing method is done
   either by manual configuration (in case of PVCs) or by B-ISDN
   signalling procedures (in case of Switched VCs).  The details of B-
   ISDN signalling are still under study in CCITT [5].  It can, however,
   be assumed that B-ISDN signalling messages include a "Low layer
   compatibility" information element, which will allow negotiation of
   AAL5 and the carried (encapsulation) protocol.

3.  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:


















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