Access Nodes in Ethernet Aggregation Networks Overview

The access node is the first aggregation point in the digital subscriber line (DSL) access network and apart from terminating the DSL physical layer signals, it also terminates the user ATM layer. The access node contains an Ethernet uplink to provide connectivity to the aggregation network. When ATM is supported on the DSL line, the access node provisions an interworking function between the ATM layer on the user side and the Ethernet layer on the network side, encompassing protocol translation, access loop identification, QoS, security, and OAM attributes. This functionality might require the access node to snoop, modify, or terminate protocols in layers above the ATM Adaptation Layer 5 (AAL5) encapsulation. The access node operates as an Ethernet switch, while it also offers enhanced functionality for protocol interworking, multicast support, and customization for support of access networks (such as ARP and IGMP processing, and user identification and isolation).

Figure 38 illustrates the interface stack for this configuration.

Figure 38: Example of PPPoE over ATM Stacking

Example of PPPoE over ATM Stacking

The subscriber's access node indicates to the B-RAS application running on the router whenever a PPPoE session carries interworked PPPoA or PPPoE over ATM traffic. This indication enables the B-RAS application to modify its behavior for interworked PPPoE sessions. To denote that a PPPoE session contains interworked traffic, the PPPoE client or the host includes the IWF-Session DSL Forum VSA (26-254) in the unicast PPPoE Active Discovery Request (PADR) packet that it transmits to the PPPoE access concentrator to which it wants to connect.

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