Historically, DSL has been an asymmetric service (ADSL), evolving into a symmetric one (G.SHDSL) designed to replace E1 TDM circuits and provide voice, ATM, raw IP, and ISDN transport.
DSL copper cables are terminated at a central office (CO) DSLAM port (digital subscriber access line multiplexer). The DSLAM serves two purposes:
One is to physically terminate the subscriber line and separate the voice band from the data bands utilizing an integrated splitter device similar to the one on the customer end; the voice signal is delivered directly to the PSTN network on OSI Layer 1.
The second purpose is to relay the data traffic to an IP backbone, usually based on ATM or Ethernet. Aggregation and service-selection gateways constitute the distribution layer of modern DSL provider architectures.
Almost all open-source UNIX operating systems provide mature PPTP support required for the PPPoA architectures that are popular in some European countries. Linux, OpenBSD, and FreeBSD support native PPPoE. PPPoA or PPPoE support of your favorite operating system usually requires a modified/patched version of the PPP toolset. Discussion goes beyond the scope of this book, but you can find easily several cookbooks for setup via your favorite search engine or Linux repository. Several DSL NICs are also available (ATM25, splitterless operation). Some of their important characteristics are as follows:
DSL modes of operation: PPPoA, PPPoE, bridging mode
DSL flavors: ADSL, HDSL, SDSL, G.SHDSL, G.Lite, VDSL, and so on
Software requirements of DSL access: PPPoE or PPPoA stack support, PPTP (for example, via Netgraph/mpd daemon under FreeBSD)