DSL Protocols

DSL itself provides a Layer 1 transmission path between two endpoints, in some ways like the Layer 1 service that analog modems and ISDN modems provide. However, DSL uses some additional protocols to support data transfer. For instance, DSL uses ATM as the Layer 2 protocol between the DSL router or DSL modem in the home and the ISP router. Additionally, DSL uses a protocol called PPP over ATM (PPPoA). PPP and ATM are both data-link protocols, but they serve different purposes. PPP provides dynamic address assignment by the ISP for the DSL modem and some basic authentication with Challenge Handshake Authentication Protocol (CHAP). (CHAP is covered in the CCNA ICND Exam Certification Guide and the ICND exam.) Also, depending on the gear installed at a site, DSL might require PPP over Ethernet (PPPoE) for traffic between a PC in the home and the DSL modem/router in the home.

Figure 15-14 shows a typical installation using an ADSL router, like the Cisco 827H series. The 827H acts as both a router and a DSL modem.

Figure 15-14 Protocols Used with a DSL Router

Andy's PC

DHCP Server

DSL Router

DHCP Client


ATM VC-Layer 2

Local Loop-Layer 1 DSL

Figure 15-14 shows a typical installation using an ADSL router, like the Cisco 827H series. The 827H acts as both a router and a DSL modem.

Layer 1-SONET



Andy's House

Mayberry CO

Any PCs at the home can connect to the DSL router using Ethernet. In fact, as is common with many DSL routers, the Cisco 827H DSL router includes a four-port Ethernet hub built into the router, so you can just cable a PC directly to the 827H. Alternately, you can create an Ethernet any way you want, as long as there is Ethernet connectivity from the PC to the DSL router. A straight-through Ethernet cable would be used to connect the PC directly to the DSL router.

The PC can be configured just like it would be on any other Ethernet, thinking of the DSL router like any other router. The PC would point to the DSL router's Ethernet IP address as its default gateway. The PC even can use DHCP to acquire an IP address, with the DSL router providing the DHCP server feature.

DSL provides the Layer 1 encoding features for high-speed transmission over the local loop, but it also references ATM as the data-link protocol to use over the DSL link. Back in Chapter 4, "Fundamentals of WANs," you read about the basic features of ATM and SONET, and how ATM sends and receives ATM cells at Layer 2, with SONET providing the Layer 1 transmission details. DSL defines how you can use ATM cells over DSL lines, instead of over SONET, with the ATM cells being forwarded over the DSL connecting from the home to the DSLAM.

The ISP's router needs to receive the data traffic, not the DSLAM in the local CO. So, the DSLAM forwards the ATM cells over the link to the ISP router, probably using an optical cable and Sonet at Layer 1. The receiving ISP router can reassemble the cells and extract the IP packet.

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