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.
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.
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
-
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
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.
xDSLs:
A Snapshot
Distance
#
of Pairs of Wires
Speed
Downstream
Speed
Upstream
Dedicated
Voice
Bandwidth?
Line
Coding
DSL (SDSL)
1.54 Mbit/s
1.54 Mbit/s
No
DSL (IDSL)
bit rate DSL (HDSL)
regenerators)
each loop
each loop
DSL (ADSL)
(ideal to 9,000 ft)
high bit rate DSL (VDSL)
(ideal to 3,000 ft)
42 Mbit/s
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.
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
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