ADSL
ADSL modems, as specified in the ANSI and ITU standards, make use of technology that is very different from ITU V-series modems.
ANSI ADSL uses the Discrete Multi-Tone (DMT) technique. The DMT line code sends multiple tones of data over the line allocating more data to the lower frequencies where there are less analog impairments.
ADSL modems also split incoming data into two streams - a fast stream and an interleaved stream. Interleaving a portion of the data facilitates error correction by giving protection against noise bursts at the cost of higher latency.
ADSL Integration and Software Implementation
The integration of ADSL and V.90 modems is made possible by the POTS channel that ADSL technology provides. This integration technique, used by GAO ADSL/V.90 from GAO Research Inc., can be applied to both splitter based ADSL which complies with the ANSI T1.413 standard and splitterless ADSL which will comply with the forthcoming G.Lite standard. As illustrated in Figure 1, ADSL channel splitting gives wide bandwidth for Internet applications and doesn't affect regular telephone service. The V.90 modem function offers bit rates of up to 56 kbps downstream and 33.6 kbps upstream. It makes use of the POTS channel and serves as a fallback if the ADSL service fails or is not available. Telephone service may also be accessed through the POTS channel if the V.90 modem is not used.
V.90 technology takes advantage of the fact that digital transmission facilities have been rapidly replacing analog transmission facilities on the Public Switched Telephone Network (PSTN). Because Internet service providers (ISP) are digitally connected both to the Internet and to a telephone company's central office (CO), a conversion from digital to analog lines in the connection from ISP to CO is avoided.
Whether integrated with V.90 or not, OEMs must make sure ADSL products are upgradable to the final version of the ITU G.dmt standard. They also need to get their products to market quickly in order to gain a large market share. Software implementations of ADSL modems will be much easier to upgrade than hardware implementations. If the decision is made to go with a software implementation of ADSL, OEMs must either design that software in-house or license it from a software vendor. Modem software is extremely complicated and requires a great deal of expensive engineering time to develop. Time to market is critical in the modem industry - if a product is released too late, customers will have lost interest in favor of the next version. Thus, licensing software is the fastest and least expensive option for OEMs developing ADSL modems.
Characteristics to look for if you are licensing modem software:
- It must comply with the current or draft standard and be upgradable to the final standard.
- The software must be tested according to the appropriate standards.
- It must use the least possible memory and MIPS.
In order to reduce memory and MIPS requirements, ADSL modems such as GAO ADSL/V.90 can run at reduced rates on today's processors. As processors get faster and more powerful, it will become feasible to upgrade and run full-rate ADSL software modems.
For further information on ADSL see:
G.992.1 (ADSL)
Lite Speed, an article by Dr. Gao on splitterless ADSL modems
High Speed Access Technology for the Last Mile: Integrated ADSL/V.90 Modem
