Asymmetrical digital subscriber line (ADSL) an in-depth study

Asymmetrical Digital Subscriber Line (ADSL) is one member of a group of broadband access technologies that uses the existing copper-based local loop of the analog PSTN for high-speed digital data transmission. One feature of ADSL is that it permits analog voice POTS transmissions to continue uninterrupted over the same wiring. Specifically, POTS continues to use the 0 to 4 KHz frequency range of the copper wiring, while ADSL uses bandwidth starting at 25 KHz and extending up to approximately 1.1 MHz for data transmission. The term asymmetrical refers to the fact that data rates downstream (to the user) and upstream (from the user) are not the same. Typical ADSL data rates range from 1.536 to 6.144 Mbps downstream and from 16 to 640 Kbps upstream. Local loop length, wire size, and the presence of devices to improve voice communication such as bridged taps and loading coils all affect ADSL data rates. Digital data is coded by one of two methods: Discrete Multitone Modulation (DMT) or Carrierless Amplitude and Phase Modulation (CAP). Echo control is also accomplished by one of two methods: Frequency Division Multiplexing (FDM) or echo cancellation. This paper consists of four sections: 1) A technical review and comparison of the CAP and DMT line encoding technologies. 2) A market review of the presence of CAP and DMT technologies in customer premise equipment (CPE) such as modems and routers. 3) A review of the POTS physical layer that exists between the ADSL subscriber and the Telco CO, and its impact on ADSL availability and quality of service (QOS). 4) A technical review of the newer, splitterless, G.Lite technolog