Digital Subscriber Line Modems

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Many internet connection types make use of the national and global telecommunications network referred to as the public switched telephone network (PSTN).

• core of the PSTN is fiber optic, but at its edge, it is still often composed of legacy two-pair copper cabling
  • This low-grade copper wire segment is referred to as the plain old telephone system (POTS), local loop, or last mile

Digital subscriber line (DSL) uses the higher frequencies available in these copper telephone lines as a communications channel

• use of advanced modulation and echo cancelling techniques enable high bandwidth, full duplex transmissions
• various “flavors” of DSL:
  • Asymmetrical DSL (ADSL)
    • provides a fast downlink but a slow uplink
    • various iterations of ADSL
      • latest (ADSL2+) offering downlink rates up to about 24 Mbps and uplink rates of 1.25 Mbps or 2.5 Mbps
  • Symmetric DSL
    • offer the same uplink and downlink speeds
    • of more use to businesses and for branch office links, where more data is transferred upstream than with normal Internet use
• customer network is connected to the telephone cabling via a DSL modem
  • might be provisioned as a separate device or as an embedded function of a SOHO router
    • On a standalone DSL modem, the RJ11 WAN port on the modem connects to the phone point
    • The RJ45 interface connects the modem to the router
  • A filter (splitter) must be installed to each phone socket to separate voice and data signals
    • can be self-installed on each phone point by the customer
    • Modern sockets are likely to feature a built-in splitter