What you need to know about ADSL (broadband Networks #1)

ADSL !!!

I can hear you say "I know everything about ADSL, its a mature technology and I already know it inside out, why is he  writing about ADSL now?"

To be honest I am also surprised by writing about ADSL after working with it for about 7 years now and the rest of the world is talking about FTTx technologies.However, I found it a good start for a series of posts about broadband technologies. Although ADSL is a mature technology, it is still growing even after the spread of fiber technologies and I believe it has the potential to grow more in the coming years.

ADSL was born of the need for speed internet access coupled with the desire for low cost dedicated connections. ADSL is still satisfying both needs for both data carriers and subscribers. ADSL data rates are sufficient for almost  all internet applications available today at low cost compared to other technologies and using the same infrastructure that was built 100 years ago (PSTN).

Lets start by covering DSL technology without getting into much telecommunication details that I forgot myself few years after my graduation from college.

What is DSL?

DSL stands for digital subscriber lines, different xDSL technologies exist ( SDSL, SHDSL, VDSL, ADSL and others ). All of them turn twisted pair copper lines (PSTN lines) into a high speed telecommunication channels without the need for an infrastructure upgrade.

DSL operates over normal telephone lines originally intended to provide voice communication. These lines are named local loops and connect the subscriber site to a PSTN central office or CO.

Today fiber is replacing DSL. However, the high cost and resources involved into installing fiber links delayed the wide implementation of fiber technologies.

DSL Technologies are very scalable and extendable, as they support almost all available network protocols (Frame relay, ATM,TDM, Ethernet, IP, …etc).

A typical DSL network consists the following components:

  • DSL modems at the subscriber side.
  • Local loop copper lines.
  • DSL access multiplexers (DSLAMS) at the CO.

Asymmetric Digital Subscriber line (ADSL):

ADSL is a way to transmit digital signals over analog carriers; the majority of ADSL services coexist on the same physical cable with telephone services (POTS). This is achieved using Frequency division  multiplexing (FDM), FDM splits the two services into two frequency bands the 300Hz to 3.4KHz band for voice and above 3.4K band for data.

The name ADSL stands for Asymmetric digital subscriber line. This name comes from the difference between the downstream and upstream data rates. ADSL has a high downstream data rates vs lower data rates for upstream due to the higher demand for download traffic at subscriber sites.

ADSL operation:

ADSL employs the use of two transceivers one at the subscriber site and another at the CO. These transceivers perform modulation and demodulation operations. The subscriber requires a modem/router and a splitter (more about this later) which connects the local loop to then to the phone set and computer .

DSLAMS  (DSL Access Multiplexers)are used  at the service provider side to connect the multiple ADSL lines to backbone network of the service provider.

ADSL requires a signal splitter or a (low pass high pass filter) that is integrated into the modem, DSLAM or as standalone element. The splitter divides the bandwidth available on the telephone line into two virtual channels to separate voice and data.

ADSL supports rates up to 24Mbps (ADSL2+ ITU G.992.5 Annex M) in the downstream direction and  upto 3.5Mbps in the upstream direction. Its important to mention that ADSL speeds depends on the length  and the quality of the local loop.

That was a brief review of ADSL to refresh our minds because I am planning to visit broadband solutions again very soon.

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