G.992.2 (ADSL G.Lite)

Introduction

ADSL (Asymmetric Digital Subscriber Line) is a new modem technology that promises to revolutionize online commerce, communications, and entertainment. ADSL transforms the existing copper telephone lines into an instant high-speed connection for Internet access, video conferencing, remote access, and multimedia applications.

The existing ADSL installation techniques require a splitter in the subscriber's home to separate the POTS (Plain Old Telephone Service ) channel from the ADSL channel. To install the splitter, the telephone company must send a technician to the customer's premises to set up the device. In addition, a second pair of wires may be required to connect the splitter and the ADSL modem. These truck rolls create significant cost to the telephone companies.

Also, to receive ADSL services, the consumer will need to purchase an ADSL ATU-R (ADSL Transmission Unit-Remote), a modem- like piece of equipment, as well as paying for the modem, line provisioning cost, inside wiring fee, NIC (Network Interface Card), and the technician services fee. Thus, the up-front cost of getting an ADSL line can be very high, possibly even prohibitive, for many residential users.

Because of the above-mentioned obstacles (high cost and technical complexity) and the fact that most users may not actually need to use all the available bandwidth offered by full- rate ADSL implementation, efforts were made to develop reduced-rate ADSL which sacrifices speed in favor of operating without a splitter. Thus, a new version of ADSL that would be consumer installable was born. The Universal ADSL Working Group (UAWG) was formed in January 1998 to address these issues and develop a proposal for submission to the International Telecommunications Union (ITU) for adoption as a standard called G.Lite, which was adopted in October 1998. The formal ratification will take place in June 1999. The transmission speed of G.Lite (G.992.2) is 1.5Mbps downstream and 512Kbps upstream.

Figures 1 and 2 below illustrate the configuration differences between splitterless ADSL Lite and splittered, full-rate ADSL.

Splitterless ADSL Lite

Splittered Full Rate ADSL

How ADSL Works

ADSL modems use discrete multi-tone modulation (DMT). In this scheme, the whole transmission band (1.104 MHz for ADSL Lite, 2.208 MHz for full-rate ADSL) is subdivided into a number of independent, parallel, equally spaced sub channels (128 for ADSL Lite, 256 for full-rate ADSL). A quadrature-amplitude modulated (QAM) signal is transmitted in each sub channel. With properly chosen sampling and DMT symbol frequencies the Fast Fourier Transform can be used for fast and efficient calculation of the time-domain signal waveform.

The diagram below illustrates the bandwidth distribution for DMT based ADSL. The frequency distribution of POTS, ADSL Lite, and full-rate ADSL are all depicted. It is because ADSL occupies different frequencies than POTS that both a telephone and an ADSL modem can be used at the same time. Full-rate ADSL uses high power and bandwidth, so it is prone to bleeding into the POTS frequencies. As a result, a splitter is necessary to make sure this doesn't occur. ADSL Lite uses less power and bandwidth than full-rate ADSL. Therefore, it doesn't bleed into the POTS frequencies and a splitter isn't required. Less power and bandwidth also means lower speed. This is why splitterless ADSL has a maximum speed of 1.5 Mbps as compared to 8 Mbps for full-rate ADSL.

ADSL DMT Bandwidth Distribution

G.Lite Principles

Like full-rate ADSL, ADSL Lite uses the Discrete Multi-Tone (DMT) modulation technique. The DMT line code uses a set of tones to send data over the line and allocates more data to those frequencies where there are less analog impairments. The data is then reassembled by the modem at the other end of the line. ADSL Lite modems also interleave the serial bit stream of data, thus facilitating error correction by providing protection against noise bursts at the cost of higher latency.

Transmitter

The following is a description of the main parts of the ADSL Lite transmitter as specified by the ITU G.992.2 standard.
ADSL Lite Modem Transmitter

Figure 1: Transmitter of the ADSL Lite Modem

FEC Encoder/Interleaver:

The interleave buffer convolutionally interleaves the Reed-Solomon code words. The depth of the interleaving varies, but it is consistently a power of 2. The FEC coding can reliably correct occasional errors if the data is interleaved.

Constellation Encoder:

This encoder is similar to a V.Series modem quadrature amplitude modulation (QAM) encoder. One way to improve system performance is by block processing Wei's 16-state, 4-dimensional trellis code. An algorithmic constellation encoder constructs QAM constellations with a minimum of 8 bits and a maximum of 15 bits per DMT channel (10 bits per channel @ a V.34 constellation).

Modulation:

The inverse fast Fourier transform (IFFT) modulator combines many QAM constellations. The channel analysis signal allows the use of a maximum of 127 carriers (at frequencies n´f, n = 0 to 127, f = 4.3125 kHz). In reality, about 97 carriers or channels are usable. Channel estimation determines the range of usable n. It is interesting to note that the downstream transmission of 97 QAM constellations is equivalent to approximately 97 V.34 (simplex) modems running in parallel.

Cyclic Prefix:

The cyclic prefix constitutes the guard band. If the channel's response is not longer than the cyclic prefix, then use of the cyclic prefix results in DMT symbols that are free of inter-block interference.

Receiver

The ITU G.992.2 standard does not specify the receiver, leaving much to the discretion of the modem designer. Many of the receiver functions such as the cyclic prefix removal, demodulator, decoder, descrambler, and deinterleaver perform the inverse of the transmitter functions. The components unique to the receiver include the time domain equalizer, frequency domain equalizer, and automatic gain control. The role of some of the receiver functions in an ADSL Lite modem is to recover and process the transmitted signal for presentation to the demodulator. The demodulator's task is to recover the bit stream that forms the transmitted message.
ADSL Lite Modem Receiver

Figure 2: Receiver of the ADSL Lite Modem

Barriers to Performance

Other voice and data services can affect the performance of ADSL Lite. For example, basic rate interface (BRI) is an ISDN service that can cause interference if it is bundled in the same cable with ADSL Lite services. This interference is called cross talk.

Cross talk is line distortion caused by interference from wire pairs in the same bundle being used for separate signal transmission. The interference is created when the cross talk induced signals combine with the signals meant for transmission over the copper wire loop. If the effects of the cross talk are not too significant, the ADSL Lite systems can accurately reconstruct the original signal; but, when there is too much cross talk, bit errors occur when the signals are misinterpreted at the far end.

Physical line conditions can also cause problems for ADSL Lite services. Bridged taps and load coils are examples of two such barriers. A bridged tap is any part of the local loop that is not in the direct transmission path between the CO and the service user. If there are too many bridge taps or if they are too close together, ADSL Lite services cannot function. Load coils are placed on longer phone lines to improve the voice frequency response characteristics. A coil is an inductor that acts like a low pass filter - it doesn't allow high frequencies through. Since ADSL Lite uses higher frequencies, it does not work on lines that have load coils.

Environmental conditions near the home can also interfere with ADSL Lite performance. For example, wire resistance goes up as temperature rises and goes down as temperature falls. After handshaking has taken place and an ADSL Lite modem connection has been established, a sudden, significant change in wire resistance can throw the modems off, possibly causing them to drop the connection. Induced voltage from electrical appliances in the home, including hairdryers and air conditioners, can also cause ADSL Lite modems to drop their connection.

It is the task of modem designer to try to combat as many of these impairments as possible.

Software Implementation

The conventional modem and fax solution for product designers has been the use of data pump chip sets. In the past, this approach offered a significant price advantage because identical, fixed function chip sets can be produced at a high volume to gain price competitiveness. However, the functions these chip sets offer are not programmable by OEM product designers.

Once a chip set has been created, the programming capability of the OEM's platform (digital signal processor or microprocessor) is lost, and thus the chip set cannot be programmed to perform other functions when the ADSL Lite modem software is not being used. The fixed function nature of the chip set also makes it impossible to upgrade an ADSL Lite modem product without discarding the hardware. Upgradability is crucial for ADSL Lite modems since it is an emerging technology and thus market conditions, technology, and standards are evolving rapidly. Furthermore, the same chip sets are available to all modem manufacturers and make it difficult for one manufacturer to differentiate itself from its competitors.

Due to the above-mentioned problems with chip set implementations of communications functions, interest in modem and fax software for DSPs and microprocessors has increased substantially in recent years. ADSL Lite software modems that are based on these processors may offer better cost performance ratios than fixed-function modem chip sets. Software implementations also offer flexibility in bug fixing, functional upgrades, and product design. A general-purpose programmable processor can be used to replace several dedicated functional chips that are required in a traditional design. Several different variations within a product line, each with distinct features, can all be based on the same processor. With all the signal-processing functions implemented as software on a single processor, costs go down, size decreases, and upgrades can be done either by simply downloading the software through the host processor or by replacing the existing ROM with one containing the upgrade.

Once an OEM has decided to go with a software implementation of an ADSL Lite modem, there is a second, and equally important, decision to be made. That is whether to design the software in-house or license it from a vendor. Time to market is very critical in the communications industry. If a product is released too late, the planned market share gain may not be achievable. This might well defeat the very purpose of introducing that particular product.

ADSL Lite modem software is complicated, difficult to program and requires a great deal of expensive engineering time to develop. Creating algorithms and software modules that have good performance and minimal memory and MIPS requirements is a challenging task, and understanding signal processing functions and their requirements is crucial for the success of a product design which includes ADSL Lite modem software.

At present, GAO Research and Consulting Ltd. is probably the only software vendor that has ADSL Lite software modules and packages ready for licensing at a fraction of the cost & time of developing them in-house. The logical and practical conclusion then, is that licensing ADSL Lite modem software from GAO Research and Consulting Ltd. is the fastest and least expensive option for OEMs developing communications products based on ADSL G.Lite technology.

BENEFITS FOR END USERS:

High Speed - Data rates of up to 1.5 Mbps downstream and 512Kbps upstream
-- Nearly 60 times faster than 24.4Kbps modems
-- Nearly 30 times faster than 56Kbps modems
-- Nearly 12 times faster than 128Kbps ISDN modems
Always on-line and connected - The modem is always connected so no time is wasted dialing up the service several times a day and waiting to be connected. In other words, it saves you time and eliminates frustration. Working on the Internet becomes a more enjoyable experience.
Continuous access to your telephone - You can use your telephone for normal conversation or send a fax while you are surfing the Internet.
Private and secure - Unlike existing modems where you share the bandwidth with others, you get a dedicated line of your own which ensures privacy and security for the communications between you and the service provider. Also, because it is your own dedicated line, transmission speeds are not affected. With cable modems, transmission speeds do drop as more users go on line.
Splitterless - Very simple and convenient for you to install; no visit to your premises is required. Saves you time and money.

BENEFITS FOR SERVICE PROVIDERS:

Do-it-yourself technology - Being splitterless reduces the complexity of installation and avoids the need for new wiring at the user's premises. Thus, GAO's G.Lite modem makes it possible for customers to enjoy the benefits of increased bandwidth on a more cost-effective basis. It is expected that residential users, small businesses, and SOHO operators will rush to get more powerful access to the Internet and corporate LANs.
Can avoid rolling a truck to install - Saves time and money.
Enables the telephone company to utilize its existing copper wires to deliver high-speed remote access to the Internet, corporate networks, and on-line services over ordinary phone lines - Enables a quick time-to-market advantage without added new infrastructure, costly outside plant additions and reinvestments. Increases your profitability many-fold.
Enables the telephone company to leverage the existing infrastructure - As both business and residential properties around the world are running out of spare lines on presently installed telephone cables, the availability of GAO's G.Lite modem technology gives service providers the capability to provide new data services to their customers while maintaining the telephone service on the same line.
Enables customers to enjoy new, real-time, interactive, multimedia and broadcast-quality applications like video-conferencing, distance-learning, video-on-demand, collaborative computing etc.
Provides the telcos with the ability to offer a private, secure channel for communication to their customers - Unlike existing modems where your customers share the bandwidth with others, they get a dedicated line of their own which ensures privacy and security for the communications between you and them . Also, because it is their own dedicated line, transmission speeds are not affected. With cable modems, transmission speeds do drop as more users go on line.
Enables service providers to offer a guaranteed , high-speed data rates of up to 1.5 Mbps (downstream) and 512 Kbps (upstream) to their customers which is:
- --Nearly 60 times faster than 24.4Kbps modems
- --Nearly 30 times faster than 56Kbps modems
- --Nearly 12 times faster than 128Kbps ISDN modems
Always on-line and connected - The modem is always on and connected - just like a standard telephone or a cable TV. So, no time is wasted dialing up the service several times a day and waiting to be connected. In other words, it saves your customers time and eliminates their frustration. Makes working on the Internet an enjoyable experience for your customers. GAO's G.Lite is on standby, waiting ready for use whenever your customer is ready.
Continuous access to the telephone - Your customers can use their telephone for normal conversation or send a fax while they are surfing the Internet.