The Great Divide

Forget nationwide networks. Global roaming is the next must-have. So how will the industry reconcile ANSI-41's and GSM's differences?

Millions of years ago, Europe and North America were joined as one continent. At about the same time, telecom standards were completely compatible. Wireless is no exception to the rule that European and North American standards are incompatible, with the Americas largely relying on ANSI-41 networks using TDMA, CDMA or analog technology, and Europeans using the GSM network. Can the gulf between ANSI-41 and GSM be bridged?

ANSI-41 is the network standard that allows cellular or PCS systems operating analog, TDMA (IS-54 and ANSI-136) or CDMA (ANSI-95 and IS-2000) radio equipment to interoperate, providing intersystem handoff, call delivery, validation and authentication. Without ANSI-41, these systems couldn't allow phones to operate outside their home systems. GSM has parallel network protocols to provide similar capabilities between different systems running GSM radios in the cellular and PCS bands.

GSM and ANSI-41 evolved in different environments, and it shows in their capabilities. GSM evolved in an environment where many countries had systems that needed to interconnect. As a result, it has better international-roaming capabilities than ANSI-41, which evolved in the massive U.S./Canadian market, where international roaming was unnecessary in wireless' early years. Although ANSI-41 systems didn't consider international roaming at first, their requirements for national roaming were much more stringent. At first, the U.S./Canadian market was composed of hundreds of independent companies, whereas European countries assigned nationwide licenses. Consequently, ANSI-41 capabilities for inter-MSC roaming are superior.

Other differences between GSM and ANSI-41 are more random in nature. Authentication protocols are utterly different, largely because two different groups of engineers were in charge of developing them.

First Attempt
TIA standards subcommittee TR-46 was set up for the development of PCS standards but soon became focused on GSM issues as PCS providers with systems compatible with ANSI-41 migrated back to TR-45. One of their major projects was TIA document IS-129, which described how interworking could be accomplished. Unfortunately, this standard addressed only the easy problems, such as inter-technology call delivery, and then only at a sparse level of detail. Tougher issues such as intersystem handoff and short messaging were avoided entirely.

When GSM became established in North America, companies such as BellSouth and Southwestern Bell ended up with two incompatible TDMA systems in their networks: ANSI-136 using ANSI-41 as a network standard and PCS 1900 using a GSM backbone network. These companies have made considerable efforts to provide a higher level of interworking through a project known as the GSM/ANSI-41 interoperability team (GAIT).

Compared to IS-129, GAIT has defined a broader set of services, including call forwarding, call-waiting, 3-way calling and SMS. Adaptations may be made in the handset through ANSI-136 protocol modifications or in the network through an interworking and interoperability function (IIF).

A handset modification is illustrated by the difference between ANSI-41 and GSM feature activation. ANSI-41 systems used feature code strings (e.g., *73) that are identified by digit translations, whereas the GSM protocol defines special messages to activate a list of features. A phone with GSM capabilities may have to generate the predefined string when operating in ANSI-136 mode, and when a GSM system receives a feature code, it will have to have digit translations adapted to interpret it as a command to activate or deactivate a feature.

An ANSI-136 protocol modification is illustrated by "Ghost," a special short-message teleservice that allows GSM-formatted short messages to flow through ANSI-136/ANSI-41 networks. If Ghost is unavailable, the IIF can convert ANSI-41 SMS delivery point-to-point messages to their GSM equivalent.

Even when a direct mapping exists between ANSI-41 and GSM messages, the IIF must completely analyze the incoming message, map parameters where possible and then regenerate in the different format. The IIF will perform more high-level functions, as well. For example, all ANSI-41 messages require an ESN. For each mobile, this code can be stored in the IIF, but in the case of a smart-card-equipped ANSI-136 mobile, the current ESN cannot be known. In this case, a default ESN will be provided to the ANSI-41 HLR to allow validation checks.

Routing messages raises additional complexities. Making MSCs aware of the kind of HLR that each roamer is hosted by would create a management headache. This problem can be avoided through the use of SS7 global-title routing. ANSI-136/ANSI-41 systems can use one global-title-translation (GTT) type that routes messages for ANSI-41 native mobiles directly to the HLR but routes messages for GSM native mobiles to the IIF. On the other side of the IIF, a different GTT type can be used to do exactly the opposite. After protocol conversion, the IIF then merely has to change the translation type to ensure that routing occurs correctly.

Moving to a Standard
GAIT isn't an accredited standards committee, but in November 1999, it forwarded the specification to ATIS T1P1 and TIA TR-46 and requested that they complete the development of the protocol. That work, which is to be completed this year, may result in an industrywide definition of the ANSI-41/GSM IIF. GSM/ANSI-41 interworking has largely been driven from the ANSI-136/TDMA camp in the past. The UWCC has had a strategy of driving convergence of its technology with GSM and recently announced that 3GPP would be taking over standardization of GSM, moving it out of the grasp of ETSI.

CDMA providers also have an interest in interworking with GSM systems, although their requirements are driven more by international roaming than by single providers that operate two, disparate technologies. They recently established their own GAIT (GAIT-B). Ideally, they'll use techniques and protocol modifications very similar to ANSI-136 GAIT to try to maximize compatibility, particularly at the IIF level.

Convergence is the aim of many companies in both the ANSI-136 TDMA and ANSI-95 CDMA camps. GAIT has defined a workmanlike method for bringing North American systems closer to GSM, but the ultimate solution lies in 3G systems that have an opportunity to start from scratch by designing network protocols that combine the best of ANSI-41 and GSM with new ideas for the high-bandwidth capabilities being developed.

Crowe (crowed@cnpwireless.com) is a wireless-standards consultant and editor of Cellular Networking Perspectives, a wireless-standards and -technology bulletin.