The soccer World Cup that takes place in South Korea and Japan during June will revive many old rivalries on the pitch, but is also serving to bring together two long-time rivals away from the sports arena.

For the first time, users of GSM (Global System for Mobile Communications) cellular handsets will be able to roam on South Korea's CDMA (Code Division Multiple Access) networks by slipping their handset SIM (Subscriber Identity Module) cards into specially developed CDMA terminals.

"You can make a call on our CDMA network with the (GSM) SIM card," said Kwon Chul Kuen, a spokesman for SK Telecom Co. Ltd., which along with rival KT Freetel Inc. (KTF) is offering the service.

That may not sound like a big deal, but anyone who travels regularly will tell you that switching on their cell phone and waiting to see if it works is one of the aspects of international travel that constantly brings surprise. While much of the incompatibility within standards has been ironed out, for example through the introduction of tri-band handsets for GSM, it remains difficult or impossible for users of one standard to roam on networks based on a different standard.

This year's World Cup had promised to be particularly troublesome for visitors. Between the two co-host nations, South Korea has three carriers and all use CDMA-based networks, while a single Japanese carrier uses CDMA and two others use the domestic PDC (Personal Digital Cellular) system. Most visiting fans are expected to come from countries where the GSM system is dominant.

Recognizing the problem and seeing a chance to make some extra cash (international roaming services are very lucrative because of the high charges levied on users), two of the South Korean carriers set about making it possible for these GSM users to roam on their networks. The result is a pair of custom-made handsets, one for each carrier, that allow full roaming while in South Korea for GSM users.

To get the system working, the carriers had to do a fair amount of work and bring together disparate standards and protocols.

The first problem was in the SIM card, which acts as the basic identifier for each GSM customer and also holds other information, such as the telephone book. With the exception of terminals on sale in China, CDMA handsets do not have removable cards. They rely instead on a User Identity Module (UIM) that is hard coded into the handset.

At first the carriers toyed with the idea of introducing removable UIM cards, and handsets with a reader for both UIM and SIM cards, but shelved that idea for fear the system would not be ready in time for the tournament. Instead, they decided to modify their networks to support the GSM SIM cards. This meant building a bridge between the subscriber identification and billing systems used across each standard.

"It is no different from someone using Word Perfect and Word for word processing," said Eric Hill, director of international roaming at the GSM Global Roaming Forum. "You can translate between the two formats but they are different and without a translator they are incompatible."

"They put convergence boxes, or you can call them black boxes, into the network that do protocol translation between the networks," said Hill. The result is a working service that is likely to satisfy visitors to South Korea during the championship but only goes part of the way to satisfying those in the industry because they have bigger dreams.

"What the Koreans have done is just tremendous in terms of roaming for the World Cup," said Terry Yen, leader of the international roaming team at the CDMA Development Group. "I think SK Telecom and KTF had to be relatively specific due to the time constraints they were under, so (the roaming system) works in Korea but not anywhere else."

What the groups led by Hill and Yen, which include most of the cellular-network operators, are now looking at is a specification that allows for two-way roaming.

"The next step is two-way roaming, but we are still talking about separate SIM cards. In terms of SIM-card interoperability, the best we can hope for in the short term is devices that accept differences between the two formats," said Hill. Longer term, the group is hoping to develop a smart card that contains both GSM and CDMA subscriber information and can be used in handsets built to either standard.

Beyond that, Hill is looking to what might be called the Holy Grail of cellular technology.

"Where we are trying to get is a fully functioning terminal, SIM card and network which does not require any overhead that the customer or network must manage," he said.

When might such a system be available? "We are looking at the middle to the end of 2004, but it's too early to tell," he said.