The advertising campaign for BT's home broadband service features a nice modern family, with a woman and her two adolescent children living with her new partner.

They have been using them for a while, and like many other families they are gradually moving up the curve of technology adoption and are now happily embracing wireless, online backup and even TV over the internet.

They may be forced to call for technical support this week, however, as BT has just announced that it is asking every one of its three million broadband customers to share part of their wi-fi bandwidth by joining the FON community, and explaining this to the mass market could be a bit of a challenge.

FON users, or "Foneros" as they like to be called, share their bandwidth with other people, either by using a special wireless router provided by Fon or by installing special software on other brands of router.

FON-enabled routers have two separate wi-fi channels, creating a secure private channel for the owner to use and a 512kpbs open shared channel that other people can connect to.

The secure channel is totally separate from the public one, so that home networks aren't exposed to hackers.

Free access

Subscribers get free access to the shared channel, while non-users or "Aliens" can buy a daily pass.

The money goes to the organisation, although Foneros can get a small cut depending on whether they see themselves as public-spirited "Linuses" or revenue-generating "Bills"" - named after Linux creator Linus Torvalds and Microsoft's Bill Gates.

Our fictional family is going to have to decide which of the two icons of the network age they feel most sympathy with, and configure their new FON-enabled router to offer wireless access to passers-by.

It will certainly be a challenge for the copywriters at BT's advertising agency, who will need to explain why the firmware on the Total Broadband routers has been updated and what the new service actually involves in a 30-second TV spot.

But it seems to be a pretty good move for BT, and other ISPs may well decide to sign up too if it takes off as predicted.

Offering your customers unlimited free access to three million wifi hotspots around the UK, and millions more around the world, is a pretty good deal.

And it's even legal, so there will be no need to hide in your car and pretend that you're not really reading your e-mail from someone's open wireless connection.

Several fortunes

FON was set up by Spanish telecoms entrepreneur Martin Varsavsky back in 2005.

Having made several fortunes from companies like Viatel, Jazztel and Ya.com, FON combined his long-term interest in open source and free software with his telecoms expertise, providing a community-based solution to the problem of finding open wireless connections when away from home.

When it launched late in 2005 I described it as "an interesting idea, fatally flawed by the terms of use of most internet service providers which seem to make it a breach of contract to install the software", but of course Mr Varsavsky was in this for the long term and has managed to keep most of his Foneros out of trouble by appealing to the more technically proficient geeky early adopters while negotiating with ISPs to roll out the service more widely.

There's certainly a real need for open, accessible wireless connectivity.

On my way to Geneva from London's City airport last week I tried to read my e-mail, but found that the month of wireless connectivity I'd expensively acquired from T-Mobile would not let me roam onto the network provided at the airport, so if I wanted to surf the web or catch up on work I'd have to pay another 10 pounds.

This experience is both common and frustrating, as airports and also hotels seem to see network access as a way to gouge money out of customers.

Public provision

Hotels, used to raking in money for phone calls, charge over the odds for net access because they assume that business users will just claim the cost on expenses and those paying their own way will be so desperate that, as with the minibar, they'll pay what is asked.

Public provision has varying degrees of success. In the US large-scale public wireless projects have been falling apart under political pressure, although over here cities like London, Norwich and Manchester seem to have more success, perhaps because there is a stronger tradition of public provision of vital services - like health care - than in the US.

Services like FON, or the plan to add wireless transmitted to the bridge of Venice, offer another way, one that is driven by the community and rewards altruism with access, providing clear benefits to users and the wider world.

It undercuts the commercial offerings of cafes and hotels, but no business is guaranteed unfettered access to the market - ask the music industry.

And if all of us currently paying monthly rates to have our home broadband access lie unused for hours a day can make it easily and securely available to other members of the community, why shouldn't we?

Bill Thompson is an independent journalist and regular commentator on the BBC World Service programme Digital Planet.