Fibre-based broadband will deliver much faster speeds to customers
Homes and businesses in Cardiff and Taffs Well are among the first in the UK to have access to super-fast broadband from BT.
An upgrade of a local network serving 16,000 premises is being carried out as part of a £1.5bn programme by the firm.
The installation of fibre-based broadband will lead to speeds in the two communities of about five times faster than current speeds.
MPs in south Wales have called the scheme an "important development".
The fibre to the cabinet (FTTC) broadband technology will deliver download speeds of up to 40 megabits per second (Mbps) on the Cardiff and Taffs Well network - with the prospect of 60Mbps in the future.
This is in comparison to first-generation speeds of up to eight megabits.
Virgin Media also offers speeds of up to 50Mbps for homes in parts of Cardiff and Taffs Well.
Cardiff and Taffs Well are at the forefront of the first major phase of BT's plans to roll out fibre-based broadband to at least 40% of the UK - including 10 million homes - by summer 2012.
A total of 16,000 homes and businesses in these areas will have access to the fast-speed broadband within weeks. The work is being carried out by BT's local network business Openreach.
Ann Beynon, BT director for Wales, said: "Once again, Wales is at the forefront of the broadband revolution in the UK.
"The upgrading of the network and BT exchanges in central Cardiff and Taffs Well is the latest vital step in ensuring that Wales benefits from exciting, leading-edge technology capable of delivering a wide range of innovative new applications, such as high definition video conferencing and 3D television over broadband."
South Wales MPs Kim Howells, Jenny Willott, Alun Michael, Kevin Brennan and Julie Morgan welcomed the arrival of the technology in parts of their constituencies.
In a joint statement they said: "The arrival of super-fast broadband is a very important development for our communities and it represents a significant investment in this area. We welcome the opportunity for local householders and businesses to be at the forefront of benefiting from this exciting technology."
Exchanges in Barry, Caerphilly, Penarth, Llanedeyrn and Llanishen will be upgraded next, providing super-speed broadband to a further 48,000 Welsh homes and businesses by late summer.
The programme in Wales follows successful pilots in Whitchurch, Cardiff, Glasgow and London.
More than 99% of UK homes and businesses now have access to first generation broadband capable of delivering up to eight megabits per second. By 2012, speeds of up to 100 Mbps will be available in some parts of the UK.