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Just about everyone with a cellular or PCS phone can go roaming through the gloaming (or in the morning, afternoon or night as well) without even really being aware of how complex is the process they are initiating. Landline phone systems do not have to worry about the whereabouts of their phones. In fact, you can unplug a wired phone and the phone system will just think that the phone is busy. Wireless phones, by comparison, have to continually report their whereabouts, as this information is needed to route incoming calls to them.
Mobility management is an essential part of roaming, at least if roamers want to be able to receive calls, and not just originate them. This process is simplified by keeping knowledge about the location of a mobile in only one place – the HLR (Home Location Register). Any telephone system can find out where a mobile is by querying the HLR, making locating a mobile a two step process – first ask the HLR where the mobile is, and then route a call there. Since an HLR can be located based on the MIN, IMSI or directory number associated with a wireless phone, there is no difficulty in doing this.
Unfortunately, this process has a glaring inefficiency for roamers because calls are also routed through the home system. When a wireless subscriber is in a home system this has little impact, but when roaming elsewhere it can significantly increase the trunking required to set up a call. For example, if an American wireless subscriber has business in Argentina and an Argentinian colleague calls them, the call will be routed from Argentina to the home system in the United States and then back again. A call across the street has been turned into two international long-distance calls! This inefficiency also occurs within a country. Calling my cellphone from a pay phone in Miami results in a call from Miami to Calgary and then to wherever I am currently located – which could be across the street in Miami!
Is it possible to eliminate this inefficiency? Well, yes ... but only a qualified yes. Even if carriers started working on solutions today, it would be several years before implementations were widespread.
Why bother to eliminate this inefficiency if it is so difficult? Since roamers still receive calls, and the complex routing is transparent, what difference does it make? There are actually several reasons why consumers would benefit from optimizing routing to roamers. First, by reducing trunk utilization, it would reduce the cost of calls to roamers, and this cost is either directly charged to the mobile subscriber, or built into the cost of a single-rate plan. Secondly, calls may take considerably more time to set up with two unnecessary long-distance legs. And, thirdly, the more trunks involved in the call, the more likely a call is to be refused due to a lack of trunks. Fourthly, the quality of the call may suffer, particularly if not all facilities are digital.
From the viewpoint of a carrier, optimal routing could make them more competitive through reduced prices and could increase the quality of service that they could offer to their subscribers when roaming.
Assuming that I have convinced you that optimized routing to roamers is worthwhile, why is it so difficult? There are several components to this – numbering difficulties, wireless/landline integration issues and international protocol interworking issues.
If our caller in Argentina is using a pay phone, how would the local switch know that he is dialing a mobile phone? All the switch sees is an international dialing access code, a country code, and a bunch of digits that it does not interpret. Even the toll switches in Argentina are unlikely to look any further through the dialed digits than the country code. The Argentinian switches would have to maintain a complete database of U.S. area codes and office codes to be able to make this distinction, and with local number portability, even this would not work.
But, there is a potential solution. The SS7 ISUP standard has defined a new capability known as “Release to Pivot” (RTP) which is designed to allow a call to be set up to one destination, and then yanked back and set up to another destination. This is designed for services like enhanced directory assistance (411), where the call can be set up to the number provided from the directory (at an extra charge of course). Forwarding the call from the directory assistance center is possible, but would result in all their trunks being tied up, whereas RTP allows the directory assistance trunk to be re-used once the call is diverted.
In the optimal routing context, the Argentinian pay phone would initiate a call to the wireless phone’s home system, which would then divert the call to a phone number (known as a Temporary Local Directory Number or TLDN) provided by the visited system in Argentina. At this point, the home system could drop out of the call, which would be re-routed, by the local switch connected to the pay phone, to the appropriate Argentinian wireless switch.
Unfortunately, there is still a problem that stems from the lack of coordination between wireless and landline standards bodies. RTP was designed to support landline services, without consultation with wireless standards bodies. Consequently, it comes close to meeting the needs of wireless, but not close enough.
The difficulty is that there is a period of time during wireless call setup when it is not known where the call will end up. If, in our call scenario, the roamer in Argentina did not answer his phone, or if it was busy, the Argentinian landline switch would not know how to forward the call appropriately.
What is needed is the ability to maintain two call legs until either the roamer answers his phone or a call forwarding condition occurs. If the roamer answers, the leg to the home system can safely be dropped. If a call forwarding condition is detected, the leg to the visited system can be dropped, a new leg can be established to the call-forward number and, if further call forwarding is not possible, the leg to the home system can be dropped.
Telecommunications standards suffer not only from parochial divisions into landline versus wireless (with the critically important SS7 standard being artificially pigeonholed as ‘landline’), but telecom standards are generally defined on a national basis. So, even though a U.S. standards body (ATIS T1S1) has defined RTP for ANSI SS7 ISUP, other national SS7 protocols may not yet support it, or may support some variants of this.
If telecom carriers make the jump from SS7 to IP as the underlying transport protocol, they have an opportunity to gain one method of interconnecting telecommunications systems that works between countries and that works equally well between wireless and landline systems. This could enhance the level of services that consumers receive and increase the acceptability of wireless services. Trying to standardize and consistently implement a capability like RTP in multiple national SS7 variants would be much more difficult.
One of the biggest overlooked issues in telecommunications is the lack of wireless/landline integration. This is the reason why Calling Party Pays has never been widely deployed in the U.S., it is the reason why operator services are generally not available from wireless phones, and it is also the reason why, for a phone call to get across a Camino in Buenos Aires, it may have to travel to the United States and back again.
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