With a Mobile WiMax service about to soft-launch in Seoul, one of South Korea's biggest telecom carriers, KT, last week demonstrated the mobile-internet service to reporters.

The service is being promoted under the brand name WiBro. WiBro technically refers to a Korean-developed technology that was folded into the IEEE 802.16e Mobile WiMax specification.

Trials of the service began in April and a limited commercial service will kick off this month. The initial service covers a single area in downtown Seoul, three areas south of the Han River in the city's IT valley, and the suburb of Bundang. A subway line and two expressways from Seoul to Bundang also have coverage.

About 150 base stations support the service now, and this is expected to expand to between 700 and 1,000 by the fourth quarter this year. At that time the full commercial service will launch in Seoul and nine other cities, according to KT's plans.

KT says the service should deliver up to 1Mbps (megabits per second) to users travelling as fast as 75mph (miles per hour). So, does it live up to this promise?

To demonstrate the service and prove it works while on the move, KT took reporters on a ride through Seoul in a bus equipped with WiBro-linked laptop PCs and PDAs (personal digital assistants).

The speed challenge was difficult to test in Seoul's congested streets - we were lucky to even reach 20mph - but the data throughput was much easier to gauge.

The system managed to cope with streaming from Google Video just fine, and browsing the web was easy. KT demonstrated a three-way video conference with one participant on the bus, one in Seoul and one in Australia, and that went well and worked simultaneously with web browsing and watching a live stream of CNN.

A broadband speed-testing website estimated the connection at about 322Kbps (kilobits per second), although the site was US-based, so there could have been some impact from the international connection.

There were few glitches, and the trial service seemed to fulfill its promise of being slower than existing wireless-LAN systems but with much greater coverage.

If KT and SK Telecom, which is also planning to launch a service, can deliver that for a cheap, flat monthly fee, it could provide an attractive consumer alternative 3G data service. They offer similar data rates but are relatively expensive.