Tutorial adsl

INTRODUCTION
The demand for high-speed data networks in the “last mile” has driven the need for robust, interoperable,
and easy to use multi-vendor Digital Subscriber Line (DSL) access solutions. DSL collectively refers to a group of
technologies that utilize the unused bandwidth in the existing copper access network to deliver high-speed data
services from the distribution center, or centraloffice, to the end user. DSL technology is attractive because it
requires little to no upgrading of the existing copper infrastructure that connects nearly all populated locations in the
world. In addition, DSL is inherently secure due to its point-to-point nature. A simple diagram of a typical DSL
system is shown in Figure 1 below:
central o ffice
(CO)
internet, ISP, business
network , etc .
home
business
campus DSL access m ultiplexers
(DSLAMs)
wide a rea
network
(W AN)
DSL "the last m ile "
Figure 1: Typical DSL system.
There are many variations of DSL, each aimed at particular markets, all designed to accomplish the same
basic goals. ADSL, or Asymmetric DSL, is aimed at the residential consumer market. ADSL provides higher data
rates in the downstream direction,from the central office to the end user, than in the upstream direction, from the
end user to the central office. Within the Internet connectivity-based residential environment, small requests by the
end user often result in large transfers of data in the downstream direction. ADSL is a direct result of the
asymmetric nature of the Internet and the needs of the end user, and was originallydesigned for video-on-demand
applications.
ADSL possesses some distinct advantages when compared to traditional analog modems. One of them is
the ability to operate alongside existing Plain Old Telephone Service (POTS) on a single pair of wires without
disruption. POTS is the basic service that provides all phone lines with access to the Public Switched Telephone
Network (PSTN). POTS provides themeans for all voice-band related applications and technologies, such as
telephony, caller identification, call waiting, analog facsimile, analog modem, etc.. ADSL systems allow the end
user to access any POTS associated services and ADSL services simultaneously. ADSL also has the ability to
dynamically adapt to varying channel conditions. ADSL systems automatically measure the characteristics ofthe
channel and decide upon an appropriate data rate that can be effectively maintained according to a predefined
acceptable bit-error rate (BER).
ADSL: ANSI T1.413-1998
The American National Standards Institute (ANSI) Telecommunications Committee created the first
standardized ADSL specification. The current version of this specification is ANSI T1.413-1998 “Network and
CustomerInstallation Interfaces – Asymmetric Digital Subscriber Line (ADSL) Metallic Interface.” ANSI T1.413-
1998 defines the minimum set of requirements for satisfactory performance of ADSL systems utilizing the Discrete
Multi-Tone (DMT) line code. The DMT line code, as defined in ANSI T1.413-1998, divides the useful bandwidth
of the standard two wire copper medium used in the PSTN, which is 0 to 1104kHz, into256 separate 4.3125kHz
wide bins called sub-carriers. Each sub-carrier is associated with a discrete frequency, or tone, indicated by
4.3125kHz * n, where n = 1 to 256, and is essentially a single distinct data channel.
A maximum of 255 sub-carriers can be used to modulate data in the downstream direction. Sub-carrier
256, the downstream Nyquist frequency, and sub-carrier 64, the downstreampilot frequency, are not available for
user data, thus limiting the total number of available downstream sub-carriers to 254. Each of these 254 sub-carriers
can support the modulation of 0 to 15 bits. Since the ADSL DMT data frame rate is 4000 frames per second, the
maximum theoretical downstream data rate of an ADSL system is 15.24Mbps. Due to limitations in system
architecture, specifically...