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Digital Subscriber Lines (xDSL)

[Parts of A DSL Network - Creative Commons]


 Asymmetric Digital Subscriber Line and DSL Broadband

Digital Subscriber Line (DSL) is a "last mile" service that allows consumers to access the Internet by using their existing telephone lines. While there are faster technologies available such as cable and fiber, DSL coverage is virtually everywhere. 

DSL is a broadband technology for delivering high-speed Internet to residences and businesses. Similar to cable and fiber connections, DSL connections bridge the “last mile” between the mainstream Internet “backbone” and customer residences. DSL bridges this distance using an unlikely candidate: copper telephone lines.

Most commonly "DSL" refers a Asymmetric Digital Subscriber Line, where the download and upload speeds are different though there are 14 different types of DSL service. However the term DSL may also refer to VDSL2 (Very-high-bit-rate digital subscriber line 2) which is the latest and fastest version of DSL that allows providers to offer integrated, HDTV, home phone and home broadband service all through one line.

Most DSL technologies use asymmetrical data transfer which means that consumers download speeds will be much faster than their upload speeds. While this isn't an issue for typical web browsing, doing any sort of transfer that requires substantial upload bandwidth (such as Skype, video gaming, and uploading Youtube videos) will be much slower than other technologies such as fiber or cable.

While both DSL and Dial-up Internet leverage existing telephone lines, DSL uses a frequency that allows calls and data to be transfered at the same time without interference allowing for users to both talk on their phone and use the Internet. 

One of the biggest drawbacks to DSL is that it even though it uses existing infrastructure, reliable service requires central offices to be placed relatively close (just over a mile) to where customers reside to receive optimum service. Chances are if you don't have DSL service available in your area, it's because DSL providers haven't established a central office close enough to where you are. 

Please refer to [Wikipedia: Digital Subscriber Line (DSL)] for more details.


- xDSL

xDSL, a term that encompasses the broad range of digital subscriber line (DSL) service options, has the potential to revolutionize the areas of Internet access and telecommuting by offering a low-cost, high-speed data transport option for both individuals and businesses. Today, with more than 750 million sites around the world wired for telephone service, the basic infrastructure for xDSL is already in place; each twisted-pair circuit is a veritable high-speed, dedicated pathway just waiting to be activated. 

xDSL is similar to ISDN inasmuch as both operate over existing copper telephone lines (POTS) and both require the short runs to a central telephone office (usually less than 20,000 feet). However, xDSL offers much higher speeds - up to 32 Mbps for upstream traffic, and from 32 Kbps to over 1 Mbps for downstream traffic. 

This xDSL technology enables telephone companies to offer broadband service without major network rewiring and can be implemented quickly and profitably, especially because it stands to benefit both the consumer (with faster data rates) and the service providers (with new revenues from old cables). The two main categories of xDSL are ADSL and SDSL. Two other types of xDSL technologies are High-data-rate DSL (HDSL) and Very high DSL (VDSL). 

Here’s how it works. Nearly all existing telephone lines can carry frequencies up to 1 MHz. But analog telephone service only requires a maximum frequency of 3.3 KHz, leaving a large amount of the bandwidth unused. xDSL makes use of this otherwise wasted space by piggybacking high-speed data traffic onto the unused bandwidth.  

By filtering the frequencies at each end of this wide-open range (4 KHz to 2.2 MHz) and isolating them from the voice-bandwidth channel, the local telco can transport both traditional telephone signals and high-speed xDSL signals over the same old four-wire telephone line that already links your home or business to their central office (CO). At least, that’s the promise of xDSL.


DSL Frequency_072420A
[DSL Frequency - Creative Commons]

- A Simplified DSL Network

The device on the customer’s side above is a DSL modem, translating signal into data for the home network. The string is a telephone cable, which carries data in the form of electromagnetic signal. The device on the Internet provider’s side is a digital subscriber line access multiplexer (DSLAM), designed to route many connections at once and communicate with the Internet “backbone.” 

Real-world applications will vary and be a bit more complex, but the overall concept is the same.


  • Copper phone line: Copper cable made by twisting two copper wires around one another, decreasing interference between the wires and repelling electromagnetic interference from outside.
  • DSL Modem: A device installed in a subscriber’s residence, designed to “translate” the electrical signal from the DSL provider into data for a computer or router.
  • DSLAM (digital subscriber line access multiplexer): Device that receives, decodes, and transmits electrical signals to many phone lines at once.
  • DSL Filter: A small device that plugs into a phone jack and separates low-frequency phone signal from high-frequency broadband signal.
  • Extender: Also called a “loop extender,” DSL extenders are placed between the DSLAM and subscriber modem to boost the signal within the copper cable, allowing signal to reach rural customers situated far from the local DSL office.


- DSL Frequency

DSL broadband is able to travel via copper telephone cables because data is transmitted via separate, higher frequency bands than those used by voice communication. This is similar to how radio and satellite communications divide the spectrum to deliver content - but rather than happening over the air, it’s happening within a wire. 

To visualize this, we can picture the DSL cable as a highway. The “lane” for traditional telephone traffic is only 4Khz wide - but the width of the highway is about 3000 times wider than that, stretching all the way up to about 12,000Khz. While this is “narrower” than the frequencies possible in a fiber or coaxial broadband connection, it still allows for quite a bit of flexibility in transmitting data.

[More to come ...]



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