Chapter 7: Second Level Commands June 30, 2006
7-46 SG1-UM-8500-03
PPPoE support
SG-1 PPP over Ethernet (PPPoE) support enables multiple hosts at a remote site to connect through the same
customer premise access device. It also provides access control and billing functionality in a manner similar to dial-
up services using PPP. In many access technologies, the most cost effective method to attach multiple hosts to the
customer premise access device, is via Ethernet. In addition, it is desirable to keep the cost of this device as low as
possible while requiring little or no configuration.
This feature of the SG-1, provides the ability to connect a network of hosts over a simple bridging access device to a
remote Access Concentrator. With this model, each host utilizes its own PPP stack and the user is presented with a
familiar user interface. Access control, billing and type of service can be done on a per-user, rather than a per-site,
basis. To provide a point-to-point connection over Ethernet, each PPP session must learn the Ethernet address of
the remote peer, as well as establish a unique session identifier. The PPPoE protocol includes a discovery protocol
that provides this.
The main aim of this feature is to provide ISPs with xDSL support with an easy consumer end adoption. This model
is preserving the point-to point session, which is familiar to both the end-user and to the ISP.
A PPPoE session establishment performs the following scenario:
• Discovery stage:
– Host broadcasts a PPPoE Active Discovery Initiation (PADI) packet.
– When SG-1 receives a PADI, it replies by sending a PPPoE Active Discovery Offer (PADO) packet to the
host.
– The host then sends a single PPPoE Active Discovery Request (PADR) packet to the SG-1.
– When the SG-1 receives a PADR packet, it prepares to begin a PPP session. It generates a unique
SESSION_ID for the PPPoE session and replies to the host with a PPPoE Active Discovery Session-
confirmation (PADS) packet
– PPP session stage
Usage Scenarios
1.SG-1 as xDSL aggregator using ATM network:
In this scenario, a PPP session is initiated on an Ethernet-connected client through a standard ADSL modem.
The session is transported over the ATM DSL link via RFC 1483 Ethernet-bridged frames and is terminated by
the SG-1 which is acting as an xDSL aggregator (using the SCC-ETH ATM card).
Scenario highlights:
• PVCs (Permanent Virtual Connection) are defined between the DSLAM and the SG-1.
• The SG-1 terminates the PPPoE sessions initiated at users PCs and grants a service.
• User packets are passed as follows:
– The user PC, which runs PPP client, encapsulates the PPP packet with Ethernet.
– The PPPoE packet is being sent from the user PC to the xDSL modem.
– The xDSL modem encapsulates it with DSL.
– The DSLAM de-encapsulates the DSL ATM switches to pre-configured PVC to the SG-1.
– The SG-1 de-encapsulates the ATM de-encapsulates the Ethernet and terminate the PPP session.