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    A Mini project training on


    Submitted in partial fulfillment of the requirements for the award of theDegree of





    M.RAMYA 09P31A0469


    Mr. N.RAJESH BABUM.Tech , Associate Professor,ECE



    (Affiliated to JNTUK, Kakinada& Approved by AICTE, New Delhi)

    Surampalem, East Godavari District

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    Department of

    Electronics and Communication Engineering


    This is to certify that the mini project report entitled


    being submitted by

    M.RAMYA 09P31A0469

    This is to certify that the mini project work titledBROADBAND ADSL/VDSL

    TECHNOLOGYIN BSNL is a bonafide work ofM.RAMYA carried out in partialfulfillment of the requirements under our guidance.

    Project guide Head of the Department


    Associate Professor , ECE M.Tech(Ph.D.)

    HOD, ECE

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    Performing Mini Project is an important role in shaping up an

    Engineering student for practical knowledge and to be update with latest

    Technologies. First of all, I would like to express my attitude towards Head of

    the Department of Electronics and Communication Engineering

    Mr.R.V.V.KRISHNA, M.Tech(Ph.D.), Associate Professor and HOD,ECE for his

    guidance throughout our Mini Project work. With great pleasure we want to

    take this opportunity to express our heartfelt gratitude to all the people who

    helped in making this Mini Project work a grand success.

    We would like to thank Mr.N.RAJESH BABU , M.Tech ,Associate Professor,

    ECE as my mini project guide for giving valuable suggestions.

    First of all we are highly indebted to Principal Dr. CH.SRINIVASA

    RAO, M.Tech, Ph.D., FIETEfor giving us the permission to carry out this Mini

    Project.We would like to thank the HOD & Other Teaching Staff of ECE

    Department for sharing their knowledge with us.

    We thank Sri K.SURESH KUMAR S.D.E and Co-Staff Members, BSNL,

    Rajahmundry for extending their utmost support and cooperation in providing

    successful completion of the Project.

    We thank Sri V.RAMESH BABU, Assistant General Manager(admn.) of

    BSNL, Rajahmundry for extending their utmost support and cooperation in

    providing all the provisions for the successful completion of the Project.

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    Digital Subscriber Line (DSL) technology is a modem technology

    that uses existing twisted-pair telephone lines to transport high-bandwidth

    data, such as multimedia and video, to service subscribers. The term

    Xdsl covers a number of similar yet competing forms of DSL , including

    ADSL , SDSL , HDSL , RADSL , and VDSL . xDSL is drawing significant

    attention from implementers and service providers because it promises to

    deliver high-bandwidth data rates to dispersed locations with relatively

    small changes to the existing telco infrastructure .

    xDSL services are dedicated , point-to-point , public network access over

    twisted-pair copper wire on the local loop (last mile) between a

    network service provider (NSPs) central office and the customer site , or

    on local loops created either intra-building or intra campus. Digital The

    benefits Subscriber Line (DSL) technologies has revolutionized Internet

    access of DSL technology, coupled with the deregulation of the tele-

    communications industry, have caused in increase in the number ofservice providers (xSP) offering DSL services.

    Everyone from ILECs to CLECs to ISPs are offering DSL services to

    homes and Businesses-with Asymmetric Digital Subscriber Line (ADSL)

    currently being the most common and cost effective choice. Research

    into DSL technologies has produced variants of ADSL to help resolve

    issues users are faced with today, as well as plan for future

    implementations. One of these variants is Very High Bit-Rate Digital

    Subscriber Line (VDSL). VDSL differs from the other DSL technologies

    primarily in the areas of speed and distance. Lower costs, competition

    with other technologies and forward thinking for future bandwidth

    requirements are contributing to making VDSL a variable technology for

    even wider implementation. Currently the primary focus in xDSL is the

    . development and deployment of ADSL and VDSL technologies and


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    1. Asymmetric Digital Subscriber Line (ADSL) 6-11

    1.1ADSL capabilities

    1.2ADSL technology

    1.3ADSL standards and associations

    1.4ADSL market status

    2. Very High Data Rate Digital Subscriber Line (VDSL) 12-14

    2.1VDSL projected capabilities

    3. ADSL 15-22



    3.3Interleaving and fast path

    3.4Installation problems

    3.5Transport protocols

    3.6ADSL standards

    4. Digital Subscriber Line Access Multiplexer 23-26

    4.1Path taken by data to DSLAM

    4.2 Role of DSLAM

    5. Conclusion 27

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    Asymmetric Digital Subscriber Line (ADSL)

    ADSL technology is asymmetric. It allows more bandwidth

    downstream from an NSPs Central office to the customer sitethan

    upstream from the subscriber to the central office. This asymmetry,

    combined with always-on access (which eliminates call setup), makes

    ADSL ideal for Internet/intranet surfing, video-on-demand, and remote

    LAN access. Users of these applications typically download much more

    information than they send. ADSL transmits more than 6 Mbps to a

    subscriber, and as much as 640 kbps more in both directions (shown in

    Figure ). Such rates expand existing access capacity by a factor of 50

    or more without new cabling. ADSL can literally transform the existing

    public information network from one limited to voice, text, and low-

    resolution graphics to a powerful, ubiquitous system capable of bringing

    multimedia, including full motion video, to every home this century.

    FIGURE 1

    1.1 ADSL Capabilities

    An ADSL circuit connects an ADSL modem on each end of a twisted-

    pair line, creating three information channels - a high speed downstream

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    channel, a medium speed duplex channel, and a basic telephone service

    channel. The basic telephone service channel is split off from the

    digital modem by filters, thus guaranteeing uninterrupted basic telephone

    service, even if ADSL fails. The high-speed channel ranges from 1.5 to

    6.1 Mbps, and duplex rates range from 16 to 640 kbps. Each channel can

    be submultiplexed to form multiple lower-rate channels.

    ADSL modems provide data rates consistent with North American T1

    1.544 Mbps and European E1 2.048 Mbps digital hierarchies (see Figure

    1.2) and can be purchased with various speed ranges and capabilities.

    The minimum configuration provides 1.5 or 2.0 Mbps downstream and a

    16 kbps duplex channel; others provide rates of 6.1 Mbps and 64 kbps

    duplex. Products with downstream rates upto 8 Mbps and duplex rates

    up to 640 Kbps are available today ADSL modems accommodate

    Asynchronous Transfer Mode (ATM) transport with variable rates and

    compensation for ATM overhead, as well as IP protocols. Downstream

    data rates depend on a number of factors, including the length of the

    copper line, its wire gauge, presence of bridged taps, and cross-coupled

    interference. Line attenuation increases with line length and frequency

    and decreases as wire diameter increases. Ignoring bridged taps ADSL

    performs as shown in Table 1.1.

    Table 1: Claimed ADSL Physical-Media Performance








    1.5 or




    0.5mm 5.5km

    1.5 or




    0.4mm 4.6km

    6.1Mbps 24


    0.5mm 3.7km

    1.5 or 2




    0.4mm 2.7

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    Table 2: This chart shows the speeds for downstream bearer and duplex

    bearer channels

    Although the measure varies from telco to telco , these capabilities can

    cover up to 95% of a loop plant, depending on the desired data rate.

    Customers beyond these distances can be reached with fiber-based

    digital loop carrier (DLC) systems . As these DLC systems become

    commercially available , telephone companies can offer virtually

    ubiquitous access in a relatively short time.

    Many applications envisioned for ADSL involve digital compressedvideo . As a real-time signal, digital video cannot use link- or network-

    level error control procedures commonly found in data communications

    systems . ADSL modems therefore incorporate forward error correction

    that dramatically reduces errors caused by impulse noise . Error

    correction on a symbol-by-symbol basis also reduces errors caused by

    continuous noise coupled into a line.

    Downstream Bearer Channels

    n*1.536 Mbps 1.536 Mbps

    3.072 Mbps

    4.608 Mbps

    6.144 Mbps

    N*2.048 Mbps 2.048 Mbps

    4.096 Mbps

    Duplex Bearer Channels

    C Channel 16 kbps

    64 kbps

    Optional Channel 160 kbps

    384 kbps

    544 kbps

    576 kbps

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    1.2 ADSL Technology

    ADSL depends on advanced digital signal processing and creative

    algorithms to squeeze so much information through twisted-pair telephone

    lines. In addition, many advances have been required in transformers, analog

    filters, and analog/digital (A/D) converters. Long telephone lines may

    attenuate signals at 1 MHz (the outer edge of the band used by ADSL) by as

    much as 90 dB, forcing analog sections of ADSL modems to work very hard

    to realize large dynamic ranges, separate channels, and maintain low noise

    figures. On the outside, ADSL looks simple transparent synchronous data

    pipes at various data rates over ordinary telephone lines. The inside, where all

    the transistors work, is a miracle of modern technology.

    To create multiple channels, ADSL modems divide the available bandwidth of

    a telephone line in one of two ways-frequency division multiplexing (FDM)

    or echo cancellation-as shown in figure 1.3. FDM assigns one band for

    upstream data and another band for downstream data .The downstream path is

    then divided by time-division multiplexing into one or more high-speed

    channels and one or more low-speed channels .The upstream path is alsomultiplexed into corresponding low-speed channels .Echo cancellation assigns

    the upstream band to overlap the downstream, and separates the two by means

    of local echo cancellation, a technique well known in V.32 and V.34 modems.

    With either technique, ADSL splits off a 4khz region for basic telephone

    service at the DC end of the band.

    Figure 2: Graph of Upstream and Downstream

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    An ADSL modem organizes the aggregate data stream created by multiplexing

    downstream channels, duplex channels, and maintenance channels together into

    blocks, and attaches an error correction code to each block. The receiver then

    corrects errors that occur during transmission up to the limits implied by the

    code and the block length. The unit may, at the users option, also create

    superblocks by interleaving data within subblocks; this allows the receiver to

    correct any combination of errors within a specific span of bits. This in turn

    allows for effective transmission of both data and video signals.

    1.3 ADSL Standards and Associations

    The American National Standards Institute (ANSI) Working Group

    T1E1.4 recently approved an ADSL standard at rates up to 6.1 Mbps (ANSI

    Standard T1.413). The European Technical Standards Institute (ETSI)

    contributed an annex to T1.413 to reflect European requirements. T1.413

    currently embodies a single terminal interface at the premises end. Issue II,

    now under study by T1E1.4, will expand the standard to include a multiplexed

    interface at the premises end, protocols for configuration and network

    management, and other improvements.

    The ATM Forum and the Digital Audio-Visual Council (DAVIC) have both

    recognized ADSL as a physical-layer transmission protocol for UTP media.

    The ADSL Forum was formed in December 1994 to promote the ADSL

    concept and facilitate development of ADSL system architectures, protocols,

    and interfaces for major ADSL applications. The forum has more than 200

    members, representing service providers, equipment manufacturers, and

    semiconductor companies throughout the world. At present, the Forums

    formal technical work is divided into the following six areas, each of which is

    dealt with in a separate working group within the technical committee:

    ATM over ADSL (including transport and end-to-end architecture aspects)

    Packet over ADSL (this working group recently completed its work)

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    CPE/CO (customer premises equipment/central office) configurations and



    Network management

    Testing and interoperability

    1.4 ADSL Market Status

    ADSL modems have been tested successfully in more than 30 telephone

    companies, and thousands of lines have been installed in various technology

    trials in North America and Europe. Severaltelephone companies plan market

    trials using ADSL, principally for data access, but also including video

    applications for uses such as personal shopping, interactive games, and


    Semiconductor companies have introduced transceiver chipsets that are

    already being used in market trials. These chipsets combine off-the-shelfcomponents, programmable digital signal processors, and custom ASICs

    (application-specific integrated circuits). Continued investment by these

    semiconductor companies has increased functionality and reduced chip count,

    power consumption, and cost, enabling mass deployment of ADSL-based


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    Very-High-Data-Rate DigitalSubscriber Line (VDSL)

    It is becoming increasingly clear that telephone companies around theworld are making decisions to include existing twisted-pair loops in their

    next-generation broadband access networks. Hybrid fiber coax (HFC), a

    shared-access medium well suited to analog and digital broadcast, comes up

    somewhat short when used to carry voice telephony, interactive video, and

    high-speed data communications at the same time. Fiber all the way to the

    home (FTTH) is still prohibitively expensive in a marketplace soon to be

    driven by competition rather than cost. An attractive alternative, soon to be

    commercially practical, is a combination of fiber cables feeding neighborhood

    optical network units (ONUs) and last-leg-premises connections by existing or

    new copper. This topology, which is often called fiber to the neighborhood

    (FTTN), encompasses fiber to the curb (FTTC) with short drops and fiber to

    the basement (FTTB), serving tall buildings with vertical drops.

    One of the enabling technologies for FTTN is VDSL. In simple terms, VDSL

    transmits high speed data over short reaches of twisted-pair copper telephone

    lines, with a range of speeds depending on actual line length. The maximum

    downstream rate under consideration is between 51 and 55 Mbps over lines up

    to 1000 feet (300 m) in length. Downstream speeds as low as 13 Mbps over

    lengths beyond 4000 feet (1500 m) are also common. Upstream rates in early

    models will be asymmetric, just like ADSL, at speeds from 1.6 to 2.3 Mbps.

    Both data channels will be separated in frequency from bands used for basic

    telephone service and Integrated Services Digital Network (ISDN), enabling

    service providers to overlay VDSL on existing services. At present the two

    high-speed channels are also separated in frequency. As needs arise for higher-

    speed upstream channels or symmetric rates, VDSL systems may need to use

    echo cancellation.

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    2.1 VDSL Projected Capabilities

    Although VDSL has not achieved ADSLs degree of definition, it has

    advanced far enough that we can discuss realizable goals, beginning with data

    rate and range. Downstream rates derive from submultiples of the SONET

    (Synchronous Optical Network) and SDH (Synchronous Digital Hierarchy)

    canonical speed of 155.52 Mbps, namely 51.84 Mbps, 25.92 Mbps, and 12.96

    Mbps. Each rate has a corresponding target range:

    Table 3:Target Ranges







    12.96-13.8 4500 1500

    25.92-27.6 3000 1000

    51.84-55.2 1000 300

    Upstream rates under discussion fall into three general ranges:

    1.62.3 Mbps.

    19.2 Mbps

    Equal to downstream

    Early versions of VDSL will almost certainly incorporate the slower

    asymmetric rate. Higher upstream and symmetric configurations may only be

    possible for very short lines. Like ADSL, VDSL must transmit compressedvideo, a real-time signal unsuited to error retransmission schemes used in data

    communications. To achieve error rates compatible with those of compressed

    video, VDSL will have to incorporate forward error correction (FEC) with

    sufficient interleaving to correct all errors created by impulsive noise events

    of some specified duration.

    Interleaving introduces delay, on the order of 40 times the maximum length

    correctable impulse. Data in the downstream direction will be broadcast to

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    every CPE on the premises or be transmitted to a logically separated hub that

    distributes data to addressed CPE based on cell or time-division multiplexing

    (TDM) within the data stream itself. Upstream multiplexing is more difficult.

    Systems using a passive network termination (NT) must insert data onto a

    shared medium, either by a form of TDM access (TDMA) or a form of

    frequency-division multiplexing (FDM). TDMA may use a species of token

    control called cell grants passed in the downstream direction from the ONU

    modem, or contention, or both (contention for unrecognized devices, cell

    grants for recognized devices). FDM gives each CPE its own channel,

    obviating a Media Access Control (MAC) protocol, but either limiting data

    rates available to any one CPE or requiring dynamic allocation of bandwidthand inverse multiplexing at each CPE. Systems using active NTs transfer the

    upstream collection problem to a logically separated hub that would use

    (typically) Ethernet or ATM protocols for upstream multiplexing.

    Migration and inventory considerations dictate VDSL units that can operate at

    various (preferably all) speeds with automatic recognition of a newly

    connected device to a line or a change in speed. Passive network interfaces

    need to have hot insertion, where a new VDSL premises unit can be put on the

    line without interfering with the operation of other modems.

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    3.1 Overview

    Figure 3: MODEM

    A gateway is commonly used to make an ADSL connection.

    ADSL differs from the less common symmetric digital subscriber line

    (SDSL) in that bandwidth (and bit rate) is greater toward the customer

    premises (known as downstream) than the reverse (known as upstream).This

    is why it is called asymmetric. Providers usually market ADSL as a service

    for consumers to provide Internet access in a relatively passive mode: able to

    use the higher speed direction for the download from the Internet but not

    needing to run servers that would require high speed in the other direction.

    There are both technical and marketing reasons why ADSL is in many places

    the most common type offered to home users. On the technical side, there is

    likely to be more crosstalk from other circuits at the DSLAM end (where the

    wires from many local loops are close to each other) than at the customer

    premises. Thus the upload signal is weakest at the noisiest part of the local

    loop, while the download signal is strongest at the noisiest part of the local

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    loop. It therefore makes technical sense to have the DSLAM transmit at a

    higher bit rate than does the modem on the customer end. Since the typical

    home user in fact does prefer a higher download speed, the telephone

    companies chose to make a virtue out of necessity, hence ADSL. On the

    marketing side, limiting upload speeds limits the attractiveness of this service

    to business customers, often causing them to purchase higher cost leased line

    services instead. In this fashion, it segments the digital communications

    market between business and home users.

    3.2 Operation

    Currently, most ADSL communication is full-duplex. Full-duplex ADSL

    communication is usually achieved on a wire pair by either frequency-

    division duplex (FDD), echo-cancelling duplex (ECD), or time-division

    duplex (TDD). FDD uses two separate frequency bands, referred to as the

    upstream and downstream bands. The upstream band is used for

    communication from the end user to the telephone central office. The

    downstream band is used for communicating from the central office to the

    end user.Figure 4:Graph of frequency ranges of Upstream and Downstream

    Frequency plan for ADSL. Red area is the frequency range used by normal

    voice telephony (PSTN), the green (upstream) and blue (downstream) areas

    are used for ADSL.

    With standard ADSL (annex A), the band from 26.000kHz t

    137.825kHz is used for upstream communication, whilecommunication.




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    Under the usual DMT scheme, each of these is further divided into smaller

    frequency channels of 4.3125kHz. These frequency channels are

    sometimes termed ''bins''. During initial training to optimize transmission

    quality and speed, the ADSL modem tests each of the bins to determine the

    signal-to-noise ratio at each bin's frequency. Distance from the telephone

    exchange, cable characteristics, interference from AM radio stations, and local

    interference and electrical noise at the modem's location can adversely affect

    the signal-to-noise ratio at particular frequencies. Bins for frequencies

    exhibiting a reduced signal-to-noise ratio will be used at a lower throughput

    rate or not at all; this reduces the maximum link capacity but allows the

    modem to maintain an adequate connection. The DSL modem will make a

    plan on how to exploit each of the bins, sometimes termed "bits per bin"

    allocation. Those bins that have a good signal-to-noise ratio (SNR) will be

    chosen to transmit signals chosen from a greater number of possible encoded

    values (this range of possibilities equating to more bits of data sent) in each

    main clock cycle. The number of possibilities must not be so large that the

    receiver might incorrectlydecode which one was intended in the presence of

    noise. Noisy bins may only be required to carry as few as two bits, achoice

    from onlyone of four possible patterns, or only one bit per bin in the case of

    ADSL2+, and very noisy bins are not used at all. If the pattern of noise versus

    frequencies heard in the bins changes, the DSL modem can alter the bits-per-

    bin allocations, in a process called "bitswap", where bins that have become

    more noisy areonly required to carry fewer bits and other channels will be

    chosen to be given a higher burden. The data transfer capacity the DSL

    modem therefore reports is determined by the total of the bits-per- bin

    allocations of all the bins combined. Higher signal-to-noise ratios and more

    bins being in use gives a higher total link capacity, while lower signal-to

    noise ratios or fewer bins being used gives a low link capacity.

    The total maximum capacity derived from summing the bits-per-bins is

    reported by DSL modems and is sometimes termed ''sync rate''. This will

    always be rather misleading, as the true maximum link capacity for user data

    transfer rate will be significantly lower; because extra data are transmitted

    that are termed ''protocol overhead'', reduced figures for PPPoA connections of

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    around 84-87 percent, at most, being common. In addition, some ISPs will

    havetraffic policies that limit maximum transfer rates further in the networks

    beyond the exchange, and traffic congestion on the Internet, heavy loading on

    servers and slowness or inefficiency in customers' computers may all

    contribute to reductions below the maximum attainable. When a wireless

    access point is used, low or unstable wireless signal quality can also cause

    reduction or fluctuation of actual speed.

    The choices the DSL modem make can also be either conservative, where the

    modem chooses to allocate fewer bits per bin than it possibly could, a choice

    which makes for a slower connection, or less conservative in which more bits

    per bin are chosen in which case there is a greater risk case of error should

    future signal-to-noise ratios deteriorate to the point where the bits-per-bin

    allocations chosen are too high to cope with the greater noise present. This

    conservatism, involving a choice of using fewer bits per bin as a safeguard

    against future noise increases, is reported as the signal-to-noise ratio ''margin''

    or ''SNR margin''. The telephone exchange can indicate a suggested SNR

    margin to the customer's DSL modem when it initially connects, and the

    modem may make its bits-per-bin allocation plan accordingly. A high SNR

    margin will mean a reduced maximum throughput, but greater reliability and

    stability of the connection. A low SNR margin will mean high speeds,

    provided the noise level does not increase too much; otherwise, the connection

    will have to be dropped and renegotiated (resynced). ADSL2+ can better

    accommodate such circumstances, offering a feature termed ''seamless rate

    adaptation'' (SRA), which can accommodate changes in total link capacity

    with less disruption to communications.

    Vendors may support usage of higher frequencies as a proprietary extension to

    the standard. However, this requires matching vendor-supplied equipment on

    both ends of the line, and will likely result in crosstalk problems that affect

    other lines in the same bundle. There is a direct relationship between the

    number of channels available and the throughput capacity of the ADSL

    connection. The exact data capacity per channel depends on the

    modulation method used.

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    ADSL initially existed in two versions (similar to VDSL), namely CAP and

    DMT. CAP was the ''de facto'' standard for ADSL deployments up until 1996,

    deployed in 90 percent of ADSL installs at the time. However, DMT was

    chosenfor the first ITU-T ADSL standards, G.992.1 and G.992.2 (also called

    ''G.dmt'' and ''G.lite'' respectively). Therefore all modern

    installations of ADSL are based on the DMT modulation scheme.

    3.3 Interleaving and fast path

    ISPs (rarely, users) have the option to use interleaving of packets to counter

    the effects of burst noise on the telephone line. An interleaved line has a

    depth, usually 8 to 64, which describes how many Reed-Solomon codeword

    are accumulated before they are sent. As they can all be sent together, their

    forward error correction codes can be made more resilient. Interleaving adds

    latency as all the packets have to first be gathered (or replaced by empty

    packets) and they, of course, all take time to transmit. 8 frame interleaving

    adds 5 ms round-trip-time, while 64 deep interleaving adds 25 ms. Other

    possible depths are 16 and 32.

    "Fastpath" connections have an interleaving depth of 1, that is one packet is

    sent at a time. This has a low latency, usually around 10 ms (interleaving adds

    to it, this is not greater than interleaved) but it extremely prone to errors, as

    any burst of noise can take out the entire packet and so require it all to be

    retransmitted. Such a burst on a large interleaved packet only blanks part of

    the packet, it can be recovered from error correction information in the rest of

    the packet. A "fastpath" connection will result in extremely high latency on a

    poor line, as each packet will take many retries.

    3.4 Installation Problems

    ADSL deployment on an existing plain old telephone service (POTS)

    telephone line presents some problems because the DSL is within a frequency

    band that might interact unfavourably with existing equipment connected to

    the line. Therefore, it is necessary to install appropriate frequency filters at the

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    customer's premises to avoid interference between the DSL, voice services

    and any other connections to the line, for example in support of intruder

    alarms "Red Care" being an example in the UK. This is desirable for the voice

    service and essential for a reliable ADSL connection.

    In the early days of DSL, installation required a technician to visit the

    premises. A splitter or ''micro filter'' was installed near thedemarcation point,

    fromwhich adedicated data line was installed. This way, the DSL signal is

    separatedas close as possibletothe central office and is not attenuated inside

    the customer's premises. However, this procedure was costly, and also caused

    problems with customers complaining about having to wait for the technician

    to perform the installation. So, many DSL providers started offering a "self-

    install" option, in which the provider provided equipment and instructions to

    the customer. Instead of separating the DSL signal at the demarcation point,

    the DSL signal is filtered at each telephone outlet by use of a low-pass filter

    for voice and a high-pass filter for data, usually enclosed in what is known as a

    micro filter. This microfilter can be plugged by an end user into any 'phone

    jack: it does not require any rewiring at the customer's premises.

    Commonly, microfilters are only low-pass filters, so beyond them only low

    frequencies (voice signals) can pass. In the data section, a microfilter is not

    used because digital devices that are intended to extract data from the DSL

    signal will, themselves, filter out low frequencies. Voice telephone devices

    will pick up the entire spectrum so high frequencies, including the ADSL

    signal, will be "heard" as noise in telephone terminals, and will affect and

    often degrade the service in fax, data phones and modems. From the point of

    view of DSL devices, anyacceptance of their signal by POTS devices mean

    that there is a degradation of the DSL signal to the devices, and this is the

    central reason why these filters are required.

    A side effect of the move to the self-install model is that the DSL signal can be

    degraded, especially if more than 5 voiceband (that is, POTS telephone-like)

    devices are connected to the line. Once a line has had DSL enabled, the DSL

    signal is present on all telephone wiring in the building, causing attenuation

    and echo. A way to circumvent this is to go back to the original model, and

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    install one filter upstream from all telephone jacks in the building, except for

    the jack to which the DSL modem will be connected. Since this requires

    wiring changes by the customer, and may not work on some household

    telephone wiring, it is rarely done. It is usually much easier to install filters at

    each telephone jack that is in use.

    DSL signals may be degraded by older telephone lines, surge protectors,

    poorly-designed micro filters, radio-frequency interference, electrical noise,

    and by long telephone extension cords. Telephone extension cords are

    typically made with small-gauge, multi-strand copper conductors which do

    not maintain a noise-reducing pair twist. Such cable is more susceptible to

    electromagnetic interference and has more attenuation than solid twisted-pair

    copper wires typically wired to telephone jacks. These effects are especially

    significant where the customer's phone line is more than 4km from the

    DSLAM in the telephone exchange, which causes the signal levels to be lower

    relative to any local noise and attenuation. This will have the effect of

    reducing speeds or causing connection failures.

    3.5Transport Protocols

    ADSL defines three "Transmission protocol-specific transmission convergence

    (TPS-TC)" layers:

    * Synchronous Transport Module (STM), which allows the transmission of

    frames of the Synchronous Digital Hierarchy (SDH)

    * Asynchronous Transfer Mode (ATM)

    * Packet Transfer Mode (starting with ADSL2, see below)

    In home installation, the prevalent transport protocol is ATM. On top of ATM,

    there are multiple possibilities of additional layers of protocols(two of them

    are abbreviated in a simplified manner as "PPPoA" or "PPPoE"), with the all-

    important TCP/IP at layer 4 of the OSI model providing the connection to the


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    Table 4:ADSL Standards

    Standard name Common




    Upstream rate




















    ADSL2 12Mb






    Annex J

    ADSL2 12Mb






    Annex L









    ADSL2+ 24Mb






    Annex L









    Annex M

    ADSL2+ 28Mb




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    Digital Subscriber Line Access Multiplexer

    Figure 5: Siemens DSLAM SURPASS hiX 5625

    A digital subscriber line access multiplexer (DSLAM, often

    pronounced dee-slam) is a network device, often located in the telephone

    exchanges of the telecommunications operators. It connects multiple customer

    digital subscriber line (DSL) interfaces to a high-speed digital

    communications channel using multiplexing techniques. By placing additional

    DSLAMs at locations remote from the telephone exchange, telephone

    companies provide DSL service to locations previously beyond effective


    4.1 Path taken by data to DSLAM

    1.Customer premises: DSL modem terminating the ADSL, SHDSL or VDSL

    circuit and providing LAN interface to single computer or LAN segment.

    2.Local loop: the telephone company wires from a customer to the telephone

    exchange or to a serving area interface, often called the "last mile" (LM).

    3.Telephone exchange:

    Main distribution frame (MDF): a wiring rack that connects outside subscriber
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    lines with internal lines. It is used to connect public or private lines coming

    into the building to internal networks. At the telco, the MDF is generally in

    proximity to the cable vault and not far from the telephone switch.

    xDSL filters: DSL filters are used in the telephone exchange to split voice from

    data signals. The voice signal can be routed to a POTS provider or left unused

    whilst the data signal is routed to the ISP DSLAM via the HDF (see next entry).

    Handover distribution frame (HDF): a distribution frame that connects the last

    mile provider with the service provider's DSLAM

    DSLAM: a device for DSL service. The DSLAM port where the subscriber

    local loop is connected converts analog electrical signals to data traffic

    (upstream traffic for data upload) and data traffic to analog electrical signals

    (downstream for data download).

    4.2 Role of the DSLAM

    Figure 6: xDSL Connectivity diagram

    The DSLAM equipment collects the data from its many modem ports and

    aggregates their voice and data traffic into one complex composite "signal"

    via multiplexing. Depending on its device architecture and setup, a DSLAM

    aggregates the DSL lines over its Asynchronous Transfer Mode (ATM), frame

    relay, and/or Internet Protocol network (i.e., an IP-DSLAM using PTM-TC

    Packet Transfer Mode - Transmission Convergence) protocol(s) stack.

    The aggregated traffic is then directed to a telco's backbone switch, via an

    access network (AN) also called a Network Service Provider (NSP) at up to

    10 Gbit/s data rates.
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    The DSLAM acts like a network switch since its functionality is at Layer2 of

    the OSI model. Therefore it cannot re-route traffic between multiple IP

    networks, only between ISP devices and end-user connection points. The

    DSLAM traffic is switched to a Broadband Remote Access Server where the

    end user traffic is then routed across the ISP network to the Internet.

    Customer-premises equipment that interfaces well with the DSLAM to which

    it is connected may take advantage of enhanced telephone voice and data line

    signaling features and the bandwidth monitoring and compensation

    capabilities it supports.

    A DSLAM may or may not be located in the telephone exchange, and may

    also serve multiple data and voice customers within a neighborhood serving

    area interface, sometimes in conjunction with a digital loop carrier. DSLAMs

    are also used by hotels, lodges, residential neighborhoods, and other

    businesses operating their own private telephone exchange.

    In addition to being a data switch and multiplexer, a DSLAM is also a large

    collection of modems. Each modem on the aggregation card communicates

    with a single subscriber's DSL modem. This modem functionality is integrated

    into the DSLAM itself instead of being done via an external device like a

    traditional computer modem.Like traditional voice-band modems, a DSLAM's

    integrated DSL modems usually have the ability to probe the line and to adjust

    themselves to electronically or digitally compensate for forward echoes and

    other bandwidth-limiting factors in order to move data at the maximum

    connection rate capability of the subscriber's physical line.

    This compensation capability also takes advantage of the better performance of

    "balanced line" DSL connections, providing capabilities for LAN segments

    longer than physically similar unshielded twisted pair (UTP) Ethernet

    connections, since the balanced line type is generally required for its hardware

    to function correctly. This is due to the nominal line impedance (measured in

    Ohms but comprising both resistance and inductance) of balanced lines being

    somewhat lower than that of UTP, thus supporting 'weaker' signals (however

    the solid-state electronics required to construct such digital interfaces is more


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    Because of the extremely high speeds that VDSL can accommodate, it is

    being looked at as a good prospective technology for accommodating high

    bandwidth applications like VoIP telephony and even HDTV transmission,

    which ADSL is not capable of. Another very useful feature of VDSL stems from

    thefact that it uses 7 different frequency bands for the transmission of data.

    The user then has the power to customize whether each frequency band

    would be used for download or upload. This kind of flexibility is very nice in

    case you need to host certain files that are to be downloaded by a lot of

    people. The most major drawback for VDSL is the distance it needs to be

    away from the telephone exchange. Because of this, ADSL is still preferable

    unless you live extremely close to the telephone exchange of the company

    that you are subscribed to. Due to the limitations of VDSL and its high price,

    its expansion is not as prolific as that of ADSL. VDSL is only widespread in

    countries like South Korea and Japan. While other countries also have VDSL

    offerings, it is only handled from a few companies; mostly one or two in most

    countries. In comparison, ADSL is very widely used and all countries that offer

    high speed internet offer ADSL. Hence VDSL is faster than ADSL and is not

    widespread as ADSL. But still ADSL is better for homes that are much farther

    from the DSLAM.

    ADSL was born of the need for speed coupled with the desire for low cost

    dedicated remote network access. There is no doubt that ADSL will

    revolutionize the way we see the World Wide Web, and quite possibly

    witness the demise of home entertainment as we know it. As the phoenix

    from the flames we will see ADSL emerge heralding the coming of a new age

    of remote multimedia. There is little doubt that ADSL will be around for a long

    time to come, albeit under another name.

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    If we are to truly realise the potential of the cyberspace concept we will need

    to access it with as much convenience as turning on the television. With the

    internet influencing our lives more and more each day, it will be high speed

    ADSL connections that power the revolution. In the future people will view

    ADSL like they view cable TV. That such a small object as an ADSL card may

    wield such an influence over our lives may seem a little unbalanced, or is that


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    1. . Thomas; E. Gray (January 2001), RFC 3037: LDP Applicability, IETF2. de Ghein, Luc, MPLS Fundamentals, pp. 249326

    3. Raza et al., Online routing of bandwidth guaranteed paths with local restoration usingoptimized aggregate usage information, IEEE-ICC 2005, retrieved 2006-10-27.

    4. "Deploying IP and MPLS QoS for Multiservice Networks: Theory and Practice" by JohnEvans, Clarence Filsfils (Morgan Kaufmann, 2007, ISBN 0-12-370549-5)

    5. Rick Gallaher's MPLS Training Guide (ISBN 1932266003).

    6. S. Bryant; P. Pate (March 2005), RFC 3985: Pseudo Wire Emulation Edge-to-Edge (PWE3)Architecture, IETF

    7. de Ghein, Luc, MPLS Fundamentals, pp. 249326.8. "AT&TFrame Relay and IP-Enabled Frame Relay Service (Product Advisor)", Research

    and Markets, June 2007.








    Text book

    Telecommunication and switching systems and networks by thiagarajan viswanathan

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