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ADSL High-Speed Access to the Internet

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VoicenData Bureau
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Asymmetrical Digital Subscriber Line (ADSL) is the

transmission technique used on the line from the modem to the service provider.

Beyond the point at which the subscriber's line is terminated in the exchange,

other technologies are responsible for data transmission.

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ADSL defines how data can be transmitted between a user's

premises (home or office) and the local telephone exchange over normal telephone

wiring. The telephone companies call this telephone wiring 'the local loop' as

the telephone receiver is connected across the two wires, causing them to appear

as a loop when viewed from the local exchange. It is only at the local loop that

ADSL communications actually take place.

Getting data to and from the local telephone exchange is not,

in itself, of much use. The purpose of ADSL services is to enable high-speed

access to the Internet. Although we assume that ADSL is used to carry data using

Internet protocols, how this is done is not, in fact, part of the ADSL

specifications. This gives rise to some of the variations that occur in

practical implementations of ADSL.

ADSL was originally devised as a way of delivering digital

television over telephone wires and this may be a significant application in the

future. For now, the main use of ADSL exploits the unused analogue bandwidth

that is potentially available in the wires that run from the user premises to

the local exchange. This wiring was designed to carry that portion of the

frequency spectrum that is occupied by normal speech. The wires can, however,

carry frequencies above this rather limited spectrum. This is the portion that

ADSL uses. We can now see how voice and ADSL data can share the same telephone

line. In fact, splitters are used to ensure that the data and voice do not

interfere with each other.

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The frequencies that the local loop can carry, and hence, the

amount of data transmission capacity that is available, depend on a number of

factors such as:

  • the distance from

    the local exchange

  • the type and

    thickness of wires used

  • the number and

    type of joins in the wire

  • the proximity of

    the wire to other wires carrying ADSL, ISDN and other non-voice signals

  • the proximity of

    the wires to radio transmitters.

ADSL vs. ISDN vs. Dial-up

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  • PSTN and ISDN are

    dial-up technologies. ADSL is 'always-on'.

  • ADSL is

    un-metered and charged at a flat rate.

  • PSTN and ISDN

    allow you to choose the Internet Service Provider you want to use. ADSL

    connects you to a pre-defined ISP.

  • ISDN runs at

    64kbps or 128kbps. ADSL can potentially download at 8Mbps.

  • Many home ADSL

    services are provided at around 512kbps.

  • PSTN stops you

    using your phone. ADSL allows you to surf and phone at the same time.

The ADSL modem is connected to the telephone wiring (called

the 'local loop') that connects you to the local exchange equipment. It uses a

combination of several advanced signal processing techniques in order to achieve

the required throughput speeds on ordinary telephone wiring at distances of up

to several miles from the local exchange. ADSL works by implementing many modems

in parallel, each of which uses its own slice of the available bandwidth. The

amount of data that can be transmitted by each modem depends on the

characteristics of the line at the frequency allocated to that modem. Some

modems may not work at all because of interference from an outside source such

as another local loop or a radio station. Modems at the higher frequencies

typically transmit less data than the others because attenuation (losses) is

greater at higher frequencies, especially over long distances.

Implementing ADSL

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Within the block that was previously identified simply as

'Service Provider', there are three important components:

  • DSL Access

    Multiplexer - DSLAM

  • Broadband Access

    Server - BAS

  • Internet Service

    Provider - ISP

The DSLAM is the piece of equipment at your local exchange

that is at the other end of your ADSL connection. It houses a bank of ADSL

modems on one side and has a single fibre-optic data connection on the other.

The DSLAM consolidates a number of ADSL user connections - perhaps as many as

several hundred - onto a single fibre connection. This fibre will normally be

connected to a BAS, but it may not be a direct connection. The BAS can be

located anywhere. The BAS is the piece of equipment that sits between the DSLAM

at the telephone exchange and the ISP that connects the user to the Internet. It

may be at the local exchange or it may be elsewhere in the service provider's

network. A single BAS will probably handle connections from several DSLAMs. The

purpose of the BAS is to unwrap the various protocols inside which data travels

over the ADSL connection. It also makes the connection to the ISP appear exactly

as if you had connected using a dial-up modem or ISDN.

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xDSLs:

A Snapshot

 

Distance

#

of Pairs of Wires

Speed

Downstream

Speed

Upstream

Dedicated

Voice



Bandwidth?

Line

Coding

Symmetric

DSL (SDSL) Up to 28,000 ft 1 160 Kbit/s to

1.54 Mbit/s 160 Kbit/s to

1.54 Mbit/s

No

2B1Q (PAM) ISDN

DSL (IDSL) Up to 36,000 ft 1 64 to 144 Kbit/s 64 to 144 Kbit/s No 2B1Q (PAM) High

bit rate DSL (HDSL) 12,000 ft (with

regenerators) 2 768 Kbit/s on

each loop 768 Kbit/s on

each loop No 2B1Q (PAM) HDSL2 12,000 ft 1 1.54 Mbit/s 1.54 Mbit/s No OPTIS (PAM) Asymmetric

DSL (ADSL) Up to 18,000 ft

(ideal to 9,000 ft) 1 7.0 Mbit/s 640 Kbit/s Yes DMT G.lite Up to 18,000 ft 1 1.54 Mbit/s 384 Kbit/s Yes DMT Very

high bit rate DSL (VDSL) Up to 6,000 ft

(ideal to 3,000 ft) 1 2.5-52 Mbit/s 800 Kbit/s to

42 Mbit/s No DMT/CAP

As we noted earlier, ADSL does not specify the protocols that

are used to construct the connection to the Internet. The result of this is that

there are at least five different ways in which data can be carried between the

PC and the BAS. The method used by the PC and the modem must be the same as that

used by the BAS for the connection to work. The BAS are connected to an ISP.

This is the place where the connection to the Internet is made. The ISP treats

ADSL connections exactly the same as connections made using ordinary dial-up

modems or ISDN.

Data Communications Protocols that are Used on the ADSL

Connection



When you access the Internet, you use protocols that run over the TCP/IP

transport layer. This process is the same for analogue modems, ISDN and ADSL.

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Protocols used between the modem and BAS



When you dial in to an ISP with an ordinary modem or ISDN device, you use a

protocol called PPP to carry the TCP/IP data and to check your username and

password. In ADSL, PPP is also often used for username/password checking, and

Asynchronous Transfer Mode (ATM) is almost always used at the lowest level.

ATM is used as the low-level



transport for ADSL. This is because it is a flexible and convenient way for the
telephone companies to extend the data connection from the DSLAM (where the ADSL

connection ends) to the BAS, so that the BAS can be located anywhere in the

network.

ATM configuration issues

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There are two parameters that must be configured correctly on

the ADSL modem for communication at the ATM level with the DSLAM to be

successful:

  • VPI - the Virtual

    Path Identifier

  • VCI - the Virtual

    Channel Identifier

PPP is the protocol used to carry Internet traffic to the ISP

across modem and ISDN links. PPP incorporates authentication - username/password

checking - and this is the main reason for its use with ADSL. Although the BAS

handles the PPP and performs the authentication, it often does this by accessing

the subscriber databases that reside at the ISP. This way, the ISP knows that

connections routed to it by the BAS have been authenticated against the ISP's

own subscriber database.

PPP configuration issues

The only options are the username and password. If either of

these are incorrect, then connection to the ISP would be denied, just as would

happen if a dial-up modem or ISDN connection was used.

Practical issues

  • The essential

    parameters to know about when configuring an ADSL connection

  • Intelligent and

    passive ADSL modems - the significant differences

  • How voice and

    data share the telephone line

  • The factors that

    govern the actual speed available, as opposed to the theoretical maximum

  • The meaning of

    SHOWTIME in relation to an ADSL Internet connection

  • Ownership of the

    various components in an ADSL connection between you and the Internet

  • The significant

    boundary points which might aid fault diagnosis

An intelligent ADSL modem has all the required communications

protocols built-in. A passive ADSL modem relies on the PC's operating system to

provide the protocols. Only Windows 98SE, Windows Me, and Windows 2000 include

Microsoft's ATM implementation, thus limiting the use of passive modems to these

platforms. As the protocol support is in a completely different location, the

configuration procedures for active and passive ADSL modems differ significantly

- the passive modem will require more configuration on the PC. A passive modem

could be connected to the PC using USB, or it could be a PCI card installed

inside the PC. Splitters that separate the high frequencies used by ADSL from

the low frequencies used by voice are situated at each end of the local loop. At

the user’s end of the connection, the low frequencies go to the phone and the

high frequencies go to the ADSL modem. At the local exchange, the low

frequencies go to the normal telephone network while the high frequencies go to

the service provider. The speed of the connection achieved between the ADSL

modem and the DSLAM depends on how far you are from the DSLAM, and the maximum

allowed speed for your connection configured in the DSLAM.

What Determines Access Speed

  • The number of

    other users connected to the same DSLAM and how many of these users are

    actively using their connections

  • The speed of the

    connection between the DSLAM and BAS

  • How many other

    DSLAMs are connected to the same BAS and how many of these users are active

  • The speed of the

    connection between the BAS and the ISP

  • How many other

    BAS are connected to the same ISP and how many of these users are active

  • The speed of the

    ISP's connection to the Internet

  • How many of the

    other users of the ISP (using dial-up modems and ISDN as well as ADSL) are

    active.

  • Whether the ISP

    already has the information cached so that it is not necessary to access the

    Internet.

The condition when a successful connection from the user's

ADSL modem is made to the ADSL equipment at the local telephone company is known

as showtime. This indicates that the ADSL modem is 'talking' to the DSLAM. It

says nothing about any of the other components in the connection to the

Internet.

Although you may have 'showtime', the ATM configuration or

other settings may be incorrect.

The connection type (PPPoE / PPPoA / RFC1483B / RFC1483R) may

be incorrect.

The username / password that you are using (if you are using

a PPP-based protocol) may be incorrect, so that you cannot get from the BAS to

the ISP.

Service Providers

  • The local or

    national telephone company who owns the local loop

  • Another company

    who rents the local loop from the telephone company

  • The Internet

    Service Provider

  • The Internet

    Service Provider might themselves rent the local loop from the telephone

    company or another company.

Thus, it is possible that two or three organizations are

responsible for the connection to the Internet.

As we have seen previously, the success of an end-to-end

connection from an ADSL user to the Internet depends upon numerous components in

the chain. The configuration, location, and in particular the ownership of these

elements can vary quite significantly. The support implications of this are that

when a failure to connect occurs, deciding where the fault is located, and who

is responsible for resolving the issue, may not always be clear. Between the

ADSL modem and the DSLAM, the diagnostic situation is relatively

straightforward, since there is a known standard (ADSL itself) and not too many

different parties or components are involved.

However, as stated previously, showtime indicates only that a

successful DSLAM connection has been made. Beyond this, there are no easily

accessible standards or reference points to determine where any potential fault

may lie. In addition, of course, the fault may be with any one of a number of

parties, which adds still further complications.

V Srivathsan



Business Development Manager — India, Eicon Technology Corporation


Principal Member - DSL Forum, California - www.dslforum.org

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