components of an ADSL network. Most DSL networks use ATM as the data-link layer protocol. In basic terms, a DSLAM is an ATM switch with DSL interface cards (ATU-Cs). The DSLAM terminates the ADSL connections and then switches the traffic over an ATM network to an aggregation router. The aggregation router is the Layer 3 device where IP connections from the subscriber terminate. There are three ways to encapsulate IP packets over an ATM and DSL connection: In RFC 2684 Bridging, the ADSL CPE bridges the Ethernet frame from the end user’s PC to the aggregation router, where integrated routing and bridging (IRB) provides connectivity to the IP cloud. RFC 2684 Bridging has security and scalability issues, making it unpopular. ISPs are now opting for PPPoE and PPPoA, both of which are much more scalable and secure but involve a more complex implementation.

Note that the connection from the ATU-R to the ATU-C is using DSL at Layer 1 to send and receive ATM cells. The connection from the DSLAM to the aggregation router is over an ATM-based network that may include a fiber ring at Layer 1. This allows an ATM PVC to be established between the DSL modem and the aggregation router that can be used to carry Ethernet frames (RFC 2684 Bridging). The DSLAM essentially performs as an ATM switch. If the CPE equipment has an ATM interface this connection still uses DSL as layer-1 but at layer-2 it establishes an ATM PVC directly to the aggregate router (PPPoA).
Content 2.5 Deploying ADSL 2.5.5 PPPoE Point-to-Point Protocol over Ethernet (PPPoE) is a network protocol for encapsulating PPP frames in Ethernet frames. It is used mainly with ADSL services. It offers standard PPP features such as authentication, encryption, and compression. Figure summarizes these points and shows the PPPoE and TCP/IP protocol stack. In an ADSL deployment, the CPE bridges the Ethernet frames from the end user’s PC to an aggregation router over ATM with an Ethernet frame carrying a PPP frame. A PPP session is established between the subscriber device with PPPoE client support (either an end user PC with PPPoE client software or the CPE router configured as the PPPoE client) and the aggregation router. Figure shows a router as a client. Depending on the deployment you choose, either the PC or the router can be the PPPoE client. In the PPPoE architecture, the PPPoE client functionality connects to the ADSL service. The PPPoE client first encapsulates the end-user data into a PPP frame, and then the PPP frame becomes encapsulated inside an Ethernet frame. The IP address allocation for the PPPoE client uses the same principle as PPP in dial mode, which is through IP Control Protocol (IPCP) negotiation with Password Authentication Protocol (PAP) or Challenge Handshake Authentication Protocol (CHAP) authentication. The aggregation router that authenticates the users can use either a local database on the aggregation router or an authentication, authorization, and accounting (AAA) server.PPPoE client functionality can be available as a software PPPoE client application on the end-user PC. With this model, PPPoE provides the ability to connect a host over a simple bridging CPE to an aggregation router. A host uses its own PPP stack, and the user is presented with a familiar user interface (using the PPPoE client software) similar to establishing a dialup connection. Unlike PPPoA, access control, billing, and type of service can be controlled on a per-user, rather than a per-site, basis.
Content 2.5 Deploying ADSL 2.5.6 DSL and PPPoE Deployment Options When deploying PPPoE and DSL, three options are available based on the equipment used, DSL termination, and PPPoE client functionality:
Content 2.5 Deploying ADSL 2.5.7 PPPoE Session Establishment Usually, PPP only works over a point-to-point connection. Using PPP over an Ethernet multiaccess environment requires additional enhancements. PPPoE has two distinct stages as defined in RFC 2516: When a PPPoE client (end-user PC or router) initiates a PPPoE session, the client must first complete a discovery process to identify which PPPoE server can meet the client request. Then, the host must identify the Ethernet MAC address of the peer and establish a PPPoE session ID. Although PPP defines a peer-to-peer relationship, discovery is inherently a client-server relationship. In the discovery process, the PPPoE client discovers an aggregation router (the PPPoE server). Based on the network topology, there can be more than one PPPoE server, with which the PPPoE client can communicate. The discovery stage allows the PPPoE client to discover all PPPoE servers and then select one to use. There are four steps to the discovery stage : Step 1 The PPPoE client (end-user PC or router) broadcasts a PPPoE Active Discovery Initiation (PADI) packet. This packet includes the service type that the client is requesting. The destination MAC address is set to broadcast. Step 2 The PPPoE server (aggregation router) sends a PPPoE Active Discovery Offer (PADO) packet that describes which service the server can offer. The destination MAC address is the unicast address of the client (end-user PC or router). The source MAC is the unicast address of the PPPoE server. Step 3 The PPPoE client sends a unicast PPPoE Active Discovery Request (PADR) packet to the PPPoE server. Step 4 The PPPoE server sends a unicast PPPoE Active Discovery Session-confirmation (PADS) packet to the client. When discovery is successfully completed, both the PPPoE client and the selected PPPoE server have the information that they will use to build their point-to-point connection over the Ethernet. After the PPPoE session begins, PPP goes through the normal link control protocol (LCP) and Network Control Protocol (NCP) process. A PPPoE Active Discovery Terminate (PADT) packet can be sent anytime after a session has been established to indicate that a PPPoE session has been terminated. Either the PPPoE client or the PPPoE server can send the packet. More information on the PPPoE specification can be obtained in RFC 2516. As specified by RFC 2516, the maximum receive unit (MRU) option must not be larger than 1492 bytes because Ethernet has a maximum payload size of 1500 octets. The PPPoE header is 6 octets and the PPP protocol ID is 2 octets, so the PPP maximum transmission unit (MTU) must not be greater than (1500 – 8 =) 1492 bytes. An Ethernet and PPPoE frame contains one of these Ethertypes:
Content 2.5 Deploying ADSL 2.5.8 Data over ADSL: PPPoA Point-to-Point Protocol over ATM (PPPoA), is a network protocol for encapsulating PPP frames in ATM AAL5. It is used mainly with cable modem, DSL and ADSL services. PPPoA provides authentication, encryption, and compression. If it is used as the connection encapsulation method on an ATM based network it has slightly overhead than PPPoE. PPPoA also avoids issues related to having a MTU lower than that of standard Ethernet transmission protocols that affect PPPoE. PPPoA is a routed solution, unlike RFC 1483 Bridged and PPPoE. Figure shows the