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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 standards 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 inter-system handoff, call delivery, validation and authentication. Without ANSI-41, these systems could not allow wireless phones to operate outside their home system.
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, and therefore has better international roaming capabilities than ANSI-41, which evolved in the massive U.S./Canadian market, where international roaming was not at all necessary in early years. Although ANSI-41 systems did not 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, but largely because two different groups of engineers were in charge of developing them. The differences make as much sense as the fact that Americans and the British speak differently.
TIA standards subcommittee TR-46 was set up for the development of PCS standards, but soon became focused on GSM issues as PCS carriers with systems compatible with ANSI-41 migrated back to TR-45. One of their major projects was a TIA document, IS-129, that described how interworking could be accomplished. Unfortunately, however, 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 inter-system handoff and short messaging were avoided entirely.
When GSM became established in North America, some companies (mainly SouthwesternBell and BellSouth) ended up with two incompatible TDMA systems in their network: ANSI-136 using ANSI-41 as a network standard, and PCS1900 using a GSM backbone network. These companies have made considerable efforts to provide a higher level of interworking through a project known as GAIT (GSM/ANSI-41 Interoperability Team).
GAIT has defined a broader set of services than IS-129, including call-forwarding, call-waiting, 3-way calling and short message services. 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. Alternatively, if GHOST is not available, the IIF can convert ANSI-41 SMDPP messages to their GSM equivalent.
Even when a direct mapping exists between ANSI-41 and GSM messages, the IIF will need to 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 can be stored in the IIF, but in the case of a “Smart Card” (UIM)-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 to be satisfied.
Routing of messages raises additional complexities. Making MSCs aware of the kind of HLR by which each roamer is hosted would create a management headache. This can be avoided through the use of SS7 global title routing. ANSI-136/ANSI-41 systems can use one global title translation 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 global title translation type can be used to do exactly the opposite. The IIF, after protocol conversion, then merely has to change the translation type to ensure that routing occurs correctly.
GAIT is not an accredited standards committee, but in November 1999, they forwarded the specification to ATIS T1P1 and TIA TR-46, requesting that they complete the development of the protocol. That work, which is to be completed this year, will hopefully result in an industry-wide 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 more regional grasp of ETSI.
CDMA carriers also have an interest in interworking with GSM systems, although their requirements are more driven by international roaming than by single carriers that operate two disparate technologies. Recently they established their own GAIT (GAIT-B). Ideally, they will use very similar techniques and protocol modifications to ANSI-136 GAIT, to try to maximize compatibility, particularly at the level of the IIF.
Convergence is the aim of many companies in both the ANSI-136 TDMA and ANSI-95 CDMA camps. GAIT has defined a workman-like 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.
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