More than 150,000 users in 160 countries have downloaded free "web accelerator" software over the last two months from a US start-up founded by ex-Microsoft personnel.

The initial rollout of NetSonic, a Microsoft Windows 95 utility from Web 3000, uses proprietary technology to sample, update and cache a user's favorite Internet sites. NetSonic stores the main graphic elements of each page and only retrieves the content that has changed. This reduces download traffic and time.

A free version of the basic NetSonic can be downloaded from the company's Web site, www.web3000.com. The deluxe edition, called NetSonic Professional and due in late July, will include additional configuration options. There will be a charge for this edition, expected to be around the $30 mark.

"What NetSonic does that is unique is to use a combination of caching and background validation, coupled with a proprietary method of refreshing only those elements on the page that are needed to build a completely up-to-date page," said Gene Kavner, Web 3000 founder and chief executive.

NetSonic runs alongside the browser, rather than in a proxy-based architecture common to other products. This design means that NetSonic does not require any reconfiguration of browsers and can work with all popular makes.

Kavner, who was involved in developing the Microsoft Network for Microsoft, outlined how most of the other products on the market that call themselves accelerators do what's dubbed pre-fetching - where they try to anticipate where a user wants to click next and attempt to load links ahead of the user by using the modem idle time.

What sets this product apart from competitors, like Connectix's Surf Express and Peak Soft's Peak Jet, is that NetSonic does not use a local proxy server. NetSonic minimises the time a user waits for a Web page to be displayed by intelligently caching Web content for immediate display while at the same time, in the background, checking for and retrieving any changes to the Web page.

By analysing the actual graphical resources on each Web page, intelligently deciding how likely a resource is to change, then, requesting updates for only those resources, NetSonic reduces the amount of round-trips and data transmitted over the Internet.

Web 3000 has a deal in place with Hayes Microcomputer Products to bundle NetSonic with various of Hayes modems.