PPP over ATM AAL5 (PPPoA) describes the use of ATM Adaptation Layer 5 (AAL5) for framing PPP encapsulated packets.

The Point-to-Point Protocol (PPP) provides a standard method for transporting multi-protocol datagrams over point-to-point links.

ATM AAL5 protocol is designed to provide virtual connections between end stations attached to the same network. These connections offer a packet delivery service that includes error detection, but does not do error correction.

Most existing implementations of PPP use ISO 3309 HDLC as a basis for their framing. When an ATM network is configured with point-to-point connections, PPP can use AAL5 as a framing mechanism.

The PPP layer treats the underlying ATM AAL5 layer service as a bit-synchronous point-to-point link. In this context, the PPP link corresponds to an ATM AAL5 virtual connection. The virtual connection MUST be full-duplex, point to point, and it MAY be either dedicated (i.e. permanent, set up by provisioning) or switched (set up on demand). LLC encapsulated PPP over AAL5 is the alternative technique to VC-multiplexed PPP over AAL5.

When transporting a PPP payload over AAL5, an implementation:

• MUST support virtual circuit multiplexed PPP payloads as described in section 5 below by mutual configuration or negotiation of both end points. This technique is referred to as "VC-multiplexed PPP".
• MUST support LLC encapsulated PPP payloads on PVCs as described in section 6 below by mutual configuration or negotiation of both end points. This technique is referred to as "LLC encapsulated PPP".
• For SVC set up, an implementation MUST negotiate using the Q.2931 [9] Annex C procedure, encoding the Broadband Lower Layer Interface (B-LLI) information element to signal either VC- multiplexed PPP or LLC encapsulated PPP.

Protocol Structure

Virtual Circuit Multiplexed PPP Over AAL5. The AAL5 PDU format is shown below:

AAL5 CPCS-PDU Format

The AAL5 CPCS-PDU payload field is encoded as shown below:

1. LLC header: 2 bytes encoded to specify a source SAP and destination SAP of routed OSI PDU (values 0xFE 0xFE), followed by an Un-numbered Information (UI) frame type (value 0x03).
2. Network Layer Protocol IDentifier (NLPID) representing PPP, (value 0xCF).
3. The PPP protocol identifier field, which can be either 1 or 2 octets long.
4. Followed by the PPP information field.

Related protocols: PPP, PPPoE, ATM, ALL5

Sponsor Source: PPPoA is defined by IETF (http://www.ietf.org) RFC 2364.

Reference: http://www.javvin.com/protocol/rfc2364.pdf: PPP Over AAL5