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.