By Frank Gao, Ph.D
GAO Research Inc.
Editor's note: In the race to providing broadband solutions, or at least wider band solutions, for homes and small businesses, the cable TV industry and the telephone industry are squared off in an interesting competitive stance. (AT&T's impending acquisition of TCI is a bridge across these troubled waters, since it combines a telco with a cable company, but that is a story for another day.) As we have reported on before in Techonline Review, the cable company weapons are cable modems, which deliver a couple of megabits on a shared access technology using the coax cable to the home. The telco weapons are a family of digital subscriber line (DSL) solutions. The family is known collectively as xDSL, where x can take on one of many variables. Perhaps the most discussed solution is asymmetrical DSL or ADSL. ADSL, unlike the others, has been standardized (see companion report from Ken Krechmer). Two versions are available: the full ADSL and a "lite" version, which is a compatible subset.
ADSL Lite is the latest digital subscriber line technology to make a stir. This form of DSL has become popular as a last mile solution mainly because it is splitterless. Without a splitter, an ADSL Lite modem can simply be plugged into a phone jack in the customer's home. In practical terms, this means no installation visit is required from the telephone company, which reduces costs and will probably facilitate a ubiquitous roll out of the service. Figures 1 and 2 below illustrate the configuration differences between splitterless ADSL Lite and splittered, full rate ADSL.
There has been a real push over the last year to produce industry standards for DSL modems. Finally, in October 1998, the International Telecommunications Union (ITU) completed several DSL standards. The standard for ADSL Lite is designated G.992.2. This draft standard specifies a lower speed, splitterless version of the G.992.1 standard for full-rate ADSL. G.992.2 stipulates data rates of up to 1.5 Mbps downstream and 512 Kbps upstream while still providing a POTS channel for voice communications in a lower frequency spectrum on the same physical wire.
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 fewer 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.
The following is a description of the main parts of the ADSL Lite transmitter as specified by the ITU G.992.2 standard.
The following algorithm is used.
where dn is the n-th output from the buffer (scrambler input), and dn' is the n-th output from the corresponding scrambler.
The interleave buffer convolutionally interleaves the Reed-Solomon codewords. 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.
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).
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.
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.
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.
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.
t is the task of modem designer to try to combat as many of these impairments as possible.
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 himself from its competitors.
Due to the above-mentioned programmability and flexibility issues with chip set implementations for 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 programmable DSP 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 single manufacturer's 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 for 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 critical in the communications industry - if a product is released too late, a narrow window of market opportunity will be lost. 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.
For companies like GAO Research and Consulting Ltd., designing, developing, and supporting such software is our only business. We provide a communications software expertise that few companies have in-house and can deliver software modules and modem packages at a fraction of the cost and time of developing them in-house.
About the Author
Dr. Frank Gao is the founder and President of GAO Research Inc., a leading supplier of software modem, fax, speech, and telephony technologies. Dr. GAO received his Ph.D. degree from the University of Toronto and was employed by Bell Northern Research Ltd. before founding GAO. Dr. Gao can be reached at: email@example.com.