ATM Option System Reference - Comtech EF Data

1.3.5 PPP Over AAL5
Product Overview
- In LLC Encapsulation this information is encoded in an LLC header placed in
front of the carried PDU.
• RFC-1483 deals only with protocols that operate over LLC Type 1 (unacknowledged
connectionless mode) service.
- In LLC Type 1 operation, the IEEE 802.2 LLC header consists of three oneoctet
fields: DSAP, SSAP and CTRL.
LLC Encapsulation is required when several protocols are carried over the same VC. The
LLC header is different for different protocols:
• For routed non-ISO protocols, the CTRL field specifies an Unnumbered Information
Command PDU, and an IEEE 802.1 Sub-Network Attachment Point (SNAP)
header immediately follows the LLC header. The SNAP header identifies the
routed protocol.
IP is considered a routed non-ISO protocol, and is supported by the NetPerformer.
Routed ISO protocols are not supported.
• Bridged PDUs are encapsulated and the type of bridged protocol is identified in
the SNAP header. The Protocol Identifier (PID) in this header also indicates
whether the original Frame Check Sequence (FCS) is preserved within the
bridged PDU.
PPP over AAL5 is implemented on the NetPerformer based on RFC-2364, which uses the
same frame structure and mechanisms as RFC-1483. With RFC-2364, PDUs are
encapsulated within the Payload field of AAL5 CPCS-PDU.
When transporting a PPP payload over AAL5, two techniques may be used:
• LLC Encapsulation: The protocol type of the payload is explicitly identified by
an LLC header consisting of three one-octet fields: DSAP, SSAP and CTRL.
Refer to the section “LLC Encapsulation (RFC-1483)” on page 1-22.
- The LLC header value indicates that a routed ISO PDU follows. The routed
ISO protocol is PPP, which is identified by a one-octet NLPID field following
the LLC header.
- The PPP payload consists of three one-octet fields: PID, Information and Padding,
which follow the NLPID field in the CPCS-PDU.
• VC Multiplexing: The payload protocol is implicitly determined for each virtual
circuit.
- A VC-multiplexed PPP frame constitutes the CPCS-PDU payload.
Authentication
On the NetPerformer, PPP over ATM includes support of two authentication methods,
Password Authentication Protocol (PAP) and Challenge Handshake Authentication
Protocol (CHAP), both of which protect PPP connections from unauthorized use. Select
the desired authentication method using the Password Type parameter for the PVC.
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