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The TIA/EIA-41 standard for intersystem operations (previously called IS-41) has become the backbone of the cellular network. The increasing array of services offered to roamers is due almost entirely to the growing installation of ever more capable versions of this standard. However, there is one capability that is conspicuous by its absence – Gateway MSCs. If implemented, this concept would reduce the costs of roaming and allow greater integration of wireless and wireline switching.
The TIA/EIA-41 network reference model identifies several elements that are needed to deliver services to roamers. The MSC, or Mobile Switching Center, does the call processing for all mobiles that are physically in its service area. The HLR, or Home Location Register, stores the identification, authentication, validation and profile information for home subscribers, allowing serving systems to ensure that the right platter of services is provided to each roamer (which may be no services, in the case of non-paying subscribers or attempted fraudulent access).
Call delivery uses both the HLR and MSC elements. Take an example of a call from Vancouver to a mobile homed in Los Angeles that is currently roaming in Miami. First the HLR in Los Angeles has to be queried to find out where the mobile is, and whether the mobile has any features or restrictions that would prevent or redirect the call delivery attempt. Following that, the HLR queries the serving MSC in Miami and, assuming that the mobile is not busy, switched off or otherwise unavailable, receives a TLDN – a Temporary Local Directory Number. The TLDN is one of the tricks of TIA/EIA-41, used to solve a problem of routing to a mobile through a network that relies on geographical information stored in the phone number (e.g. the area code). Routing to a roamer using their phone number would send the call to Los Angeles and not Miami. Since a TLDN belongs to the Serving MSC (Miami in this case), the PSTN will use the TLDN like a homing pigeon to direct the call to the right destination.
One of the confusing things about call delivery in the TIA/EIA-41 standard is that the network element that first queries the HLR is an “Originating MSC”. Why would an MSC be involved on the originating end of a land-to-mobile call? It doesn’t appear to make sense. The reason is that an HLR, by definition, does not have voice or switching facilities. When a call is directed from the wireline network to a mobile, it has to terminate somewhere that does have these capabilities, and that is usually the “Home MSC”, defined as the switch that the landline “PSTN” network will route to when given the mobile’s directory number. This MSC queries the HLR and then, later, after receiving the TLDN, forwards the call to the correct serving system. The Home MSC obviously uses its switching and trunking capabilities during the call, but does not need to allocate any of its radio facilities for this type of call. In the vast majority of cases, the “Originating MSC” is the same as the “Home MSC” – although it doesn’t have to be. In our scenario, both the HLR and the Home MSC (which is also the Originating MSC) would be located in Los Angeles. The call path would be from Vancouver to Los Angeles, where all the queries would take place, and then on to Miami. Obviously, this is not the most efficient route from Vancouver to Miami!
Let’s take a different scenario. Assume that the Vancouver wireline switch knows that it is terminating a call to a mobile. It could then take on the role of Originating MSC and issue the query (known as a LocationRequest) to the HLR itself. The advantage would be that the TLDN would also be returned to Vancouver, resulting in a single long distance call from Vancouver to Miami, bypassing the Los Angeles home system. One long distance call in this scenario replaces two in the standard scenario.
This originating MSC does not need to have any radio facilities. It just has to be a standard switching platform with the ability to initiate a TIA/EIA-41 query. Virtually any inter-exchange carrier switch, and many local exchange carrier switches could do this, at the price of a modest software upgrade. Because this switch straddles the boundary between wireline and wireless, it is called a “Gateway MSC”.
If it was easy, everybody would already be doing this. The first problem with the use of a Gateway MSC is how it would know that the dialed number is wireless. Unlike many other parts of the world, the North American numbering plan does not allocate separate area codes for different types of phones. Consequently, a large table would be required, with an entry for every block of numbers allocated to wireless phones. Number portability appears to make this job impossible, because numbers can be ported from a wireline to a wireless carrier, so that phone numbers from one block may belong to a mix of wireless and wireline phones. However, number portability will also provide queryable databases that allow a correlation to be made between a directory number and the currently associated carrier. This database could also contain information on whether the carrier supports TIA/EIA-41 queries or not (although such a capability does not currently exist). The only problem with this is that the financial model defined by the FCC encourages queries at the terminating end of the call (i.e. Los Angeles, in our example) and not at the beginning. By the time the database is queried, it is often too late to implement optimal routing. However, if business arrangements can be made, to ensure compensation of the originating carrier for the query, the Number Portability database could become a viable means for distinguishing wireless numbers from others.
Another problem is how to bill the two parties to the call. In our first scenario, the landline caller would be billed for a call from Vancouver to Los Angeles, and the roamer would be billed for the call from Los Angeles to Miami. One solution is to continue to bill this type of call in the same way, although consumer groups and regulatory agencies might not be happy (and consider the scenario where our Los Angeles roamer visits Vancouver and two long distance calls are converted into a single local call!). Another alternative would be to bill the caller who dialed a Los Angeles number for a call to Miami, but this might be impossible to implement (apart from being unfair to the caller), especially from payphones. Another alternative is to split the difference somehow, perhaps charging the caller for the call to Los Angeles, and the mobile for the difference, if any.
The Gateway MSC concept has a lot of potential for improving the efficiency of calls being delivered to roaming mobile phones. If anyone can overcome the two hurdles of reliably identifying when mobile numbers are dialed and can provide a fair and practical billing arrangement, the cost of roaming will be reduced, creating leeway for price reductions. Given that the cost of roaming is a significant barrier to many wireless phone users, this could increase the number of people who turn their phones on to receive calls while roaming and be of benefit to carriers and phone users alike.
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