Will VoIP really drive convergence?

Voice doesn't necessarily require a lot of bandwidth, it just requires that bandwidth to be guaranteed. And in some cases, the cheapest way to do that is to run it over a separate network.

Siemens's Bishop says: "10 Mbps ethernet is dirt cheap and adequate for a voice only network."

This is an approach which particularly works for companies mainly interested in IP to cut down on the costs of provisioning employees, he says.

"We have some clients in professional services who are often moving teams and phones and the costs of moving and changing are extraordinary for them," says Bishop.

Many companies considering the idea of converging all data onto one network can't quite believe it will be reliable enough.

Rightly or wrongly, data networks aren't seen as being the most reliable things in the world. And if something should happen to the data network, workers want to be able to know they can pick up the phone and call the help desk. When they pick up the phone and there's no dial tone, it becomes a very serious problem.

So there's a perception, justified or not, that phones which are running on a potentially dodgy data network won't have that cast-iron, unshakable, 'fine-nines' reliability that we're used to from conventional telephone networks.

Of course, the reliability of any given network doesn't just depend on the technology. It's as much if not more a function of network design. The more redundancy and fail-safe features you build in, the more reliable it will be.

But that comes at a price and obviously the more you spend on a network, the less attractive the cost-benefit analysis becomes - as saving cash was the reason for converging your networks it in the first place.