Company bans on flying and the fear of further hijacks are leading people to take another look at video conferencing. The good news is that the technology has moved on a long way from its flickering, garbled past. The bad that the high-speed networks over which video must travel are still not an affordable option for most homes and offices.

Shares in video conferencing companies surged after the attacks on the World Trade Centre, as Wall Street brokers anticipated video meetings replacing face-to-face gatherings. And that was despite the fact that some companies held back from promoting themselves in the immediate aftermath.

Martyn Lewis, the former BBC newscaster who chairs the UK video conferencing company Teliris, says: "We stopped marketing till George Bush started urging everyone to get back to business as usual."

Nevertheless, the company has seen a fourfold increase in demand for its products. PictureTel, a Nasdaq-listed video conferencing company, says calls to its 1-800 freephone number trebled after the attacks, hits on its website doubled, and its resellers are selling through their stock much faster than usual.

The company, which is to merge with US rival Polycom, now expects sales for 2001 to be more than 20% higher than last year. "Before September 11, sales were sort of flat," says Ned Semonite, PictureTel's executive vice president of marketing.

Phil Sewell, managing director of the UK company Global VideoCom, says: "Enquiries have tripled. Many businesses that were sitting on the fence or trying to complete complex cost justifications have just gone ahead and bought."

Companies that have started to use video conferencing for the first time have been pleasantly surprised. David Bell, a director of Pearson and chairman of the Financial Times, started to use GlobalTable, Teliris's system, two weeks ago for communications between Pearson's headquarters in central London and its New York office.

He says: "A scheduled two-hour meeting between London and New York involving seven people lasted for almost four hours, with everyone's reactions and attitudes echoing precisely those of a meeting in the same room. Our executives could not believe how natural and relaxing it was."

And George Russell, chairman of the UK venture capital firm 3i, which will use the same system to communicate with its Palo Alto, California office this week, says: "The real return on investment is not the air fare saved but the extra, high quality thinking time it is going to give my top executives."

Part of the reason why the system seems more lifelike is that people at remote video conferencing sites are displayed on 42-inch plasma screens with no more than four people to a screen. Lewis says: "With old-style video conferencing systems, a single monitor is sometimes split into many windows. A recent newspaper photo of George Bush shows him peering at a single screen in which loads of people he can barely see are displayed in four windows."

Bigger screens mean better views of boardroom battles without the need to use that strange video conferencing option of displaying just the person who is shouting the loudest. "We wanted executives to feel as if they have eye contact with each other, even if they are on the opposite sides of the world," says Lewis, "so we have added virtual vectoring software. Now, when someone in the Paris office turns to look at the monitor showing a colleague in London, the New York office will see that."

There is also no time delay when people are speaking, as is usual with the ISDN lines commonly used for video conferencing. This is because GlobalTable uses dedicated fibre-optic lines operating at speeds of 4.5 megabits a second in each direction. But this is a high-end system, with near-DVD quality costing $120,000 to $750,000 for a dedicated meeting-room system and line-rental of $13,000 a month for unlimited usage.

"Many companies find even ISDN too expensive," says GlobalVideoCom's Sewell. "Three ISDN lines providing a total speed of 384 kilobits a second - which gives good-quality video -will cost £150 a hour for a video link between Europe and New York. And the technology is not that reliable.

"In the first few days after the World Trade Centre attacks, it was almost impossible to get an ISDN circuit to New York, and it is not unusual for lines to drop. Asymmetric digital subscriber line technology [ADSL, which expands the capacity of ordinary phones lines] might be an alternative, but there have been delays in local loop unbundling and problems with contention."

Sewell says such obstacles mean many companies want to add video conferencing to their virtual private networks (VPNs), telecoms networks provided by large telecoms carriers that operate as if they were the company's own. Global VideoCom used to use ISDN but now saves by linking its Edinburgh and London offices to head office in Berkshire at 768kbit/s over a £60,000-a-year VPN that it also uses for phone calls.

Sewell leaves monitors on displaying the regional offices permanently so that he can make sure phones are being answered, and that staff are happy. "There is an area near the monitors with couches and we usually go and sit there at lunchtime and have a chat with whoever is around in the other offices," he says.

To add video conferencing to a VPN costs around £400 a user, which includes a camera and other hardware and software for each PC. The company can also link a dedicated video conferencing monitor to the VPN. Once on the company network, conferencing can be integrated with other applications. Global VideoCom provides email software that integrates with Microsoft Outlook so users can check when colleagues are free.

Such innovation means that the surge in interest in video conferencing could outlast the current crisis. Ovum, the consultancy, says the worldwide market for video conferencing is worth more than $6bn and this is expected to rise by about 20% this year.

But the real winner in the short term is likely to be audio conferencing - using an ordinary phone to talk to many people at different sites. David Alexander, an analyst at the US market researchers Frost & Sullivan, has revised his forecast for US audio conferencing up from 6.5bn minutes to 7.8bn for 2001.

"With webcasting, people can look at PowerPoint or other presentations online and some webcasters are adding video," says Alexander.

Most of the video conferencing shares which soared after September 11 are now below their recent highs, yet Nasdaq-listed Gentner, which derives a lot of its revenue from audio conferencing, is still buoyant. Shares in the company hit $22.80 on Tuesday and Nigel Gonsalves, a telecoms analyst at US-based H C Wainwright, has them on a strong buy after they passed his previous price target of $18.

Indeed, Gonsalves has just one worry. "The only thing that inhibits video conferencing from becoming a mass market tool is that executives won't get to enjoy themselves on business trips," he notes.