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It used to be that if you knew what IS-41 was, you were, by definition, a wireless telecom nerd. But, on the 10th anniversary of the publication of the first version of this standard, you have to know the difference between IS-41 and ANSI-41 to qualify.
IS-41 provides inter-system operations between 800 MHz cellular systems that may be manufactured by different companies. It can also interconnect 1800 MHz PCS systems that are based on TIA cellular standards – CDMA (IS-95), TDMA (IS-136) or even analog (IS-91) systems, if they are ever adapted to the PCS band. IS-41 does not, in fact, really care about the frequency of the wireless calls that it supports – only minor adaptations are required to support a new frequency band.
There are several good reasons to interconnect wireless systems, to provide services and capabilities that seemed like luxuries during the dawn of cellular service in 1984, but are now essential elements of any wireless network – intersystem handoff, call delivery, feature portability, validation and authentication.
Intersystem handoff was the only feature supported in the first version of IS-41 (Revision 0). It allows a wireless call connected to one MSC (Mobile Switching Center) to be extended to a neighboring MSC, as long as a data connection and sufficient trunks are provisioned between them. The connection to the landline phone network is maintained at the original MSC in the call, known as the “Anchor MSC”. A variant, known as Handoff Back, allows the inter-MSC connection to be disconnected mid-call if the mobile hands back to a cell in the first MSC.
Fully automatic call delivery was first provided in IS-41 Revision A (TIA standards are numbered 0, A, B, C ... for some unknown reason). This capability is more complex than handoff, involving several phases and a fully interconnected network rather than just point-to-point connections. First, the wireless system has to continually track the mobile. Every time it enters a new system, a report has to be sent back to the home system (known as a registration). When a call is attempted to the mobile, another IS-41 message must be sent to the MSC that last reported the mobile’s presence, to obtain a special routing number (known as a TLDN – Temporary Local Directory Number). The home system can send this TLDN into the telephone network knowing that the call will be routed to the correct MSC. Once this call arrives, the TLDN can be set aside for a future call delivery attempt by another mobile.
Feature portability piggy-backs on the registration that is already required to track a mobile to enable call delivery. In response to the registration, the home system (known as an HLR – Home Location Register) sends a profile of the mobile’s features to the system that is hosting the roamer. This allows call waiting, three way calling and call forwarding to work seamlessly for roamers. Another little known aspect of feature portability allows features, such as call forwarding, to be enabled and disabled while roaming.
Validation and Authentication are essential and powerful tools that IS-41 provides for managing fraud. Validation ensures that the MIN and ESN transmitted by a mobile match the information stored in the HLR. Because this process occurs in real-time, it can be done during call setup, and before fraudulent mobiles obtain any service. The required exchange of information is also part of the registration process. Verifying that a MIN and ESN match might not seem like a very powerful capability – but there was a day, not long ago, when validation occurred after the first call was completed – guaranteeing that fraudulent mobiles could get at least one call with every fresh MIN/ESN pair. The fraudulent technique of “tumbling” was a method of getting around post-call validation, which has been virtually eliminated by IS-41 validation.
IS-41 validation leaves only one possibility for radio interface fraud, and that is cloning – a tough nut to crack indeed. Cloners provide a legitimate MIN/ESN pair to the system, and cannot be detected by IS-41 validation, although the IS-41 registration information can be a powerful information source for detecting cloning fraud based on heuristic call pattern monitoring techniques. Authentication was designed to clamp down on cloning, using sophisticated encryption technology. It is limited at present because many switches do not yet support this technology and, more importantly, most mobiles cannot support it. However, as non-authenticating mobiles gradually are outnumbered by newer authenticating mobiles (both analog and digital), authentication will draw an ever tightening circle around cloners.
Authentication is just one of several new capabilities that are supported by the newest, and biggest, version of IS-41 – Revision C. It is just now being implemented by major infrastructure vendors. It has so many new capabilities that only a few are being implemented by each vendor. Some of the apparent favorites are Short Message Service, Calling Number Display and Dialed Digit Triggers. The “Trigger” capability allows many Wireless Intelligent Network features to be implemented, such as the extension of a private PBX 3, 4 or 5-digit dialing plan to wireless systems and the ability to connect calls to voice recognition devices. Other capabilities in this revision are either invisible to users of wireless phones (such as improved border cell and digital mobile handling capabilities) or are so exotic that it may be years before they are implemented.
Now I would like to qualify you for true standards-nerd status. If IS-41 was the only standard you had ever heard of, you might conclude that the acronym “IS” stood for “Inter-System”. But it doesn’t stand for that, nor for “Industry Standard,” another common misconception. “IS” actually stands for Interim Standard, which explains why it is now called ANSI-41 (actually ANSI/TIA/EIA-41). An interim standard, by Telecommunications Industry Association rules, is supposed to live for no more than five years, but IS-41 managed to claim enough extensions to that rule, so that it was 10 years old before it became a full ANSI standard. Becoming ANSI is not necessarily a dramatic change for a standard, but it is a sign of maturity and industry acceptance. The required change in name has, however, introduced more potential for confusion. ANSI-41 Revision 0 is actually equivalent to IS-41 Revision C, and is dramatically more complex than I S-41 Revision 0. Normally an ANSI standard does not have the same number as the corresponding TIA interim standard, but in this case, the number 41 was available. This explains another mystery – the rise and fall of the name: ANSI/TIA/EIA-689. This was the original ANSI number given to IS-41, which was quickly abandoned once it was realized that ANSI/TIA/EIA-41 was available.
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