DSL SERIES: The Varieties of DSL

Many developments took place after the first DSL technology was introduced,
to make E1/T1 services affordable. Today, one sees at least half a dozen
variants available in the market. What are these and how do they stack up
against each other.

HDSL

In the early 1990s, following the experiments on basic rate ISDN, some
vendors encouraged the use of the 2B1Q line code at a higher speed as an
alternative to provisioning T1 and E1 services, without repeaters. The technique
consisted of splitting the 1,544,000 bps service into two pairs (four wires),
each of which ran at 784,000 bps. By splitting the service across two lines and
increasing the bits per baud, per line speed and the resulting need for
frequency spectrum, could be reduced to allow longer loop reach. This technique
was referred to as High-bit-rate Digital Subscriber Line (HDSL). The result was
that an HDSL-based DS-1 service could be implemented over carrier serving area
with specified loops up to 12,000 feet long (assuming 24 gauge; or 9,000 feet
with 26-gauge wire), with no repeaters.

SDSL offers HDSL speeds on a single pair

Transceiver systems can now achieve an entire T1 or E1 line speed on a single
loop at distances approaching, and in some scenarios exceeding, the conventional
two-loop HDSL systems. This single-pair implementation of T1 or E1 HDSL is
referred to as Symmetric Digital Subscriber Line (SDSL). Due to the lack of
formal naming convention in the industry, the term SDSL has become more generic
over time. And is also used to refer to symmetric service at a variety of rates
over a single loop.

In principle, the trade-off between four-wire HDSL and two-wire SDSL systems
is the loop reach. By splitting the information across two loops, HDSL systems
can operate in lower frequencies than SDSL, resulting in a slight loop reach
advantage for HDSL.

New generations of symmetric services

Refinement and development of new line codes for symmetric DSL services has
continued, even as HDSL and SDSL have been deployed rapidly, and in mass.
Particularly, there are two emerging standards for symmetric DSL, which are
beginning to enter the market:

• G.shdsl: A new standard-based replacement for SDSL. This multi-rate
  replacement for proprietary SDSL offers symmetric bandwidth between 192 Kbps
  to 2.3 Mbps, with a 30 percent longer loop reach than SDSL and improved
  spectral compatibility with other DSL variants within the network. G.shdsl
  is expected to be applicable worldwide.
• HDSL2: A single-pair, ANSI standard-based replacement for HDSL. HDSL2
  offers the same 1.544 Mbps bandwidth as traditional four-wire HDSL
  solutions, with the advantage of requiring only a single copper pair, plus
  the additional advantage of being a standards-based solution with
  multi-vendor interoperability. HDSL2 is expected to be applicable in North
  America only.

ADSL

Maximizing the loop reach with various line codes resulted in an extensive
study of the characteristics of the loop plant itself. This study revealed that
we could transmit a signal to a greater distance from the Central Office (CO) to
a remote home office than could be achieved in the opposite direction. This was
due to the effects of crosstalk, which are more dominant on the telephone
company side of the copper wire loops than on the remote subscriber side. This
phenomenon is due to the fact that more copper wires are combined in large
bundles as they get closer to enter the CO. Conversely, as we traverse the loop
from the CO out to the end service user, the loops tend to branch- off for
connection, resulting in fewer copper wire loops. Therefore, less aggregated
crosstalk is introduced by the transmitters at the far-end wire bundles. Another
advantage of the telephone plant is by ensuring that lower frequencies are used
to transmit towards the CO.

Such devices are called Asymmetric Digital Subscriber Line (ADSL) devices.

A brief history about ADSL line codes

ADSL was originally envisioned as a residential service that
would independently co-exist with the already provisioned POTS. Therefore, the
passband attributes were considered a pre-requisite to frequency separation or
FDM between POTS, an upstream channel from the service user to the network, and
a downstream channel from the network to the service user. In addition to the
above FDM implementation, some DSL technologies, including certain
implementations of DMT, were designed to provide echo-cancellation of the
upstream and downstream channels to minimize the use of higher frequencies and
optimize loop reach.

ADSL standards

In 1992 and early 1993, the ANSI T1E1.4 Working Group moved
towards a selection of a single line code for an ADSL Video Dial Tone standard
(VDT). The focus then shifted towards achieving the maximum loop reach at
specified data rates optimized for video.

DMT was the first line code to demonstrate actual support for
6 Mbps service and was selected as the official ADSL standard for VDT services.
While DMT was selected as the official standard, CAP-based systems were used
worldwide to implement many ADSL and VDT trials, and commercial deployments,
effectively established CAP as a competing de facto ADSL standard. During this
time, the threat of the cable TV industry offering telephony services in the US,
largely subsided. Internationally, VDT applications garnered and continue to
hold interest.

The final standard for ADSL, approved by the ITU G.dmt or
G.992 and ANSI (T1.413 Issue 2), was a DMT-based system and is the basis of most
of the new ADSL deployments today.

Application switch from video to data

Through the course of these lengthy VDT trials, the industry
came to recognize that many data applications were actually asymmetric in
nature. The best example of this is Internet access. The biggest complaint from
the users was that it took too long to download files at dial-modem or even ISDN
data rates. Hence, a new service need and new technology were soon coupled, and
ADSL was re-focused to support Internet access.

Video has not disappeared entirely as an application for DSL.
However, IP-based video delivery, utilizing systems, such as RealMedia or
Windows Media, has become increasingly popular and sophisticated. Using
compression schemes, such as the industry standard MPEG-2 or newer schemes that
allow even greater compression of video, IP-based video delivery continues to be
a viable application of DSL.

RADSL

Both CAP and DMT transceivers were modified to optimize
service on a per loop basis, and this implementation was called Rate Adaptive
Digital Subscriber Line (RADSL).

RADSL technology supports the option of allowing the
transceiver to start automatically by increasing the line speed to the highest
attainable data rate, which can be reliably achieved over a given loop. While
this feature was designed primarily to simplify service installation, it also
gives service providers the option of a graceful service degradation in the
event of degrading loop conditions. Today, there are other DSL technologies that
also support rate adaptation, and service providers interested in this
functionality should examine the degree to which it is supported within the
different technologies.

RADSL standards

The industry and technologies have evolved dramatically,
since the ADSL VDT standards decision in March 1993. In recognition of this,
ANSI’s T1/E1 Working Group has established a RADSL standard known as ANSI
TR59. The FCC has specifically cited RADSL as a technology that is spectrally
compatible with both voice and other DSL technologies within the local loop.

True Interoperation

While many vendors meet the same line-coding standard at the
physical layer, many others are not aligned to the networking model at the
logical layer. Essentially, systems manufactured by different vendors do not
automatically interoperate across the DSL link today. In reality, line codes are
only
important across the local loop. To interoperate, the equipment at the wire
center, the end-point must, at least, utilize the same line code. Today, the
different line codes are less problematic, as the industry continues to take
advantage of the standards for interoperability.

Continuing developments in DSL

The onslaught of new vendors is sure to bring continued
variations in the DSL technology. Even as mass deployments of ADSL and SDSL
reach the market, new variants are being developed and marketed to fulfill the
needs of certain segments of the DSL market.

IDSL provides DSL over ISDN

In some cases, DSL concepts have been applied to existing
technologies. For example, ISDN-DSL or IDSL, first emerged as a new spin of the
1980’s technology. IDSL is simply ISDN customer premises equipment, talking to
ISDN-compatible line cards that reside on the other end of the copper wire loop,
and terminate the ISDN signal independent of the telephone switch. In this
scenario, as with all DSL variations, the data service is directed to a WAN data
service rather than a switched network.

While IDSL builds on a proven technology, it is functionally
a subset of ISDN, in that it forgoes any ability to support telephone service
and switched connectivity in general. One key benefit of IDSL is to the service
providers seeking to move long duration ISDN data connections to the Internet or
remote LAN access servers of the switched network. Another benefit is that it is
able to transmit data over copper pairs that are served by digital loop
carriers. These devices are often connected to the CO by fiber optic private
lines and, as such, cannot carry other DSL signals, such as ADSL and SDSL.

Multirate symmetric DSL

For symmetric applications, Multirate SDSL (M/SDSL) has
emerged as a valuable technology in meeting carrier requirements to deliver TDM
services on a near ubiquitous basis. Building on the single-pair
SDSL technology, M/SDSL supports changing operating line rates of the
transceiver and thus, the operating distance of the transceiver.

G.lite for the consumer market

In January 1998, the Universal ADSL Working Group (UAWG) was
announced. The result of this group’s work is a new, standards-based subset of
ADSL known as G.lite. G.lite was approved as a standard by the ITU (G.992.2) in
June 1999 and can offer speeds of up to 1.5 Mbps downstream and up to 512 Kbps
upstream. Significantly, G.lite was designed to provide this service over the
existing phone lines, without the POTS splitter, usually required by full-rate
ADSL solutions. A part of the G.lite standard is a technique known as ‘fast
retrain’, which limits the upstream power of the G.lite signal when a
telephone handset is in use. In order to minimize interference, it then restores
the power when the phone is back on hook.

ReachDSL–another option for business and residential
customers

ReachDSL is a symmetric DSL technology that addresses
subscriber demand for high-speed DSL services at extended distances.
Complementing standard ADSL technology (DMT/G.lite), ReachDSL products support
speeds ranging from 128 Kbps to 1 Mbps and have been designed to work over a
wider range of line conditions and in wiring premises. ReachDSL solutions are
capable of sharing lines, impervious to bridged taps, and are ideal for
business, residential and even private network environments.

VDSL delivers video and higher bandwidth

The newest emerging variant of DSL is Very High Speed DSL (VDSL).
VDSL systems are still being developed, so the final capabilities are not yet
firmly established, but propose standard call for downstream bandwidths of up to
52 Mbps and symmetric bandwidths of up to 26 Mbps. The trade-off of these
bandwidths is a much shorter loop reach.

But, the high speed offered by VDSL will bring opportunities
for service providers to offer the next generation of DSL services, with video
being seen as a prime application. At 52 Mbps, a VDSL line can offer a customer
multiple channels of full-quality MPEG-2 video streams and even offer one or
more channels of full-quality High Definition Television (HDTV). Some service
providers have begun trial deployments of VDSL systems providing these services,
with the VDSL endpoint appearing in the residence as a cable TV-like set-top box
with an Ethernet or other data interface for connecting to PCs for simultaneous
data services.

The DSL series is brought to you in association with Paradyne Corp.