14th March, 1998.
The 56K modem confusion is over - or is it? The International Telecommunications Union (ITU) V.pcm working group certainly hopes so. There is now a 56K standard - V.90 - but how well production devices conform to this standard remains to be seen.
On 5th February, the ITU V.pcm working group produced a 56K standard, officially called V.90. It represents a compromise between the two technology camps, X2 and K56flex, and inherits elements of both systems. A final specification will not be published until September this year, but most manufacturers will produce designs based upon the February draft.
The ITU specification defines two types of modem: V.90 analogue for the user, and V.90 digital for the ISP. The theoretical performance remains the same: 56 kbit/s from ISP to user, 33.6 kbit/s in the other direction. Details of the protocol have yet to become clear, but it appears to draw upon technologies from both K56flex and X2. There were apparently few technical difficulties in drafting the standard; final agreement was delayed by disputes over licensing Intellectual Property. It was Intel that helped broker a deal in December last year, clearing the way for the specification in February.
The whole point of an ITU standard is to ensure that all manufacturers' modems are compatible. However, given that the new devices will based on existing X2 or K56flex designs, it was felt that compatibility testing was needed. The three main players - 3Com, Lucent and Rockwell - have already completed "Interoperability Testing" on their implementations. This involved the client modems from one manufacturer calling the server modems from the other, and passing data in both directions. The first manufacturer then tested its server modems with the second's client modems.
3Com has said it will offer new V.90 modems at the end of March, and simultaneously release new software for ISP's X2 modem racks. Chip-sets from Rockwell and Lucent are also expected soon. Most 56K modems should be upgradeable to the new standard; 3Com has stated that it will be possible to upgrade all US Robotics modems purchased after 15th August, 1997. Backwards compatibility is also promised - but only with the originally supported system. Thus 3Com's V.90 modems will still be able to use X2, whilst Lucent and Rockwell's will support K56flex.
Technical details of the V.90 standard are currently unclear. It is clear, however, that elements from both X2 and K56flex have been used. Partly, this ensures that companies from both camps have to redesign their products to achieve V.90, but the major cause was financial. For a telecommunications company, to have your ideas embedded in an international standard is a very valuable prize. Therefore, everyone involved was fighting to have their techniques included, and claim their share of the licensing revenue.
Intel's December compromise seems to have consisted of adopting one data transfer technique from either side. "Modulus Conversion", a form of data coding, was adopted from X2, whilst K56flex provided "Convolutional Spectral Shaping", a controlled-ISI method. In addition, Dr. Brent Townshend agreed to license the technology contained in a patent he has applied for. Dr. Townshend, a mathematician from Stanford University, is credited by some sources as having invented the PCM modem concept.
Until production units are available for review, little can be said about the V.90 modem's performance. Both X2 and K56flex were much criticised for not being able to achieve the promised 56 kbit/s; given its ancestry, V.90 is unlikely to be very much better. However, the modem manufacturers have had time to hone their 56K software, and hopefully this will tell in the new devices. Some have warned about adopting this new standard too quickly: as with any new technology, there will be a period of bug-fixes and performance tweaking.
Hopefully, the compatibility testing that has been carried out will ensure that two manufacturers' modems will not refuse to talk to each other. This is no guarantee, though, that certain combinations of devices will not work better than others. Caveat Emptor.
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