Wireless carriers in the United States have woven a tangled web of networks, technologies and standards, and the situation only will get more complicated.

Several carriers have announced plans to install GSM/GPRS overlay networks in addition to their existing TDMA voice networks. Cingular Wireless (www.cingular.com) has operated two networks for several years. AT&T Wireless will overlay (www.attws.com) its existing TDMA/CDPD network with a GSM/GPRS platform, which means that it will operate two networks as well.

As carriers march toward GPRS services via GSM and TDMA, they need a road to get there. GAIT can help. The GAIT specification, named from the GSM/ANSI-136 Interoperability Team, a standards group created by the UWCC (www.uwcc.org) and North American GSM supporters, provides a definition of the interoperability between GSM and ANSI-41 (TDMA) networks. This allows carriers to provide service in their technology of choice while enabling their subscribers to roam on networks of different technologies.

That's why GAIT seems like a natural evolution, an obvious choice and a foregone conclusion in the wireless industry. Carriers want it, need it and supposedly are preparing their networks for GAIT interoperability now.

But no one is sure when GSM and TDMA networks will be GAIT-interoperable or when GAIT handsets will be available.

“It's really important to (carriers) that this initiative goes through and there's a level of interoperability,” said Adam Guy, The Strategis Group (www.strategisgroup.com) senior analyst, mobile wireless research. “The challenge is just whether or not there are enough TDMA subscribers to warrant the production of the equipment.”

Roaming Impetus

The GAIT specification has been under development since 1999, through the combined efforts of the GSM North America group and the UWCC. From the beginning, the goal was GSM and TDMA network interoperability.

“Worldwide, it made sense that there be a specification made where a TDMA and GSM phone could work as one phone,” said Chris Pearson, UWCC executive vice president.

Pearson said the objective was to create a technical specification that would enable one wireless device, supported by a TDMA or GSM carrier, to provide service to a customer whether he turned on his phone in a TDMA or GSM system here or anywhere in the world.

Earlier this year, the GSM Association (www.gsmworld.com) and the UWCC agreed to include TDMA interoperability with GSM as a core component of the GSM Global Roaming Forum (www.gsmworld.com) and bring the GAIT program under the umbrella of the Roaming Forum to gain worldwide support for the specification. By joining forces in the development of interoperability standards for global roaming, GSM and TDMA carriers will be able to offer seamless international service to their customers and create a roaming revenue stream.

“From a global standpoint, the potential is seen in terms of expanding roaming capabilities from TDMA to GSM on a domestic basis, as well as an international basis,” said Kris Rinne, Cingular Wireless vice president of technology and product realization. She said this was primarily U.S.-driven, but for the South American countries that are converting from TDMA to GSM, or operate GSM in some territories and TDMA in others, it will be important, too.

According to Guy, the industry will have to rely on interoperability to enable global roaming.

“We're never going to get everyone to agree on standards or frequency bands. There's just too many commitments and too much political feeling around a lot of these strategies,” he said. “But the question is, how important is it? Is it worth charging an extra $100 for a handset?”

In a recent Strategis Group survey of 500 cellular and PCS users, 12% said they needed a handset with international roaming capabilities. Among those users who spend more than $100 a month for wireless services, 24% said they needed such a handset.

“It's the high-end user that's interested in wireless data that's going to have a need for international roaming,” Guy said. “So if it's from these subscribers that carriers are going to get a rate of return on their spectrum and network-deployment investments, global roaming is something that they're going to want to provide, one way or another.”

Cingular, AT&T Wireless and VoiceStream (www.voicestream.com) all have been active in the GAIT-standard work, because the specification is especially important for carriers that operate both GSM and TDMA networks.

GAIT-ed Networks

Many carriers around the world currently are operating or have equity investments in both TDMA and GSM systems.

“The GAIT project becomes more important because as a carrier, you want customers to stay on your own networks. You don't want to pay your competitors roaming fees,” Pearson explained. “Not only is (GAIT) going to be good for international users, but it's also going to bring together networks as kind of the glue that holds together carriers operating TDMA and GSM networks, sometimes in their own countries but also even in other countries.”

Cingular's Rinne agreed.

“For us, specifically, because we operate both GSM and TDMA, it's very important because it allows us to provide our customers the option to use our networks transparently,” she said.

In the Carolinas, for example, where Cingular has GSM properties surrounded by its own TDMA properties, GAIT will allow the carrier to provide transparent services to subscribers using its own networks, as opposed to roaming on competitors' networks.

According to Rinne, deployment of GAIT also allows carriers to offer TDMA-GSM roaming in areas not currently covered.

GAIT also can help carriers in the capacity department. Even where carriers overlay their TMDA networks with GSM, GPRS and the next generation of high-speed data, the goal is to maintain the TDMA networks for the voice-centric user.

“One of the downsides of the GPRS-to-EDGE-to-WCDMA migration path is that there's more of a compromise to the voice capacity than with the CDMA migration path,” Guy explained. “So it's going to be really important for AT&T and Cingular to continue to utilize and let the TDMA network be the workhorse for the voice traffic, which, at least in the near future, is going to continue to dominate the usage.”

Enabling a GPRS subscriber to roll over to the TDMA network when there's a lot of data traffic can help reduce capacity issues “because there's a zero-sum gain in terms of voice versus data capacity in just a GSM/GPRS solution,” Guy said. “It's the same thing with EDGE, but not so with WCDMA. Once we get to WCDMA, we start to see some increase in voice capacity along with the enhanced data rate.”

Making networks GAIT interoperable won't be difficult or expensive. Rinne said basically there are three GAIT-network requirements: a network-conversion element, handsets and billing systems. Only one network element must be installed to enable GAIT interoperability.

“When a GSM customer is roaming in a TDMA market, it is the TDMA HLR,” she explained. “The reverse is also true. It does that conversion for you, from IS-41 to the GSM intersystem standards. You've got to add that network element, and essentially Cingular will run all of our networks with two platforms.”

Pearson said the infrastructure element is easy to install and basically translates the signaling networks of the two different networks.

“It translates GSM MAP to ANSI-41 and ANSI-41 to GSM MAP so that when a user turns on, whether it's a GSM or TDMA system, it registers with the home carrier's network so they understand for billing,” he said.

GAIT billing will be easy for Cingular because it already has billing support for GSM and TDMA. But Rinne said all carriers have to go through the conversion process from the GSM CDR process to TDMA and vice versa. If capacity is going to be as big a problem as expected, carriers are going to be glad they have that extra capacity to maintain the voice-only subscribers, Guy said.

GAIT Challenges

Despite the obvious carrier support for GAIT, little handset support exists today.

During CTIA Wireless 2001, Siemens (www.usa.siemens.com) revealed preliminary details about the S47, its dual-mode, GPRS-enabled GSM/TDMA handset that will enable domestic and international roaming.

The S47, targeted for U.S. availability in 4Q01, would be the first phone to support two frequencies for TDMA (TDMA 800MHz and 1.9GHz) and two for GSM (GSM 900MHz and 1.9GHz), but it's not a true GAIT handset. According to a Siemens spokesperson, it can roam in both networks' coverage areas, and it performs 95% of the functions, but that doesn't make it a GAIT phone.

AT&T Wireless said Siemens and Motorola (www.motorola.com) are manufacturing GAIT handsets for its network, but neither company would confirm this. Pearson said he expects Siemens to continue the evolution of the S47 and to manufacture true GAIT phones in the future. Other major handset manufacturers involved in the GAIT program and the GSM and TDMA markets will be making GAIT phones, too, and carriers reportedly are working with them for a 4Q01 or 1Q02 U.S. release, he said.

Rinne said GAIT handsets will support GSM 1900, TDMA 1900, TDMA 850 and AMPS 850, and they need to support networks from a software standpoint as well as a radio standpoint.

“The initial (GAIT) handsets will be voice-centric,” she said. “The standards are being finalized for GPRS interoperability so in future versions in 2002, we'll see GPRS on the GSM side. At Cingular, where we've launched both circuit-switched GSM data and circuit-switched TDMA data, the GAIT handset would support that.”

According to Guy, GAIT's biggest problem right now is estimating the demand and whether manufacturers can sell enough handsets to warrant chipset development and the additional production schedule.

“Is there sufficient demand for this?” he asked. “If you're already making 500 million handsets, is it worth making 20 million GAIT handsets? I don't know. That's the burning question.”

Rinne said the network element GAIT requires is not totally unproved because several carriers, including Cingular, have had a GSM-to-analog product for some time, so that piece of it has been tested. But now, carriers are moving from TDMA to GSM, and that surely will bring new challenges.

There are provisioning issues as well, she said, because GAIT handsets will include SIM cards and point-of-sale systems, and distribution processes for a typical TDMA market don't anticipate SIM cards.

But the GAIT committee, sanctioned by both the UWCC and GSM Alliance, has broad carrier support and active support from SIM-card, SMS-seed-platform, infrastructure and handset manufacturers.

“(GAIT) will face the challenges that a new technology typically faces, but the standard is well-written and has been completed for quite some time,” Rinne said. “We're allowing ourselves quite a bit of interoperability testing to ensure we've worked those things out.” She said Cingular has begun GAIT interoperability testing in a lab, with the goal of launching the initial markets at the end of 2001.

Pearson said the biggest challenges are behind GAIT.

“It's moving from development and building consensus to actual production and deployment of a new technology,” he said.

GAIT Interoperability Phases

According to a GAIT spokesperson, GAIT Phase 1 will begin as soon as all parts of the network and handset are completely tested. Testing already has begun, and a launch could occur in late 2001 or early 2002.

GAIT Phase 1+ will require six to 12 more months after Phase 1. Phase 1+ adds support for GPRS when a GAIT phone is homed to a TDMA network and roaming in a GSM network, TTY support in both modes (a government mandate in the United States) and enhancements to network selection and WAP.

GAIT isn't a certifying organization, but the spokesperson said most GAIT handsets can be certified by existing groups. The GSM portion can be certified by the appropriate GSM Association group (PTCRB or GCF), and the TDMA portion can be certified by CTIA (www.wow-com.com). Certain aspects of the GAIT handset require a waiver from CTIA for it to receive certification. This is because the GAIT handset supports short messaging and over-the-air activation in a different way than a TDMA-only handset.

A few aspects of the GAIT handset, including network selection, fall outside existing certification groups. This is unique for a GAIT handset because it scans for two technologies, and today's certification groups don't account for that. However, carriers can test on their own.

Phase 1	Phase 1+
-Voice	-GPRS (Packet Data) in GSM Mode
-Circuit-Switched Data	-Enhanced WAP
-SMS	-TTY Interoperability
-Basic WAP	-Enhanced Network Selection
-Basic Supplementary Services