GSM/ANSI-136 Interoperability Team.
GAIT, the GSM/ANSI-136 Interoperability Team, was a group composed of North American TDMA and GSM operators, and the Universal Wireless Communications Consortium (UWCC). They developed a standard (also referred to as "GAIT"), which allows seamless roaming between TDMA and GSM networks.
TDMA (ANSI-136) and GSM are two of the three main digital technologies for mobile phones and networks. (CDMA is the other.)
The driving force behind this standard is the fact that two major US carriers - AT&T Wireless and Cingular - are both transitioning their networks from TDMA to GSM technology. During the transition period, they have been operating both types of networks at the same time. Therefore, it is beneficial to have phones that work seamlessly on both types of networks.
The GAIT standard consists of upgrades to the networks, and also new "GAIT" phones. With both in place, roaming and features like SMS text messaging and voice mail are seamless across both types of networks, making the transition smoother for users.
GAIT technology does NOT provide seamless handoffs between TDMA and GSM. For example, when moving from a TDMA area to a GSM area (such as on a highway) a call in progress will drop. GAIT only allows certain features to work similarly on both types of networks. Without GAIT, TDMA users would not be able to access SMS while on a GSM network, and vice-versa.