International Roaming Using CDMA

• The ability to make a call (‘Mobile Origination’). While this sounds simple, this fundamental service requires a wireless system to verify that the customer's subscription is valid, that the phone is not stolen, that it is not illegally emulating another phone, and to ensure that the type of call being made is not restricted.
• The ability to receive a call (‘Mobile Termination’ or ‘Call Delivery’). This is technically much more complex, and requires the roamer's phone to first register in the system in which it wishes to receive calls. This is done automatically, and causes an exchange of information over the SS7/ANSI-41 network to the home system. When a call comes in to the home system it already knows where the mobile is, and can route a call to it.
• Inter-system handoff. This allows a mobile call to continue uninterrupted when the mobile crosses the boundary between two cellular systems.
• Short Message Service. When an SMS message comes in to the home system it will be forwarded to the mobile, wherever it is.
• Calling Name/Number Presentation. When receiving a call, the number of the caller or even their name will be displayed on their handset.
• International dialing. Some phones provide a “+” key or equivalent menu option that makes it easy to place an international call without knowing the local access number.

• Base Station. Contains the radio equipment for one or more cells.
  
• MSC (Mobile Switching Center). Connects mobiles to other mobiles or to phones in the telephone network or on other cellular systems.
  
• HLR (Home Location Register). Contains information about a subscription, including the types of services which are to be provided.
  
• AC (Authentication Center). Contains cryptographic information that allows the network to determine that a mobile is valid. Usually contained within an HLR.
  
• MC (Message Center). Stores and forwards short messages.

• Support for international digit strings in IS-41 Revision C (1996)
  
• Support for International Mobile Subscription Identifiers (IMSI) in TIA/EIA/IS-751 (1998).
  
• Assignment of International Roaming MIN's by IFAST (1998).
  
• Support for SS7 global titles in TIA/EIA/IS-807 (1999).
  
• Enhancements for calling number identification, ‘+’ code dialing and callback (2001).

• Call forward no-answer/busy can be handled more efficiently by ANSI-41 systems. Calls are forwarded from the home system, with the connection to the serving system being released. GSM systems forward from the serving system, often resulting in calls looping from home system to serving system and back to the home system.
  
• Inter-system handoff is supported more efficiently in ANSI-41. It is performed directly by neighboring MSC's, without requiring a special gateway MSC, as in GSM.
  
• ANSI-41 supports mobiles that can operate in multiple technologies (e.g. analog and CDMA).
  
• Authentication of mobiles can be done locally. In GSM authentication calculations must be performed by the AC, requiring one transfer of authentication data for every call. In practice, GSM carriers often avoid this, which reduces the level of security that their systems provide.

• Transition to global title. This will simplify routing between network elements in different countries. Current international roaming systems work well, but cannot use standard international SS7 signaling gateways.
  
• International TLDN (Temporary Local Directory Number). This is very important to routing and should be an internationally formatted phone number, as allowed by IS–41–C and TIA/EIA–41–D. The use of national numbers requires some complex digit translations.
  
• Transition to IMSI (International Mobile Station Identity). The use of IMSI will allow each country to assign identifiers to its mobile phones independently. The use of the IRM (International Roaming MIN) requires coordination of each block of one million mobile identifiers through the International Forum on ANSI-41 Standards Technology (IFAST) organization.
  
• Roaming with GSM. Several groups are working at improving the services that can be provided to subscribers who roam from a CDMA area, including the GSM Global Roaming Forum (GGRF) G-95 group. This involves interworking of signaling (ANSI-41 and GSM MAP's) as well as billing issues and many business and implementation issues.

How did ANSI-41 Evolve?

ANSI-41 has grown incrementally through a number of major revisions. Between each revision a number of application-specific interim standards (IS's) have been produced. The major revisions are:

• 1983 – AMPS analog cellular started commercial service as standalone systems in Chicago and Washington/Baltimore. It quickly spread throughout the US, into Canada and into other countries. This provided a single standard cellular protocol while Europe had a large number of incompatible standards, each available in only a handful of countries.
  
• 1988 – IS-41 Rev. 0 provided inter-system handoff and subscriber validation capabilities. These capabilities were not, in the grand scheme of things, all that important. What was important was that inter-system operations were a reality. They worked and worked well. This standard was published in 1988 and the first field trials were in 1989.
  
• 1991 – IS-41 Rev. A added true networking, through the use of SS7 protocols, and the all-important capabilities of location management (letting the HLR know where a mobile is), call delivery, subscriber validation and profile transfer. It was published in January, 1991.
  
• 1991 – IS-41 Rev. B was an incremental release over Revision A. The most important advance was to add forward/backward compatibility capabilities to ensure that a mixture of revision levels could co-exist. This was published in December, 1991 and is still in widespread use in systems that just did basic roaming capabilities - making and receiving calls.
  
• 1993 – The first CDMA digital standard (IS-95 Revision 0) was published. IS-41 was quickly adapted to provide support for CDMA systems. Although there were now two different digital systems in North America (CDMA and TDMA), nationwide coverage was ensured by dual-mode analog/digital phones, with seamless roaming provided by IS-41.
  
• 1996 – IS-41 Rev. C was a major advance over previous revisions, including the ability to incorporate ‘Intelligent Network’-like capabilities. This allows the development of services such as PBX-dialing extended worldwide. By querying the HLR, an MSC anywhere can translate an office extension into the real telephone number on a subscriber-by-subscriber basis. Itwas published in 1996. This version incorporated further support for CDMA digital systems.
  
• 1997 – TIA/EIA-41 Rev. D was the first version to be approved by ANSI. It included only incremental improvements over IS-41-C. It was published in 1997.
  
• 2002 – TIA/EIA-41 Rev. E is nearing completion. Several parts of this very large standard are already being balloted. It further extends the international capabilities of ANSI-41. It includes many enhancements, but notably incorporates IS-751 (IMSI) and IS-807 (global title recommendations), as well as the Wireless Intelligent Network (WIN), Calling Name presentation, data services, over-the-air programming and other capabilities that are currently available as separate IS documents
  
• 2002/3 – TIA/EIA-41 Rev. F is being planned. It is likely that this will have enhancements to support better interworking with GSM, packet data support and location-based services.

About the Author

David Crowe is the editor of Cellular Networking Perspectives and Wireless Security Perspectives, monthly technical bulletins available by subscription. He is a well known wireless standards consultant, and is a columnist for the Canadian Wireless Telecommunications Association's magazine Wireless Telecom.