DSL(Digital Sudscriber line)

Discussion in 'Computing and Networks' started by Raheel Niaz, Aug 27, 2010.

  1. Raheel Niaz

    Thread Starter New Member

    Aug 6, 2010
    plz help me out in following questions. I have little bit knowledge but i need more knowledge.
    1. Differences between dialup connection and DSL?
    2.Working of ADSL?
    3.DSL Setup (customer and telephone company site)?
    4.Comparison Of types of xDSL?
    I m waiting for answers. plz help me
  2. retched

    AAC Fanatic!

    Dec 5, 2009
  3. bertus


    Apr 5, 2008
  4. dataman19


    Dec 26, 2009
    First of all - a DSL is always on....
    The modem is not...
    DSL uses a digital carrier to carry the digital signal down the telephone line..
    This requires a DSLAM (Digital Subsriber Line Asych Multiplex)at the telephone company interconnect site. This device puts the Digital carrier on the phone line.
    The DSL Modem at the subscriber's premises is the actual modem/router. A DSL Router/Modem will only connect o to one port of a DSLAM and will not connect to another DSL Modem. The DSLAM provides the Asynchronous timing to make it work - the DSLAM is "master" and the DSL Modem is slave.
    ADSL is Advanced Digital Subscriber Line and provides dual carriers to give additional reliability on longer cable runs.
    The problems with DSL are that the line conditions affect everything. If someone taps into the line the loading will cause the carrier to go out of sync and the Data line is lost. (This is why you get Filters for the phones).
    There are at least a dozen versions of DSL at this time... But they all work essentially the same..
    Incidentally - DSL was not accidental...
    Back in the 1970's Bell Labs (Phoenix, Arizona) and the National Security Agency teamed up to resolve a security issue with the Subscriber Telephone Units (STU) used to provide secure encrypted telephone service for State Department and Embassy communications. the cold war was in full swing and wire tapping was a constant threat. STUs were limited to secure facilities and off site connectivity in foreign nations was out of the question.
    1970's Encryption consisted of a crude voice scrambler. This sounded like garbled audio. Any foreign agent that detected this garbled noise would simply taped it on a small reel-to-reel (or a highly coveted thing called a cassette deck). Then they could take it to another location where they had equipment to fiter and rearrange the audio and essentialy descramble it. This was a major problem in the foreign services. Many key personnel were not always in the Embassy, and many times they were in buildings and facilities physically located out of the embassy compound. Stringing telephone lines in foreign countries was troublesome and sometimes expensive. Not to mention the fact that it was obvious.
    Even the Microwave and Radio traffic was subject to intercept.
    A bell labs Engineering Team was tasked with researching viable recommendations to address the issues. In Phoenix, Arizona a Bell Labs team teamed up with a Motorola Team to integrate a new Digital carrier technology to see if it would fit the problem at hand.
    The result was a highly secretive device that applied a digital asynchronous carrier onto a regular phone line. This allowed a digital Subscriber Telephone Unit to be placed onto a regular phone line. The STU was using a digital carrier so when a phone was picked up it sounded line line noise. The neat characteristic of this set up was that it allowed the user to instantly know when someone tapped into the line. Any second phone picked up on the line caused the asynchronous line to go unstable. In essence - this gave intrusion detection.
    This system was prototyped and handed over to the folks at Langley Virginia. The Bell Labs project was transferred to the Bell labs facility in Virginia as well. Motorola maintained thir research in Phoenix, Arizona.
    The project was perfected and the system went on-line in the mid 70s.
    The break up of the Bell Labs and the selling of assets to Lucent Technologies resulted in the transfer of the old working group engineering notes. The project was deemed worthless and onsolete - the engineering notes were sold in a transfer of assets to GTE and Norcent Technologies.
    GTE took up the project and was abruptly shut down by the NSA - no reason given. Then in the late 1980's the Digital OverGuard Carrier Subset Subscriber Telephone Unit Project was declassified. By this time the Government was into the STU-III and STU-V and the new Encryption KG-40s were in full swing. The early system was deemed obsolete and declassified as such.
    GTE took the bull by the horns and the Norcent Folks jumped back onto the band wagon. The rationale was that if a digital carrier could carry a digitized Encrypted signal over a phone line for 30,000 feet - it could conceivably carry a digital LAN signal. This would mean that CAT 3 cable wouldn't have to be strung from pole to pole and that actual networking could come out of the closet.
    The Internet was a gleam in eveyone's eyes back in 1988. In order to be part of the Research net you had to be a Terminal Mainframe Site on either the DDN or UEIN nets. The DDN was the US Government and The UUNet was the University equivalent. The only way to get a 64K link on the net was to become a node on the NET. This meant a mainframe with TCP/IP Stacks configured, and a UNIX Software suite and multiple Serial Modem Cards, also a T-1 data modem multiplexer was required to interconnect.
    In order to get Internet you needed to set-up multiple modem pools with a Bulletin Board System to allow people to dial-in to your system. But high speed residential internet would be expensive.
    Enter the Norcent and GTE Digital Subscriber Line... It was an instant hit. It allowed the phone company to interconnect multiple subsribers over existing copper phone cables. Now the masses had a high-speed connection to the up and coming Internet Super Highway.
    The rest is History...
    Hope this helps you along
    Dave R. Mason
    Phoenix Computer Labs