ADSL technology is evolving rapidly with newer standards and other performance optimizations. The concept of Multi-Carrier Modulation is a standard in ADSL. Think of this as the band of non-voice frequencies being broken down into several bins, each of which can carry a certain number of bits. The information to be sent is broken down between these bins. There are some bins that can carry more information than the other depending on the channel conditions. The aggregate of the bits carried in the bins every second is a measure of the data rate in Kilobits per second.
Performance is improved by increasing the number of bins which can carry meaningful data. This led to an evolution of ADSL2 and ADSL2+ standards. The latter innovation has now lead to data rates as high as 30Mbps in the receive direction, also called as 'downstream' from the Central Office. The 'upstream' rates to the Central Office are as high as 1.2Mbps. So there is a great push to keep extending the range of Downstream data rates, since there is more and more demand for Video and Audio over DSL. This means more downstream information. It is likely when operators deploy ADSL2+ at competitive rates, DSL will gain much more popularity. In emerging markets like in China , Taiwan and India , majority of the work is in ADSL1, which has matured considerably in terms of standards specifications.
In North America, the Plain Old Telephone Service (POTS) is a familiar medium for carrying DSL traffic, in Europe , ISDN is more popular. So, to have a world wide footprint, CPE and CO software need separate tuning and logic to accommodate these specific conditions. There are also additional specifications for extremely long loop lengths from the CO, such as those that exceed 18 Kft.
As someone working in the CPE side, interoperability is a term I need to be concerned about, very frequently. This means that the CPE modem that we manufacture, should be compliant and work with the Central Office equipment from several vendors. Just like there are several companies that make CPE modems, there are also companies that manufacture CO Chipsets. These have got to work with each other consistently. For instance, an operator in China may be running into issues using an Analog Devices CO chipset against a Texas Instruments CPE chipset. On the other hand a Texas Instruments CPE would perform very well against a Texas Instruments CO the Analog Devices CPE may not perform as well. This calls for performance tuning against the Analog Devices CO. Similarly CO vendors frequently have to tune their devices against several CPEs.
Thus DSL is an exciting field which involves plenty of Signal Processing at the Physical layer and embedded software at the ATM and Networking layers. There are several companies that manufacture CPE modem chipsets like Analog Devices, Conexant, Infineon, Broadcom and of course Texas Instruments. There are also several companies that manufacture CO chipsets like all of the above companies plus Alcatel, Lucent, NEC, Nokia, etc.