The V.90 standard is neither x2 nor K56Flex, although it does use techniques from both. It is actually two standards in one, the specification defining a digital modem and analogue modem pair capable of transmitting data at up to 56 Kbit/s downstream and up to 33.6 Kbit/s upstream. In this case, downstream means from the digital to the analogue modem. The former is connected to the PSTN via an ISDN line and will usually be part of a bank of modems connected to a multiple-line ISDN at an ISP. The analogue modem plugs into the PSTN at the subscriber’s end.
The key to V.90’s 56 Kbit/s capability is the PCM coding scheme introduced by the standard’s proprietary forerunners. PCM codes are digital representations of audio signals and are the telephone system’s native language. The exchange generates these on receipt of analogue signals from the subscriber’s handset. They’re eight bits long and are transferred at a rate of 8,000 per second – a total throughput of 64 Kbit/s. A V.90 digital modem uses a large subset of these code to encode data and delivers them to the telephone system via an ISDN link. At the subscriber’s end, the codes are converted to an analogue signal by the exchange – as if they had been created in the usual way – and these tones are sent to the subscriber’s modem.
Most of the work in creating V.90 went into he line-probing and signal-generation schemes. When a V.90 connection is first established, the two modems send each other a list of their capabilities. If V.90 communication is possible, the analogue and digital modems send test signals to each other to check the quality of their connection and establish whether there are any digital impairments in the telephone system that might prevent the PCM codes from arriving correctly. For example, on some long distance or international calls, the 64 Kbit/s signal is compressed to 32 Kbit/s (or more) for reasons of economics – and this ruins V.90.
If there are no impairments, the analogue modem analyses the signals from the digital modem and informs it how best to encode its data. The two modems also sort out what the round-trip delay is and work out what equalisation to apply to the line to get the best possible frequency response.
Coding the information into PCM is a complex business. The telephone system doesn’t treat PCM codes linearly. Instead, it allocates more PCM codes to lower signal levels and fewer codes to higher levels. This corresponds with the way the human ear responds to sound, but it also means that the receiving modem might not be able to distinguish between some of the adjacent codes accurately. Also, the signal synthesised by the digital modem must be able to be accurately converted to analogue and sent through the analogue parts of the telephone exchange.
Error connection and detection systems also limit the sequential permutations possible. In short, there are sequences of codes that can’t be sent and others that must be sent, but these are dependent on the data being transmitted. A final complication is that the American and European telephone systems use different sets of PCM codes.
The V.90 standard was formally ratified on 15 September 1998, following a several-month approval process. Beyond V.90, an ITU study group is looking into the next generation of PCM modems, with the intention of achieving a 40 Kbit/s to 45 Kbit/s transmission speed from the analogue modem.