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What is ADSL? How does it work?

Asymmetric Digital Subscriber Line (ADSL) is a loose set of protocols for transmitting data at high speeds over ordinary telephone copper without the cost of ISDN, and with much higher possible speeds. This is possible because signals are sent digitally instead of translated into analog waves (sounds), riddled with static, corrected, retransmitted, and translated back to digital. The digial data is transmitted at higher frequencies than analog voice lines allow, due to noise distortion. Digital data has a much higher noise tolerance than analog data, so it passes in a frequency range where analog modem technology doesn't stand a chance.

ADSL uses frequency ranges that previously existed but were never used. The regular analog line or POTS line was designed to carry traffic in the frequency range for voice transmissions, 0 to 4 khz. Outside of that range, additional bandwidth exists that can be used for data. For example, ISDN generally exists in the frequency range from 0 to 80 khz. ADSL exists outside the range of voice, but it does conflict with ISDN (which is why ADSL and ISDN can not be provisioned on the same line). ADSL uses the 26 khz to 1.1 mhz range.

There are two separate modulation schemes for ADSL: CAP and DMT. It is these modulation schemes that enable the data to be transmitted over these higher frequencies thereby increasing the bandwidth. For example, downstream cable modem connectivity uses a modulation scheme called Quadrature Amplitude Modulation or QAM, and upstream cable modem connectivity uses Quadrature Phase Shift Key or QPSK. ADSL is currently experiencing a standards battle between CAP: Carrierless Amplitude and Phase and DMT: Discrete Multi-Tone. Unlike cable modem connectivity the modulation scheme for either CAP or DMT is used for both upstream and downstream.

Basically, a customers line is provisioned for ADSL by opening up their copper pair to the higher frequencies, and routing the digital data in these frequencies to a DSLAM (DSL Access Multiplexer). The DSLAM converts the DSL signal to ATM packets, which are sent to our Redback servers. The Redback server acts as an ARC does for modem dialup connections. It forwards the authentication request to RADIUS, assigns an IP and DNS to the client, and acts as the customer's PPP gateway to route all TCP/IP traffic onto the Internet.


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