So what is V90?
V90 is the name given to the new ITU standard for 56k modems. It is neither X2 nor K56flex, but a combination of features from both.
There has been much debate about interoperability between the systems. Basically, It will depend on what modem you have. X2 modems will upgrade to V90 and retain their X2 code as well; in fact an updated version will probably be supplied. So once upgraded, they will connect to either a V90 server or an X2 server. Rockwell flex modems however cannot, at present, hold both the flex code and the V90 code; it's one or the other. (This is due to the limited size of the flash memory). So a V90 modem will only achieve 33600bps to a flex server. A flex modem should retain its flex connection speed to a V90 server as backward compatibility will be included.
The essential message is, if you have a flex modem, DO NOT flash upgrade to V90 until your ISP is ready for it, you will lose speed.
Also, it is true to say that problems have been reported with some of the V90 flashes. This was to be expected, but a reputable manufacturer should not make the code public until it is reasonably reliable. One difficulty seems to occur when screensavers interrupt the flash. Make sure you disable these as well as closing any other programs before attempting an upgrade (screensavers are very processor-intensive!).

V90 news Information is just starting to filter through about V90. So far the transition is less painful than the introduction of flex, with most people reporting improvements in connection speeds or reductions in line drops. Most UK providers are now running final versions of the code.

Well I said it didn't I? Yes, for a lot of people it works well. But for some it doesn't. It seems that the promised interoperability has spectacularly failed to materialise, and there are some people who can't connect at V90 at all! I could only get a V90 connect to one of my ISP's three V90 numbers, and then it was hit and miss. One way round is to add a command to your init string to force the modem onl to use V90. For the Rockwell hardware modem, that's +ms=12. For the Rockwell SoftK56, it's +ms=v90. The PCtel chipset is a little obscure in this respect, but s38=1n0s37=14 does it (that is a zero in the middle). For USR modems, use s32=34.
Should you upgrade? If you have X2, definitely yes. If you have flex, make sure your ISP supports it (there are still a few Luddite ISPs out there!). If you get good stable flex connections, V90 probably won't help much. Although if your connection at present is not what you would like, you'll do no harm by upgrading, you may even benefit! Most flex-V90 flash upgrades provide the facility to flash back to the old code if V90 proves to be a disappointment.
Some odd reports have been noted so far though. Some ex-flex modems connect more successfully to V90-X2 modem racks! My HSP V90/flex will connect to V90-X2 at one ISP but not at another. Work it out?
Known issues
Few issues have been found yet. One is that upgraded modems sometimes report port speed instead of connect speed. Usually this is because the inf files must be updated at the same time as the firmware. The connect speed reported by Dial Up Networking comes from the inf file, and when the connect speed is not found, port speed is reported instead. V90 includes some new speeds compared with X2 and flex. Once again unbranded modems are found to have poorly written inf files. I have created my own version (modeminf.exe)for Rockwell chipset hardware modems which is reported to be largely successful. One "strange but true" trick has already emerged. Several people have reported improved connection speeds by adding 2, 3, or 4 commas to the end of the dial-up number. Not sure why this works, but it does! No doubt future firmware will be written to include this.
3com/USR V90 upgrades
It has been reported that connection speeds are lower with this upgrade. In actual fact USR modems were known to over-estimate initial connect speeds! The new speeds reported are simply more accurate. Actual throughput of data is more important, and most users find that this improves under V90.