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    Digital Subscriber Line (DSL)

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    Junaid Inam
    DSL technologies like ADSL, VDSL, and HDSL provide higher internet speeds over traditional phone lines by utilizing higher bandwidths that existing copper wiring supports. They work by installing DSL modems at the customer's home that separate voice and data signals on the phone line. At the telephone exchange, a DSLAM converts customer data traffic to formats like ATM to transmit to internet providers. ADSL in particular is asymmetric to provide more downstream than upstream bandwidth needed for common internet use.

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    Digital Subscriber Line (DSL)

    Uploaded by

    Junaid Inam
    204 views17 pages
    DSL technologies like ADSL, VDSL, and HDSL provide higher internet speeds over traditional phone lines by utilizing higher bandwidths that existing copper wiring supports. They work by installing DSL modems at the customer's home that separate voice and data signals on the phone line. At the telephone exchange, a DSLAM converts customer data traffic to formats like ATM to transmit to internet providers. ADSL in particular is asymmetric to provide more downstream than upstream bandwidth needed for common internet use.

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    Original Title

    Lecture DSL

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    DSL technologies like ADSL, VDSL, and HDSL provide higher internet speeds over traditional phone lines by utilizing higher bandwidths that existing copper wiring supports. They work by installing DSL modems at the customer's home that separate voice and data signals on the phone line. At the telephone exchange, a DSLAM converts customer data traffic to formats like ATM to transmit to internet providers. ADSL in particular is asymmetric to provide more downstream than upstream bandwidth needed for common internet use.

    Copyright:

    Attribution Non-Commercial (BY-NC)
    Download as PPT, PDF, TXT or read online from Scribd
    Download as ppt, pdf, or txt
    204 views17 pages

    Digital Subscriber Line (DSL)

    Uploaded by

    Junaid Inam
    DSL technologies like ADSL, VDSL, and HDSL provide higher internet speeds over traditional phone lines by utilizing higher bandwidths that existing copper wiring supports. They work by installing DSL modems at the customer's home that separate voice and data signals on the phone line. At the telephone exchange, a DSLAM converts customer data traffic to formats like ATM to transmit to internet providers. ADSL in particular is asymmetric to provide more downstream than upstream bandwidth needed for common internet use.

    Copyright:

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    Download as PPT, PDF, TXT or read online from Scribd
    Download as ppt, pdf, or txt
    of 17

    Digital Subscriber Line (DSL)

    • After traditional modems reached their peak data


    rate, telephone companies developed another
    technology called DSL to provide higher-speed
    access to the internet.
    • DSL is a family of point-to-point technologies to
    provide high-speed data transmission over
    traditional telephone lines. These technologies
    are ADSL, VDSL, HDSL and SDSL.
    Using existing local loops
    • DSL uses existing local loops. But the question arises
    how does DSL reach a data rate that was never
    achieved with traditional modems??
    • The answer is that twisted-pair local loop is actually
    capable of handling bandwidths up to 1.1 MHz. But the
    filters installed at the end of the line by the telephone
    company limits the bandwidth to 4 KHz (sufficient for
    voice communication.
    • Therefore conversion for POTS to DSL usually requires
    just changing the equipment and removing the filter at
    the exchange and not the wiring of the local loop. This is
    the factor which makes DSL technology so attractive.
    Equipment used in DSL
    • Only the equipment at the subscriber end and at
    the exchange are replaced for DSL. The wiring
    remains the same.
    • At the subscriber’s end:
    • Line Splitter: It splits up the voice and data
    transmission for telephone and DSL modem
    respectively. It may consist of just a low-pass
    and bandpass filter. Thus voice transmission
    continues even if the data equipment fails.
    DSL Equipment
    • DSL Modem: It usually uses Ethernet
    10BaseT data format so that it can be
    connected to an Ethernet port of a PC,
    router or hub etc. DSL modem does not
    convert analog to digital signals or vice
    versa. It just converts the incoming data
    into Ethernet format and vice versa. So in
    reality, its not a modem in a strict sense. It
    just converts one standard into another.
    Equipment at the Local Carrier’s
    End
    • Main Distribution Frame (MDF): Using local
    loop, many customers get connected to the Main
    Distribution Frame/Facility (MDF) which acts as
    a splitter. It sends voice traffic to the Voice
    Telephone Network and data traffic to DSLAM.
    • DSLAM: DSL Access Multiplexer (DSLAM)
    converts the data into the ATM (Asynchronous
    Transfer Mode) data format to be transmitted to
    the ATM switch which distributes it to the ISPs
    (Internet Service Providers) for access to the
    Internet.
    High-Bit-Rate Digital Subscriber
    Line (HDSL)
    • HDSL was designed as an alternative to the T-1
    line (1.544 Mbps).
    • Standard T1 line uses alternate mark inversion
    coding (AMI) which occupies a bandwidth of 1.5
    MHz and is very susceptible to attenuation. This
    limits the length of the line to 1km and repeaters
    have to be used for longer distances.
    • HDSL uses the 2B1Q coding scheme to provide
    a data rate of 2 Mbps over 2 twisted pair lines
    within a bandwidth that extends only up to about
    196 kHz. This enables a range of about 3.7 km
    to be achieved.
    HDSL
    • A long string of 0s in the case of AMI might de-
    synchronize the receiver. Another advantage of
    2B1Q is that it also includes timing information in
    the data being transmitted which means that in
    2B1Q, there are transitions in the signal that alert
    the receiver to the beginning, middle or end of the
    pulse. If the receiver’s clock is out of
    synchronization, these alerting points can reset the
    clock.
    Bipolar AMI Encoding
    Symmetric Digital Subscriber Line
    (SDSL)
    • SDSL uses the same coding scheme as
    HDSL i.e. 2B1Q.
    • As the name suggests, its symmetric (like
    HDSL) i.e. it divides the available
    bandwidth equally between the
    downstream and upstream directions.
    • SDSL was developed to provide the same
    type of service as HDSL but over a single
    twisted-pair line.
    Asymmetric Digital Subscriber Line
    (ADSL)
    • ADSL, as the name suggests is asymmetric,
    which refers to the fact that ADSL provides more
    capacity downstream (from the carrier’s central
    office to the customer’s site) than upstream
    (from customer to carrier).
    • Typically while using Internet, the user requires
    far more capacity for downstream than for
    upstream transmission. Thus, ADSL provides a
    perfect fit for the Internet requirement.
    • ADSL is carrier-based unlike HDSL or SDSL
    which use coding.
    ADSL Design
    • Frequency division multiplexing
    – Lowest 25kHz for voice
    • Plain old telephone service (POTS)
    – Use echo cancellation or FDM to give two
    bands (upstream and downstream).
    – Use FDM within bands
    • Range 5.5km
    ADSL Channel Configuration
    Echo Cancellation in ADSL
    • Echo cancellation is a signal processing
    technique that allows transmission of digital
    signals in both directions on a single
    transmission line simultaneously. In essence, a
    transmitter must subtract the echo of its own
    transmission from the incoming signal to recover
    the signal sent by the other side.
    • When echo cancellation is used, the entire
    frequency band for the upstream channel
    overlaps the lower portion of the downstream
    channel. This has 2 advantages:
    Echo Cancellation in ADSL
    • Advantages
    1.The higher the frequency, the greater the attenuation. With the use of
    echo cancellation, more of the downstream bandwidth is in the
    “good” part of the spectrum.
    2.The echo cancellation design is more flexible for changing upstream
    capacity. The upstream channel can be extended upward without
    running into the downstream: instead the area of overlap is
    extended.

    • Disadvantage
    The need for echo cancellation logic on both ends of the line adds to
    the cost and complexity of the equipment.
    Discrete Multitone (DMT)
    • Standard modulation technique for ADSL which
    combines QAM and FDM.
    • Multiple carrier signals at different frequencies
    • Some bits on each channel
    • 4kHz subchannels
    • Send test signal and use subchannels with better signal
    to noise ratio
    • 256 downstream subchannels at 4kHz (60kbps)
    – 15.36Mbps
    – Impairments bring this down to 1.5Mbps to 9Mbps
    DMT Transmitter
    Very-High-Bit-Rate DSL(VDSL)
    • An alternative approach to ADSL
    • Uses DMT
    • Bit rate of 50-55 Mbps downstream and
    1.5-2.5 Mbps upstream.

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